Durability of geopolymers and geopolymer concretes: A review

1 Introduction

Concrete has become the primary global building material, but the carbon dioxide emissions of the construction industry where concrete is the main material are tremendous, accounting for only 5% to 7% of the global total [1, 2]. Compared with ordinary Portland cement concrete (OPC), geopolymers are more environmentally friendly because it can reduce the pathway of carbon generated by excessive use of OPC [3, 4]. The amount of CO₂ produced by the comparative concrete containing 100% OPC binder was about 9% higher than that of geopolymer concrete (GPC) [5]. Therefore geopolymers are suitable binders for OPC to manufacture green concretes.

Geopolymers are the three-dimensional networks made by reacting materials containing alumina and silica with alkaline liquids [6]. Due to Al³⁺ of (AlO₄)⁻¹ is four-fold coordination, Na⁺ in the alkaline activating solution balances the surplus negative charge [7]. Water of geopolymer as a catalyst generally does not participate in the reaction. Water of calcium silicate hydrate gel is totally different. It creates a part of C-S-H. Through Figure 1, it could be found the schematic structure of OPC and geopolymer [8].

Figure 1

Schematic molecular structure of geopolymer in contrast to OPC [8]

One type of concrete uses geopolymer as a binder, namely geopolymer concrete. The strength of GPC resorts to polymerization, while the strength of OPC comes from the hydration of Portland cement [9]. OPC's strength increase and hardening mechanisms are significantly different from those of geopolymers. Geopolymers have strong mechanical properties, and their compressive strength can exceed 100 MPa [10].

Being a medium of environment, the durability of concrete exerts an imperative part in the structure 's service life containing it. The structure of concrete must ensure that the concrete can resist chemical and physical erosion and other mechanical stress during its expected service life [11]. It is extremely resistant to acid, alkaline silica and fire. Geopolymers have an inorganic structure and cannot be burned as easily as organic polymers. Besides, geopolymers are non-toxic and smoke-free, and their processing temperature is lower than other ceramic composites [12]. Geopolymers can be used as environmentally friendly building materials, which can achieve the purpose of sustainable development. Bakharev [13, 14] found that geopolymer concrete has strong acid and sulfate resistance. The chloride ion permeability of OPC and GPC was studied by Rajamane and other scholars [15]. Sathia et al. [16] studied the resistance and absorption properties of GPC to acids, and he found that this property could improve its durability.

In this paper, the mechanism and research progress regarding the carbonation resistance, structural fire resistance, corrosion resistance, permeation properties and frost resistance of geopolymer concretes are reviewed, and the main problems with the durability of geopolymer concretes are discussed.

2 Sample Preparation

2.1 Materials

Geopolymers can be made by activating a variety of materials rich in aluminosilicates with strong alkaline solutions at ambient or slimly raised temperatures [17, 18]. Metakaolin and fly ash are the main sources of aluminosilicates.

Metakaolin is an anhydrous aluminosilicate material made from calcined natural clay mineral kaolinite [Al₂Si₂O₅(OH)₄]. All the while the dihydroxylation, structured water is lost. Under this condition, a naturally amorphous and disordered structure (metakaolin) [19] will be formed, as shown in Figure 2a [20] and Figure 2b [21]. In alkaline earth/alkali solutions, the reactivity of this amorphous structure is intense [22, 23]. Besides, adding metakaolin to recycled aggregate GPC can improve the abrasion resistance, but adding nano-SiO₂ has the opposite result [24, 25].

Figure 2

The main by-product of thermal power plants is fly ash. Because fly ash consists of an amorphous alumina-silicate framework, it has been identified as a low-cost, readily available geopolymer polymer [26]. Fly ash-based geopolymers have great advantages. For example, low shrinkage, low permeability, high-temperature resistance, and high mechanical strength [27, 28]. Besides, lower CO₂ emission and decreasing energy needs in the process of manufacture make the fly ash-based geopolymers potential replacement materials for the general Portland cement [17, 18, 29]. Figure 3 summarizes CO₂ emissions of the entire manufacturing 1m³ concrete process [5]. It was found that fly ash-based geopolymers look to has been extensively used in the industry of construction [30]. The other fields applying it involve immobilization of toxic metals [30, 31], adsorption of dyes [27, 32,33,34], and the adsorption of heavy metals [26, 28, 35, 36].

