2. Freeze-Thaw Cycle Resistance
One of the durability problems of concrete in its life cycle application is freeze-thaw cycle resistance. During the freeze-thaw process, water enters the concrete and expands under freezing conditions, creating stress; at a certain temperature, the ice then melts into water. In multiple freeze-thaw processes—namely freeze-thaw cycles—concrete tends to suffer such damage as crack formation, mass loss, increased porosity, and decreased mechanical properties
[8]. It has been reported
[9] that, compared with ordinary concrete, foam concrete has superior freeze-thaw cycle resistance. This is because the freeze-thaw damage of ordinary concrete is caused by the freezing expansion of water in the capillary pores. However, due to the large pores present in foam concrete, if the water in those pores is not saturated, and even if the water freezes and expands in the larger pores, no freeze-thaw damage will occur
[9][10]. Therefore, foam concrete with a well-designed pore system will enjoy better freeze-thaw cycle resistance
[11].
It is very important to test and evaluate the freeze-thaw cycle resistance ability of foam concrete in order to maximize the longevity of foam concrete members. In terms of freeze-thaw test methods for foam concrete, the critical degree of saturation test
[12][13], top surface freezing test
[14], standard test for the resistance of concrete to rapid freezing and thawing in ASTM Standard C666
[15], and a modified freeze-thaw test method based on ASTM Standard C666
[16] have been developed and used to test and evaluate the freeze-thaw cycle resistance of foam concrete. The critical degree of saturation test and top surface freezing test have the disadvantage of not providing a complete method of freeze-thaw durability measurement, while ASTM Standard C666 fails to limit the moisture content of foam concrete during freeze-thaw cycles. Meanwhile, a modified freeze-thaw test method based on ASTM Standard C666 has provided favorable evaluation results with foam concrete.
In OPC foam concrete, freeze-thaw cycle resistance is reported to depend on the depth and rate of water absorption and the compressive strength of the foam concrete, but to have little relationship with permeability and density
[16]. Dhir et al. reported that lower density foam concrete exhibited higher freeze-thaw resistance because larger pores could accommodate the expansion of water and osmotic pressure
[17]. However, Shon et al. tested foam concrete for performance degradation after 60 freeze-thaw cycles (30 days) and reported that lower density foam concrete did not always possess higher freeze-thaw resistance
[18]. Additionally, She et al. demonstrated that lower-density foam concrete with larger and more interconnected pore structures and weaker slurry had lower freeze-thaw resistance
[19]. Indeed, the connectivity, size, and stability of the pores was much more closely related to the true impact of density on the freeze-thaw resistance of foam concrete
[18]. It was claimed that increasing the quantity of consistently small (below 300 µm) air bubbles or air voids in foam concrete was essential for increasing its resistance to the freeze-thaw cycle
[20]. It was also reported that foam concrete samples with an appropriate water-binder ratio have more closed pores and improve freeze-thaw cycle resistance
[21].
The recycling of waste in foam concrete affects freeze-thaw cycle resistance. Replacing cement with fly ash reportedly reduces the number of small pores in foam concrete, resulting in decreased freeze-thaw cycle resistance due to the low pozzolanic effect of the fly ash used and the unburned carbon in fly ash, which may affect the stability of smaller pores
[18]. The freeze-thaw cycle resistance initially increased and subsequently declined with increased replacement quantity when fly ash (which has a relatively active pozzolanic action) was employed as a cement replacement
[21]. Simultaneously replacing cement with lime and fly ash can enhance the freeze-thaw resistance of foam concrete due to the fact that lime promotes the hydration of fly ash, resulting in reduced pore size
[22]. In addition, Zhang et al.
[19] reported that the freeze-thaw cycle resistance of foam concrete first increased and then decreased with increased slag replacement (although the impact was not significant), while silica fume as a cement replacement had a considerable impact on freeze-thaw cycle resistance. Although a small amount of silica fume could promote the volcanic ash reaction and physical pore filling effect and enhance freeze-thaw cycle resistance, after reaching the most suitable replacement amount, freeze-thaw cycle resistance gradually decreased with increased replacement amount
[21]. Bayraktar et al.
[23] and Gong et al.
[24] also showed that the freeze-thaw cycle resistance of foam concrete rose initially and then declined with the increase in slag and silica fume replacement, respectively. Rice husk ash
[25] and silt soil
[26] (as partial replacements for cement) enhanced the resistance of foam concrete to the freeze-thaw cycle.
When used as a substitute for fine sand in foam concrete, fly ash can more effectively reduce strength loss than fine sand as a fine aggregate, while fine sand can reduce mass loss more effectively than fly ash, because sand can limit surface spalling and water expansion. However, the inside of sand is more vulnerable to freeze-thaw cycle damage, while fly ash can improve the pore size and distribution of foam concrete
[19]. Similar conclusions were reported by Hilal et al.
