Applications of micro-nanobubble and its influence on concrete properties: An in-depth review

2.1 Stability

The stability of MNBs refers to their ability to maintain their shape, size, and properties over time. Several factors influence the stability and reactivity of NBs, including bubble size and characteristics, zeta potential, and surface properties. In addition, the properties of NBs are influenced by the solution’s characteristics, the type of gas used, and the energy inputted into the system for nanobubble production. Solution properties such as temperature, pressure, ion type, ion concentration, pH, presence of organic substances or impurities, presence of surfactants (surface activators), and the concentration of saturated gas have a significant impact on the characteristics of NBs. Moreover, the specific gas employed and its solubility and reactivity can also affect the bubble’s properties [18,21,22,23].

2.2 Characteristics of MNBs and their controlling factors

2.2.1 Bubble size

The size of air bubbles is the most important characteristic associated with them. Generally, MBs are bubbling whose diameter is about micrometers, but in studies, their diameter ranges are different. Bubbles can be classified into macro bubbles, MBs, sub-micron bubbles, or NBs corresponding to ordinary or big bubbles, and fine and ultrafine bubbles based on their size [11,16].

Macro bubbles are larger and more influenced by buoyancy and speed, which makes them prone to instability during measurement. On the other hand, longer stability time in liquid makes MNB characterization more feasible. It is worth noting that the size of the bubbles is dynamic and by merging or shrinking, the bubbles can transform into each other [11,24]. In Figure 4, the size distribution of MNBW bubbles is depicted based on three measured parameters: volume, number, and intensity.

Figure 4

The air bubble size distributions of MNBs in water [25]. (a) Air bubble size distribution in MNBs by volume, (b) air bubble size distribution in MNBs by number, and (c) air bubble size distribution in MNBs by intensity.

The size dispersion of MNBs was assessed using the nanoparticle size analyzer tool on specimens aged 1 day, employing the cumulant analysis method that considers three parameters: volume, number, and intensity of specimens. This analysis was conducted at the central laboratory of Ferdowsi University of Mashhad in Iran. Figure 4a–c depicts the size dispersion of MNBs based on volume, number, and intensity parameters, respectively [25].

2.2.4 Zeta potential

Zeta potential is a physical property possessed by every particle in a suspension, and it can be utilized to enhance and optimize the creation of suspensions and emulsions [16]. Zeta potential refers to the electric potential difference between the moving scattering medium and the static layer attached to the scattering particle. The value of zeta potential is strongly influenced by the stability of emulsions or colloids, in both the short term and the long term [11]. In the case of MNBs, their zeta potential describes their charge characteristics, which impact their dispersion behavior. In the case of MNBs, their zeta potential describes their charge properties, which have an impact on the dispersion behavior of particles. High absolute values of zeta potential are suitable for improving MNB stability. When particles possess a high negative potential, they typically exhibit a mutual repulsion. Conversely, particles with low zeta potential lack the necessary force to prevent them from approaching each other [28]. Many factors affect the zeta potential of bubbles, such as solution pH, gas, ionic strength, additive concentration, and temperature. Generally, increasing the pH will decrease the zeta potential [29]. The zeta potential of NBs is influenced by the gas’s ability to generate hydroxyl ions on the bubble’s surface. Solutions with high pH, low temperature, and low salt concentration promote a high negative zeta potential for bubbles. In long-term experiments, the zeta potential of bubbles tends to diminish as the bubble size increases [18]. In Zhang et al.’s research [30], a correlation between the zeta potential of NBs and their size has been identified, indicating a special relationship (Figure 5). It is a graph that shows the changes in zeta potential for water containing MNBs at the age of 1 day.

Figure 5

The variation of zeta potential for water containing MNBs at the age of 1 day [25].

Zeta potential stands out as a pivotal parameter of MNBs, playing a crucial role in predicting the stability of particles within the solvent. The surface zeta potential of water containing MNBs is assessed using the zeta compact cad instrument. Figure 5 demonstrates the variations in zeta potential of water containing MNBs at the age of 1 day, spanning from −10 to −40 mV experimented at the central laboratory of Ferdowsi University of Mashhad in Iran [25].

2.2.6 Rheological behavior

Rheology is the scientific field that investigates how matter deforms and flows when subjected to external forces. It can be categorized into two branches: surface rheology and bulk rheology. In measuring the rheological properties of a material, it is important to know the flow properties such as shear stresses and shear rates. The relationship between shear stress and shear rate is shown graphically in a flow curve. Fluids can be identified by their flow curves. Various models – or constitutive equations – have been developed to idealize flow curves. Six of the most common constitutive relationships associated with concrete are shown in Figure 6 [35].

Figure 6

Six common models for determining the rheological behavior of concrete [35].

The yield stress and plastic viscosity are the two primary parameters measured by rheometers when applying the Bingham model. Equation (1) presents the rheological equation that defines the Bingham model [9].