Figure 3

Summary of CO₂-e for Grade 40 concrete mixtures with OPC and geopolymer binders [5]

3 Carbonation-resistant geopolymer concrete

Carbonation of concrete is a complex physical and chemical course. It contains the diffusing of CO₂ in the gaseous stage to the pores of concrete, its dissolution in the aqueous films on the pores, solid Ca(OH)₂ dissolution in the water in the pores, diffusing of dissolved Ca(OH)₂ in the pore water, CO₂ reaction with C-S-H, its reaction with the dissolved CO₂, with the yet unhydrated C₂S and C₃S. What's more, there is a paralleling course, which contains the reduction of concrete porosity and the hydration of cementitious materials [37].

The microstructure of general silicate concrete is diverse from that of geopolymer concrete. and it is impossible to utilize the means of analyzing the carbonation for general concrete for the experiment [38]. Meanwhile, carbonation proof performance of geopolymer concrete is not all better compared with that of general concrete.

Pasupathy et al. [39] surveyed the durability of a precast fly ash-based geopolymer concrete exposed to the outside environment for 8 years. Major specimens from GPC culverts were checked to decide the influence of carbonation, the durability, the pore-size distribution, and the permeation properties was contrasted with that of general Portland cement concrete exposed to the same environment. It was discovered that the carbonation resistance of OPC concrete is higher than that of GPC.

Li Zhuguo et al. [40] researched the carbonation depths of wide variety of GPC and GP mortars by the test of accelerated carbonation at different times in the GP mortars and GP concrete, the aluminosilicate materials are composed by ground blast furnace slag and fly ash. Figure 4 tells the testing results of the relations among the elapsed time for the concrete of GP and carbonation depth(C-depth) [40]. The depth of carbonation enhanced with the elapsing time in the first 3 weeks, but after three weeks, the increase was decreasing [40].

Figure 4

Relationship between the carbonation depth of GP concrete and the elapsed time [40]

The affecting elements for the resistance of carbonation of blast furnace slag(BFS)-based and fly ash(FA)-based GPC were analyzed by comparatively utilizing the rate coefficients of carbonization. It was found that the resistance of carbonation of BFS-based and FA-based GPC cured at the room temperature was lower than for a typical concrete that comprising general Portland cement. The resistance of carbonation strengthened with increased BFS ratio in the active fillers, NaOH content in the active activator solution, and BFS fineness or with decreased water/AF ratio and AS/AF ratio. What's more, a retarder using and heat curing benefited the resistance of carbonation of BFS-based and FA-based GPC.

The durability of fly ash geopolymer concretes was influenced by the contents of Ca²⁺ [41]. The popularly applied theory is a pattern of the depth of concrete carbonation based on Fick's first law [42]. Resistance of the carbonation of GPC is linked to the strength of compression. Concrete with an increasing strength normally probably has an increased resistance of carbonation. Nevertheless, other factors are affecting the resistance of the carbonation of GPC, including types of alkaline activator. It has shown from the results of experiment in this research that the coefficient of carbonation rate of BFS-based and FA-based GPC was not always decreasing along with its strength of compression, as indicated in Figure 5 [40].

Figure 5

Relationship between the a value and compressive strength [40]

Metakaolin/slag-based GPC (w/b ratio 0.47) was studied by Bernal et al. [43]. Through an accelerated carbonation testing utilizing a CO₂ concentration of 3.0 ± 0.2% at 20 °C for 28 days. It was found that the strength of compression was decreasing monotonically along with the process of carbonation. The relations among the extent of carbonation and volume of the pore was as same as that of sample with different metakaolin percentages, opposite to the samples of slag-based concrete. It suggested that the loss of strength of a carbonated binder is not just controlled by one parameter, the porosity. Because of the gel chemistry of binder, a convoluting effect could determine the residual level of strength after accelerated carbonation.

