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Research on the carbonation resistance and improvement technology of fully recycled aggregate concrete

  • Shifang Wang , Yongsheng Wang , Ji Yuan EMAIL logo , Ruixin Wang , Jun Wei Feng , Wei Lin , Haijie He , Xiongwei Dai , Wen Xu and Zhicheng Zhang
Published/Copyright: July 13, 2024
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Science and Engineering of Composite Materials
From the journal Science and Engineering of Composite Materials Volume 31 Issue 1

Abstract

The aim of this study is to enhance the carbonation resistance of fully recycled aggregate concrete through diverse measures in an effort to enhance solid waste disposal, reduce the consumption of natural aggregates, and broaden the utilization of recycled aggregate concrete. Six sets of fully recycled aggregate concrete specimens were prepared and subjected to rapid carbonation tests. Carbonation depth and compressive strength measurements were taken at different ages (3, 7, 14, and 28 days). Subsequent calculations and analyses were conducted on both parameters for each set of specimens. Results indicate that the incorporation of microspheres and high-toughness polypropylene fibers (HTPP) substantially improves the carbonation resistance of fully recycled aggregate concrete, leading to a 48% reduction in carbonation depth by the 28th day. Furthermore, a relative compressive strength model for fully recycled aggregate concrete post-carbonation was established based on the strength data of each specimen group. This model accurately depicts the growth pattern of compressive strength after carbonation. Additionally, a carbonation depth prediction model was developed through fitting analysis of carbonation depth data, effectively foreseeing the depth of carbonation in fully recycled aggregate concrete. Based on the carbonation depth, the carbonation life of fully recycled aggregate concrete was predicted. The carbonation life of recycled aggregate concrete with added microspheres and HTPP fibers can be increased by up to 278%. Finally, scanning electron microscopy (SEM) was employed to examine the microstructure of fully recycled aggregate concrete, revealing the mechanisms by which various methods enhance its carbonation resistance. The carbonation resistance improvement technology of fully recycled aggregate concrete is selected through this study characteristics such as simplicity, convenience, and cost-effectiveness, which are crucial for the widespread application of recycled aggregate concrete in building structures.

Keywords: fully recycled aggregate concrete; microsphere; HTPP fibers; super absorbent resin; carbonation; prediction model; microstructure

Nomenclature

HTPP

high-toughness polypropylene

PCE

polycarboxylate superplasticizer

PVA

polyvinyl alcohol

SAR

super absorbent resin

SAP

superabsorbent polymer

SEM

scanning electron microscope

1 Introduction

Recycled aggregate concrete is derived from the crushing and processing of waste concrete, with recycled aggregates replacing natural aggregates either partially or entirely in the mixture. This technology enables the recycling of waste concrete, restoring its original properties to create new construction materials. Such practices not only address environmental concerns but also optimize resource utilization, aligning with the sustainable development goals of the construction industry. It represents a crucial measure in the advancement of green and ecological concrete [1,2,3,4,5,6]. In 2021, China’s construction solid waste emissions reached an alarming 3 billion tons, with waste concrete accounting for approximately 40%. Traditional disposal methods not only consume significant land resources but also fail to fully harness the potential of waste materials. Annually, at least 200 million tons of waste concrete are generated, exacerbating the issue through land occupation and pollution from landfilling [7]. As industry and construction continue to expand, the demand for natural sand and stone has surged, straining already scarce natural resources. China alone produces about 1.8 billion cubic meters of commercial concrete each year, consuming around 3.2 billion tons of natural sand and stone, and 650 million tons of cement, contributing to severe environmental degradation. The adoption of recycled aggregate concrete not only addresses environmental challenges arising from construction waste but also facilitates waste recycling and mitigates against the depletion of natural resources [6]. Hence, there is a pressing need to prioritize recycled aggregates over natural aggregates in concrete production. However, due to the presence of old cement mortar and the formation of numerous micro-cracks during aggregate crushing, recycled aggregates exhibit drawbacks such as high water absorption rates and crushing indices, which hinder their practical application in engineering [8,9,10]. The durability of recycled aggregate concrete is severely compromised, with concrete carbonation emerging as a primary factor contributing to steel reinforcement corrosion [11,12]. Repurposed concrete blocks possess abundant pore structures, which, upon transformation into recycled aggregate concrete, diminish its resistance to carbonation. The combined effects of carbon dioxide and chloride ions accelerate steel reinforcement corrosion within recycled aggregate concrete, significantly reducing its compressive strength. Moreover, rapid population growth and industrialization have elevated atmospheric CO2 concentrations by approximately 25% over the past century [12], intensifying the carbonation of reinforced concrete structures and posing substantial challenges to their durability [13]. Given its inherent porosity, recycled aggregate concrete exhibits lower resistance to carbonation. Therefore, enhancing its carbonation resistance is crucial for expanding its application scope. Many materials, such as fibers [14,15], resins [16], mineral admixtures [17], coatings, and internal curing agents, have the potential to enhance the carbonation resistance of recycled concrete. Therefore, many scholars have conducted research on improving the carbonation resistance of recycled concrete.

