A systematic review of metakaolin-based alkali-activated and geopolymer concrete: A step toward green concrete

Diyar N. Qader; Ary Shehab Jamil; Alireza Bahrami; Mujahid Ali; Krishna Prakash Arunachalam

doi:10.1515/rams-2024-0076

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A systematic review of metakaolin-based alkali-activated and geopolymer concrete: A step toward green concrete

Diyar N. Qader , Ary Shehab Jamil , Alireza Bahrami , Mujahid Ali und Krishna Prakash Arunachalam

Veröffentlicht/Copyright: 17. Februar 2025

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Abstract

Expanding the world’s infrastructure drives up demand for building materials, particularly ordinary Portland cement (OPC) concrete, whose high carbon dioxide (CO₂) emissions have a detrimental effect on the environment. To address this issue, researchers looked into employing alternative supplementary cementitious materials (SCMs), including metakaolin (MK), which is derived from calcined kaolin clay with pozzolanic properties, to partially or completely replace OPC in concrete. This review article examines the MK’s application in alkali-activated materials (AAMs) and OPC-based concrete. By interacting with calcium hydroxide, MK functions as a pozzolanic additive for OPC concrete, enhancing its mechanical qualities and durability. The use of MK as a source material in AAMs, a newly developed class of sustainable binders, is also covered in this article. The effects of different combinations of MK with additional SCMs, including fly ash (FA), ground granulated blast furnace slag (GGBFS), silica fume, and rice husk ash, on the characteristics of alkali-activated concrete both in its fresh and hardened states, are compiled. The majority of the articles considered in this study are from the past decade, while some relevant articles from 2014 and earlier are also taken into account. The results showed that adding MK to concrete in combination with FA or GGBFS has excellent synergistic effects on microstructural development, pozzolanic activity, and strength increases. In particular, the MK–FA mix demonstrated the most encouraging performance gains. Because of its large surface area, the use of nano-MK helped achieve a denser geopolymer structure and improve mechanical properties. The best curing temperatures for MK-based geopolymers to gain strength were found to be between 40 and 80°C for a total of 28 days. The review also pointed out that the compressive strength and geopolymerization process of MK-based geopolymers were enhanced by increasing the mass ratio of Na₂SiO₃ to NaOH and NaOH concentration. Nevertheless, geopolymerization was hampered by unnecessarily high alkali concentrations. Moreover, the compressive strength was increased by partially replacing MK with TiO₂ or GGBFS. The synergistic effects of combining MK with other SCMs to improve concrete performance highlight the potential of MK-based solutions in lowering the environmental footprint of concrete buildings.

Keywords: concrete; metakaolin; alkali-activated material; supplementary cementitious material; sustainability; environmental impact; pozzolanic activity

1 Introduction

The demand for building materials is increasing owing to global infrastructure development. The concrete industry generates around 12 billion tons of concrete yearly [1,2]. The primary elements of construction and building materials are natural coarse and fine aggregates and ordinary Portland cement (OPC). According to the study by Reddy et al. [3], primary aggregate consumption in the United Kingdom climbed from 110 million tons in 1960 to over 275 million tons in 2006. Comparably, the United States generates 2 billion tons of aggregate a year, and by 2024, that amount was predicted to reach nearly 3 billion tons [3]. However, because of the quick urbanization and industrialization, OPC is used in abundant forms, causing fast depletion of natural resources and producing a carbon footprint [1]. OPC is one of the most widely used mineral binders; however, its manufacture produces significant CO₂ emissions. Indeed, around 0.94 tons of CO₂ are released into the environment while manufacturing one ton of cement [4].

As reported by Scrivener and Kirkpatrick [5], the CO₂ released from the cement industry affect the natural environment and are responsible for 5–8% of global CO₂ releases that cause climate change. Moreover, OPC is the cause of excessive energy use and CO₂ emissions in developing nations. In accordance with Usón et al. [6] and Summerbell et al. [7], around 5% of anthropogenic CO₂ emissions and 13–16% of all industrial energy use worldwide are attributed to the conventional cement industry. Not only does cement manufacturing emit CO₂ but it also emits sulfur trioxide (SO₃) and nitrogen oxides, which can contribute to the greenhouse impact and acid rain [8].

Thus, the researchers seek alternative supplementary cementitious materials (SCMs) to partially [9] or completely [10] replace OPC. These alternatives, usually derived from different waste/by-products and other naturally abundant available materials, aim to make concrete more sustainable and environmentally friendly [11] lessening the environmental impact of OPC while continuing building progress [12]. This approach is usually known as green concrete. As reported by Suhendro [13], green concrete is generally defined as concrete incorporating waste materials as a replacement for natural resources to reduce the environmental effects and promote sustainable construction materials. Besides, Asghar et al. [12] conducted a systematic review on geopolymer concrete to promote green construction where they claimed eco-friendly materials, i.e., geopolymer concrete that not only reduces CO₂ releases but is also a suitable solution to natural resource depletion. Due to their lower carbon footprint and superior properties, alkali-activated materials (AAMs) are inorganic cementitious materials that could replace conventional OPCs in the coming decades [14].

Palm oil fuel ash (POFA), coal bottom ash (CBA), fly ash (FA) [15], metakaolin (MK) [16], silica fume (SF), rice husk ash (RHA), and ground granulated blast furnace slag (GGBFS) [17] are some of the most often utilized SCMs [18]. One of the most important methods to lessen the adverse environmental impacts of the cement industry is to use MK and other SCMs to replace some of the cement or as a source of entirely new cementless materials [19]. The hydration of cement and MK interact chemically to alter the paste microstructure [20]. MK not only helps the environment but it also enhances the durability and mechanical characteristics of concrete. MK pozzolan is an aluminosilicate material that produces cementitious compounds when chemically combined with calcium hydroxide (CH) when water is present [21]. Similarly, alkali-activated metakaolin (AAMK) known as future cement has lately been employed to lessen the environmental effect of cement manufacturers and enhance overall strength properties [22]. The environmental effects of cement manufacturing are depicted in Figure 1.

Figure 1

Contribution of cement production to CO₂ emissions [23].

Several studies have been carried out to explore the use of other SCMs as a cement replacement in the concrete mix. MK is a pozzolanic cementitious substance used as a SCM [24]. It has been observed that conventional OPC, tricalcium silicate (C₃S), and tricalcium aluminate (C₃A) can activate it [25]. The most reactive compound in OPC is C₃A, and its hydration process significantly influences the initial workability and strength of concrete. Isothermal conduction calorimetry was used to evaluate the early hydration phase of pastes containing MK [26]. Differential thermal analysis [27], Fourier transform infrared spectroscopy [28], and X-ray diffraction [29] were utilized to determine reaction products and track CH consumption [30]. Porosity and compressive strength were also assessed. The results show that strätlingite (C₂ASH₈) and calcium-hydrate-silicate (CSH) are abundant in the microstructure, that CH is rapidly consumed, and that the pore size distribution is altered toward smaller values [31].

MK is a highly reactive cementitious additive that can be used in mineral binders, mortars, and concrete. It is a result of the calcination of kaolin clay and is classified as a pozzolanic mineral additive. Because MK-architecture concrete finishing plasters have such a unique visual appearance, high-strength concrete, self-compacting, and self-leveling concrete are the most appropriate applications of MK in cement-based composites [32]. Dadsetan and Bai [33] investigated concrete’s interfacial transition zone (ITZ), which can be improved by adding MK, which positively affects the concrete’s strength and increases durability in an aggressive environment. Ferreira et al. [34] conducted an investigation on MK. They demonstrated that it improves concrete’s resistance to chloride penetration, while Saboo et al. [35] examined that the inclusion of MK increases matrix density while reducing porosity. They concluded that increasing the MK content at 2% in concrete caused lowered porosity by 10%. The ingredients, compositions, ideal calcination, admixtures, curing conditions, and compressive strengths of the AAMs that the researchers have previously studied are listed in Table 1.

Table 1

Raw materials, compositions, optimal calcination, admixtures, curing conditions, and compressive strengths of AAMs that researchers have previously investigated