[27][28]. Gencel et al.
[25] and Bayraktar et al.
[23] reported that foam concrete using recycled waste marble powder as a partial replacement for fine sand improved freeze-thaw cycle resistance. Moreover, the replacement of fine sand with waste tire rubber can also increase freeze-thaw cycle resistance
[29]. However, the higher the amount of fine sand replaced by expanded perlite, the greater the mass loss and the lower the freeze-thaw cycle resistance of foam concrete, which can be attributed to the higher porosity of expanded perlite than that of fine sand
[30]. Moreover, under the same density, reducing aggregate content and increasing cement content was found to be beneficial to the freeze-thaw resistance of foam concrete
[31].
Furthermore, the addition of fiber can considerably influence the freeze-thaw cycle resistance of foam concrete. It was reported
[31] that the freeze-thaw cycle resistance of foam concrete increased with increased polypropylene fiber content. However, the addition of basalt fiber reduces the freeze-thaw cycle resistance of foam concrete, possibly due to the presence of fibrous materials reducing the integration of paste
[32]. Tang et al.
[33] indicated that foam concrete containing recycled polyester fiber possessed greater freeze-thaw cycle resistance than polyester fiber-containing foam concrete, while both had greater resistance to the freeze-thaw cycle than foam concrete without fiber.
3. Elevated Temperature Resistance
After exposure to elevated temperature or fire, concrete will undergo a series of irreversible processes, such as loss of free and chemically bound water, phase transformation, cracking, spalling, and so on, leading to failure
[34]. At 150 °C, most free water or physically bound water will evaporate
[35]. When the temperature reaches 400 °C to 600 °C, the calcium hydroxide in the concrete will decompose and the water produced will become vapor and disappear. If the water vapor has insufficient escape channels, it will cause internal stress in the concrete, leading to cracking and spalling
[36]. Foam concrete has acceptable elevated temperature resistance due to its porosity, making it generally superior to the elevated temperature resistance of ordinary concrete
[37]. Nevertheless, the water in foam concrete undergoes high evaporation under elevated temperature, leading to the excessive shrinkage of foam concrete
[38]. Lia et al.
[39] concluded that foam concrete walls can provide more evacuation time under fire through numerical simulation. Elevated temperature can cause changes in the mechanical properties and stiffness of foam concrete, and even cracks. The compressive strength of foam concrete tends to increase during a temperature rise to 400 °C, as the chemical reaction of the binder is promoted by elevated temperature. Once the temperature has surpassed 400 °C, the compressive strength gradually decreases, due to the decomposition of calcium hydroxide and other substances
[25][40][41]. In addition, regardless of the density of foam concrete, when the temperature reaches approximately 90 °C, foam concrete exhibits a loss of stiffness due to the generation of microcracks
[42]. After the temperature reaches 400 °C, macroscopic cracks tend to appear on the surface of foam concrete. The higher the temperature of foam concrete, the greater the number of cracks, and the higher the density, the greater the number of cracks at the same temperature
[43]. It is reported that foam concrete that cools slowly in air after exposure to elevated temperature is more likely to crack, resulting in a greater loss of strength than foam concrete that cools rapidly in water
[25].
The density of foam concrete is an important factor affecting elevated temperature resistance.
Figure 1 shows the relationship between OPC foam concrete density and fire duration. Generally, within a certain density range, the elevated temperature resistance of foam concrete increases proportionally as density decreases. It has been reported that foam concrete with a density of 950 kg/m
3 can withstand a fire of 3.5h, while foam concrete with a density of 1200 kg/m
3 can only withstand a fire of 2h
[44]. Jones and McCarthy
[45] conducted fire resistance tests on 100 mm-thick foam concrete slabs with densities of 930 kg/m
3 and 1250 kg/m
3, respectively, and the results showed that the former withstood fire for a longer duration and had superior elevated temperature resistance. However, the opposite occurs at lower densities. According to Vilches et al., the elevated temperature resistance of ultralight foam concrete with a density of 150 kg/m
3 was more than three times lower than that of ultralight foam concrete with a density of 400 kg/m
3 [46]. It was reported that density-related pore structures in foam concrete were only minimally affected by elevated temperature
[42].
Figure 1. Relationship between OPC foam concrete density and fire duration.
In addition, the mix proportion and composition of foam concrete are important factors affecting elevated temperature resistance. Foam concrete produced using cement with an Al
2O
3/CaO ratio higher than 2 has been reported to show no signs of spalling or explosion at 1450 °C, whereas dense ordinary concrete does
[47]. The addition of Pozzolanic additives can increase the strength of foam concrete with increased temperature within a certain temperature range and improve the elevated temperature resistance of foam concrete
[20]. Foam concrete partially replacing cement with fly ash has a higher elevated temperature resistance than foam concrete without fly ash within the density range of 600–1400 kg/m
3 [48]. It was reported that the addition of rice husk ash resulted in the elevated temperature resistance of foam concrete at 200–800 °C surpassing that of foam concrete without rice husk ash, and that the high temperature resistance increased with increased rice husk ash addition within a certain range
[25].