Regarding the addition of mineral admixtures, a study by Xiao et al. [18] has demonstrated that including mineral admixtures significantly reduces the carbonation depth of recycled aggregate concrete. Qin et al. [19] observed that the carbonation depth of recycled aggregate concrete increases with the rise in recycled coarse aggregates and fly ash content. Furthermore, some scholars have observed that the addition of nano-kaolinite enhances the mechanical properties of recycled aggregate concrete and densifies its internal microstructure [20]. Zhong et al. [21] found that incorporating nano TiO2 can notably enhance the carbonation resistance of recycled aggregate concrete. He also investigated the impact of active admixtures, mineral powder, fly ash, and air-entraining agents. His findings highlight that mineral powder has the most significant effect on carbonation depth, followed by fly ash, air-entraining agents, and others. Regarding fiber additions, research by Yan et al. [22] suggests that including steel fibers in recycled coarse aggregate concrete improves its carbonation resistance. Zhou et al. [23] discovered that discarded fibers effectively prevent the formation of detrimental pores in structures, establishing a relationship between the carbonation depth of recycled aggregate concrete with discarded fibers and the fractal dimension of pore volume. Ding et al. [24], using rapid carbonation methods, found that the carbonation resistance of recycled aggregate concrete initially increases and then decreases with the increase in the content of basalt fibers. Regarding recycled aggregate treatment, Li et al.’s research [25] indicates that soaking recycled aggregates in slurry enhances the carbonation resistance of recycled aggregate concrete, reducing the carbonation depth by approximately 50%. Additionally, both domestic and international scholars have explored physical enhancement methods for recycled aggregates, such as particle reshaping, heating-grinding, and microwave heating. They have also investigated the chemical enhancement methods, including immersion in nano SiO2 solution, water glass solution, acidic solution, polyvinyl alcohol (PVA) emulsion, and CO2 carbonation [26,27]. Regarding surface protective coatings, Zhu et al.’s research [28] demonstrates that utilizing silane surface waterproofing treatments enhances the durability of recycled aggregate concrete.

Although these methods effectively enhance the carbonation resistance of recycled aggregate concrete, some may present challenges in practical engineering. Moreover, previous scholars have not conducted comparative studies. In engineering applications, it is crucial not only to achieve favorable results but also to ensure ease of construction. Therefore, conducting comparative research on these methods is necessary to select the most suitable approach for improving the carbonation durability of recycled aggregate concrete. Furthermore, in previous studies on recycled aggregate concrete, most only incorporated a portion of recycled coarse aggregates (typically replacing 30–70%). Research on concrete using entirely recycled aggregates is relatively scarce. Full recycled aggregate concrete [29] employs 100% recycled coarse aggregates, increasing waste disposal capacity and reducing the use of natural aggregates, thereby contributing to energy conservation and carbon reduction. However, the carbonation resistance of full recycled aggregate concrete is significantly reduced. Enhancing the durability of full recycled aggregate concrete, especially its carbonation resistance, would be advantageous for expanding the application scope of recycled aggregate concrete. Additionally, it would help reduce carbon emissions in construction and accelerate the development of an environmentally friendly society.

This study aims to investigate different methods to enhance the carbonation resistance of recycled aggregate concrete, with a primary focus on researching the carbonation resistance of full recycled coarse aggregate concrete. The methods include adding mineral admixtures, applying external coatings, adding high-toughness polypropylene (HTPP) fibers, and incorporating superabsorbent polymer (SAP). The objective is to compare these various approaches for improving the carbonation resistance of full recycled aggregate concrete, aiming to provide a theoretical foundation for engineering applications.