Solid activators	Alumino silicate precursor	Aggregates	Admixtures and fibers	Calcination	Optimum molar ratios						Curing		L/S	28-day UCS* (MPa)	14-day UCS (MPa)	7-day UCS (MPa)	Ref.
Solid activators	Alumino silicate precursor	Aggregates	Admixtures and fibers	Calcination	H₂O/Al₂O₃	(Ca + Mg)O/SiO₂	Na₂O/Al₂O₃	Na₂O/SiO₂	SiO₂/Al₂O₃	H₂O/Na₂O	T (°C)	RH (%)	L/S	28-day UCS* (MPa)	14-day UCS (MPa)	7-day UCS (MPa)	Ref.
NaOH or Na₂CO₃	Albite	—	—	1,000°C/0.5 h	N.A	N.A	N.A	N.A	N.A	N.A	25	100	0.3	44.2	38.5	32.3	[36]
Dolomite, Na₂CO₃	Bentonite	—	—	1,100°C/3 h	N.A	2.1	1.84	0.4	4.6	N.A	80 + 20	100	0.35	38.3	N.A	N.A	[37]
NaOH or Na₂CO₃	Bentonite	—	—	850°C/3 h	N.A	N.A	N.A	N.A	N.A	N.A	80 + 20	100	0.3	≈25	N.A	≈30	[38]
NaOH, Na₂CO₃	BFS	—	—	—	N.A	N.A	N.A	N.A	N.A	N.A	37	100	0.27	N.A	N.A	N.A	[39]
CaO or Ca(OH)₂	BFS	—	—	—	N.A	N.A	N.A	N.A	N.A	N.A	25	99	0.4	42	34	31	[40]
(Na₂SiO₂)nO	BFS	Expanded clay granule sand	Polycarboxylic-based water-reducing admixture	—	N.A	N.A	N.A	N.A	N.A	N.A	N.A	N.A	0.4	53.8	N.A	49.3	[41]
(Na₂SiO₂)nO, NaOH	BFS or FA	Sand	—	—	N.A	N.A	N.A	1.2–1.8	N.A	N.A	23	70	0.5	49.6	N.A	38.5	[42]
(Na₂SiO₂)nO	BFS or FA	Sand	—	—	24.94	1.29	1.31	0.26	5.08	19.01	23	70	0.5	51.28	N.A	47.08	[43]
(Na₂SiO₂)nO	BFS or FA	Sand	—	—	N.A	N.A	N.A	N.A	N.A	N.A	23	70	0.3	71.6	N.A	64.5	[44]
Na₂SiO₃	BFS, FA	—	PVA fibers	—	N.A	N.A	N.A	N.A	N.A	N.A	23	N.A	0.35	52.5	N.A	N.A	[45]
Na₂SiO₃	BFS, FA	—	PE or PVA fibers	—	N.A	N.A	N.A	N.A	N.A	N.A	23	N.A	0.35	48.7	N.A	N.A	[46]
(Na₂SiO₂)nO	BFS, FA	—	Hydrophosphate (retarder)	—	N.A	N.A	N.A	N.A	N.A	N.A	N.A	N.A	0.28	80.13	N.A	67.38	[47]
Na₂SiO₃, NaOH, Ca(OH)₂	BFS, FA	—	PCE	—	N.A	N.A	N.A	N.A	N.A	N.A	23	N.A	0.3	36.9	N.A	33.9	[48]
Na₂CO₃, Slaked lime	BFS, FA	Dolomite sand and stone	Sodium lignosulfonate	—	N.A	N.A	N.A	N.A	N.A	N.A	25	>90	0.35	≈50	N.A	N.A	[49]
Maize cob ash	Fayalite slag	Std. Quartz sand	—	—	N.A	N.A	N.A	N.A	N.A	N.A	20	100	0.28	16.4	N.A	N.A	[50]
(Na₂SiO₂)nO, NaOH	FA	Std. Quartz sand	—	—	18	N.A	1.5	0.8	1.8	12	40	N.A	0.25	N.A	≈57	≈42	[51]
CaO, NaOH, MgO	FA	Natural sand and granite	—	—	N.A	N.A	N.A	N.A	N.A	N.A	23	100	0.4	≈35	≈25	≈17	[52]
Ca(OH)₂, Na₂SiO₃, NaAlO₂, Al₂O₃	FA	—	PVA fibers	—	N.A	N.A	N.A	N.A	N.A	N.A	23	N.A	0.2	N.A	N.A	N.A	[53]
Na₂SiO₃	FA	—	PVA fibers	—	N.A	N.A	N.A	N.A	N.A	N.A	23	N.A	0.35	48.7	N.A	N.A	[54]
Na₂SiO₃	FA	—	—	—	N.A	N.A	N.A	N.A	N.A	N.A	20	100	0.4	11.29	6.04	2.35	[55]
Red mud or NaOH	FA (loss on ignition > 6%)	—	—	—	N.A	N.A	N.A	N.A	N.A	N.A	23 + 60	N.A	0.34–0.56	≈1.6	≈1.6	≈1.6	[56]
NaAlO₂	Geothermal silica	—	—	—	7–12	N.A	1.00	3.00–4.99	0.75–1.2	7–12	40	100	0.214–0.386	N.A	N.A	N.A	[57]
NaAlO₂	Geothermal silica	Sand	Al₂O₃, ZnO, or ZrO₂	—	N.A	N.A	N.A	N.A	0.75	7	40	100	N.A	N.A	≈26	≈22	[58]
Maize cob ash	Kaolin	Std. quartz	—	700°C/1 h	N.A	N.A	1.28	0.76	3.07	N.A	20 + 80	60	0.3	N.A	N.A	40	[59]
NaOH or Na₂CO₃	MK	—	—	950°C/3 h	N.A	N.A	N.A	N.A	N.A	N.A	80 + 25	> 90	0.3	N.A	N.A	N.A	[60]
NaOH, KOH	MK	—	—	550°C/4 h	N.A	N.A	N.A	N.A	N.A	N.A	≈23	N.A	N.A	N.A	N.A	≈1	[58]
Maize cob ash sand	MK	Std. quartz	—	700°C/1 h	N.A	N.A	1.28	0.76	3.07	N.A	20 + 80	60	0.3	N.A	N.A	40	[61]
NaOH, (Na₂SiO₂)nO	MK	Std. quartz sand	H₂O₂, surfactant	—	28	N.A	2	0.88	1.75	14	60 + 20	100	0.75	N.A	≈65	≈45	[62]
Ca(OH)₂, NaOH	OPC, FA, metakaolin	Sand	Lignosulfonate	6,500°C/140 min	N.A	N.A	N.A	N.A	N.A	N.A	N.A	N.A	0.35	15–27	12–25	N.A	[63]
NaOH	Red mud	—	—	800°C/1 h	N.A	N.A	N.A	N.A	N.A	N.A	20	100	0.5	≈1.8	≈2.2	≈2.5	[64]
NaOH	Red mud	—	—	800°C/1 h	N.A	N.A	N.A	N.A	N.A	N.A	20	95	0.6	≈2	N.A	10	[65]
NaOH	Red mud, SF	—	Lignosulfonate	800°C/1 h	N.A	N.A	N.A	0.42	3.45	12.96	20	100	0.45	31.5	N.A	≈13	[66]
NaAlO₂	RHA	Std. quartz sand	—	—	17.78	N.A	1.27	0.98	1.25	14	40	100	0.308	N.A	≈18	≈8	[67]
NaAlO₂	RHA	—	—	—	11.81	N.A	1	0.29	3.48	11.85	80	80	0.5	N.A	N.A	30.1	[68]
NaAlO₂	Silica residue	—	—	—	N.A	N.A	1.2	2.92	3.5	11.3	70 + 23	100	0.6	N.A	N.A	≈7	[69]
NaAlO₂	Silica residue	—	—	—	14.36	N.A	0.98	0.18	5.35	14.64	80	80	0.5	N.A	N.A	N.A	[70]
NaAlO₂	Silica residue	—	—	—	15.51	N.A	0.98	0.16	6.02	15.82	80 + 23	80 + 50	0.34	N.A	N.A	N.A	[71]

*Note: UCS is uniaxial compressive strength.

1.1 Review objectives

The objectives of this review article can be summarized as follows:

To examine earlier studies on the use of MK and SCMs in alkali-activated and geopolymer concrete and summarize results, gaps, and findings.
To provide a summary of recent research on the environmental effects of cement production, SCMs, MK, and their use in green and sustainable practices by using waste materials to substitute cement.
To evaluate how particular waste materials influence the strength and decrease of CO₂ emissions in concrete.
To offer recommendations for improving concrete sustainability and performance, even more, using waste material-based SCMs and MK.

1.2 Review methodology

The following is the process that has been employed to identify, filter, and review research articles:

Literature search: Academic databases including PubMed, Scopus, and Web of Science were used to conduct a comprehensive literature search.

Inclusion criteria: The study’s inclusion criteria were research articles that directly addressed the use of MK and SCMs in concrete and were published in peer-reviewed journals between 2010 and 2023. Research on durability, strength characteristics, and environmental effects received more attention.

Exclusion criteria: Research articles that did not relate to concrete materials, lacked empirical data, or did not meet the predetermined criteria were excluded.

Screening and selection: To determine the relevance of the articles collected, the titles and abstracts were assessed as part of the screening and selection process. Articles that satisfied the requirements underwent another evaluation to see if they qualified.

Data extraction: From the selected articles, pertinent information about the objectives, methods, results, and conclusions of the studies was taken out.

Analysis and synthesis: To find trends, benefits, limitations, and unexplored study topics, the data from the selected articles were analyzed. The results were combined to provide an understanding of how MK and SCMs are used in the concrete manufacturing process.

Presentation: To guarantee transparency and consistency, the publication includes the review process’s methodology together with its conclusions and implications.

Moreover, during the search engine, several keywords are used for the natural resources, by-products, and waste materials in concrete. The use of keywords is depicted in Table 2. In addition, those studies that promote green and sustainable concrete materials by incorporating waste materials to partially replace OPC to enhance the strength characteristics and reduce the environmental effects of the concrete industry are included in the current review.

Table 2

Keywords for search engine

Items	Keywords
Natural resources	Portland cement, fine aggregate, and coarse aggregate
Waste materials	FA, POFA, CBA, MK, SF, RHA, and GGBFS
Strength properties	Compressive strength, splitting tensile strength, flexural strength, workability, and hydration

2 Properties of MK

Several factors, including the manufacturing process, purity, and source material, can affect the characteristics of MK. Nonetheless, due to its tiny particle size, high pozzolanic activity, and amorphous nature, it works well as a mineral addition to enhance concrete’s performance. The characteristics of MK and how they vary are presented in Table 3. Furthermore, Figure 2 displays the geographical distribution of researchers who investigated MK.

