Durability of Geopolymers and Geopolymer Concretes: a Review 1 Introduction Received Mar 08, 2020; Accepted May 05, 2020

Rev. Adv. Mater. Sci. 2021; 60:1–14

Review Article

Tian Lingyu, He Dongpo, Zhao Jianing, and Wang Hongguang* Durability of geopolymers and geopolymer concretes: A review https://doi.org/10.1515/rams-2021-0002 1 Introduction Received Mar 08, 2020; accepted May 05, 2020

Abstract: Geopolymers are green materials with three- Concrete has become the primary global building mate- dimensional silicon and aluminum tetrahedral structures rial, but the carbon dioxide emissions of the construction that can be serving as environmentally friendly construc- industry where concrete is the main material are tremen- tion materials and therefore have the potential to contribute dous, accounting for only 5% to 7% of the global total [1, 2]. to sustainable development. In this paper, the mechanism Compared with ordinary Portland cement concrete (OPC), and research progress regarding the carbonation resistance, geopolymers are more environmentally friendly because structural fire resistance, corrosion resistance, permeation it can reduce the pathway of carbon generated by exces- properties and frost resistance of geopolymer concretes sive use of OPC [3, 4]. The amount of CO2 produced by are reviewed, and the main problems with the durabil- the comparative concrete containing 100% OPC binder was ity of geopolymer concretes are discussed. Geopolymers about 9% higher than that of geopolymer concrete (GPC) [5]. possess the superb mechanic property and their compres- Therefore geopolymers are suitable binders for OPC to man- sion strengths could be higher than 100 MPa. Generally, ufacture green concretes. the higher the GPC strength, the better the carbonation- Geopolymers are the three-dimensional networks resistant. GPC has excellent fire resistance, due to geopoly- made by reacting materials containing alumina and silica 3+ −1 mers are acquired an inorganic skeleton which is affected by with alkaline liquids [6]. Due to Al of (AlO4) is four-fold the alkali content, alkali cation, and Si/Ai ratio. There are coordination, Na+ in the alkaline activating solution bal- a large number of Al-O and Si-O structures in geopolymers. ances the surplus negative charge [7]. Water of geopolymer Geopolymers do not react with acids at room temperature as a catalyst generally does not participate in the reaction. and can be used to make acid-resistant materials. Besides, Water of calcium silicate hydrate gel is totally different. It GPC owning low porosity volume shows good resistance to creates a part of C-S-H. Through Figure 1, it could be found permeability. The freezing-thawing failure mechanism of the schematic structure of OPC and geopolymer [8]. geopolymer concretes is mainly based on hydrostatic and One type of concrete uses geopolymer as a binder, osmotic pressure theory. GPC has poor frost resistance, and namely geopolymer concrete. The strength of GPC resorts to the freezing-thawing limit is less than 75 times. polymerization, while the strength of OPC comes from the hydration of Portland cement [9]. OPC’s strength increase Keywords: geopolymer; preparation; carbonation resis- and hardening mechanisms are significantly different from tance; fire resistance; corrosion resistance; permeation those of geopolymers. Geopolymers have strong mechani- properties cal 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 *Corresponding Author: Wang Hongguang: School of Civil Engi- and other mechanical stress during its expected service neering, Northeast Forestry University, Harbin 150040, China; Key life [11]. It is extremely resistant to acid, alkaline silica and Laboratory of Bio-based Material Science and Technology (Ministry fire. Geopolymers have an inorganic structure and cannot of Education), Northeast Forestry University, Harbin 150040, China; be burned as easily as organic polymers. Besides, geopoly- Email: [email protected] Tian Lingyu: School of Civil Engineering, Northeast Forestry Univer- mers are non-toxic and smoke-free, and their processing sity, Harbin 150040, China temperature is lower than other ceramic composites [12]. He Dongpo, Zhao Jianing: School of Civil Engineering, Northeast Geopolymers can be used as environmentally friendly build- Forestry University, Harbin 150040, China