(1) τ = τ 0 + μ · γ ̇ τ ≥ τ 0 ,

where τ is the shear stress in Pa, τ 0 is the yield stress in Pa, μ is the plastic viscosity in Pa.s, and γ ̇ is the shear rate in 1/s.

Some researchers have recommended the Herschel–Bulkley (H-B) model (equation (2)) to describe the shear thickening behavior of cementitious mixtures. In this model, the shear thickening behavior occurs when n > 1, and the shear thinning behavior occurs when n < 1. The value of n indicates the degree of shear thickening response.

τ = τ 0 + k · γ ̇ n τ ≥ τ 0 ,

where τ is the shear stress in Pa, τ 0 is the yield stress in Pa, and γ ̇ is the shear rate in 1/s. n is the flow index, and k is the consistency factor in Pa s ⁿ .

Bingham’s model was modified to describe the shear thickening and thinning behavior (equation (3)). In this regard, the shear thickening behavior occurs when c/μ > 0, and the shear thinning behavior occurs when c/μ < 0 [36].

τ = τ 0 + μ · γ ̇ + C γ ̇ 2 τ ≥ τ 0 .

MNBs primarily due to bubble coalescence and coarsening are thermodynamically unstable systems. Previous research has demonstrated the significance of the rheological characteristics of the gas–liquid interface in determining the stability of MNBs [14]. In a study conducted by Shen et al. [37], the stability and rheological behavior of bubble suspensions coated with concentrated food emulsifiers were investigated. MBs were generated using flow concentration, where a food-grade emulsifier was employed. The emulsifier formed a thin shell around the MNBs, resulting in their stabilization. The diameter of the produced MBs ranged from 120 to 200 μm and could be controlled by adjusting the flow rate of the liquid and gas phases. To assess the rheological properties of the MBs suspension, a rotational rheometer was used. Gas retention in the experimental sample was determined through weight difference measurements. Shen et al. observed that MNBs remained stable over time without significant changes in size. Furthermore, it was discovered that as the shear stress increased, the viscosity of the system decreased. This observation suggests that the MNB suspension displayed viscoelastic properties. The reduction of static yield stress and plastic viscosity of SCC using MNB water were higher than tap water with the same amount of superplasticizer, concrete age, and temperature [9]. Based on convexity in the diagram, shear thinning behavior was observed in concrete samples with MNBs. The c/μ (negative) ratio went up by increasing the superplasticizer dose. This means increasing the intensity of shear-thinning behavior by increasing the dosage and potential for segregation and bleeding [9].

4.1 Literature survey

This section presents the findings from a range of studies that have explored the use of MNBs in both conventional concrete and SCC.

Zhang et al. [76] studied the impact of MNBs on the rheology of concrete, focusing on bubble characteristics such as size, specific surface area, and distance factor. The research used 11 different types of air bubble agents and conducted gray correlation and classification analyses. Their findings suggest that bubble characteristics play a more important role in concrete rheology than air content alone. Smaller bubbles (10–200 µm) lead to a greater improvement in concrete slump and yield stress, as they distribute more evenly in the mortar, enhancing the balling and lubrication effect. Larger bubbles (beyond 200 µm) have less influence on cohesion. Classification analysis indicated that smaller bubbles (10–600 µm) result in greater slump increases compared to larger bubbles (1,600–600 µm). Overall, the research highlights the importance of bubble size and distribution in optimizing the rheological properties of fresh concrete. The presence of bubbles in the mortar improves the flow and rheological properties of fresh concrete by uniformly distributing small bubbles in the mortar. These bubbles fill spaces between aggregates and cement particles, trapping free water and releasing it to form concrete lubrication. The introduction of bubbles thickens the mortar layer, increasing the distance between coarse aggregates, and reducing collision and frictional resistance, which significantly enhances concrete’s overall performance.

Grzegorczyk-Frańczak et al. [77] examined the impact of using MNBs containing O₂ and O₃ gases in concrete. The study found that MNBs reduced concrete porosity, resulting in increased bulk and specific density, with a more pronounced effect from O₂ bubbles. Concretes with MNBs, particularly those with ozone, had lower water absorption and higher compressive strength at 14 and 28 days. After 150 freeze–thaw cycles, concrete with O₂ MNBs showed 44% less weight loss than the reference concrete, while O₃ MNBs demonstrated even less weight loss. The reduction in compressive strength was less for concretes with MNBs than for reference concrete. In addition, concretes with MNBs had higher thermal conductivity and lower total surface voids and apparent density, with the best performance seen in concretes with O₃ MNBs. Arefi et al. [78] investigated the effect of adding MNBs to water before mixing it with aggregate and cement. The results showed that concrete with MNBs had lower workability, with a slump about 10 mm less than conventional concrete. MNBs led to early or immediate hydration and lower temperature changes during setting. The setting time was reduced, and the 7- and 28-day compressive and tensile strength of concrete with MNBs were higher than those of conventional concrete.