Table 3

Properties of MK with their variation [72,73,74,75,76,77]

Property	Variation
Composition	Primarily consists of SiO₂ and alumina Al₂O₃; minor amounts of other oxides like Fe₂O₃, CaO, MgO, and K₂O may be present
Particle size	Typically ranges from 0.5 to 5 μm in diameter
Surface area	High specific surface area, typically ranging from 10 to 25 m²·g⁻¹
Pozzolanic activity	Highly pozzolanic, reacting with Ca(OH)₂ from cement hydration to form additional C–S–H
Color	Generally off-white or light gray
Density	Slightly lower density than cement, typically around 2.5 g·cm⁻³
Reactivity	Highly reactive due to its amorphous nature and high surface area
Water demand	Increased water demand due to its high surface area and pozzolanic reaction

Figure 2

Geographical distribution of researchers who studied MK.

2.1 Fineness

The fineness of the materials utilized as binders to produce alkali-activated concrete (AAC) is vital on which the physical and mechanical characteristics are based. The fineness of MK is crucial in determining the fresh and required properties of AAC. The chemical compound MK is produced when kaolin clay is calcined at temperatures ranging from 500 to 900°C. This substance is made up of particles that have an angular shape and a porous structure. It is composed of amorphous aluminosilicate and highly reactive natural pozzolan. There is a possible range of 1–20 μm for the average particle size. In addition, the specific gravity of MK was somewhere in the range of 2.20–2.60 [78]. Even though SiO₂ and Al₂O₃ make up the majority of MK, the chemical composition of MK can vary greatly depending on its source. Potassium oxide, calcium oxide, magnesium oxide, and ferric oxide comprise the remainder of the ingredients [79]. The following is a list of some of the chemical requirements that must be met for MK to be classified as Class N (raw or calcined natural pozzolan) following the definition provided by ASTM C618, and it is necessary that SiO₂ + Al₂O₃ + Fe₂O₃ ≥70%, SO₃ <4%, moisture content <3%, and loss on ignition of >10% [80]. The process of grinding kaolin and firing significantly impacts determining the shape and size of the granules. As shown in Figures 3 and 4, the scanning electron microscopic (SEM) images display the shape of the particles in the kaolin stage and after thermal treatment to become MK.

Figure 3

SEM of kaolin [81].

Figure 4

SEM of MK [82].

Several investigations have evaluated the effect of MK nanoparticles on the characteristics of geopolymer concrete. The incorporation of 4–6% nano-MK aided in attaining a denser geopolymer structure and enhanced mechanical characteristics. This is due to the homogenous dispersion of nano-MK and the creation of the geopolymerization reaction’s core material. In addition to increasing the microstructure’s impermeability, transport characteristics and durability are enhanced [83,84]. Figure 5 depicts the microstructure of nano-MK using the SEM image.

Figure 5

SEM of nano-MK [85].

2.2 Workability

The workability of geopolymer concrete is an essential property that influences all other characteristics. The control of this property influences not only the setting time and flow but also the early mechanical characteristics of geopolymer concrete, as well as the properties of fluid and gas transport through geopolymer concrete [86]. The type and quantity of binders utilized in the production of geopolymer concrete and any additives used to improve geopolymer concrete’s properties all impact geopolymer concrete’s fresh properties [87]. Therefore, incorporating MK into the preparation of geopolymer concrete has a discernible effect on the material’s workability [88]. MK’s chemical and physical properties are the foundation for understanding how its inclusion affects the workability of geopolymer concrete. One reason for the faster initial and final setting times of geopolymer pastes is the chemical composition of MK [89]. Both Al₂O₃ and SiO₂, present in high concentrations in MK, are the substances that speed up the reaction and give geopolymer mixtures an early initial hardening. The early rapid hardening hurts geopolymer paste’s workability and flow characteristics [90]. In accordance with reviews, the content of Al₂O₃ and SiO₂ found in MK ranges from 17.8–46.9% and 50.6–74.3%, respectively [88]. Accordingly, increasing the SiO₂ and Al₂O₃ contents in high calcium FA-based geopolymer mixtures accelerates the setting time. Thus, the time spent setting geopolymer based on MK content will be reduced [91]. The relation between the flow of fresh geopolymer paste and the Si/Al ratios is illustrated in Figure 6. An increase in the Si/Al ratio leads to an increase in the flow diameter of geopolymer paste for several mixtures consisting of different binders-to-solution ratios. On the other hand, MK has a distinct particle shape closer to the plates and a tiny average particle size, as demonstrated in Figure 6. Workability is adversely affected as a result of these physical characteristics. Nuaklong et al. [82] showed that the slump flow of geopolymer concrete mixture decreased with the increase of the MK content because of the high fineness and angular shape of the MK particles; these two characteristics are responsible for the significant decrease in workability. The results indicate, for example, that using MK as a partial replacement for FA-based geopolymer concrete containing limestone (B) and recycled concrete (C) at rates of 0, 10, 20, and 30% resulted in lower slump flow in the 609, 546, and 473 mm mixtures, respectively, as demonstrated in Figure 7.

Figure 6

(a) Impact of Si/Al ratio on flow paste of geopolymer concrete [91]. (b) Mini-slump flow of geopolymer concrete.

Figure 7

Influence of MK content on workability of geopolymer concrete [82].

2.3 Curing condition

Curing methods and conditions have a direct and significant effect on the development of the properties of geopolymer concrete [92]. These properties include the concrete’s strength and durability. In addition, the circumstances of the mixing stage, as well as the posttreatment, have a significant impact on the early development of the microstructure as well as the subsequent durability, strength, and transport properties of the material [92]. Generally, when low calcium geopolymers are prepared and cured in typical laboratory conditions, they take longer to reach initial hardening and gain early strength. In contrast, geopolymer blends, which can be produced at high temperatures and then cured at those temperatures, have properties that lead to faster hardening and increased strength [93]. Therefore, geopolymers based on low-calcium binders are usually heat-treated to accelerate the development of strength properties. Almost maximum strength can be reached quickly depending on the temperature and duration of curing and the chemical composition of the binders in geopolymer [94].

Several studies investigated the implications of early heat treatment methods under different conditions. The conditions applied to geopolymer concrete varied in terms of heat curing time, temperatures applied, start time of heat curing, etc. According to Nasvi et al. [95], using a high curing temperature had a favorable effect on the compressive strength of geopolymer concrete for 24 h. The compressive strength reached its maximum value when the curing temperature was raised to 80°C, and the compressive strength had a linear increase pattern in the curing temperature range that ranged from 20 to 80°C. Many researchers concluded that the temperature range of 40–85°C was ideal for curing to get the highest possible strength [96,97,98]. Using different types of activators and varying degrees of alkalinity in the tested steroids could account for these conflicting findings [95]. It is well known that the temperature at which aluminosilicates are cured plays a significant part in activating these compounds [99]. The polymerization is influenced by the diffusion of hydroxide and silicate ions inside the geopolymer gel, primarily regulated by the temperature at which the gel is cured. The slower geopolymerization that occurs at lower temperatures might result in delay and reduced compressive strength [96,100].

The impact of curing temperatures on the compressive strength of geopolymer concrete is shown in Figure 8. When the processing temperature is higher upto 100°C, the compressive strength decreased, as shown in Figure 9. The reason for this is likely to be the shrinkage resulting from the loss of moisture at 100°C, which leads to a weakening of the microstructure and the formation of micro and large cracks [101]. Therefore, the polymerization process’s continuation requires moisture to build a concrete structure that achieves good mechanical properties [95,102]. Moreover, accelerating the development of geopolymer concrete strength may be accomplished by curing MK-based geopolymer for 28 days at temperatures ranging from 40 to 80°C [98,100]. Another study assessed the relationship between cure time and temperature and the compressive strength of AAMK-based geopolymers. Temperatures ranging from 20 to 100°C were used, with treatment periods ranging from 24 to 168 h. After 24 h of heat treatment, all geopolymer concrete samples made using MK putty consolidated to form solid structures. The solid samples attained compressive strength within 11 MPa after 24 h of treatment at 20°C. When the curing duration was raised to 168 h, the compressive strength of the samples rose to 37 MPa, confirming that the polymerization rate of MK-based geopolymer mortar was extremely sluggish at relatively low temperatures and required a more extended period for solidification. In addition, upon curing at 60°C for 168 h, the AAMK-based geopolymer specimens attained compressive strength findings of up to 52 MPa [103].

Figure 8

Effect of curing temperature on compressive strength of geopolymer concrete [101].

Figure 9

Effect of curing time on compressive strength of geopolymer concrete [103].

The heat curing period of geopolymer concrete is crucial for accelerating the polymerization process and gaining strength early. The results indicated by Singh and Middendorf [104] suggest that the influence of the heat treatment period is significant for achieving better strength in concrete prepared from alkali-activated binders. Samples exposed to extended heat treatment periods exhibited higher strength attributes. Also, the increased curing time of geopolymer concrete, which exceeds 24 h, does not significantly impact its properties. Increasing the curing time will enhance the polymerization process, significantly improving the yield of N–A–S–H gel and hydroxy sodalite. This is because the polymerization reaction requires a certain amount of time to complete [103]. Furthermore, increasing the curing temperature of geopolymer concrete based on FA/MK to 65°C leads to an increase in strength after a period ranging from 1 to 3 days. In addition, curing geopolymer concrete at temperatures higher than 80°C for a period more extended than 24 h may result in the loss of strength because of thermal dehydration [98,100].