2 Experimental overview

2.1 Raw material

Cement used is ordinary Portland cement, while the sand utilized is washed sea sand with an apparent density of 2,630 kg/m³ and a fineness modulus of 2.60. Mixing water was tap water. Recycled coarse aggregates are sourced from demolished C30 concrete, ranging in particle size from 5 to 20 mm, meeting the requirements outlined in GB/T 25177-2010 “Recycled Coarse Aggregates for Concrete” [30]. Fly ash is obtained from Taizhou Tianda Environmental Building Materials Co., Ltd, and is of Grade II quality. Microspheres [27,28,29,30,31,32,33,34] are manufactured by Tianjin Zhucheng New Material Technology Co., Ltd, featuring an apparent density of 2,520 kg/m3, a bulk density of 760 kg/m3, and a specific surface area exceeding 1,300 m2/kg. Polycarboxylate superplasticizer (PCE) with a solid content of 20% is employed as the water reducer. The SAP, sized at 120 mesh, is procured from Yixing Trustworthy Chemical Trading Co., Ltd. HTPP fibers [35,36] are sourced from Ningbo Shike New Material Technology Co., Ltd, boasting a tensile strength of 750 MPa, an elongation at break of 14%, a length of 12 mm, a diameter of 0.15–0.2 mm, a draw strength of 800–1,200 MPa, and an elastic modulus of 8–10 MPa, with some of the raw materials depicted in Figure 1.

Figure 1 Partial raw materials. (a) Recycled aggregate. (b) Microsphere. (c) SAP. (d) HTPP fibers.
Figure 1

Partial raw materials. (a) Recycled aggregate. (b) Microsphere. (c) SAP. (d) HTPP fibers.

2.2 External coating material

HM1500 inorganic waterproofing agent is produced by Guangzhou Anbaijia New Materials Co., Ltd, using JC/T1018-2006 “Waterborne Penetrating Inorganic Waterproofing Agent [37].” This product is widely used in waterproofing, corrosion resistance, and weathering protection in areas such as building construction, airports, pool bridges, and other engineering projects. HM1500 waterproofing agent is an inorganic solution containing highly efficient catalysts and carriers. It possesses strong penetration and diffusion capabilities, allowing it to permeate the interior of cementitious concrete (mortar) structures and react with alkali substances in the cement mixture. This reaction forms insoluble branched crystals, creating an inherent waterproof protection. It also fills internal voids, seals capillary channels, increases density, and forms a reliable permanent waterproof layer. Additionally, it can enhance the mechanical strength of cement structures and provides functions such as mold resistance, weather resistance, and protection against acid and alkali erosion. Compared to similar materials, the application of HM1500 waterproofing agent is simple. It can be directly sprayed or brushed onto the concrete substrate without the need for a leveling layer or any protective coating.

The silane impregnating agent is produced by Beijing Jiasheng Building Materials Co., Ltd. It is a colorless and transparent liquid that, when applied, does not alter the natural color of concrete. It exhibits excellent adhesion to the concrete substrate and can penetrate into the interior of the concrete, forming a waterproof and breathable protective layer. It offers outstanding durability, good resistance to ultraviolet radiation, excellent breathability, and provides effective protection against erosion from seawater, salt spray, de-icing agents, and freeze-thaw cycles for concrete. The silane impregnating agent provides waterproofing, chloride ion resistance, and UV resistance while maintaining breathability. It effectively prevents substrate deterioration due to water infiltration, sunlight, acid rain, and seawater, which can lead to corrosion, loosening, peeling, and mold growth in both concrete and internal steel reinforcement structures. This enhances the lifespan of buildings. When applied to the concrete surface, it can penetrate the substrate, undergo a chemical reaction with the moisture within, forming a network of silicon resin polymer protective structures. Eventually, it creates a strong hydrophobic layer on the substrate’s surface, preventing and resisting steel corrosion and concrete decay. This results in improved durability for concrete structures. Silane impregnating agents are commonly used in various projects, including marine docks, nuclear power plants, high-speed railways/roads, bridges, tunnels, and projects in freeze-thaw environments.

2.3 Preparation of test pieces

This study involved the design of six sets of fully recycled aggregate concrete specimens. Among them, specimen 1# serves as the blank control group. Compared with specimen 1#, specimen 2# has its outer surface coated with a silane impregnating agent, while specimen 3# has its outer surface coated with HM1500 waterproofing agent. The other three groups of specimens are designed to enhance carbonation resistance by incorporating additional materials. Specimen 4# has SAP added during mixing, while specimen 5# replaces 10% of the cementitious material with microbeads, and specimen 6# incorporates HTPP fibers at a volume fraction of 0.6%. Other materials are the same as those in group 1#.