Grzegorczyk-Frańczak et al. [79] studied the impact of MNBs of different gases (O₂, O₃, and CO₂) on the physical and mechanical properties of lime-cement mortars. They found that adding MNBs decreased the workability of fresh mortar, especially with CO₂ MNBs. Higher replacement percentages of MNBs also led to a greater decrease in workability. Concretes with 100% MNB replacement had higher specific densities and turbidity than those with 50% replacement. Volumetric density increased with CO₂ but decreased with O₂ and O₃ MNBs. The highest compressive strength after 28 days was achieved with the 50% O₂ mortar, showing a 31% improvement compared to the reference sample. Flexural strength generally increased in most mortars after 56 days, ranging from 8.3 to 34% higher than the reference sample, except for a 2.6% decrease in the 50% O₃ mortar.

Asadollahfardi et al. [80] replaced drinking water with MNBs and metakaolin as mineral substitutes in concrete. This change accelerated the hydration process and increased the 28-day compressive strength by 8.82 and 13.2% for 50 and 100% MNB replacements, respectively, although these gains were slightly reduced over 90 days. Tensile strength also improved by 10–19% with MNBs, while 42-day flexural strength rose by 3.97 and 5.7%. Water absorption was significantly reduced at 28 days and continued to decrease slightly at 90 days. The 28-day rapid chloride permeation test (RCPT) showed that 50% MNB replacement greatly reduced chloride penetration, with a slight increase at 90 days. Replacing water with MNBs also increased the concrete’s electrical resistance. Mohsen Zadeh et al. [81] investigated the impact of using pozzolans and MNBW in concrete mixtures. Replacing 50% and 100% of water with MNBs increased the 28-day and 90-day compressive strength. The 28-day tensile strength increased by 10 and 19.6% for 50 and 100% MNBs, respectively, while the 42-day flexural strength increased by 2.85 and 5.48% with 50 and 100% replacements. Water absorption decreased significantly at 28 days with reductions of 16 and 20% for 50 and 100% MNBs, respectively, and continued to decrease slightly at 90 days. Chloride permeability also reduced significantly at 28 and 90 days with 50 and 100% MNBs. In addition, MNBs reduced the pH of concrete.

Yahyaei et al. [9] studied the impact of MNBs on SCC. They found that adding MNBs slightly reduced slump flow and increased V funnel time, leading to faster slumping. MNBs also improved permeability and increased the J-Ring test value, but had a little effect on L-box and air percentage results. MNBs enhanced compressive strength but reduced it in mixtures with silica fume. MNBs improved concrete’s impermeability and specific electrical resistance. SEM results showed well-distributed MNB particles under 10 µm. MNB size under 50 nm affected shrinkage but not compressive strength. Kim et al. [82] conducted research in which they used water containing high concentrations of hydrogen nanobubbles as mixing water for cement mortar. They explored the effects of substituting regular mixing water with hydrogen nanobubble water (HNBW) in proportions of 40 and 80%. The nanobubbles used in the study had diameters of less than 200 nm.

TGA is the most widely used method to determine the degree of hydration. In this research, according to Bhatty’s method [76], temperature ranges and types of hydrations were determined. This approach identifies cement hydrates by measuring the weight loss that occurs over a specific temperature range. They concluded that the flexural strength of cement mortar increases with the replacement of nanobubbles. As the amount of substitution increases, the bending strength also increases. Nanobubble has not had a significant effect on the compressive strength of mortar for 3 and 7 days. While the 28-day compressive strength has increased by 6.5 and 11% for 40 and 80% replacement, respectively. Furthermore, by comparing the XRD patterns of cement paste, the pastes made with 0, 40, and 80% substitution of hydrogen nanobubbles had similar patterns. With increasing nanobubble concentration, although there was no difference in the type of crystalline phase, a slight difference in the peak intensity of scattered X-rays was observed. As a result, it was confirmed that the new crystalline phase was not formed by the nanobubbles. In addition, it was confirmed that the reaction speed of nanobubbles with cement particles affects the improvement of mortar strength. Moreover, TGA test results show that:

• During initial heating (28–105°C), the weight loss of all samples was similar.

• As the concentration of bubbles increased during heating up to 1,000°C, the weight loss increased gradually.

• In each temperature range, a sudden change in the “weight-loss-time” curve occurs.

• The amount of total hydrate of cement (C–S–H, Ca (OH)₂, CaCO₃) increases with the increasing concentration of hydrogen nanobubbles. The increase of CaCO₃ for 40 and 80% substitution is almost similar, and the increase in the other two hydrates causes a difference for these two substitution percentages.