In terms of aggregates, ordinary concrete will produce significant partial differential shrinkage, expansion, and cracking at elevated temperature due to the presence of coarse aggregates, while foam concrete has substantial elevated temperature resistance because it contains only fine aggregate
[49]. Gencel et al. improved the elevated temperature resistance of foam concrete by partially replacing fine sand with waste marble powder, with elevated temperature resistance increasing with increased replacement within a certain amount of replacement
[25]. However, the use of expanded polystyrene particles as fine aggregates in foam concrete reduces elevated temperature resistance
[50]. Additionally, it was reported
[48] that polypropylene fiber melts at close to 160–170 °C, forming small channels for water vapor escape and reducing the internal vapor pressure of foam concrete at elevated temperature. Thus, increasing elevated temperature resistance and the addition of polypropylene fiber can reduce shrinkage cracks at elevated temperatures and thus reduce spalling.
In terms of GFC, its elevated temperature resistance is superior to that of geopolymer concrete due to its porous structure
[51]. Changes in compressive strength and the elevated temperature resistance of GFC at elevated temperature are affected by the type, proportion, and quantity of precursors and activators. It was reported that the strength of fly ash-based GFC decreases at 100–400 °C, but when the temperature exceeds 400 °C, the unreacted fly ash will be sintered. This results in strength increase, depending on the Si/Al ratio in the binder—the higher the Si/Al ratio, the higher the strength increase
[40][51][52][53]. The compressive strength of GFC in which the fly ash is partially replaced with slag gradually increased with temperature up to 100 °C, but decreased once the temperature exceeded 100 °C, indicating reduced elevated temperature resistance
[40]. However, the compressive strength of GFC in which fly ash is partially replaced with slag gradually increased at temperatures up to 100 °C, and then declined once the temperature exceeded 100 °C, indicating reduced temperature resistance
[40]. Furthermore, metakaolin-based GFC was said to enjoy excellent elevated temperature resistance
[54][55][56][57][58][59]. Peng et al. exposed one side of a 20 mm-thick metakaolin-based GFC sample to a temperature of 1100 °C. After three hours, the measured temperature on the opposite side was lower than 250 °C and the pore structure did not collapse, indicating good elevated temperature resistance
[60].
The addition of fibers can greatly improve the elevated temperature resistance of foam concrete
[8]. It was reported that the addition of basalt fiber increased the compressive strength of GFC under elevated temperature, increasing more significantly with increases in the amount of addition
[61][62]. The elevated temperature resistance of polyvinyl alcohol fiber-reinforced GFC increased with increasing fiber content when no more than 2% polyvinyl alcohol fiber was added. However, when the dosage exceeded 2%, its elevated temperature resistance reduced to below that of unadded GFC at temperatures above the melting point of polyvinyl alcohol fiber (200 °C), which was caused by excessive porosity generated by immoderate polyvinyl alcohol fiber melting
[63]. In addition, Masi et al.
[64] showed that the elevated temperature resistance of polyvinyl alcohol fiber-reinforced GFC was superior to that of basalt fiber-reinforced GFC. Polypropylene fiber with a low melting point (160–175 °C) can produce small channels in GFC for water vapor escape after melting, which reduces the internal vapor pressure of the GFC at elevated temperature and increases elevated temperature resistance
[59]. According to the research of Li et al.
[65], wollastonite fiber-reinforced metakaolin-based GFC offers high elevated temperature resistance.
4. Carbonation Resistance
Concrete is carbonized when exposed to the atmosphere, which is a process by which cement hydration products, such as calcium hydroxide (Ca(OH)
2), calcium silicate hydrate (CSH), and calcium aluminate hydrate (CAH), chemically react with CO
2 to form calcium carbonate (CaCO
3)
[66]. Since these reactions cause the pH of the porous solution of concrete to decrease, the destruction of the protective layer of oxide film on the surface of the rebar, and eventually the corrosion of the rebar, results. Loon et al. showed that OPC foam concrete has higher carbonation compared to normal concrete after one and a half years of carbonation testing
[67]. Foam concrete was reported to have larger crystals after carbonation as well as more closed pores, due to the formation of modified silica gel and CaCO
3 [68]. The porosity and pore morphology of foam concrete are important factors affecting carbonation resistance. Loon et al.
[67] and Jones et al.
[45] both reported the higher porosity (the lower the density, the higher the carbonation rate) of foam concrete, with the presence of more open pores resulting in a higher carbonation rate
[7].