2.4 Hydration (geopolymerization)

Previous studies that have contributed to our understanding of the geopolymerization process of MK have made it easier to understand. An alkaline activator is used in the first step of this process, which separates alumina and silica from the precursor [105]. The alkaline attack on the structure of MK resulted in the release of silicates and aluminates into the solution. Since the bonds between Al and O are less stable than those between Si and O, Al will first enter the solution as Al(OH)₄ complexes. Because of this, the Si-tetrahedra that have been split will now be more susceptible to OH− attack, and consequently, there will be fewer and fewer units that contain SiO groups [106]. This could be explained by taking into account that the aluminum layers of MK have a larger lattice stress than the silicon layers. Hence, the creation of aluminosilicate oligomers occurs due to interactions between dissolved microspecies and includes any silicates initially delivered by the activating solution [107]. The dissolution process will continue until the concentration of dissolved aluminates reaches a high enough level to destabilize the silicate solution. At this time, the precipitation of the dissolved species will begin to form a gel. The reaction processes continue for some time since they depend on geopolymer mixture’s composition and the curing conditions. Depending on these factors, the first setting and hardening time may range from almost immediate to several days [108]. Because the alkaline solution causes the aluminosilicate species to dissolve, a three-dimensional network of SiO₄ and AlO₄ tetrahedra related to one another by sharing all oxygens is produced, as depicted in Figure 10. It is essential to note that the framework contains cations such as Na⁺, K⁺, and Ca⁺ to counteract the negatively charged Al 3 + Al₃ ⁺ ions. The reaction that takes place between geopolymer precursor and alkaline solution results in the production of water. This water, called rheological water, is vital in the process. Rheological water is essential in improving geopolymer concrete’s workability [109,110]. There is a significant disparity in the MK sources used in geopolymerization regarding the crystallinity of the kaolinite from which they were generated, the particle size, and the degree of purity. Because each of these factors is significant in the production of geopolymers using MK, it is doubtful that there will ever be a specific “recipe” optimal for geopolymer derived from various sources [107,111].

Figure 10

Polymerization stages [112].

2.5 Compressive strength

The crystalline formations of the kaolinite change into reactive, amorphous structures when heated. This process produces highly reactive pozzolan materials, which influence the overall strength of geopolymer concrete [113]. This is because when kaolin is subjected to temperatures as high as 700°C, the hexagonal kaolinite layer is destroyed, and an atomic composition is formed. This composition changes the kaolin’s hexagonal coordinate ions into pentagonal and quaternary coordinates, increasing geopolymer-based concrete’s strength and durability [114]. Several parameters determine geopolymers’ compressive strength, the most important of which are the constituent materials, the mixing proportions, and the curing conditions. The following will be discussed as the primary considerations.

2.5.1 Mass ratio of Na₂SiO₃/NaOH

In accordance with the findings of many studies, altering the proportion of Na₂SiO₃ to NaOH in geopolymer concrete may considerably affect the material’s ability to produce desirable mechanical characteristics. Based on Abdul Razak et al. [115], increasing the mass ratio of Na₂SiO₃ to NaOH results in an increase in the material’s compressive strength. Figure 11 illustrates the effect of the mass ratio of Na₂SiO₃ to NaOH on the compressive strength of geopolymer concrete. This could be because the formation of the aluminosilicate gel and the geopolymerization reaction is facilitated by an increase in the concentration of silicon species. The increase of mechanical strength and the creation of geopolymer matrix are also results of this reaction [116]. Increasing the proportion of Na₂SiO₃/NaOH helps increase the compressive strength of geopolymer mixtures because silicates play a vital role in accelerating silica and alumina dissolution in MK, thus enhancing the geopolymerization process. Moreover, the increased Na₂SiO₃ concentration in the geopolymer mixture aids in synthesizing geopolymers by acting as charge-neutralizing ions [117]. However, if the ratio of Na₂SiO₃ to NaOH rises above 3.0, this may result in a reduction in compressive strength since more liquid/solid materials are present and there is a higher concentration of NaOH [118]. Also, the creation of polymeric linkages between the substrate and the alkali activator, which should happen in the Al–Si phase, is prevented by increasing the excessively high alkali concentration, which adversely influences the geopolymerization process [119].

Figure 11

Effect of mass ratio of Na₂SiO₃ to NaOH on compressive strength of geopolymer concrete [120].

2.5.2 Solid/liquid mass ratio

The solid/liquid mass ratio correlates with reaction rate and viscosity in alkali-activated and MK-based mixtures. In addition, the solid/liquid mass ratio affects flow properties and slurry formation within the molds [118]. Therefore, the viscosity, flow capacity, and filling capacity are mainly determined by the density of geopolymer mixtures and the proportion of voids. Strength characteristics are typically correlated with the density of the mixtures; having a higher density (fewer pores) achieves higher strength properties [121]. The compressive strength of samples of MK-based geopolymer concrete may decrease when the solid/liquid (S/L) mass ratio rises. This may be explained by how the geopolymer paste’s viscosity and workability have changed. According to Aouan et al. [120], the S/L ratio has increased from 1.2 to 2.5. The presence of imbalanced proportions of binder and alkaline content, which resulted in a slow rate of source material breakdown, caused a reduction in the strength of MK-based geopolymer concrete. Furthermore, incomplete particle sedimentation occurs at high S/L ratios. MK degrades slowly, resulting in a weaker structure and reduced compressive strength. In accordance with the study by Jaya et al. [122], an increase in S/L that is more than 0.8 reduces the compressive strength of MK geopolymers, as shown in Figure 12.

Figure 12

Impact of solid/liquid mass ratio on compressive strength of MK geopolymers [122]. Annotations: Mix 1 (S/L = 0.6), Mix 2 (S/L = 0.7), Mix 3 (S/L = 0.8), Mix 4 (S/L = 0.9), and Mix 5 (S/L = 1).

2.5.3 Ratio of Si/Al and NaOH concentration

One of the compositional characteristics that significantly impacts the mechanical performance of geopolymers is the ratio between silicon and aluminum, in addition to the contents of aluminates and silicas [24]. As a result, aluminates and silicates are released into the environment because of the breakdown of the solid aluminosilicate source by alkaline hydrolysis. Several studies concluded that the ratio of Si to Al in the MK binders significantly affects the polymerization processes in the AAMK-based mixtures. The Al amount determines the polymeric reaction process rate at the setting and solidification time of MK-based geopolymer mixtures. Higher amounts of Al lead to shorter setting and early solidification times [71,91].

Previous investigations have evaluated NaOH concentration’s effect on AAC’s characteristics. Figure 13 shows the effect of increasing the concentration of NaOH on the mechanical characteristics of MK-based concrete with NaOH solution concentrations ranging from 4 to 12 mol·L⁻¹. As a result, raising the NaOH concentration enhances and accelerates the geopolymerization process and the strength properties of the alkali-activated MK-based concrete. In contrast, due to the low concentration of NaOH, geopolymerization is minimal and inadequate because of the insufficient dissolution for the breakdown of Si and Al from MK compounds [115]. Therefore, it is advisable to utilize a highly concentrated NaOH solution to generate geopolymer samples of concrete with high mechanical and physical characteristics.

Figure 13

Effect of NaOH concentration on compressive strength of MK-based geopolymer [115].

The compressive strength of MK-based geopolymers can improve its properties by adding different types of pozzolanic or SCMs. In accordance with the study by Sanalkumar and Yang [123], it is possible to increase the compressive strength of geopolymer concrete based on MK from 43 to 62 MPa by adding TiO₂ at a rate of 10% of the mass of MK after a test has been conducted on the material for 28 days, as shown in Figure 14. Meanwhile, Chen et al. [124] investigated the influence of GGBFS addition as a partial replacement for MK on the compressive strength of MK-based geopolymer concrete. The test findings demonstrate that the compressive strength can be increased by 38.8–66.4% at GGBFS substitution amounts ranging from 10 to 40% of the MK mass compared to the reference mixture, as shown in Figure 15. Many other materials can be used as partial substitutes or complements to help improve the behavior of alkali-activated geopolymer-based concrete; TiO₂ and GGBFS are mentioned as examples.

Figure 14

Compressive strength of MK-based geopolymer and TiO₂ modified [123].

Figure 15

Compressive strength of MK-based geopolymer and GGBFS modified [124].

2.6 Tensile strength

Tensile strength, in general, is one of the characteristics of hardened concrete, which is directly influenced by the properties of compressive strength. Therefore, any improvement in compressive strength is associated with an improvement in tensile strength and vice versa, indicating that tensile strength and compressive properties are closely related. They are primarily affected by the same factors [125]. This is observed in many previous studies, which display the strong relationships between the tensile and flexural strength and the compressive strength of plain geopolymer concrete [126,127]. This does not apply to fiber-reinforced geopolymer concrete, whose tensile strength and flexural strength are positively impacted by fibers. The tensile and flexural strengths of plain geopolymer concrete are much lower than that of fiber-reinforced concrete. Other factors, such as fiber type, size fraction and fiber shape, direction and distribution of fibers, etc., play a role in determining geopolymer concrete’s tensile and flexural strength [128,129]. In light of this, the tensile strength of AAC is assessed in several scenarios, both with and without fibers. According to the findings of several studies, the tensile strength of AAMK-based concrete can fluctuate depending on the mix design and curing conditions utilized. An MK-based concrete without fibers with a compressive strength of 27.5 MPa was found to have an approximate tensile strength of 3.8 MPa [130]. SiO₂ helps the formation of complementary C-single bonds, the C-single matrix, and the N-a single mono bond matrix, and it accelerates the polymerization process [131]. The resulting polymerization of complementary and single bonds increases concrete’s strength, particle coverage, and grouting. Figure 16a and b depict the interrelationship between compressive and tensile strengths of geopolymer concrete based on MK [132].

Figure 16

Compressive and tensile strengths of geopolymer concrete based on MK [132]. (a) Compressive strength. (b) Tensile strength.

To sum up, several research studies that looked into adding MK to different concrete mixtures to improve their characteristics have been reviewed. Relevant data have been compiled into Table 4 to offer a better understanding of the MK content and its percentage utilization throughout these experiments. This table presents a summary of the precise MK percentages used in the corresponding concrete mixes.