• With the increase in the concentration of nanobubbles, the increase in C–S–H is greater, and this means that according to the bubble concentration,

• C–S–H (3CaO·SiO₂·3H₂O) developed more actively than CH(Ca(OH)₂).

• The hydration and pozzolanic reactions are continuously promoted by hydrogen nanobubbles, leading to an overall increase in the degree of hydration, which, in turn, improves the mechanical strength of the cement mortar.

• Increasing nanobubble concentration increased the formation of Ettringite and C–S–H crystals at the same treatment dose.

Wan and He [83] investigated the impact of MNBs and an aluminum sulfate nonalkaline accelerator on the volumetric stability of cement-based materials (shotcrete) over 180 days. The addition of MNBW significantly reduced shrinkage deformation, including self-shrinkage and drying shrinkage, by over 10% in 28 days and about 6.5% in 180 days. MNBW improved internal moisture, reduced porosity by 16.9%, and decreased average pore size by 22.9%, increasing density. MNBs also lowered surface tension and negative capillary pressure, reducing the risk of shrinkage and cracking. MNBW enhanced early resistance development and improved elastic modulus, minimizing the crack width and increasing the ability to resist cracking.

Sheikh Hassani et al. [25] examined the effects of using water with MNBs on cement mortar and concrete at different ages and water-to-cement (W/C) ratios. MNBW slightly increased concrete temperature and pH while boosting water’s electrical conductivity and turbidity. It also shortened the initial and final setting times of cement pastes. MNBW reduced the flowability of cement mortar by 18.75% and decreased slump by 50%. MNB concrete showed lower slump loss over time and increased compressive strength by 16% at 7 days and 7% at 28 days. MNBs enhanced early hydration product formation, leading to less strength increase beyond 28 days. Scanning electron microscopy revealed well-formed crystals and dense solid masses with fewer micropores in MNB concrete.

Lan et al. [84] studied the impact of water-containing nanobubbles and nano silica on air content, freeze–thaw resistance, and hydration in cement materials at different air pressures (0.4, 0.7, and 1 atmosphere). They used analytical techniques like SEM, XRD, and TG-DSC to evaluate hardened cement paste samples. All nanobubbles were smaller than 90 nm, with an average size of 45 nm. Nanobubbles increased the solidified air content in cement paste by 12.3–22% across different pressures, reduced the distance factor, and varied cavity diameter and density depending on pressure. Nanobubble water accelerated cement hydration, increasing calcium silicate hydrate and other cement hydration products. SEM examination showed smoother, more compact pore walls in cement paste with nanobubble water. Compressive strength increased by 0.3–3.9% based on air pressure. Although low pressure increased weight loss, especially after 50 freeze–thaw cycles, nanobubble water improved the freeze–thaw resistance of air-bubbled and nonbubbled samples at low pressure.

Kim et al. [85] investigated the impact of using hydrogen nanobubbles with high concentrations on the performance, durability, water tightness, and microstructure of cement mixtures. By using osmosis for 40 and 80 min, they increased the concentration of hydrogen nanobubbles in water, resulting in more stable states and particle sizes decreasing with longer osmosis time. This increased concentration of nanobubbles led to higher compressive strength, especially after 7 days, though it reduced workability. The higher nanobubble concentration also decreased porosity and increased the density of the mortar, forming a denser cavity structure. The results showed changes in pore distribution that supported a solid structure in the cement paste. SEM tests revealed that as HNBW concentration increased, the internal structure of the cement paste became denser, with more hydration reactions like ettringite and C–S–H crystal formation. Khoshroo et al. [86] studied the impact of replacing traditional concrete with MNBs at 30, 60, and 100% levels on concrete properties. They found that MNB concrete reduces fluidity due to accelerated hydration but can be managed by adding a superplasticizer. MNB concrete has higher density and packing, with higher replacement percentages leading to greater packing. Compressive strength increased by 6, 9, and 13% at 28 days for 30, 60, and 100% replacement levels, respectively, due to faster hydration and improved homogeneity from particle bubbles. However, a decrease in compressive strength was seen at 90 days likely because most hydration occurred within the first 28 days.

Tayebi Jebeli et al. [87] studied the enhancement potential of using MNBW as a replacement for waste foundry sand (WFS) in concrete mixtures. They replaced natural sand with varying levels of WFS (10–40%) and MNBW (50–100%) in concrete mixtures. The optimal mixture was found to be 40% WFS concrete containing 100% MNBW. Adding WFS reduced slump compared to a reference sample, especially with MNBW. The sample with 20% WFS showed the highest increase in compressive strength across different time points, but higher levels of WFS replacement led to decreased compressive strength. In general, MNBW improved compressive strength, particularly at 90 days. Ultrasonic pulse velocity (UPV) results showed no significant effect on concrete quality with MNBW, although water absorption decreased slightly up to 20% WFS replacement. The study found improved chloride penetration up to 20% WFS replacement, with no significant changes beyond this point. SEM tests indicated a reduction in the number and size of pores with WFS, but cracks on the surface increased in width with higher WFS levels and in mixtures with MNBW.