4 Structural fire-resistant geopolymer concrete

It is extremely important to protect the structures from fire. As a new material, a geopolymer can be widely used in many fields. Compared with Portland cement adhesive, it has superior inherent fire resistance. Geopolymers are not as flammable as organic polymers, which rely on their internal inorganic structure [12, 44]. It has advantages over traditional ceramic composites because it has lower processing temperatures and is non-toxic and smoke-free [45]. The reason why geopolymers can withstand high temperatures is that their performance and properties are determined by their internal structural composition and synthetic design [46].

In the large-scale application of geopolymers in the construction industry as refractories, it is necessary to investigate and inspect the thermal properties of geopolymers from macroscale, mesoscale, and microscale aspects [47]. This idea can be explained in Figure 6 [7]. Microscopically, the transformation and phase transition of nanostructures of geopolymers at high temperatures require precise observation. Its phase transition activity at high temperatures has been analyzed and reported by some researchers [48,49,50,51,52]. The chemical stability of geopolymers is certainly very high [53,54,55,56,57]. Therefore, chemical stability is closely related to microscopic activities, and microscopic properties are more conducive to the stability of matter at higher scale levels (that is, at mesoscale and macroscale). At high temperatures, the thermally induced cracking ability and volume deformation ability of a substance is called moderate thermal stability. The resistance to spalling and the ability of materials to withstand high temperatures is related to macro-stability [46].

Figure 6

Methodology to ensure good structural performance of geopolymers subject to high temperature heating [7]

The microscopic properties of geopolymers have been reported by some researchers. It has also been found that geopolymers are chemically more stable than OPC hydrated products, and their chemical structure is easier to be destroyed because when exposed under the temperature, OPC will get severely deteriorated. Factors as alkali contents, alkali cation types and Si / Al ratio play imperative roles in deciding the chemical structures of geopolymers when exposing to the raising temperatures. These factors shall then be tailored to accomplish a suitable mix of geopolymer for the structural fire applications.

When geopolymers are exposed to high temperatures, thermal expansion or thermal contraction will occur, causing macroscopic cracks. It is essential that the water content of the geopolymer mixture can be controlled to adjust for thermal deformation. For example, for processing purposes, fly ash-based geopolymers require less water than metakaolin geopolymers, so if choose a structural polymer that requires high fire resistance, metakaolin-based geopolymers are not a good answer [58]. Moreover, the addition of PVA and basalt fibers to GPC can reduce strength and weight loss to enhance structural fire-resistant under high-temperature environments [59, 60]. Adding microencapsulated phase change materials (MPCM) to GPC can improve the porosity to enhance the performance of heat capacity and decrease compressive strength [61, 62]. The types of alkali metal cations and the alkali-activating solution have a huge effect on thermal deformation. For example, comparing to using sodium as an alkali cation, thermal shrinkage could be better reduced by using potassium as an alkali cation [56]. When applying a mixture of geopolymers at high temperatures, these factors must be carefully considered.

Rickard et al. [63] studied the microstructure of two geopolymer mixtures: one was a dense microstructure and highly reactive geopolymer, and the other mixture did not react due to the large amount of fly ash. Therefore, the strength and density of the polymer are relatively low. Because of the dense microstructure, low-strength polymers are stronger than high-strength polymers. Due to macro-cracks and dehydration damage, high-strength geopolymers have increased strength losses, reduced thermal performance, and increased dimensional instability. Studies have found that low-strength geopolymers will be slightly damaged because of dehydration, which can better adapt to volume changes, and its strength increases when exposed to heat. The model in Figure 7 speeds up these results [63].

Figure 7

Schematic depiction of the proposed micro-structural changes in geopolymer paste upon firing [63]

5 Corrosion-resistant geopolymer concrete

5.1 Resistance to acids

Acidic substances react with alkaline substances in calcium-containing cement base materials, such as CaCO₃, CaSO₄, and ettringite, causing volume expansion, and the gaps are created inside the matrix to make it easier for acidic materials to enter. The damage arises from acid erosion of the concrete. Acid erosion affects geopolymer concrete, but there are a large number of Si-O and Al-O structures in its structure. Therefore, acid does not easily react with geopolymers at room temperature, so it can be used to make acid-resistant materials.