Table 4

Summary of MK content and combinations used in previous studies

Study reference	Pure MK or combined	MK quantity/percentage	Other materials
Al-Akhras [78]	Pure	5, 10, and 15%	—
Tafraoui et al. [79]	Pure	25%	—
Nuaklong et al. [82]	Combined	10–30%	High calcium FA (HCFA) (70–90%)
Görhan et al. [92]	Combined	10–40%	FA (60–90%)
Rovnaník [100]	Pure	19.8%	—
Caballero et al. [113]	Pure	44.5 and 46.6%	—
Sanalkumar and Yang [123]	Combined	90%	TiO₂ (10%)
Chen et al. [124]	Combined	60–90%	GGBFS (10–40%)

3 Effect of MK blended with other cementitious materials

3.1 MK blended with FA

FA is an unburned residue that is left behind after burning pulverized coal in an electricity-producing unit. When mixed with OPC and water, this finely divided amorphous aluminosilicate with variable calcium content reacts with OPC’s hydration product (CH) to produce calcium-aluminate-hydrate (CAH) and CSH [133]. On the other hand, high-purity kaolinite clay material is calcined at temperatures between 700 and 850°C to create MK [134]. It is made up of silica and reactive alumina, which combine with CH produced during the hydration process to form crystalline compounds called CAH and CSH [76,135,136,137].

MK content in concrete is suggested to be 5–10% by weight of cement. A compressive strength of 106 MPa reached a 10% replacement level of MK but declined to a replacement level beyond 10% [138,139]. A higher MK content (such as microsilica) increases the water demand of concrete mixes, necessitates more superplasticizers, and reduces the fracture resistance of concrete [140]. The presence of FA, whose glassy surface has a plasticizing effect, can help minimize this disadvantage of MK [141]. Composite ash-MK additives are anticipated to increase strength significantly, have a moderate water demand, and improve pozzolanic activity and surface energy [32]. This is especially important for low-cement concrete. Numerous studies [32,142,143,144,145] describe cements that contain mineral additions, such as FA and MK mixes. Table 5 presents the chemical and physical compositions and characteristics of mineral additive materials with OPC.

Table 5

Chemical and physical characteristics of mineral additives [76,112,133,141,146,147,148]

Mineral	Chemical properties	Physical properties
FA	Primarily consists of SiO₂, Al₂O₃, and Fe₂O₃ May also contain CaO, MgO, SO₃, and trace elements	Finely divided, powdery material Particle size typically ranges from 0.5 to 100 μm Specific gravity between 2.1 and 2.9 Specific surface area in the range of 300–500 m²·kg⁻¹
MK	Composed of SiO₂ and Al₂O₃ Typically contains around 50–55% SiO₂ and 40–45% Al₂O₃	Fine, white to off-white powder Particle size typically ranges from 1 to 15 μm Specific gravity between 2.2 and 2.5
OPC	Primarily consists of SiO₂, Al₂O₃, and Fe₂O₃ May also contain CaO, MgO, and SO₃	Fine, powdery material Particle size typically ranges from 1 to 150 μm Specific gravity between 2.1 and 2.8
GGBFS	Composed of CaO, SiO₂, Al₂O₃, and MgO Typical composition is around 30–50% CaO, 30–50% SiO₂, 5–15% Al₂O₃, and 0–15% MgO	Glassy, granular material Particle size typically ranges from 1 to 45 μm Specific gravity between 2.8 and 2.9
Lime	Primarily consists of CaO	White, porous, and crystalline solid Specific gravity between 3.3 and 3.4
SF	Primarily composed of SiO₂ May also contain trace amounts of other oxides such as Al₂O₃, Fe₂O₃, and CaO	Extremely fine, spherical particles Particle size typically ranges from 0.1 to 0.5 μm Specific gravity between 2.2 and 2.3
RHA	Composed of SiO₂, typically around 85–95% May also contain small amounts of other oxides such as Al₂O₃, Fe₂O₃, CaO, and MgO	Finely divided, porous, and lightweight material Particle size typically ranges from 1 to 100 μm Specific gravity between 2.0 and 2.3

Coal-sourced FA is one of the most researched and often utilized mineral admixtures in cement concrete [149]. FA’s mild pozzolanic activity (10–50 mg CaO/g) is mainly influenced by the vitreous phase’s quantity, composition, and dispersion. To pass entirely through a No. 008 sieve, FA from the Pridneprovskaya thermal power station in Ukraine, for instance, had a concentration of roughly 30 mg·g⁻¹ before grinding and 63 mg·g⁻¹ following the process [150]. Yet, such tiny ash crushing to increase activity is exceedingly energy-demanding and challenging. As an active addition to cement concrete, FA offers various benefits over other additive materials in cement concrete, such as decreased water needs and improved workability [151]. On the other hand, in comparison to pure OPC-based paste, cement-based paste adding FA has a lower hydration heat [152]. Moreover, the permeability of concrete containing FA is reduced [35], and a high amount of FA improves durability in maritime environments [153].

On the basis of the power law, Dvorkin et al. [32] concluded that adding MK to FA increases the total surface energy. At MK 0.2 volume content, the wetting heat (surface energy parameter) rises to 55–57%; at MK 0.4 volume content, it can reach up to 75%. When 20–30% MK is added to FA, its pozzolanic activity doubles [154]. This effect is more significant than when ash is re-grinded to a surface area of 420–450 m²·kg⁻¹. The enhanced activity of ash-MK mixes can drastically boost the cement hydration’s degree and the low-basic hydrosilicates amount in the products of its hydration [32,155]. After 3 days, the cement’s degree of hydration is already 0.65, 25% greater than cement without MK. However, after 28 days, the addition of 30% MK raises it to 0.8 [156]. Furthermore, using Mk with FA in cement concrete paste can enhance the compressive strength by 20% compared to the compressive strength of cement concrete without additives. With the addition of 70% FA Mk admixture at 28 days of curing, the compressive strength reaches 60 MPa, which is 20% higher than the standard cement concrete [32]. Saboo et al. [35] concluded that the optimum range of FA substitution in previous concrete is discovered to be 5–15%. The synergistic impact of FA and MK was examined at various substitution rates. In contrast to alternative proportions, the results showed that an 80:10:10 mix of cement, MK, and FA produced the densest microstructure with the most excellent durability [157].

3.2 MK blended with slag

GGBFS is a byproduct of blast furnaces utilized to manufacture pig iron. It has been used essentially in many nations worldwide, delivering several technical benefits in the building industry [33]. When the GGBFS concentration rises, the water-to-binder ratio falls for the same consistency, indicating that GGBFS positively influences the consistency. Meanwhile, by increasing the percentage of GGBFS, the compressive strength of GGBFS-containing concrete mixtures rises [158]. Uysal and Sumer [159] evaluated the fact that increasing the GGBFS rate increases compressive strength and workability. The 60% OPC and 40% GGBFS combination provided the best resistance to magnesium and sodium sulfate assaults.

ASTM C989 [160] classifies slag for usage in mortar and concrete into three classes depending on activity index: 80, 100, and 120. At 7 and 28 days, Grade 120 has the maximum compressive strength. The most prevalent grades are 100 and 120 [73]. Alumina, magnesia, silica, and lime are the primary components of slag. The specific composition varies although silica and lime are the most common phases [161,162]. Slag must be dampened in either air or water to generate a usable cementing material, resulting in a substance with a significant amorphous percentage. Granulated slag is formed when slag is quenched in water, whereas pelletized slag is generated when slag is quenched in air [163,164]. Slag is usually ground to a Blaine fineness of 400–500 m²·kg⁻¹. FA and SF include spherical particles, whereas grinding creates smooth angular particles [165].

The replacement of slag for cement resulted in decreased heat and hydration. Slag hydration becomes exceedingly sluggish at low temperatures, and its usage in cold-weather concreting is not suggested. At ordinary temperature, including slag causes a 1-h delay in setting time compared with the control mixture [166,167,168]. Additionally, it has been noted that slag enhances the mix’s cohesiveness and workability and improves concrete’s bleed capacity with a slight increase in the bleed rate [167,169,170].

3.3 MK blended with SF

Microsilica, also called SF, is a pozzolanic byproduct of the manufacture of silicon metal. It comprises small, spherical, amorphous silica particles ranging in size from 0.15 to 0.2 μm with a specific surface area of 25,000 m²·kg⁻¹ [147,171]. Only 50% of SF produced during the production of ferrosilicon alloys is acceptable for use as pozzolanic material. The tiny particle size of SF (100 times smaller than cement) improves the packing between aggregate surfaces and cement grains, lowering overall porosity, enhancing durability, and dramatically lowering the permeability of concrete incorporating SF [172,173]. SF improves cohesiveness and decreases bleeding. On the other hand, the probability of drying shrinkage cracking is enhanced [173]. The inclusion of SF enhances autogenous shrinkage while decreasing drying shrinkage. Creep is also minimized when the SF content increases. Morsy and Shebl [174] studied the effect of MK and SF pozzolana on the behavior of blended cement pastes against fire. They concluded that substituting 15% MK and 5% SF for OPC in cement pastes boosts thermal shock resistance by nearly 10 times. Besides, CS of blended cement rises with the increasing temperature up to 400°C but falls as treatment temperatures rise to 800°C. In addition, the impact of SF and MK on the heat of hydration and CS formation of mortar was investigated by Kadri et al. [175].