Figure 1: Schematic molecular structure of geopolymer in contrast to OPC [8] ing materials, which can achieve the purpose of sustainable 2 Sample Preparation 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 2.1 Materials and other scholars [15]. Sathia et al. [16] studied the resistance and absorption properties of GPC to acids, and he Geopolymers can be made by activating a variety of materi- found that this property could improve its durability. als rich in aluminosilicates with strong alkaline solutions at In this paper, the mechanism and research progress ambient or slimly raised temperatures [17, 18]. Metakaolin regarding the carbonation resistance, structural fire resis- and fly ash are the main sources of aluminosilicates. tance, corrosion resistance, permeation properties and frost Metakaolin is an anhydrous aluminosilicate mate- resistance of geopolymer concretes are reviewed, and the rial made from calcined natural clay mineral kaolinite main problems with the durability of geopolymer concretes [Al2Si2O5(OH)4]. All the while the dihydroxylation, struc- are discussed. tured 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 abra-

(a) XRD of Metakaolin [20] (b) SEM of Metakaolin [21]

Figure 2 Durability of geopolymers and geopolymer concretes: A review Ë 3

Figure 3: Summary of CO2-e for Grade 40 concrete mixtures with OPC and geopolymer binders [5]

sion resistance, but adding nano-SiO2 has the opposite In the design of sodium aluminum ratio, silicon alu- result [24, 25]. minum ratio and water sodium ratio, the need for a higher The main by-product of thermal power plants is fly ash. silicon aluminum ratio will lead to excessive use of alkali Because fly ash consists of an amorphous alumina-silicate activator and high cost. With the design of water binder ra- framework, it has been identified as a low-cost, readily tio, water glass modulus, and alkali equivalent, the amount available geopolymer polymer [26]. Fly ash-based geopoly- of alkali activator is less, and the best mixing ratio can be mers have great advantages. For example, low shrinkage, found in the close steps. low permeability, high-temperature resistance, and high mechanical strength [27, 28]. Besides, lower CO2 emission and decreasing energy needs in the process of manufac- 3 Carbonation-resistant ture make the fly ash-based geopolymers potential replacement materials for the general Portland cement [17, 18, 29]. geopolymer concrete Figure 3 summarizes CO2 emissions of the entire manu- facturing 1m3 concrete process [5]. It was found that fly Carbonation of concrete is a complex physical and chemi- ash-based geopolymers look to has been extensively used cal course. It contains the diffusing of2 CO in the gaseous in the industry of construction [30]. The other fields ap- stage to the pores of concrete, its dissolution in the aqueous plying it involve immobilization of toxic metals [30, 31], films on the pores, solid Ca(OH)2 dissolution in the water in adsorption of dyes [27, 32–34], and the adsorption of heavy the pores, diffusing of dissolved Ca(OH)2 in the pore water, metals [26, 28, 35, 36]. 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 con- 2.2 Methods crete porosity and the hydration of cementitious materials [37]. There are two design methods for geopolymer: The microstructure of general silicate concrete is diverse from that of geopolymer concrete. and it is impossi- 1. Design by water binder ratio, water glass modulus, ble to utilize the means of analyzing the carbonation for and alkali equivalent. general concrete for the experiment [38]. Meanwhile, car- 2. Design by sodium aluminum ratio, silicon aluminum bonation proof performance of geopolymer concrete is not ratio, and water sodium ratio. all better compared with that of general concrete. 4 Ë Tian L. et al.