The mix proportions for the six groups of specimens are shown in Table 1. The preparation of test specimens in this experiment followed the standard GB/T50081-2019 “Standard Test Methods for Mechanical Properties of Concrete” [38]. For each numbered specimen, 100 mm × 100 mm × 100 mm cubic specimens and 100 mm × 100 mm × 400 mm prismatic specimens were prepared. The prismatic specimens were used to test the carbonation depth of each age group, while the cube specimens were used to test the strength of various carbonation ages and for SEM analysis. Each set of specimens consisted of three individual specimens. The specimen preparation process was as follows: First, dry mixing of the powder and recycled aggregates in the mixer for 30 s to achieve uniform blending, followed by the addition of water and mixing for 90 s. Subsequently, the mixture was placed into molds, vibrated to shape, and covered with plastic film. After 24 h, the molds were removed, and the specimens were placed in a curing chamber with a temperature of (20 ± 3)°C and a relative humidity greater than 90%. The production and molding of the fully recycled aggregate concrete specimens are illustrated in Figure 2. The dry densities of specimens for each group are presented in Figure 3, with dry densities of the six groups of fully recycled aggregate concrete specimens fluctuating between 1,900 and 2,100 kg/m3.

Table 1

Experimental mix ratio for carbonation design of fully recycled aggregate concrete (1 m3)

No. PCE/kg Water/kg Cement/kg Fly ash/kg Sand/kg RCA/kg SAR/kg Microbeads/kg Fiber Surface
1# 7 205 303 117 610 1,130 0 0 0 Untreated
2# 7 205 303 117 610 1,130 0 0 0 Silane
3# 7 205 303 117 610 1,130 0 0 0 HM1500
4# 7 205 303 117 610 1,130 0.80 0 0 Untreated
5# 7 205 303 78 610 1,130 0 39 0 Untreated
6# 7 205 303 117 610 1,130 0 0 0.6% Untreated
Figure 2 Preparation of fully recycled aggregate concrete. (a) Fresh recycled aggregate concrete. (b) Forming of recycled aggregate concrete.
Figure 2

Preparation of fully recycled aggregate concrete. (a) Fresh recycled aggregate concrete. (b) Forming of recycled aggregate concrete.

Figure 3 Dry density of fully recycled aggregate concrete.
Figure 3

Dry density of fully recycled aggregate concrete.

2.4 Test method

2.4.1 Carbonation test

According to the “Standard Test Methods for Long-term Performance and Durability of Ordinary Concrete” (GB/T50082-2009) [39], the specimens were removed from the standard curing chamber after 26 days of curing. They were then taken out of the constant temperature and humidity chamber after 2 days of drying. Two opposite sides of the specimens were left unwaxed, while the rest of the sides were coated with paraffin. Among these 6 sets of specimens, specimen 2# is coated with a silane impregnating agent on the two exposed side surfaces, while specimen 3# is coated with MH1500 waterproofing agent on the two exposed side surfaces, and the dosage is approximately 200 g/m2, the coating process of the specimen is shown in Figure 4. The next step involved drawing parallel lines on the side surface at intervals of 10 mm to determine the positions of each measuring point. Subsequently, the specimens were placed in a carbonation chamber, maintaining a distance of at least 50 mm between them. The carbonation chamber maintains a temperature of approximately 20°C, humidity at around 70%, and CO2 concentration at about 20%. After carbonation for 3, 7, 14, and 28 days, the specimens were split open and sprayed with a 1% phenolphthalein solution, and the depth of carbonation was measured using an electronic caliper.

Figure 4 Coating on the surface of the specimen.
Figure 4

Coating on the surface of the specimen.

2.4.2 Microstructure testing

Microstructural analysis was conducted using SEM. After curing the specimens for 28 days, a small section from the center of each broken specimen was taken, coated with gold, and observed using an electron microscope scanner. Microscopic morphology of the cement matrix and interface transition zone was captured and analyzed using the SEM. Additionally, this experiment also examined the pore structure on the surface of the recycled aggregate concrete and the surface morphology of specimens treated with silane impregnating agent and HM1500 waterproofing agent.