Lee et al. [88] explored the impact of using nanobubble water in cement composites, including high-performance concrete, lightweight cement composites, and high-strength mortar. The water used contained 7% nanobubbles with an average size of 750 nm. Results showed that using nanobubble water improved the compressive strength of cement composites compared to those made with regular water, even with a lower cement ratio. The performance improvement ranged from 3 to 22%, with the greatest increase (22%) seen in ultra-high-performance concrete (UHPC). The study also found that nanobubble water enhanced the workability of the concrete, especially in UHPC, and attributed the improvement to the small size and high quantity of the nanobubbles, which interact with silica fume and prevent the ball effect. Larger nanobubbles, in contrast, tend to float up and disappear from the mixing water. He et al. [89] studied the effects of MNBs and mineral additives on the performance, durability, and mechanical properties of C60 concrete. They found that while MNBs decreased the workability of both conventional concrete and self-consolidating concrete (SCC), samples containing MNBW and silica fume exhibited about 4% higher compressive strength. In addition, the penetration depth of carbonation in samples with MNBs decreased by about 38%. Frost resistance tests showed a 53% reduction in mass loss and a 2.9% decrease in the loss of dynamic elastic modulus during ice melting cycles, indicating that MNBs improve the durability and mechanical properties of C60 concrete.

Wan et al. [90] explored the cold resistance of shotcrete concrete with additives and MNBW. They found that using MNBW reduced the size of concrete pores, increased density, and improved bonding between paste and aggregate. MNBW enhanced the 28-day compressive strength of shotcrete concrete more with an alkaline accelerator than an alkali-free accelerator. MNBW also reduced the loss of compressive strength due to freezing and thawing, particularly after 200 cycles, where it reduced mass loss by 42.2% in concrete with alkaline accelerators and 5.4% without. In addition, samples with normal water showed serious damage after 200 cycles, whereas those with MNBW maintained their relative dynamic elastic modulus. Overall, MNBW improved cold resistance and durability in shotcrete concrete.

Chang et al. [91] used MNBW in the mixing process of sulfo-aluminate cement (SAC) paste and found that MNBW enhanced hydration and improved the microstructure of the cement matrix. SAC paste mixed with MNBW showed improved mechanical properties compared to paste mixed with tap water. Compressive strength increased by 23.8% at 1 day and 14.4% at 28 days, while flexural strength increased by 16.6% at 1 day and 10.7% at 28 days. MNBW also reduced the setting time of the cement paste and lowered its plastic viscosity. MNBW facilitated earlier and higher heat release during hydration, and SEM images revealed a more uniform distribution of hydration products and fewer cracks. MNBW samples had smaller potential pore sizes and lower porosity, resulting in a denser cement matrix. The study presents a unique method for improving the performance of SAC-based materials, which can benefit various construction projects.

Chen et al. [92] examined the impact of using hydrogen MNBs on the workability and mechanical properties of concrete. The study found that adding nanobubble water improved the flowability and slump of concrete, particularly with higher water-to-cement (W/C) ratios. Concrete samples with nanobubble water exhibited increased compressive strength, particularly at 3 days (18%), and showed improvements in water absorption, electrical resistance, and chloride penetration resistance across different ages. SEM images revealed higher hydration rates and formation of hydration products in mixtures containing nanobubble water, indicating enhanced performance due to fewer pores and greater density in the concrete.

Yahyaei et al. [9] evaluated the effect of using micronanobubbles on the rheology of SCC. The results of their study have shown that the plastic viscosity and yield stress were higher in samples made with MNBs. In concrete containing MNBs, shear thinning behavior has occurred and the c/μ ratio (negative) has increased with increasing amount of superplasticizer. When the dosage of superlubricant increased from 4 to 5.5 kg/m³, the use of MNBs increased the percentage of air in SCC by 10, 5, and 4.76%.

Chen et al. [92] investigated the effects of using 150 nm HNBW in cement concrete. They found that HNBW enhanced workability and mechanical properties of concrete while maintaining compressive strength even when cement content was reduced by 10%. HNBW also improved concrete durability, including better water absorption, electrical resistance, and chloride permeability resistance. SEM imaging confirmed the improved quality of concrete with HNBW. The increased workability was attributed to the adsorption of negatively charged hydrogen nanobubbles on cement particles, leading to dispersion and cushioning. HNBW can reduce the need for cement and additives, improving sustainability. The improved mechanical properties and durability result from the weak alkalinity of HNBW and the generation of hydroxyl radicals, which promote cement hydration, compaction, and reduced internal pores.