Figure 4: Relationship between the carbonation depth of GP concrete and the elapsed time [40]

Pasupathy et al. [39] surveyed the durability of a precast fly ash-based geopolymer concrete exposed to the out- side 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 Figure 5: GP concrete, the aluminosilicate materials are composed Relationship between the a value and compressive strength [40] 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]. based on Fick’s first law42 [ ]. Resistance of the carbonation The depth of carbonation enhanced with the elapsing time of GPC is linked to the strength of compression. Concrete in the first 3 weeks, but after three weeks, the increase was with an increasing strength normally probably has an in- decreasing [40]. creased resistance of carbonation. Nevertheless, other fac- The affecting elements for the resistance of carbona- tors are affecting the resistance of the carbonation ofGPC, tion of blast furnace slag(BFS)-based and fly ash(FA)-based including types of alkaline activator. It has shown from the GPC were analyzed by comparatively utilizing the rate coef- results of experiment in this research that the coefficient of ficients of carbonization. It was found that the resistance carbonation rate of BFS-based and FA-based GPC was not of carbonation of BFS-based and FA-based GPC cured at always decreasing along with its strength of compression, the room temperature was lower than for a typical concrete as indicated in Figure 5 [40]. that comprising general Portland cement. The resistance Metakaolin/slag-based GPC (w/b ratio 0.47) was stud- of carbonation strengthened with increased BFS ratio in ied by Bernal et al. [43]. Through an accelerated carbona- the active fillers, NaOH content in the active activator so- tion testing utilizing a CO2 concentration of 3.0 ± 0.2% at ∘ lution, and BFS fineness or with decreased water/AF ratio 20 C for 28 days. It was found that the strength of compres- and AS/AF ratio. What’s more, a retarder using and heat sion was decreasing monotonically along with the process curing benefited the resistance of carbonation of BFS-based of carbonation. The relations among the extent of carbona- and FA-based GPC. tion and volume of the pore was as same as that of sample The durability of fly ash geopolymer concretes was in- with different metakaolin percentages, opposite to the sam- fluenced by the contents2+ ofCa [41]. The popularly applied ples of slag-based concrete. It suggested that the loss of theory is a pattern of the depth of concrete carbonation strength of a carbonated binder is not just controlled by Durability of geopolymers and geopolymer concretes: A review Ë 5 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 tradi- Figure 6: tional ceramic composites because it has lower processing Methodology to ensure good structural performance of geopolymers subject to high temperature heating [7] temperatures and is non-toxic and smoke-free [45]. The rea- son why geopolymers can withstand high temperatures is that their performance and properties are determined by When geopolymers are exposed to high temperatures, their internal structural composition and synthetic design thermal expansion or thermal contraction will occur, caus- [46]. ing macroscopic cracks. It is essential that the water con- In the large-scale application of geopolymers in the tent of the geopolymer mixture can be controlled to adjust construction industry as refractories, it is necessary to in- for thermal deformation. For example, for processing pur- vestigate and inspect the thermal properties of geopolymers poses, fly ash-based geopolymers require less water than from macroscale, mesoscale, and microscale aspects [47]. metakaolin geopolymers, so if choose a structural poly- This idea can be explained in Figure 6 [7]. Microscopically, mer that requires high fire resistance, metakaolin-based the transformation and phase transition of nanostructures geopolymers are not a good answer [58]. Moreover, the ad- of geopolymers at high temperatures require precise obser- dition of PVA and basalt fibers to GPC can reduce strength vation. Its phase transition activity at high temperatures and weight loss to enhance structural fire-resistant under has been analyzed and reported by some researchers [48– high-temperature environments [59, 60]. Adding microen- 52]. The chemical stability of geopolymers is certainly very capsulated phase change materials (MPCM) to GPC can high [53–57]. Therefore, chemical stability is closely related improve the porosity to enhance the performance of heat to microscopic activities, and microscopic properties are capacity and decrease compressive strength [61, 62]. The more conducive to the stability of matter at higher scale types of alkali metal cations and the alkali-activating solu- levels (that is, at mesoscale and macroscale). At high tem- tion have a huge effect on thermal deformation. For exam- peratures, the thermally induced cracking ability and vol- ple, comparing to using sodium as an alkali cation, thermal ume deformation ability of a substance is called moderate shrinkage could be better reduced by using potassium as thermal stability. The resistance to spalling and the ability an alkali cation [56]. When applying a mixture of geopoly- of materials to withstand high temperatures is related to mers at high temperatures, these factors must be carefully macro-stability [46]. considered. The microscopic properties of geopolymers have been Rickard et al. [63] studied the microstructure of two reported by some researchers. It has also been found that geopolymer mixtures: one was a dense microstructure and geopolymers are chemically more stable than OPC hydrated highly reactive geopolymer, and the other mixture did not products, and their chemical structure is easier to be de- react due to the large amount of fly ash. Therefore, the stroyed because when exposed under the temperature, OPC strength and density of the polymer are relatively low. Be- will get severely deteriorated. Factors as alkali contents, al- cause of the dense microstructure, low-strength polymers kali cation types and Si / Al ratio play imperative roles in are stronger than high-strength polymers. Due to macro- deciding the chemical structures of geopolymers when ex- cracks and dehydration damage, high-strength geopoly- posing to the raising temperatures. These factors shall then mers have increased strength losses, reduced thermal per- be tailored to accomplish a suitable mix of geopolymer for formance, and increased dimensional instability. Studies the structural fire applications. have found that low-strength geopolymers will be slightly 6 Ë Tian L. et al.