3.4 MK blended with RHA

RHA is obtained by burning rice husk at roughly 700°C. It is a superb pozzolanic material with a high concentration of silica, which provides the pozzolanic feature [148]. It should be noted that the chemical composition and crystallinity percentage of RHA samples are impacted by the nutrients available in the soil where the crop was grown, as well as the burning temperature of rice husk [176]. Khan et al. [177] concluded that employing RHS as a partial cement substitution in concrete can enhance the durability and microstructural and mechanical properties. Uche [178] used RHA and MK as a partial replacement for OPC to evaluate the physico-mechanical properties (i.e., soundness, consistency, and settings time) and concrete compressive strength. He concluded that at 60 days of curing, the best CS was achieved by substituting cement with 15 and 20% at MK and 5 wt% RHA, providing a mortar CS of 40.5 MPa. Besides, Abo-El-Enein et al. [179] examined the effect of partial substitution of OPC, MK, and RHA on the mechanical and physicochemical characteristics of the hardened OPC–RHA–MK blended cement pastes. Compared to the ordinary paste neat with OPC, the paste having 5% RHA enhances the mechanical and physicochemical characteristics of the hardened blended cement pastes. Besides, OPC–RHA–MK blends containing 5% RHA blended with 10, and 15% MK revealed high CS. Meanwhile, Shatat [180] assessed the mechanical properties and hydration behavior of blended cement comprising several amounts of RHA in combination with MK. He concluded that the compressive strength of the specimens containing ternary cement mixes with 20–15% MK and 5–10% RHA was higher than that of the control specimen without RHA.

3.5 MK blended with CH

Within the last decade, much focus has been on the interaction between MK and CH. The pozzolanic reaction of MK with CH yields C₂ASH₈ (strätlingite), CSH, C₃ASH₆ (hydro garnet), and C₄AH₁₃. Bucher et al. [181] discovered advantages when MK and limestone filler were used. The material’s structure developed fine voids that enhanced its resistance to carbonation. Weise et al. [182] tried to improve the MK reactivity in blended cement with additional CH. They concluded that a substitution rate of 30% or higher results in the maximum 28 days of CH consumption in the absence of CH addition. The amount of CH accessible from cement hydration was found to be the limit of the pozzolanic reaction of MK in these samples. Thermogravimetric analysis data on CH consumption displayed that adding CH to the samples increased their quantity, which improved MK’s pozzolanic reaction.

A combination of calcined clay and limestone is the foundation for LC3, a novel cement. Reducing half of the cement’s clinker content can lower CO₂ emissions by up to 40%, whereas LC3 is an affordable solution that does not require significant capital improvements to existing cement plants. It is produced using low-grade clays and limestone, both readily available. Furthermore, LC3 uses industrial waste materials, which raises resource efficiency and lowers the usage of limited raw materials needed to produce clinker.

The most promising combinations of MK with various SCMs are listed below, based on the findings previously discussed:

3.5.1 MK blended with FA

This combination has outstanding synergistic benefits. FA’s pozzolanic activity can be almost doubled when 20–30% MK is added, as opposed to FA alone. The pozzolanic activity, surface energy, and general performance of concrete are all enhanced by the MK–FA blend. Cement, MK, and FA mixed at a ratio of 80:10:10 to create the densest microstructure and maximum durability. A total of 20% more compressive strength was obtained with a 70% MK–FA blend than with regular cement concrete.

3.5.2 MK blended with GGBFS

The pozzolanic reaction and the formation of CSH and other cementitious compounds are improved when MK and GGBFS are combined. Concrete with an enhanced microstructure is stronger and more durable. Although precise ideal ratios are not given, mixtures of MK and GGBFS exhibit encouraging outcomes.

3.5.3 MK blended with SF

In comparison with conventional OPC pastes, the addition of 15% MK and 5% SF greatly enhanced the thermal shock resistance and compressive strength development at elevated temperatures. Concrete that is both high-performing and fire-resistant can be produced with this mix.

3.5.4 MK blended with RHA

Ternary blends with 10–15% MK and 5% RHA illustrated higher compressive strengths than control samples. The mechanical properties and durability of cement pastes and mortars were improved by the RHA–MK combination.

The results reveal that adding MK to FA or GGBFS blends well in terms of improving concrete strength, microstructural development, and pozzolanic activity. The review measured particular performance gains and found that the MK–FA blend is the most promising. Moreover, the characteristics of MK-based geopolymer and concrete are quantitatively compared in Table 6 to a defined baseline, which includes FA- and OPC-based geopolymers.

Table 6

Quantitative comparison between MK, OPC, and FA properties [81,87,152,167,183,184,185]

Property	MK-based concrete/geopolymers	OPC concrete	FA geopolymers
Compressive strength	50–70 MPa at 28 days (10–20% increase over OPC)	45–60 MPa at 28 days (baseline)	40–55 MPa at 28 days
Workability (slump test)	50–75 mm	100–125 mm	90–115 mm
Initial setting time	60–90 min	90–120 min	120–150 min
Final setting time	180–240 min	240–300 min	300–360 min
Heat of hydration	250–300 J·g⁻¹ at 72 h	300–350 J·g⁻¹ at 72 h	200–250 J·g⁻¹ at 72 h
Durability (chloride penetration)	500–1,000 C at 56 days	2,000–4,000 coulombs at 56 days	1,000–2,000 C at 56 days
CO₂ emissions	200–250 kg CO₂/m³ of concrete	300–350 kg CO₂/m³ of concrete	150–200 kg CO₂/m³ of concrete

Figure 17 illustrates the effect of MK content on the compressive strength of geopolymer concrete, based on data from Table 6 and previous studies [32,140,141] in the literature. The addition of MK first raises the compressive strength of OPC concrete, which starts at a baseline strength of 50 MPa at 28 days. At 10% MK content, a peak strength of 106 MPa is recorded [140,141], which is a notable improvement over OPC concrete. But the strength starts to decrease after this ideal point. It is interesting to note that strength stabilizes at about 60 MPa when FA is added at higher MK contents (30 and 70%) [32]. The strength of pure geopolymer concrete based on MK can be as high as 70 MPa. These results imply that although MK can significantly increase the strength of concrete, the ideal amount of this material depends on the mix’s overall composition and the presence of additional cementitious materials.

Figure 17

Compressive strength versus content of MK.

4 Repair techniques

Many structures, including water and sewage systems, pavements, offshore constructions, dams, and other concrete structures, have been built using concrete. Numerous variables, which can be categorized according to how the concrete’s mechanical, chemical, and physical characteristics vary throughout its service life, can influence the performance of concrete structures [186]. Hence, as their primary design objective, concrete structures require regular maintenance, repair, and rehabilitation to maintain serviceability. Owners of concrete buildings and structures have recently shown a preference for repairing damaged structures rather than having them completely rebuilt. Many valuable publications are accessible, which address the expanding demand for repair techniques, materials, and factors affecting this area of study. There have been constant efforts made worldwide to create environmentally friendly products. Due to their high CO₂ emissions, the steel and concrete industries are driven to reduce their CO₂ emissions by 2030. The energy cost of OPC in concrete design is significant, and investigations demonstrate that CO₂ emissions vary from 0.66 to 0.82 kg for every kg of cement produced [187]. Finding a material for concrete repair that is both affordable and effective. Geopolymer, known as AAC, is becoming more popular for building and repairing structures. The characteristics of geopolymer, such as its elastic modulus, tensile strength, and Poisson’s ratio, which are comparable to those found in OPC concrete, imply that repair materials made of geopolymer and concrete are compatible [104,188,189]. In addition, geopolymer cures at room temperature and has better early mechanical characteristics than other SCMs. Another benefit of employing geopolymer repair materials is their environmental friendliness. Previous research indicates that compared to its OPC counterparts, the manufacture of FA- or MK-based geopolymer emits 80–90% less carbon dioxide [190]. The ability to repair concrete structures is one of the developments in the use of AAMK. The concrete industry is under increasing pressure to be more environmentally friendly and minimize its carbon footprint, emphasizing the potential applications for geopolymer repair binders.

4.1 Bond and impact strengths

Using affordable and environmentally friendly materials for concrete repair has recently become a significant advancement in the building sector. The recent development of cost-effective and environmentally friendly concrete repairing materials has received considerable research attention. The successful concrete repair work implementation requires the repair materials’ superior bonding behavior [191]. The following two factors influence bond strength: (1) chemical interactions between the surface of the substrate and the repair materials and (2) physical characteristics of the interface surface that are impacted by surface roughness. It is generally accepted that the bond interface is the weakest part of the repaired concrete structure because it displays significantly worse mechanical characteristics than the old concrete matrix [190]. Hence, when adopting cementitious repair materials, a strong bond strength that can withstand enormous loads is crucial [192].

The stress applied to the bond interface is used to categorize the several available bond strength tests. The bond interface may be subjected to bending, perpendicular, or parallel stresses. Parallel stresses can be applied by direct shear testing or slant shear tests. The two primary stresses that the bond contact is subjected to in the slant shear test are parallel stresses (shear stress) and perpendicular stresses (compressive stress) [192]. Conversely, only shear or parallel stresses are applied in a direct shear. On the other side, perpendicular stress, or tensile stress, could be applied at the bond interface using split tension and pull-off tests. While the pull-off test applies direct, perpendicular tensile forces, split tension uses indirect tensile pressures. Indirect tensile or compressive stresses may be applied to the bond contact depending on its location concerning the neutral axis [193].

While the non-AA materials were generally more expensive, the bond strength with conventional concrete (CC) was improved when a calcium source, such as OPC or CH, was added to FA-based and MK-based AA materials. This resulted in a bond strength comparable to fiber-reinforced nonshrink grout, multipurpose nonshrink grout, high-performance repair mortar, and polymer-modified mortar [194]. Pacheco-Torgal et al. [195] state that geopolymer repair materials have bond strength equivalent to commercial repair materials, even at an early age. In accordance with an evaluation by Songpiriyakij et al. [196], repair geopolymer had 1.5 times stronger bond strength than epoxies. Higher bond strengths were seen in the repair of CC using either alkali-activated paste or mortar (AAM) made from class C FA, class F FA, and MK.