In addition, the mix proportion and composition of foam concrete are important factors affecting carbonation resistance. It has been reported that the higher the water-cement ratio, the lower the carbonation resistance of foam concrete due to the greater formation of capillaries
[69]. Foam concrete with fly ash partially replacing cement has higher carbonation resistance as the volcanic reaction of fly ash improves the pore structure, but foam concrete with fly ash partially replacing cement has lower carbonation resistance when the water-binder ratio is relatively high
[70]. Tian et al. stated that, in foam concrete containing phosphogypsum, concrete with the partial replacement of cement with ground granulated blast furnace slag has superior carbonation resistance to foam concrete with a partial replacement of cement with fly ash
[71]. Park and Choi investigated the carbonation resistance of foam concrete partially replacing cement with stainless steel-argon oxygen decarburization slag, wherein the results showed that the addition of stainless steel-argon oxygen decarburization slag promoted carbonation
[70]. Brady et al.
[72] and Jones et al.
[73] reported similar results, finding that foam concrete using FA as a fine aggregate offered higher carbonation resistance at lower densities and lower carbonation resistance at higher densities than fine sand foam concrete. The application of integral and non-integral surface treatment agents proved an effective method by which to delay the carbonation rate of foam concrete
[74].
GFC exhibits better carbonation resistance due to its chemical composition
[8]. Carbonation of the GFC forms sodium carbonate (Na
2CO
3) and CaCO
3 crystals, which further leads to an increase in strength and density
[75]. Porosity and pore morphology significantly affect the carbonation resistance of GFC. GFC with higher porosity exhibits a higher carbonation rate because the volume of gas penetrating the GFC is directly related to porosity
[76]. Further, more open pores result in higher carbonation rates
[77]. GFC with different precursors and activators has different carbonation responses. Mastali et al.
[75] reported that ground granulated blast furnace slag-based GFC increased density and compressive strength by up to 6% and 30%, respectively, under carbonation. Alzaza et al. also concluded that density and compressive strength increase after carbonation
[76]. In terms of SAC foam concrete, the smaller the pore size and the higher the pore integrity, the better the carbonation resistance of foam concrete, with SAC foam concrete exhibiting greater carbonation resistance after waterproofing treatment
[78]. Singh and Scrivener reported that LC
3 foam concrete accelerated carbonation when autoclaved
[79].
It is worth mentioning that the carbonation resistance and effects of the carbonation of waste recycling foam concrete, GFC, SAC foam concrete, MPC foam concrete, and LC3 foam concrete have not been widely studied, despite their substantial impact on the life of foam concrete members.
5. Efflorescence Resistance
When concrete comes into contact with water or is exposed to moist air, it can produce efflorescence—the development of white salt deposits on its surface. Although the efflorescence of ordinary concrete can cause a change in color on the concrete surface, it is nevertheless harmless and the degree of efflorescence tends to be low
[80]. However, the efflorescence of geopolymer is a serious problem because geopolymer has a large number of soluble alkalis, in which alkaline cations dissolve in water and leach to the surface with water through pores, where they react with atmospheric CO
2 to form white carbonate surface sediments
[81][82]. Little research is available on the efflorescence of OPC foam concrete because the low soluble alkali content of OPC foam concrete makes its degree of efflorescence much lower, which has little effect on the performance of OPC foam concrete
[83]. For GFC, the larger pore size and higher porosity of GFC lead to more severe efflorescence, potentially causing surface wear or even spalling
[80][84].
It has been reported that fly ash-based GFC has high porosity and a large pore size, allowing the soluble alkali to leak rapidly and resulting in the rapid efflorescence of GFC
[85]. Alzaza et al. investigated the efflorescence properties of polypropylene fiber-reinforced stone wool and metakaolin blends-based GFC, and the results showed that, after the seven-day efflorescence test, the higher stone wool content and the addition of polypropylene fiber led to more severe efflorescence, and the higher porosity resulted in more severe efflorescence due to higher water penetration and the release of alkali cations
[76]. Nevertheless, Şahin et al. showed that slag-based GFC with higher porosity exhibited lower efflorescence due to its lower solids content, resulting in a reduced quantity of leachable alkali solution
[86]. If GFC is to be applied to peripheral members exposed to moisture, its efflorescence problem must be solved. The applicability and effectiveness of current methods applied to enhancing efflorescence resistance in solid geopolymer concrete—such as reducing the number of leachable alkali cations by applying heat curing or using active precursors to increase the degree of reaction—and reducing water intrusion into concrete by increasing the hydrophobicity of the concrete surface
[87][88] have yet to be further tested in GFC
[82]. In addition, the current studies do not cover the efflorescence properties and efflorescence resistance of SAC foam concrete, MPC foam concrete, and LC
3 foam concrete, which hinders the widespread application of these foam concretes in humid environments.