Davidovits et al. [12] emphasized that when the samples were placed in 5% H₂SO₄ solution for 30 days, the mass loss of metakaolin-based geopolymers was found to be 7%. According to the report, the fly ash-based geopolymer microstructure can be retained for 3 months after being placed in HNO₃.

Temuujin et al. [64] found that the alkali and acid resistance of fly ash-based geopolymers are largely determined by their mineral composition. Fe, Si, and Al highly soluble ions are obtained from strong acid and alkali solutions. Figure 8 shows the concentration of dissolved elements in a geopolymer sample after being placed in an alkaline or acid solution [64]. The study found that when the fly ash-based geopolymer slurry was placed in a 5% H₂SO₄ and 5% acetic acid solution, the performance was superior to ordinary Portland cement slurry. The formation of zeolites and depolymerization aluminosilicate network were closely related to the deterioration in the pastes [13].

Figure 8

Concentration of elements dissolved from geopolymer samples, (a) in acid solution; (b) in alkali solution [64]

Hardjito et al. [65] discovered that the strength of compression of a fly ash-based GPC in 0.5% H₂SO₄ solution was reduced by 20% when exposed for 12 months. The value was 65% and 52% after the samples were exposed to 2% and 1% H₂SO₄ solutions, separately. Erosion and pitting on the surface of the concrete were also watched. The degradation of the geopolymer matrix is the main cause of the loss of concrete strength, not the degradation of the aggregate. The researchers found that the acid resistance of OPC concrete is not as good as that of GPC. Figure 9 depicts the change in compressive strength of a polymer sample when exposed to an acetic acid solution [13]. Figure 10 shows the change in compressive strength of polymer samples when exposed to sulfuric acid solution [13].

Figure 9

Compressive strength evolution of the geopolymer and Portland cement specimens exposed to 5% acetic acid solution [13]

Figure 10

Compressive strength evolution of the geopolymer and Portland cement specimens exposed to 5% sulfuric acid solution [13]

Ariffin et al. [66] used a mixture of palm oil fuel and powdered fuel to make GPC in a 2% sulfuric acid solution for up to 18 months. It was found that OPC concrete lost 20% of its weight and GPC lost 8% of its weight. The strength of OPC decreased by 68% after 30 days, while the strength of GPC decreased by 35% within 18 months, and OPC severely deteriorated after 18 months. N-A-S-H has no obvious effect on the structure of GPC, but C-S-H is harmful to OPC concrete.

Pasupathy et al. [67] announced that the weight damage in the concrete samples was less than 5% after 3 months of exposure to a 3% H₂SO₄ solution. Slag-based GPC (40 MPa) has reduced 33% strength in contrast to OPC concrete that has reduced 47% when exposing to an acetic acid solution (pH=4) for twelve months. Low calcium C-S-H with Ca/Si ratio of 1 and slag particles seemed more stabilized in the solution of acid in contrast to the OPC pastes constituents. In the immersion of solution of 2% H₂SO₄, the loss of strength was 11% for the GPC in contrast to 36.2% for the concrete of OPC [68].

5.2 Resistance to seawater and sulphate solutions

It was reported by Hanrahan [69] that the durability of fly ash-based geopolymer concrete is greatly governed by the internal configuration of aluminosilicate gel components in extreme environments (including 5% MgSO₄ solution and 5% Na₂SO₄ solution). As shown in Figure 11 [69], the transition of alkaline species from geopolymer to the solution might be related to some fluctuations as shown by the compression strength of OPC and GPC concrete exposed to 5% MgSO₄ and Na₂SO₄. The GPCs made with a sodium-silicate activator are less crystalline than that those made with sodium hydroxide. The geopolymer concrete activated with NaOH solution only performed better than the OPC concrete. It was reported by Criado et al. [70] that the durability and strength of the GPC improved along with the time regardless of the kinds of chemical solutions where the samples were submerged, as shown in Figure 12 [71].