The bond strength between the overlay repair mortar and the concrete substrate determines how well the repair works. Pull-off and slant shear tests are frequently used to characterize the bond strength and determine whether the repair material is suitable [197].

Today, some codes [198] have modified and adopted Arizona slant shear test [199]. Some claim that the slant shear test produces consistent results [200]. However, compared to other methods, experts believe that slant shear can more precisely represent actual stress levels in structures. The slant shear test employs a cylindrical specimen of two equal half-specimens. Every component has a diagonal surface with a vertical offset of thirty. Under axial loading, prepared samples are examined. The bond strength is evaluated under combined shear and compression pressures. Recent research has focused on pozzolans’ application in concrete repair projects and determining the bond strength [186]. The thermal decomposition of kaolin can result in the amorphous substance known as MK [201]. Furthermore, the bond strength highly depends on the casting process and the angle at which the substrate and extra layers interact. Finally, it can be said that the bond strength is also affected by the two concrete layers’ respective compressive strengths, stiffness, and shrinkage [202]. Figure 18 shows a summary of these components.

Figure 18

Quick summary of variables influencing bond strength [193].

It has been claimed that the type of alkaline activator solution and the compressive strength of geopolymer concrete affect the bond strength of the material [203]. It was discovered that the material source and the alkali activator solution impacted the GP reaction products.

Splitting tensile and slant shear tests were utilized by Alanazi et al. [204] to investigate the bonding strength of MK-based geopolymer mortar utilized in cement concrete pavement rehabilitation. Most of the specimens evaluated had adhesion failure at the contact surface. In addition, most failures happened in the substrate, indicating a strong bonding contact between geopolymer mortar and cement substrate. Moreover, Zanotti et al. [205] assessed and enhanced the substrate repair bonding between the concrete substrate and ambient-cured MK-based geopolymer mortar. The bond plane’s inclination, the curing temperature of 20–45°C, and the percentage of polyvinyl alcohol (PVA) fibers of 0, 0.5, and 1% were the variables. According to the results, the early-age cracking in the repair mortar indicated that geopolymer mortar was not very compatible. It was discovered that for both ambient and heat-cured specimens, the cohesiveness of the mixture was greatly enhanced by adding a 0.5% volume percentage of PVA fibers. By using a pull-off test, Duan et al. [183] evaluated the bonding strength of an MK-based geopolymer repair mortar waterproofed with a hydrophobic modifying agent. Because of its dense microstructure, the created repair mortar displayed a considerable interfacial bonding zone. Finally, Huseien et al. [206] evaluated the effects of switching from GGBFS to MK on the mechanical characteristics of geopolymer mortar. The test findings of the bond strength test suggested that geopolymer mortar might be substituted for cementitious repair mortar. Based on prior research, it is possible to conclude that MK-based geopolymer will eventually replace cement-based repair mortars as a developing binder. There are numerous obstacles in MK-based geopolymers despite studies on the mortar’s ability to bond and fulfill its potential as a repair material. Conversely, impact strength refers to a material’s capacity to absorb mechanical energy during the deformation and fracture process when subjected to impact loading [207]. The terms “impact strength” and “impact energy” are also applied to the amount of energy absorbed before fracture. Somasekharaiah et al. [207] examined MK’s influence on mechanical characteristics of high-performance concrete (HPC). The sample size of 150 mm diameter and 60 mm concrete was designed. Methods of drop weighting were applied. As shown in Figure 19, the 150 mm diameter cylinder discs were set on the impact tester’s base and let drop several times. The maximum effect value observed for 1.25% hybrid fiber when MK is added is 658/712 (N1/N2). At 10% replacement of MK, the impact strength without fiber is 403/423 (N1/N2). For the combination including 1.25% fiber, the most significant percentage gain in strength is 284.79/295.56 (N1/N2). The change ratio was 63.27/68.32 for the identical mix of W/B = 0.275 without fiber content when 10% of cement was replaced with MK.

Figure 19

Impact testing machine [207].

5 Repair applications of MK

Concrete repairs frequently experience premature failure, which causes severe economic, environmental, and societal losses [208]. The utilization of geopolymer as a binder for concrete rehabilitation has gained increasing attention recently. The compatibility of geopolymer repair materials with concrete is confirmed by the similarities between its properties like elastic modulus, tensile strength, and Poisson’s ratio, and those of conventional cement concrete. Furthermore, geopolymer has superior early mechanical qualities compared to other cementitious materials and can cure at room temperature. Another benefit of employing geopolymer repair materials is that they are environmentally friendly [190]. The production of geopolymers based on FA or MK releases 80–90% less carbon dioxide than its OPC [184,185,209,210]. Pressure on the concrete industry is rising. To be more environmentally friendly and minimize its carbon footprint, emphasizing the potential applications for geopolymer repair binders.

5.1 Column confinement

High-rise building columns have long been made of high-strength and HPC. Yet, in recent years, HPC has increasingly been used in bridges where durability and strength are important factors. The primary argument for implementing HPC is to generate a more economical product, a workable technical solution, or a combination of the two. Currently, the cost of an HPC cubic yard is usually higher than that of a standard cubic yard. The HPC needs additional ingredients, including cement, FA, MK, SF, water reducers, and retardants, to certify that concrete achieves its desired performance [211]. The cost of a finished project outweighs the price of a single material despite concrete being only one part of the construction. On the other hand, it should not be emphasized if using HPC would not lead to any technological or commercial advancements. In addition, it was acknowledged that achieving excellent performance can only be done effectively through more than high strength. To discover a viable substitution, in-depth research is required due to the longevity of these chemicals in various environments [212]. This involves the utilization of pozzolanic concrete-like industrial effluents and the potential for durability improvements in addition to developing HSC composites. Concrete composites can now combine high strength and excellent performance due to pozzolanic materials and superplasticizers [213].

The behavior of high-performance short and long concrete columns containing MK was investigated in an experimental investigation by Dharmaraj et al. [211]. In the range of 0–10% by cement weight, they employed MK to replace OPC. Seven proportions were cast with MK replacing cement in amounts of 0, 5, 7.5, and 10%. Fourteen columns, seven each for short and long columns, respectively, were cast and given a 28-day cure. While long concrete columns were tested with uniaxial bending and small concentration, all small concrete columns were evaluated with concentrated compression. The findings indicated that MK may produce an affordable HPC with a 28-day strength of up to 64 MPa.

5.2 Shear strengthening

The deterioration of reinforced concrete structural elements can occur for a number of reasons, such as shifting loads, modifications in the way the building framework is used, the need for additional strength to support greater loads due to preexisting damage, internal steel reinforcement corrosion, and natural or man-made disasters like fires and earthquakes. Sudden and catastrophic shear failure in reinforced concrete structures is possible [214]. As a result, strengthening the structural components is necessary to restore the load-carrying capacity lost due to the aforementioned factors. Due to the various loading combinations acting on the structural elements, choosing an appropriate strengthening strategy still presents a difficulty for structural engineers. The shear capacity of concrete beams strengthened with an FRP containing M was assessed by Vellaipandian et al. [215]. The concrete beams used in their investigation were constructed using 35% MK as cement replacement. The study’s main goal was to understand the general performance of fiber-reinforced polymer beams based on MK. Several tests were conducted using various percentages of MK, biochar, and eggshell powder to reach the desired cubic compressive strength of 30 MPa. Twelve reinforced concrete beams were cast from the developed optimum mix and strengthened with various arrangements of CFRP wrapping. In accordance with the test results, the reinforced concrete beam strengthened with complete wrapping and manufactured with MK (CFRP–FW) performed better in ductility and peak strength.

The shear strength of HPC incorporating high-reactivity MK (HRM) under direct shear was investigated by Raj and Pillai [216]. Push-off specimens of a size 150 mm × 150 mm × 450 mm were examined for shear strength with and without side face reinforcement. The findings of the push-off specimens with and without side face reinforcement were also compared to analyze the outcomes further. The M50 and M60 grade HPC with HRM contents of 2–14% via cement replacement and a control HPC without HRM content were used in the concrete mixture. The results illustrate that mixtures with an HRM replacement of 8% for cement provided the highest shear strength for HPC push-off specimens with and without side face reinforcement. Until an increase in HRM content of 8%, the shear strength of concrete increases; after that, it steadily decreases. As can be seen, the ratio of the two strengths depends on the overall degree of concrete strength. Concrete’s strength in compression and shear are closely related. In other words, increased compressive strength corresponds to higher shear strength, but the rate of shear strength increase is lower.

5.3 Flexural strengthening

MK is a valuable additive for concrete since it accelerates the filling impact, pozzolanic reaction, and hydration reaction. Partially replacing OPC with MK can greatly enhance concrete’s ITZ, compressive strength, and flexural strength [217]. Researchers widely utilized MK to produce reactive powder concrete (RPC). In ultra-high performance concrete (UHPC), 10, 20, and 30% MK was used instead of cement by Šeps et al. [218]. Based on the findings, the compressive strength improved by up to 20% MK before decreasing. The flexural strength, however, was first reduced by up to 20% before being increased (to a 30% substitution level). Dvorkin et al. [219] studied the combined usage of MK and FA as an SF replacement in RPC. Findings depicted that adding MK and FA to RPC increased its compressive and flexural capacities. The optimum MK substitution rate was 10% of the cement’s weight. Because of increased viscosity and associated water consumption by RPC, more than 10% of the MK content was inefficient.