Figure 7: Schematic depiction of the proposed micro-structural changes in geopolymer paste upon firing [63] 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].

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 sam- Figure 8: Concentration of elements dissolved from geopolymer ples were placed in 5% H2SO4 solution for 30 days, the samples, (a) in acid solution; (b) in alkali solution [64] 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 ash-based geopolymer slurry was placed in a 5% H2SO4 and 5% acetic acid solution, the performance was superior placed in HNO3. Temuujin et al. [64] found that the alkali and acid resis- to ordinary Portland cement slurry. The formation of zeo- tance of fly ash-based geopolymers are largely determined lites and depolymerization aluminosilicate network were by their mineral composition. Fe, Si, and Al highly solu- closely related to the deterioration in the pastes [13]. ble ions are obtained from strong acid and alkali solutions. Hardjito et al. [65] discovered that the strength of com- Figure 8 shows the concentration of dissolved elements pression of a fly ash-based GPC in 0.5%2 H SO4 solution was in a geopolymer sample after being placed in an alkaline reduced by 20% when exposed for 12 months. The value or acid solution [64]. The study found that when the fly was 65% and 52% after the samples were exposed to 2% Durability of geopolymers and geopolymer concretes: A review Ë 7

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 con- stituents. 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].

Figure 9: Compressive strength evolution of the geopolymer and 5.2 Resistance to seawater and sulphate Portland cement specimens exposed to 5% acetic acid solution [13] 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 Figure 10: Compressive strength evolution of the geopolymer and with NaOH solution only performed better than the OPC Portland cement specimens exposed to 5% sulfuric acid solution concrete. It was reported by Criado et al. [70] that the dura- [13] bility and strength of the GPC improved along with the time regardless of the kinds of chemical solutions where the and 1% H2SO4 solutions, separately. Erosion and pitting on samples were submerged, as shown in Figure 12 [71]. the surface of the concrete were also watched. The degra- Durability performances of fly ash geopolymer concrete dation of the geopolymer matrix is the main cause of the have been researched by Adam [72]. One new understand- loss of concrete strength, not the degradation of the aggre- ing of the durability and strength of GPCs have been offered gate. 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]. 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 Figure 11: Compressive strength of fly ash activated with sodium concrete. silicate solution and NaOH, and OPC specimens, exposed to 5% of Na2SO4 and MgSO4 [69] 8 Ë Tian L. et al.