Figure 11

Compressive strength of fly ash activated with sodium silicate solution and NaOH, and OPC specimens, exposed to 5% of Na₂SO₄ and MgSO₄ [69]

Figure 12

Mechanical strength of fly ash mortars (a) NaOH-activated, and (b) water glass-activated [71]

Durability performances of fly ash geopolymer concrete have been researched by Adam [72]. One new understanding of the durability and strength of GPCs have been offered on the chloride and carbonation resistance and the effects of the dosage of Na₂O on the compression strength of samples of GPC. Both activator modulus and Na₂O dosage play crucial parameters for the GPC production. It was discovered that fly ash geopolymer concrete shows a strength compared to that of concrete of OPC. Nevertheless, the properties of the durability of fly ash geopolymer concrete were watched to be good on the chloride and carbonation resistance than those for the concrete of OPC.

6 Permeation properties of geopolymer concrete

The permeating performance influenced by the size of permeable pores of the concrete of geopolymer means the performance of the microstructure of the produced matrix [73, 74].

A decreased water permeability (2.46–4.67 × 10–11 m/s) for a GPC (activator-fly ash ratio of 0.30–0.40 cured at 60°C for 24 h) in contrast to that for OPC has been reported by Nikraz and Olivia [75], because of its lower pore interconnectivity and denser paste. It was also reported that the most affecting parameter influencing the property of GPC was the water-geopolymer by some researchers. Fly ash geopolymers blended slag, because of their reduced porosity volume and enhanced pore size matrix, which showed good resistance to the permeability, which might progressively be improved by raising temperature curing [76].

As talked previously, the microstructure of the geopolymer matrix has been densified by metakaolin particles, which might lower the porosity [77]. After it was exposed to the sodium sulfate solution for 180 days, as indicated in Figure 13 [77], the water absorption and apparent porosity will be increasing along with the rising temperature. At raising the temperature, the water included in the matrix moving to the surface and produced micro-cracks that was increasing the absorption of water. The permeability of chloride of the concrete of the geopolymer was tested by the Rapid chloride permeability (RCP) test as the chloride of acid-soluble (total chloride), as per the ASTM C-1202 [78].

Figure 13

The porosity and pore size distribution of geopolymer incorporating various content of metakaolin after 180 days of exposure [77]

Part et al. [79] forecast that long time resistance of chloride of geopolymer concrete will be lower than that of concrete of OPC due to the lower increment of strength along the time of concrete of geopolymer. The rates of corrosion of PC concrete and the geopolymer in 3% sodium chloride solution are indicated in Figure 14 [80]. The tried survey of the long time chloride resistance of a geopolymer concrete utilizing a fast chloride permeability testing was futile, as the specimens of geopolymer showed a fast-rising in the temperature in the testing, which is opposite to the law of Ohm and showed that RCPT is not an appropriate test means to assess the resistance of chloride of concrete of geopolymer.

Figure 14

Corrosion rate of geopolymer and PC concrete specimens, immersed in 3% NaCl solution [80]

7 Frost-resistant geopolymer concrete

Deterioration of freeze-thaw-induced concrete could be contributory to the microcracks appearing at the zones of weak interfacial transition among paste/MPCM and the paste/aggregate [81].

PE-EVA-PCM and St-DVB-PCM were contained by characterization of the Microscopic Structure of the PCC and GPC after 0 and 28 freeze-thaw cycles were executed by tomography imaging of X-ray and SEM. SEM images of the samples after 28 freeze-thaw cycles are indicated in Figure 15 [81]. Metakaolin-based geopolymers with Na/AI=1.01; Si/AI=2.01, 2.32 and 2.62; Si/Al=2.62; and Na/Al=1.01, 1.26, and 1.36 were subjected to freeze-thaw cycles. The experiment shows that the geopolymer presents a decreased freeze-thaw resistance with an increased Si/Al ratio and an increased freeze-thaw resistance with an increased Na/Al ratio.