3 Results and discussion

3.1 Analysis of the impact of carbonation depth

The carbonation of the six groups of recycled aggregate concrete specimens at 3, 7, 14, and 28 days is illustrated in Figures 5–10. The carbonation depths are presented in Table 2 and Figure 11. During the carbonation test, the growth of carbonation depth was rapid in the first 7 days, gradually increasing as the carbonation age progressed. At 28 days, the maximum carbonation depth reached 25.2 mm, significantly exceeding the carbonation depth of ordinary concrete [40]. The relationship between the carbonation depth of the various groups of recycled aggregate concrete and carbonation time is depicted in Figure 11. It is evident from Figure 11 that the carbonation depth of the recycled aggregate concrete specimens in all six groups increased with the increase in the carbonation time. Initially, the growth rate was relatively fast, but it slowed down after 7 days. However, specimen 1# continued to exhibit a relatively high carbonation rate. The untreated recycled aggregate concrete specimens maintained the highest carbonation depth throughout the entire aging period. Compared to specimen 1#, the carbonation depth of specimens 2#–6# decreased by 16, 20, 36, 48, and 48% at 28 days, respectively. Microspheres and HTPP fibers significantly enhanced the resistance of recycled aggregate concrete to carbonation.

Figure 5 Specimens 1# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.
Figure 5

Specimens 1# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.

Figure 6 Specimens 2# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.
Figure 6

Specimens 2# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.

Figure 7 Specimens 3# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.
Figure 7

Specimens 3# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.

Figure 8 Specimens 4# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.
Figure 8

Specimens 4# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.

Figure 9 Specimens 5# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.
Figure 9

Specimens 5# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.

Figure 10 Specimens 6# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.
Figure 10

Specimens 6# carbonation phenomenon. (a) 3 days. (b) 7 days. (c) 14 days. (d) 28 days.

Table 2

Carbonation depth of each group of fully recycled aggregate concrete

Number Carbonation depth at different carbonation ages/mm
3 days 7 days 14 days 28 days
1 8.12 13.23 15.21 25.62
2 9.22 12.25 19.16 21.12
3 10.41 13.21 18.32 20.26
4 7.36 11.25 13.28 16.47
5 5.95 8.05 10.89 13.15
6 6.33 8.52 11.24 13.17
Figure 11 Comparison of carbonation depth.
Figure 11

Comparison of carbonation depth.

3.2 Analysis of compressive strength after carbonation

The 28-days cured strength of the six groups of recycled aggregate concrete specimens is shown in Figure 12, and the strength of each group of specimens after undergoing various carbonation stages is shown in Figure 13. It can be observed that the strengths of specimens 4#–6# are higher than those of specimens 1#–3# at all ages. The strengths of specimens 1#–3# remain relatively constant before carbonation. After carbonation, the strength of specimen 1# is higher than that of specimens 2# and 3#. This is because specimen 1# has a higher degree of carbonation, resulting in the formation of more calcium carbonate, leading to higher strength. The higher strength of specimens 4#–6# is primarily attributed to several factors. In specimen 4#, the presence of SAP enhances the curing effect inside the recycled aggregate concrete, enabling sufficient hydration reactions internally. Specimen 5# exhibits higher strength due to the microspheres improving the internal pore structure of the concrete and filling the defective areas in the interface transition zone, thereby enhancing the pore structure and optimizing the structure of the interface transition zone, leading to an increase in compressive strength. The high strength of specimen 6# is attributed to the addition of HTPP fibers, which effectively improve the micro-pore structure of recycled aggregate concrete, resulting in a denser internal structure. This significantly suppresses the extension of micro-cracks within the structure, thereby enhancing the strength of recycled aggregate concrete.

Figure 12 28 days age intensity.
Figure 12

28 days age intensity.

Figure 13 Strength of each carbonation age.
Figure 13

Strength of each carbonation age.

Additionally, the evaluation of strength growth in recycled aggregate concrete at different carbonation ages can be achieved through relative strength analysis. Relative compressive strength data of recycled aggregate concrete under carbonation, obtained from experiments, are presented in Table 3. The relationship curve depicting the relative compressive strength of the six groups of recycled aggregate concrete with carbonation age is illustrated in Figure 14. To derive accurate equations for the relative compressive strength curves, this study employed a bivariate function to fit the curves of the six sets of recycled aggregate concrete after carbonation, as shown in Figure 15. The fitted curve aligns well with the experimental curve, and the relationship of the model function is expressed as Equation (1).