Figure 13: The porosity and pore size distribution of geopolymer incorporating various content of metakaolin after 180 days of expo- Figure 12: Mechanical strength of fly ash mortars (a) NaOH- sure [77] activated, and (b) water glass-activated [71]

A decreased water permeability (2.46-4.67 × 10-11 m/s) on the chloride and carbonation resistance and the effects for a GPC (activator-fly ash ratio of 0.30–0.40 cured at60∘C of the dosage of Na O on the compression strength of sam- 2 for 24 h) in contrast to that for OPC has been reported by ples of GPC. Both activator modulus and Na2O dosage play Nikraz and Olivia [75], because of its lower pore intercon- crucial parameters for the GPC production. It was discov- nectivity and denser paste. It was also reported that the ered that fly ash geopolymer concrete shows a strength most affecting parameter influencing the property ofGPC compared to that of concrete of OPC. Nevertheless, the prop- was the water-geopolymer by some researchers. Fly ash erties of the durability of fly ash geopolymer concrete were geopolymers blended slag, because of their reduced poros- watched to be good on the chloride and carbonation resis- ity volume and enhanced pore size matrix, which showed tance than those for the concrete of OPC. good resistance to the permeability, which might progressively be improved by raising temperature curing [76]. As talked previously, the microstructure of the geopoly- 6 Permeation properties of mer matrix has been densified by metakaolin particles, which might lower the porosity [77]. After it was exposed geopolymer concrete to the sodium sulfate solution for 180 days, as indicated in Figure 13 [77], the water absorption and apparent poros- The permeating performance influenced by the size of per- ity will be increasing along with the rising temperature. meable pores of the concrete of geopolymer means the At raising the temperature, the water included in the ma- performance of the microstructure of the produced matrix trix moving to the surface and produced micro-cracks that [73, 74]. was increasing the absorption of water. The permeability of chloride of the concrete of the geopolymer was tested by Durability of geopolymers and geopolymer concretes: A review Ë 9

increased freeze-thaw resistance with an increased Na/Al ratio. 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 Figure 14: Corrosion rate of geopolymer and PC concrete specimens, nm pores can be observed with the geopolymer that had a immersed in 3% NaCl solution [80] weak resistance. Metakaolin-based geopolymer groups with Si/Al=2.00 the Rapid chloride permeability (RCP) test as the chloride and Na/Al=0.80-1.20 in addition to Na/Al=1.05 and of acid-soluble (total chloride), as per the ASTM C-1202 [78]. Si/Al=1.80-2.20 were conducted with a permeability test. Part et al. [79] forecast that long time resistance of chlo- The test results show that geopolymers with increased ride of geopolymer concrete will be lower than that of con- Si/Al ratios present a generally increased permeability, and crete of OPC due to the lower increment of strength along geopolymers with high Na/Al ratios present a decreased per- the time of concrete of geopolymer. The rates of corrosion meability. The permeability coefficient of the geopolymer of PC concrete and the geopolymer in 3% sodium chloride is correlated with porosity, effective porosity, and critical solution are indicated in Figure 14 [80]. The tried survey of pore diameter. A calculated permeability coefficient that the long time chloride resistance of a geopolymer concrete considers pore structure parameters is proposed to predict utilizing a fast chloride permeability testing was futile, as the water permeability of metakaolin-based geopolymers. the specimens of geopolymer showed a fast-rising in the Metakaolin-based geopolymer concretes have poor temperature in the testing, which is opposite to the law frost resistance, and the freezing-thawing limit is less of Ohm and showed that RCPT is not an appropriate test than 75 times. The Na2O equivalence content and activa- means to assess the resistance of chloride of concrete of tor modulus have a great effect on the freeze-thaw per- geopolymer. formance and mechanical properties of geopolymer concrete. A metakaolin-based geopolymer concrete with a Na2O equivalent weight of 16% and modulus of 1.5 was 7 Frost-resistant geopolymer the best herein, and the freezing-thawing limit reached 75 times. The frost resistance of geopolymer concretes had concrete a positive correlation with the Na2O equivalent content; the correlation coefficient was 0.845, and the correlation Deterioration of freeze-thaw-induced concrete could be with the activator modulus and other parameters was not contributory to the microcracks appearing at the zones of significant. weak interfacial transition among paste/MPCM and the Slag can shorten the setting time of the geopolymer, paste/aggregate [81]. enhance the mechanical properties of the geopolymer con- PE-EVA-PCM and St-DVB-PCM were contained by char- crete, and strengthen the tensile strength by more than acterization of the Microscopic Structure of the PCC and 20%. The frost resistance of geopolymer concrete increased GPC after 0 and 28 freeze-thaw cycles were executed by with increasing slag content. The limit freeze-thaw times tomography imaging of X-ray and SEM. SEM images of the of geopolymer concrete with a 30% slag increased from samples after 28 freeze-thaw cycles are indicated in Fig- 43 to 135. The frost resistance of geopolymer concrete was ure 15 [81]. Metakaolin-based geopolymers with Na/AI=1.01; improved in the decreasing order of sodium lauryl sulfate Si/AI=2.01, 2.32 and 2.62; Si/Al=2.62; and Na/Al=1.01, 1.26, >sodium dodecyl sulfate> sodium arsenate, which had the and 1.36 were subjected to freeze-thaw cycles. The exper- ideal mixing amounts of 0.06%, 0.08%, and 0.02%, respec- iment shows that the geopolymer presents a decreased tively. The freezing-thawing limit for geopolymer concretes freeze-thaw resistance with an increased Si/Al ratio and an could be increased from 36 times to 60-75 times by entraining air. Three particular air-entraining agents introduced 10 Ë Tian L. et al.