Figure 15

SEM images of the fracture surface of PCC and GPC [81]

The mercury intrusion porosimetry results show that the metakaolin-based geopolymer had two types of pores. The most likely pore diameter decreased with a high Si/Al ratio and increased with a high Na/Al ratio. The geopolymer presents additional small pores and an increased porosity with a decreased Na/Al ratio and large pores and a decreased porosity with an increased Na/AI ratio. The geopolymer with a weak freeze-thaw resistance shows an increased porosity and additional pores less than 50 nm. After the F-T cycles, an increasing number of gel pores and 200–400 nm pores can be observed with the geopolymer that had a weak resistance.

Metakaolin-based geopolymer groups with Si/Al=2.00 and Na/Al=0.80–1.20 in addition to Na/Al=1.05 and Si/Al=1.80–2.20 were conducted with a permeability test. The test results show that geopolymers with increased Si/Al ratios present a generally increased permeability, and geopolymers with high Na/Al ratios present a decreased permeability. The permeability coefficient of the geopolymer is correlated with porosity, effective porosity, and critical pore diameter. A calculated permeability coefficient that considers pore structure parameters is proposed to predict the water permeability of metakaolin-based geopolymers.

Metakaolin-based geopolymer concretes have poor frost resistance, and the freezing-thawing limit is less than 75 times. The Na₂O equivalence content and activator modulus have a great effect on the freeze-thaw performance and mechanical properties of geopolymer concrete. A metakaolin-based geopolymer concrete with a Na₂O equivalent weight of 16% and modulus of 1.5 was the best herein, and the freezing-thawing limit reached 75 times. The frost resistance of geopolymer concretes had a positive correlation with the Na₂O equivalent content; the correlation coefficient was 0.845, and the correlation with the activator modulus and other parameters was not significant.

Slag can shorten the setting time of the geopolymer, enhance the mechanical properties of the geopolymer concrete, and strengthen the tensile strength by more than 20%. The frost resistance of geopolymer concrete increased with increasing slag content. The limit freeze-thaw times of geopolymer concrete with a 30% slag increased from 43 to 135. The frost resistance of geopolymer concrete was improved in the decreasing order of sodium lauryl sulfate >sodium dodecyl sulfate> sodium arsenate, which had the ideal mixing amounts of 0.06%, 0.08%, and 0.02%, respectively. The freezing-thawing limit for geopolymer concretes could be increased from 36 times to 60–75 times by entraining air. Three particular air-entraining agents introduced bubble spaces and pore sizes exceeding 400 μm and 300 μm. Common air-entraining agents have poor compatibility in high-alkaline, high-viscosity fresh geopolymer concretes. Slag can improve the frost resistance of geopolymer concretes better than air-entraining agents. There is an increased improvement in the frost resistance using both slag and air-entraining agents.

The degree of saturation is the most important factor that influences the frost resistance of geopolymer concretes, and the degree of saturation is greater than the rate of the temperature decrease. The porosity of a geopolymer concrete exceeds 15%. The balance of capillary water saturation was 0.84 to 0.92 within 12 hours. Here, the geopolymer pores were filled with sodium silicate solution, and the concentration of Na+ ions was very high and reached approximately 0.6 mol/L. The pore structure of the geopolymer concrete was terrible because the pores exceeded 75% in the SEM images, and the proportion of harmful spores in the freeze-thaw process was further increased. They caused the freezing pressure and osmotic pressure to increase during the freeze-thaw process, and even salt precipitation produced a crystallization pressure, ultimately resulting in poor frost resistance. The freezing-thawing failure mechanism of geopolymer concretes is mainly based on hydrostatic and osmotic pressure theory.