The effects of magnesium oxide (MgO) and MK on the flexural capacity of UHPC were assessed by Zhang et al. [217]. At UHPC, OPC, which included 0–15% of its weight, was replaced with MK and MgO (MM) at a weight ratio of 3:2. The microstructure, strength, and fluidity of UHPC, as well as the pullout behavior of a single steel fiber in a UHPC matrix, were all investigated. As MM replacement ratios rise, the flexural capacity of UHPC initially rises and subsequently falls, according to the data. The 10% MM UHPC was the strongest after 60 days, with a flexural strength of 33.8% more than the UHPC without MM.

The potential efficacy of MK-based geopolymer mortar as a bonding substance for external reinforcement of shallow beams was examined by Menna et al. [220]. Bending tests demonstrated that beams reinforced with a high-strength steel cord system had remarkable adhesion properties, which resulted in a notable increase in flexural resistance. In addition, Colangelo et al. [221] focused on nonstructural applications while analyzing the mechanical characteristics of a thermally insulated geopolymer composite made from waste-expanded polystyrene and MK. The researchers found that incorporating marble powder and organic epoxy into the mixture increased the composite’s compressive and flexural strengths.

6 Long-term durability and ultrasound MK-based concrete

The durability of concrete refers to its resistance to disintegration and degradation. Permeability is one of the most important aspects affecting durability since it can speed up the rate of capillary absorption and enhance the resistance of concrete to potentially harmful substances. Geocrete is considered durable and sufficient to tolerate environmental effects. To evaluate whether artificial neural networks (ANNs) could accurately forecast compressive strength in concrete based on various factors, including aggregate–cement ratio, water–cement ratio, age of testing, and a ratio of cement/MK of 5 and 10%, a campaign of experiments were conducted [222]. This required the casting of 27 specimens with cross sections measuring 25 cm and 50 cm in length, as well as 162 cylindrical samples with dimensions of 10 cm in diameter and 20 cm in height. Several concrete mix proportions were used, and a longitudinal transducer with a frequency of 54 kHz was employed to measure the ultrasonic rapidity. The ANN model created to forecast compressive strength performed exceptionally well, achieving an accuracy of less than 5%. After analysis, it was concluded that the water–cement ratio, and the proportion of MK had the highest combined effects on the compressive strength forecasted by the ANN model.

Ashok et al. [223] investigated the impact of nanoparticles and nano alumina (NA), one of the advantageous cementitious (pozzolanic) components of cement, on the mechanical properties of concrete. In concrete, the bulk of the cementitious material is substituted with certain pozzolanic and nanoparticle elements. In an experiment, different concrete mixes, such as M30, M40, and M50, had their nanoparticle-containing concrete cured in water for 28, 56, and 90 days. The goal was to standardize mechanical properties including splitting tensile strength, flexural strength, and compressive strength as well as nondestructive testing methods like the ultrasound pulse velocity (UPV) and rebound hammer (RH) test on the hardened concrete. Additionally evaluated were the mechanical characteristics and workability of conventional and nanoreplaced concrete. The outcomes of experimental testing suggest that it is possible to assess the impacts of nanoparticles in concrete in both its fresh and hardened states. To prepare the Nano replacement concrete, 1 and 15% of the CC’s material were substituted with nano alumina and MK, respectively. Water cement ratios of 0.48, 0.45, and 0.4 are utilized for different grades.

NA and MK replaced 1 and 15% of the additional cementitious material in each sample. A superplasticizer solution was added to the nanoparticle concrete mix proportions at a dose of 1.5% to enhance the workability and flow properties of the material. The UPV at 90 days is shown in Figure 20 [223]. The RH and UPV test results demonstrate that NA concrete is of excellent quality than regular concrete. The density of packing created by the enormous surface area-to-volume ratio of NA particles in concrete results in a quality of over 4.0 km·s⁻¹ for NA concrete, whereas regular concrete has a quality of less than 4.0 km·s⁻¹. Adding nano alumina to concrete mixtures improved the RH test by 90 days, as Figure 21 illustrates. As a result of this enhanced surface hardness, NA concrete has a more significant rebound number than ordinary concrete.

Figure 20

UPV test at 90 days [223].

Figure 21

RH test at 90 days [223].

El Idrissi et al. [224] tracked the development of the local structure using 29Si and 27Al NMR to better understand the formation and evolution of the binding phase over time. The outcomes of this investigation were used to cast doubt on the connection between changes in mechanical characteristics at the material scale and the evolution of the chemistry of binding phases. MK and GGBFS have been combined with a small amount of FA to investigate its effects and possible benefits. The mechanical performance of GGBFS-based grouts is adversely affected by the development of their structure, as opposed to MK-based systems, which exhibit relative stability at both local and macroscopic scales. Compressive strengths of MK and activated slag (AS) after 28 and 360 days of curing, with or without FA (ASF and AMF), are displayed in Figure 22.

Figure 22

Compressive strengths of metakaolin (AM) and AS, either with or without FA (AF and AMF), after 28 and 360 days of curing [224].

Rheological properties (L-Box, slump flow, T50), mechanical properties (compressive, splitting tensile, and flexural strength), and durability properties (chloride ion penetration and water absorption) were also investigated by Sivakumar et al. [225]. The test findings showed that as the fiber content rose, workability declined. It was discovered that the compressive strength of concrete was not improved by glass fibers. Furthermore, the splitting tensile strength and flexural strength of glass-fibered SCC increased with the increasing fiber dose. Based on the durability investigations, incorporating glass fiber somewhat diminishes concrete resistance to chloride ions and water absorption. The results show that adding glass fiber and MK in the right amounts can significantly improve the self-compacting concrete’s durability and mechanical qualities. In Figure 23, MK’s SEM picture is shown.

Figure 23

SEM image of MK [225].

7 Conclusions

This review study concludes by providing a comprehensive summary of the application of AAMK as a sustainable alternative to OPC in concrete repair and building. The pressing need for environmentally friendly building materials is highlighted by the effects of OPC production on the environment, particularly its high CO₂ emissions. Several SCMs have been investigated throughout this article; MK has emerged as a beautiful choice because of its exceptional qualities and compatibility with geopolymers. An introduction to the topic highlights the environmental issues raised by the concrete industry and the need to find sustainable alternatives. It is underlined that AAMK, a cement of the third generation, presents a substantial opportunity to reduce the environmental effect of cement manufacturing.

The characteristics of MK are explored in detail in this research, including its fineness, workability, and critical role in geopolymerization. It shows how the development of the characteristics of geopolymer concrete is influenced by curing conditions, highlighting the significance of temperature and duration in attaining the intended results. It also discusses the MK-based geopolymers’ compressive and tensile strengths, highlighting their potential for high-performance uses. Meanwhile, concrete restoration techniques are reviewed, emphasizing impact and bond strength. Strong bonding between repair materials and substrates is essential, and the review emphasized how MK-based materials demonstrate potential bonding behavior. In addition, it offers insights into several repair scenarios where MK-based concrete has been successfully used to improve structural performance and longevity, including column confinement, shear strengthening, and flexural strengthening.

The emphasis on long-term durability and the application of cutting-edge methods, such as ultrasonography, to evaluate compressive strength implies the dedication to guaranteeing the longevity of structures built or restored using AAMK-based concrete. The study concludes by discussing the application of nanoparticles in concrete and delving into the chemistry of the binding phases in MK-based concrete, which advances our knowledge of the material’s characteristics and behavior.

This review article highlights the potential of AAMK as a high-performance and sustainable alternative that paves the way for a more durable and environmentally friendly future for researchers, engineers, and practitioners in construction and concrete repair. Besides, it encourages future research to replace the use of natural resources or harmful environmental materials with more sustainable materials that are abundant and environmentally friendly. Moreover, modern techniques such as artificial intelligence and machine learning should be used to examine the strength properties of SCM and AAMK-based concrete by developing several models and algorithms. The modern techniques should be compared with the conventional methods and will provide an overview of the differences.

Funding information: The authors state no funding involved.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: All data generated or analyzed during this study are included in this published article.

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Received: 2024-02-04

Revised: 2024-10-25

Accepted: 2024-11-25

Published Online: 2025-02-17

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/rams-2024-0076

Schlagwörter für diesen Artikel

concrete; metakaolin; alkali-activated material; supplementary cementitious material; sustainability; environmental impact; pozzolanic activity

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A systematic review of metakaolin-based alkali-activated and geopolymer concrete: A step toward green concrete

Artikel

Abstract

1 Introduction

1.1 Review objectives

1.2 Review methodology

2 Properties of MK

2.1 Fineness

2.2 Workability

2.3 Curing condition

2.4 Hydration (geopolymerization)

2.5 Compressive strength

2.5.1 Mass ratio of Na2SiO3/NaOH

2.5.2 Solid/liquid mass ratio

2.5.3 Ratio of Si/Al and NaOH concentration

2.6 Tensile strength

3 Effect of MK blended with other cementitious materials

3.1 MK blended with FA

3.2 MK blended with slag

3.3 MK blended with SF

3.4 MK blended with RHA

3.5 MK blended with CH

3.5.1 MK blended with FA

3.5.2 MK blended with GGBFS

3.5.3 MK blended with SF

3.5.4 MK blended with RHA

4 Repair techniques

4.1 Bond and impact strengths

5 Repair applications of MK

5.1 Column confinement

5.2 Shear strengthening

5.3 Flexural strengthening

6 Long-term durability and ultrasound MK-based concrete

7 Conclusions

References

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2.5.1 Mass ratio of Na₂SiO₃/NaOH