(1) λ = α t 2 + β t + γ ,

where λ represents the relative compressive strength. The parameter α represents the initial velocity of compressive strength for recycled aggregate concrete, while β represents the initial value of relative compressive strength. Additionally, γ represents the initial value of relative compressive strength. The variable t represents the carbonation age, with experimental values of 0, 3, 7, 14, and 28 days. The parameters obtained through fitting in this experiment are shown in Table 4. Since (2α + β) is a positive value, it indicates that the relative compressive strength of recycled aggregate concrete increases with the extension of the carbonation age, aligning with the experimental findings. By comparing the model parameters of the six sets of recycled aggregate concrete in Table 4, it can be observed that the first set exhibits a faster initial carbonation rate and a rapid increase in compressive strength, resulting in a higher rate of change in relative compressive strength compared to the other sets. This observation is consistent with the described pattern of relative compressive strength change under carbonation.

Table 3

Strength and relative compressive strength of specimens at different carbonation ages

Number Compressive strength/MPa Relative compressive strength F cu , c /MPa
0 days 3 days 7 days 14 days 28 days 0 days 3 days 7 days 14 days 28 days
A1 29.5 29.8 37.9 42.0 45.9 1.00 1.01 1.28 1.42 1.56
A2 29.6 30.1 35.9 39.7 40.1 1.00 1.02 1.21 1.34 1.35
A3 29.1 31.5 36.4 39.9 40.3 1.00 1.08 1.25 1.37 1.38
A4 40.9 41.4 43.9 49.6 54.9 1.00 1.01 1.07 1.21 1.34
A5 37.3 43.5 45.2 54.5 56.9 1.00 1.16 1.21 1.46 1.53
A6 40.3 43.8 46.6 52.8 59.7 1.000 1.09 1.16 1.41 1.48
Figure 14 Curve of relative compressive strength with age.
Figure 14

Curve of relative compressive strength with age.

Figure 15 Relationship curve between relative compressive strength and carbonation age. (a) Specimen 1#. (b) Specimen 2#. (c) Specimen 3#. (d) Specimen 4#. (e) Specimen 5#. (f) Specimen 6#.
Figure 15

Relationship curve between relative compressive strength and carbonation age. (a) Specimen 1#. (b) Specimen 2#. (c) Specimen 3#. (d) Specimen 4#. (e) Specimen 5#. (f) Specimen 6#.

Table 4

Relative compressive strength model parameters

Number a b c R 2
A1 −8.31299 × 10−4 0.04466 0.96211 0.96035
A2 −8.76061 × 10−4 0.03826 0.96794 0.99999
A3 −9.71832 × 10−4 0.04112 0.99006 0.99038
A4 −1.3285 × 10−4 0.01674 0.97947 0.98033
A5 −8.44237 × 10−4 0.04251 1.0052 0.97644
A6 −7.04076 × 10−4 0.03773 0.98172 0.97413

3.3 Prediction of carbonation depth of recycled aggregate concrete

The assumption widely accepted by researchers is that concrete is an isotropic continuous medium material, and steady-state diffusion theory can be used to simulate the diffusion process of CO2. This study establishes a carbonation depth prediction model for fully recycled aggregate concrete based on the carbonation theory of ordinary concrete [41,42,43,44,45]. The expression of this model is shown as Equation (2).

(2) H = K t ,

where H represents the carbonation depth (mm), K represents the carbonation coefficient, and t represents the carbonation time (days). The data on carbonation depth vs carbonation time were fitted, and the corresponding formulas and correlation coefficients (R 2) for the fits are shown in Table 5. The curves obtained from the fitting are illustrated in Figure 16. The obtained carbonation rate coefficients K and the fitting correlation coefficients are also presented in Table 5. Carbonation depth theoretical values compared with experimental values are shown in Table 6. From Table 6, it can be observed that the prediction model has a relatively small error compared to the measured values. Additionally, Figure 16 demonstrates that the experimental curve aligns well with the fitted curve, indicating that the carbonation depth prediction model is applicable to all recycled aggregate concrete specimens. From the coefficients K in Table 5, it can be inferred that specimen 1# exhibits the highest carbonation rate, while specimens 2# and 3# show a slight decrease in carbonation rate. Specimens 4#–6# demonstrate a significant reduction in carbonation rate compared to specimen 1#, with decreases of 26.1, 40.0, and 29.5%, respectively. The addition of microspheres to recycled aggregate concrete has the most significant effect on enhancing its carbonation resistance.