8 Conclusions

In this paper, the mechanisms and research progress regarding the carbonation resistance, structural fire resistance, corrosion resistance, permeation properties and frost resistance of geopolymer concretes were reviewed, and the main problems with the durability of geopolymer concretes were discussed. The conclusions of this review are as follows:

1. Durability of geopolymers and geopolymer concretes is influenced by the materials and design methods for geopolymer preparation. The reactivity of metakaolin-based GPC structure is intense. Besides, adding metakaolin to recycled aggregate GPC can improve the abrasion resistance. FA-based GPC has great advantages, such as low shrinkage, low permeability, high-temperature resistance, and high mechanical strength. There are two design methods for geopolymer preparation. In the design of sodium aluminum ratio, silicon aluminum ratio and water sodium ratio, the need for higher silicon aluminum ratio will lead to excessive use of alkali activator and high cost. With the design of water binder ratio, water glass modulus, and alkali equivalent, the amount of alkali activator is less, and the best mixing ratio can be found in the close steps.

2. The carbonization proof property of concrete is not poorer than that of concrete of geopolymer. Usually, the higher the GPC strength, the better the resistance of the carbonation. Nevertheless, an alkaline activator also affects the resistance of the carbonation of GPC. BFS-based and FA-based GPC resistance of carbonation is influenced by BFS ratio in the active fillers, NaOH content in the active activator solution and BFS fineness. The effect of metakaolin-based and slag-based GPC is the gel chemistry of binders. It suggested that is not just controlled by one parameter. Besides, due to the microstructure of OPC is diverse from that of GPC, it is impossible to utilize the same experiment to analyze the carbonation of concrete.

3. There is a non-organic structure in the materials of geopolymer and indicate fixed higher anti-fire to that of Portland cement binder. Elements as the alkali content, alkali cation, and Si/Ai ratio are imperative and decide the chemical structure of geopolymers when exposed to the rising temperatures. Those elements shall be tailored to accomplish a suitable geopolymer mix for the application in a structural fire. Because FA-based GPC requires less water than metakaolin-based GPC during processing, FA-based GPC is easier to adjust for thermal deformation. Comparing to using sodium as an alkali cation, thermal shrinkage could be better reduced by using potassium as an alkali cation. Besides, low-strength GPC thermal performance is better than high-strength GPC. Due to macro-cracks and dehydration damage, high-strength geopolymers have increased strength losses and increased dimensional instability. Low-strength geopolymers will be slightly damaged because of dehydration, which can better adapt to volume changes, and its strength increases when exposed to heat.

4. There are a large number of Al-O and Si-O structures in geopolymers. Geopolymers do not react with acids at room temperature and can be used to make acid-resistant materials. The durability of fly ash-based GPC is observed to be better in terms of chloride resistance than OPC. The alkali and acid resistance of fly ash-based geopolymers are largely determined by their mineral composition to form an aluminosilicate network (N-A-S-H). What's more, the degradation of the geopolymer matrix is the main cause of the loss of concrete strength in acid solution, not the degradation of the aggregate. And Na₂O dosage plays crucial parameters for the GPC.

5. Because of lowered porosity volume and enhanced matrix of pore size, the slag mixed fly ash-based geopolymer showed good resistance to permeability, which might progressively be improved by the rising temperature curing. The microstructure of the matrix of geopolymer was densified by metakaolin particles, this might lower the porosity. Geopolymers with increased Si/Al ratios generally present an increased permeability, and geopolymers with increased Na/Al ratios present a decreased permeability. The survey of the long time chloride resistance of GPC utilizing a fast chloride permeability testing is opposite to the law of Ohm and showed that RCPT is not an appropriate test means to assess the resistance of chloride of concrete of geopolymer.

6. The freezing-thawing failure mechanism of geopolymer concretes is mainly based on hydrostatic and osmotic pressure theory. Geopolymers have a decreased freeze-thaw resistance when they have an elevated Si/Al ratio and an increased freeze-thaw resistance with an elevated Na/Al ratio because geopolymers with increased Si/Al ratios present a generally increased permeability, and geopolymers with high Na/Al ratios present a decreased permeability. The frost resistance of geopolymer concretes improves with increasing slag content. Besides, the Na₂O equivalence content and activator modulus have a great effect on the freeze-thaw performance and mechanical properties of GPC. The degree of saturation is the most important factor that influences the frost resistance of geopolymer concretes and causes the freezing pressure and osmotic pressure to increase during the freeze-thaw process, and even salt precipitation produced a crystallization pressure.