Tochahi et al. [93] studied the mechanical properties and durability of concrete samples with natural metakaolin and zeolite pozzolans mixed with water nanobubbles. The samples were cured in both seawater and standard conditions. At 28 days, the sample with 100% MNBs cured in seawater showed improvements compared to the same mixture cured in standard conditions. Specifically, it exhibited increased compressive strength (6.97%), tensile strength (12.82%), bending strength (11%), and electrical resistance (14%). In addition, it showed reductions in water absorption within 30 min (10.8%) and RCPT results (10.21%).

Considering the mentioned literature reviews, MNBs improve the mechanical properties of conventional and SCC. Up to 31% improvement has been reported for the compressive strength of concrete containing MNBs. The tensile and flexural strength of MNB concrete also improves between 10–20 and 3–34%, respectively. Concrete containing MNBs also affects the durability properties, so the porosity of concrete decreased by 17%, and the size of the voids also decreased. The 28-day water absorption of MNB concrete decreased by 20%, and chloride permeability also reduced significantly; this reduction has been reported in some sources up to 67%. The presence of MNBs in concrete increases the freeze–thaw resistance of concrete. Lan et al. [84] indicated that both NBW and NS can improve freeze–thaw resistance and mechanical strength of samples at low atmospheric pressure by accelerating cement. They proved that NBWs and nano-silica can improve freeze–thaw resistance and mechanical strength of samples at low cement hydration. Finally, MNB concrete may be suitable for shotcrete [90] because it reaches acceptable compressive strength sooner than conventional concrete.

The presence of MNBs in concrete has a negative effect on workability and rheology parameters. Their presence reduces the slump flow by 5–10%. Also, the J-ring height difference and V-Funnel time experienced a slight increase in SCC. In the investigation of researchers, MNBs have no noticeable effect on the L-Box ratio. However, the workability of SCC using MNBs is in the range of European Guidelines for Self-Compacting Concrete (EFNARC) [94] (2005) guidelines for SCC production.

It is necessary to find types of workability, mechanical, and durability experiments of MNB concrete that have been performed in the world to determine the gap in research. Considering research reported in Table 4 indicates types of experimental works associated with the workability, mechanical, and durability of MNBs that have been done up to now.

Table 4 shows the effect of adding MNBs on the performance of conventional concrete and SCC and divides the effect of adding MNBs on performance into efficiency, mechanical properties, durability, etc. To evaluate the workablitiy of concrete, slump flow, J-ring rate, T₅₀, V-funnel time, L-box ratio, U-box ratio, visual stability index (VSI), air content, static segregation, dynamic segregation is used. The third column of Table 4 indicates various workability, mechanical and durability tests to evaluate the efficiency of concrete containing MNBs are listed to evaluate the mechanical properties, compression strength, flexural strength, tensile strength tests have been used in previous studies. To evaluate the durability of concrete containing MNBs, water absorption (volumetric), SEM, RCPT, electrical resistance, resistance to the freeze–thaw cycle, resistance to water penetration, UPV, resistance to water penetration, resistance to chloride penetration (RCMT), carbonation depth, and impermeability tests have been used more.

According to the British standards (BS) (2019) [95], there are four classes for the slump of conventional concrete, including S1, S2, S3, and S4, which depend on the type of application of concrete (Table 5). In addition, it is possible to use a mixture of drinking water and MNBs in the preparation of concrete, which caused an increase in slump compared to the use of only MNBs. Finally, if sufflaminate cement (SAC) is used for MNB concrete, improves the rheological properties of SAC paste [91] and environmental impact compared to using Portland cement. Kanyenze et al. [96] reached contradicting slump outcomes compared to the work of Arefi et al. [78] that they indicated an important reduction in flowability (or consistency). They claimed the difference in outcomes can be attributed to the consumption of MNB water compared to pure nanobubble water in this test, which resulted in different water characteristics and the following impacts on concrete.

5.1 Environmental assessment

The life cycle assessment (LCA) has been done by two methods, the midpoint by use with CML2000 and the endpoint by use with IMPACT2002+, in the CML2000 method, the category of effects of global warming potential (GWP), human toxicity, acidification potential (AP), and freshwater eutrophication (FE) have been investigated. The IMPACT2002+ method evaluates the results of the Life Cycle Inventory (LCI) in the form of human health, ecosystem quality, climate change, and natural resources. Figure 8 shows the damage results on the midpoint of concrete containing MNBs using the CML2000 method [97].

Figure 8

Results on the midpoint of concrete containing MNB using CML2000 method [98].