Table 5

Fitted parameters of the carbonation model

Fitting parameters Group number
1# 2# 3# 4# 5# 6#
K 4.55122 3.41615 4.38734 3.35641 2.7295 3.20791
R 2 0.98261 0.95168 0.93821 0.93998 0.95786 0.98338
Figure 16 Carbonation model’s fitted curve. (a) Specimen 1#. (b) Specimen 2#. (c) Specimen 3#. (d) Specimen 4#. (e) Specimen 5#. (f) Specimen 6#.
Figure 16

Carbonation model’s fitted curve. (a) Specimen 1#. (b) Specimen 2#. (c) Specimen 3#. (d) Specimen 4#. (e) Specimen 5#. (f) Specimen 6#.

Table 6

Experimental and theoretical values of carbonation depth

Specimen group number Carbonation age
3 days 7 days 14 days 28 days
x/mm x′/mm Error/% x/mm x′/mm Error/% x/mm x′/mm Error/% x/mm x′/mm Error/%
1# 8.12 7.88 2.96 13.23 12.04 8.99 15.21 17.03 −11.97 25.62 24.08 6.01
2# 9.22 7.65 17.03 12.25 11.68 4.65 19.16 16.52 13.78 21.12 23.36 −10.61
3# 10.41 7.49 28.05 13.21 11.44 13.40 18.32 16.18 11.68 20.26 22.89 −12.98
4# 7.36 5.81 21.06 11.25 8.88 21.07 13.28 12.56 5.42 16.47 17.76 −7.83
5# 5.95 4.73 20.50 8.05 7.22 10.31 10.89 10.21 6.24 13.15 14.44 −9.81
6# 6.33 4.84 23.54 8.52 7.39 13.26 11.24 10.45 7.03 13.17 14.78 −12.22

Note: x represents the test value and x′ represents the theoretical values; Positive error indicates that the experimental value is greater than the theoretical value, while negative error indicates that the measured value is smaller than the theoretical value.

3.4 Carbonation life

The impact of carbonation on the durability of reinforced concrete mainly manifests as the infiltration of carbon dioxide into the concrete, reacting with the alkalis in the concrete. This reaction reduces the pH value of the concrete. When the depth of carbon dioxide infiltration reaches the protective layer of reinforced concrete structures, it leads to the breakdown of the passivation film on the surface of the reinforcement, resulting in corrosion of the reinforcement. Consequently, there is a continuous decrease in the structural load-bearing capacity. Therefore, the diffusion of carbon dioxide in concrete structures is correlated with their durability and service life.

This study refers to relevant literature to investigate the carbonation life of recycled aggregate concrete components. According to the literature [46,47], when carbon dioxide diffuses to the surface of reinforced concrete, the concrete’s protective layer is completely destroyed, and at this point, the reinforcement begins to corrode. According to the “Code for Design of Concrete Structures” GB50010-2010 [48], the protective layer of reinforcement is generally 25 mm. Therefore, it is generally considered that when the carbonation depth reaches 25 mm, it represents the carbonation life of the concrete component. The carbonation life of the component under natural environmental conditions is determined by Equation (3) [49]. By using this formula, it is possible to predict the carbonation life or durability under natural environmental conditions.

(3) d 2 d 1 = t 2 . c 2 t 1 . c 1 ,

where d 1 is the carbonation depth under accelerated carbonation experiment conditions; d 2 is the carbonation depth at which the carbonation life is reached under natural conditions, which is 25 mm; t 1 is the accelerated carbonation time, which is 28 days; t 2 is the time to reach the carbonation lifespan in the natural environment; c 1 is the carbon dioxide concentration during accelerated experiments, in this experiment, the concentration of CO2 for accelerated carbonation is 20%; c 2 is the carbon dioxide concentration in the natural environment, which is 0.03%.

The calculated carbonation lifespans of the six sets of fully recycled aggregate concrete specimens are shown in Figure 17. The carbonation life of fully recycled aggregate concrete significantly increases after the addition of microspheres or HTPP fibers. Compared to fully recycled aggregate concrete without any treatment, the carbonation life increases by 278%. Additionally, the carbonation life of fully recycled aggregate concrete treated with silane impregnation and MH1500 also shows a corresponding increase, with an increase of over 47%.

Figure 17 Comparison of carbonation life.
Figure 17

Comparison of carbonation life.

3.5 Microscopic mechanism analysis

3.5.1 Effects of applying protective agents externally

Figure 18 shows SEM images of the specimen surfaces. From Figure 18, it can be observed that the surface of specimen 1# is uneven, with many small voids. Compared to specimen 1#, the surface of specimen 2# has been blocked by the silane impregnating agent, and the surface appears to be in a cohesive state, but numerous pores are still visible. Similarly, compared to specimen 1#, the surface of specimen 3# is relatively smooth, which is because MH1500 blocks the surface of the concrete; however, some cracks still appear on the surface. The SEM images of the surface explain why the carbonation rates in the 2# and 3# groups are lower than in group 1#; the surface coatings prevent the infiltration of CO2.

Figure 18 SEM image of the specimen surface. (a) Specimen 1#:3000. (b) Specimen 2#:3000. (c) Specimen 3#:3000.
Figure 18

SEM image of the specimen surface. (a) Specimen 1#:3000. (b) Specimen 2#:3000. (c) Specimen 3#:3000.

3.5.2 Effect of adding materials

Figure 19 is an SEM comparative image between specimens 1# and 4#, 5#, 6#. In Figure 19a, the width of the interface transition zone is significantly greater than that in Figures 19b and c. Figure 19b represents full recycled aggregate concrete specimens with the addition of a highly absorbent resin. The inclusion of this resin continuously releases water during the concrete curing process, promoting hydration reactions within the matrix, resulting in a denser matrix. Figure 19c depicts full recycled aggregate concrete specimens with the addition of microspheres. After adding microspheres, some of which have particle sizes in the micrometer or even nanometer range, these fine particles fill the interface transition zone, making it denser. Figure 19d shows the scanning electron microscope image of full recycled aggregate concrete with the addition of HTPP fibers. From Figure 19d, it can be observed that the fibers are tightly embedded within the cement matrix, making the cement matrix exceptionally dense. This inhibits the extension of microcracks and prevents the infiltration of carbon dioxide gas, thus significantly reducing the carbonation depth.

Figure 19 SEM image of the interior of the specimens. (a) Specimen 1#. (b) Specimen 4#:3000. (c) Specimen 5#:900. (d) Specimen 6#:90.
Figure 19

SEM image of the interior of the specimens. (a) Specimen 1#. (b) Specimen 4#:3000. (c) Specimen 5#:900. (d) Specimen 6#:90.

4 Conclusion

This study employs five methods to enhance the carbonation resistance of full recycled aggregate concrete and draws the following conclusions:

  1. Through comparing techniques for enhancing the carbonation resistance of recycled aggregate concrete, the method of adding microspheres and HTPP fibers was identified as the preferred option, demonstrating effective results, simplicity, convenience, and cost-effectiveness.

  2. The incorporation of microspheres and HTPP fibers showed the most significant effect in preventing carbonation of fully recycled aggregate concrete, resulting in a 48% reduction in carbonation depth by the 28th day.

  3. A relative compressive strength model and a carbonation depth prediction model were established for fully recycled aggregate concrete. The curves of these models showed good agreement with the measured data, providing valuable references for predicting carbonation depth and assessing strength after carbonation for such types of recycled aggregate concrete.

  4. The mechanism for enhancing carbonation resistance of fully recycled aggregate concrete were explained from a microscopic perspective. Microspheres enhanced the density of recycled aggregate concrete and the compactness of the interface transition zone, while HTPP fibers resulted in a denser internal structure, effectively inhibiting the propagation of internal microcracks within the structure.

  1. Funding information: This research was funded by Science and technology project of Department of housing and urban–rural development of Zhejiang Province (2023K217, 2023K165), Taizhou Science and Technology Plan Project (23gyb03, 22gyb07, 22gyb11), General scientific research project of Zhejiang Education Department (Y202249335), Science and technology project of the Ministry of Housing and Urban-Rural De-velopment (2021-K-125).

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and consented to its submission to the journal, reviewed all the results, and approved the final version of the manuscript. SW: writing – original draft, formal analysis and conceptualization. YW: investigation, methodology, and formal analysis. JY: conceptualization, supervision, and writing – review and editing. RW: data curation and investigation. JWF: formal analysis, writing – review and editing. WL: investigation and validation. HH: supervision and methodology. XD: writing – review and editing. WX: supervision and methodology. ZZ: writing – review and editing and data curation.

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: All data included in this study are available upon request by contacting the corresponding author.

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Received: 2024-03-25
Revised: 2024-04-22
Accepted: 2024-05-25
Published Online: 2024-07-13

© 2024 the author(s), published by De Gruyter

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

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https://doi.org/10.1515/secm-2024-0022
Keywords for this article
fully recycled aggregate concrete; microsphere; HTPP fibers; super absorbent resin; carbonation; prediction model; microstructure
Creative Commons
BY 4.0

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