Figure 8 shows that MNB concrete has caused more environmental damage in all classes of midpoint effects evaluated in Taherian’s study than conventional concrete. MNB concrete emitted 424.17 kg CO₂ eq in the global warming group, while conventional concrete emitted 386.44 kg CO₂ eq. In fact, MNB concrete emitted 37.73 kg CO₂ eq more in the global warming effect category. The total emission of conventional concrete and MNBs is considered equal to 100%, in this case, conventional concrete has 47.67% and MNB concrete has 52.33% of pollution emissions. In the floor, the acidification effect of conventional concrete is 0.89 kg SO2 eq and MNB concrete is 0.84. Therefore, in this class, out of 100% of the total emission, 51.45% belongs to MNB concrete and 48.55% belongs to conventional concrete. Notably, in the midpoint technique, the most significant disparity in pollution impact pertains to freshwater eutrophication (FE), indicating a 4.8% higher impact in concrete containing MNBs compared to conventional concrete. Conversely, the least disparity in pollution impact is observed in the human toxicity effect class, with concrete containing MNBs demonstrating only a 2.42% increase in damage compared to conventional concrete [98]. Figure 9 illustrates the damage outcomes at the endpoint of concrete containing MNBs using the IMPACT 2002+ method.

Figure 9

Damage results on endpoint of concrete containing MNB using CML2000 method [98].

The utilization of concrete containing MNBs marginally elevates environmental pollution indicators in both midpoint and endpoint methods. The total emission of production per cubic meter of conventional concrete and MNBs is considered equal to 100% emission and the contribution of each concrete is shown in Figures 8 and 9. MNB concrete has released a DALY value of 0.000142 in the group of damage to human health, while conventional concrete has released a DALY of 0.000137 pollution into the environment. Therefore, out of 100% of the total damage to human health, 50.90% of the share of concrete is MNBs and 49.10% of the share is conventional concrete. MNB concrete has caused more damage in all classes, but its amount is very close to conventional concrete and does not have a significant difference with conventional concrete, and adding MNBs to concrete slightly increases the amount of damage to the environment [98].

In the life cycle assessment using the endpoint method, the results indicate that the maximum disparity in damage indicators is associated with the climate change category, with a difference of 4.68%. Conversely, the least disparity in the assessment of damage to natural resources is observed in the production phase, where concrete containing MNBs emits only 1.02% more emissions compared to conventional concrete [98]. According to the existing studies of LCA about concrete containing MNBs, the environmental effect of MNBs in concrete is negligible. However, more studies are necessary to determine the impact of MNBs in concrete in the environment precisely.

5.2 Economic assessment

Concrete producers and consumers are mainly looking for optimal quality in mechanical strength and durability to save their costs. Considering this issue, the purpose of this analysis is to find out the economics of concrete production and compare it with the environmental burden. Also, performing economic analysis in this review is important because of the strong role of this topic in the construction industry. Of course, due to the different climatic conditions in the world and the different levels of access, we cannot expect to reach an economic balance in the world.

Taherian [98] compared five types of concretes including MNB concrete, conventional concrete, nano-silica concrete, micro-silica concrete, and geopolymer concrete economically in international. Figures 10 and 11 show the results of economic evaluation and emissions related to global warming and acidification in Taherian’s study [98].

Figure 10

Economic evaluation results and Kg CO₂ eq emissions for concrete types [98].

Figure 11

Economic evaluation results and Kg SO₂ eq emissions for concrete types [98].

The calculation of production cost is based on the rates of the original year. As shown in Figure 10, in the beginning year, the production cost of 1 cm³ of geopolymer concrete had the highest price with a production cost of 1429.322 dollars, but at the same time, by emitting 286.85 kg CO₂ eq, it has emitted less pollution than other concretes and has performed best in this respect. After geopolymer concrete, nanosilica concrete cost $625,047 per cubic meter more than other evaluated concretes. MNB concrete with a cost of $395.48 has the lowest production cost and also has the best performance in this field by releasing 495.7 kg CO₂ eq after conventional concrete and geopolymer concrete. Micro silica concrete has created the most pollution in the environment by releasing 605.32 kg CO₂ eq and then nano silica concrete has released more pollution than other concretes [98].

Figure 11 shows that the production of microsilica concrete per cubic meter has the most damage in the class of acidification effect on the environment. By introducing 1.55 kg SO₂ eq, this concrete has created more pollution than other evaluated concretes in Taherian’s study. Meanwhile, microsilica concrete has a higher production cost than MNB concrete. The production cost of each cubic meter of microsilica concrete was 495.7 dollars [98].

Conventional concrete has caused the least damage to the environment in this index. By releasing 0.84 kg SO₂ eq, this concrete has brought the least pollution to the environment, while its production cost is higher than MNBs and microsilica concrete. As shown in Figure 11, conventional, MNBs, nano silica, and geopolymer concrete have caused less damage in the acidification effect layer, while the production of each cubic meter of MNB concrete had the lowest production cost and geopolymer concrete had the highest production cost. A total of 625,047 dollars are needed to produce each cubic meter of nano-silica concrete, and in this regard, it was the second most expensive concrete among the concrete evaluated in Taherian’s study [98].

Taherian stated that the price of materials (fine and coarse aggregate, cement, MNBs, and other chemical components) for concrete containing MNBs in Iran is about 1% and in the world about 28% less than conventional concrete. However, in this work, the cost of producing MNBW is not taken into account, which will increase the price of concrete containing MNBs. It seems that more economic studies are necessary to reach the real price of MNB concrete [98].

5.3 Advantages and disadvantages of MNBs in concrete

MNBs can be useful as a method to improve the properties of concrete. In concrete with MNB, the thermal stability of concrete increases, and heat transfers from the walls to the surrounding environment. MNBs also have sound insulation properties and absorb sound and thus reduce sound penetration in the structure. In addition, MNBs increase mechanical properties such as compressive and tensile strength and show wear resistance. In general, MNBs reduce the workability of concrete slightly and improve durability and MNB concretes reduce the setting time of concrete and help develop the strength of the cement base material and improve the elastic modulus at an early age [9,25,77,86,90]. MNBs have disadvantages such as increasing the cost of making and injecting concrete and conducting more and more detailed tests to ensure the properties of concrete and the newness of its manufacturing technology. It may decrease with changes in environmental conditions (such as temperature and humidity) and reduce the workability properties of fresh concrete [7,99]. The enhancement in mechanical properties and durability of concrete is attributed to the weak alkalinity of nanobubble water and the fact that nanobubbles contract and collapse generating hydroxyl radicals, promoting the hydration reaction of cement to develop hydration products. This densifies the concrete structure and reduces internal pores. Hydrogen nanobubbles, as a nanomaterial, demonstrate desirable effects for meeting high-performance concrete requirements [92].

5.4 Limitations in the industrialization of concrete containing MNBs

One of the challenges of the industrialization of concrete containing MNBs can be the cost of producing this concrete. In addition, the production process of concrete containing MNBs requires complex and precise processes that have not yet been optimized due to the novelty of this technology. Another challenge of this concrete is its durability in the long term, and the useful life of concrete containing MNBs requires more studies to be able to accurately predict its stability and useful life. The supply of raw materials for the production of concrete containing MNBs may not be easily available in some regions of the world.

In various studies, some ambiguities have been mentioned about the basic characteristics of MNBs, including the stability of bulky MNBs, which needs more research [100]. Moreover, despite all those advances, the production of MNB with precise size and concentration control remains a major challenge, particularly for NB. Addressing this challenge may necessitate collaborative interdisciplinary research efforts [14]. Differences in outcomes may occur due to factors such as bubble fabrication methods, their lifespan, and stability. The review emphasizes the need for future iterative experiments with various mixing ratios to reduce random influences and to validate and enhance the reliability of the results [92]. The existence of stable and long-lasting NBs has been confirmed by various experimental studies. However, traditional theoretical models, such as those based on Epstein and Plesset’s theory, suggest that nanobubbles should have short lifetimes due to their small size and rapid dissolution from high Laplace pressure [101]. These models, initially designed for larger microbubbles (MaBs), may not be applicable to nanobubbles, which exhibit unique properties like electrostatic repulsion and stable motion influenced by Brownian motion. This conflict between theory and observation highlights the need for new models that account for the specific characteristics of nanobubbles to better explain their existence and stability. Inconsistencies among reports on NBs’ fundamental aspects and characteristics create challenges in understanding them. Different studies show varying findings on NBs’ size, stability, and behavior, which may result from differences in experimental conditions and methods. These discrepancies hinder a clear and consistent understanding of NBs and highlight the need for standardized experimental methods and reliable models to accurately predict and explain nanobubble behavior.

5.5 Future works

The following areas of research are crucial for future work:

1. Due to discrepancies between theoretical models and observations of MNBs, there is a need to develop models that define specific properties of MNBs more accurately.

2. Economic analyses of using MNBs in various types of concrete are necessary to assess cost-effectiveness and practical applications.

3. Since two-stage concrete (TSC) is environmentally friendly, energy-efficient, and offers benefits such as low drying shrinkage, high bonding resistance, a high modulus of elasticity, and exceptional durability [102–104], it is recommended to study the integration of TSC in MNB concrete.

4. Conducting a LCA using MNBs in different types of concretes is essential for evaluating their environmental impact and potential pollution.

5. Understanding the long-term effects of MNBs on the mechanical and durability aspects of concrete is vital for sustainable development.

6. Testing various mechanical and durability properties is required, including fracture toughness, elastic modulus, temperature effects, depth of water penetration, chloride concentration (28-day measurement from two opposite sides in percentage), the RCMT, mass loss under elevated temperature, and temperature impact.

7. In addition, examining the influence of different types of fibers on the workability, mechanical, and durability properties of MNB concrete is recommended.