Figure 15: SEM images of the fracture surface of PCC and GPC [81] bubble spaces and pore sizes exceeding 400 µm and 300 8 Conclusions µm. Common air-entraining agents have poor compatibil- ity in high-alkaline, high-viscosity fresh geopolymer con- In this paper, the mechanisms and research progress regard- cretes. Slag can improve the frost resistance of geopolymer ing the carbonation resistance, structural fire resistance, concretes better than air-entraining agents. There is an in- corrosion resistance, permeation properties and frost resis- creased improvement in the frost resistance using both slag tance of geopolymer concretes were reviewed, and the main and air-entraining agents. problems with the durability of geopolymer concretes were The degree of saturation is the most important factor discussed. The conclusions of this review are as follows: that influences the frost resistance of geopolymer concretes, and the degree of saturation is greater than the rate of the 1) Durability of geopolymers and geopolymer concretes temperature decrease. The porosity of a geopolymer con- is influenced by the materials and design meth- crete exceeds 15%. The balance of capillary water satura- ods for geopolymer preparation. The reactivity of tion was 0.84 to 0.92 within 12 hours. Here, the geopolymer metakaolin-based GPC structure is intense. Besides, pores were filled with sodium silicate solution, and the con- adding metakaolin to recycled aggregate GPC can centration of Na+ ions was very high and reached approx- improve the abrasion resistance. FA-based GPC has imately 0.6 mol/L. The pore structure of the geopolymer great advantages, such as low shrinkage, low perme- concrete was terrible because the pores exceeded 75% in ability, high-temperature resistance, and high me- the SEM images, and the proportion of harmful spores in chanical strength. There are two design methods for the freeze-thaw process was further increased. They caused geopolymer preparation. In the design of sodium the freezing pressure and osmotic pressure to increase dur- aluminum ratio, silicon aluminum ratio and water ing the freeze-thaw process, and even salt precipitation sodium ratio, the need for higher silicon aluminum produced a crystallization pressure, ultimately resulting in ratio will lead to excessive use of alkali activator and poor frost resistance. The freezing-thawing failure mech- high cost. With the design of water binder ratio, wa- anism of geopolymer concretes is mainly based on hydro- ter glass modulus, and alkali equivalent, the amount static and osmotic pressure theory. Durability of geopolymers and geopolymer concretes: A review Ë 11

of alkali activator is less, and the best mixing ratio 5) Because of lowered porosity volume and enhanced can be found in the close steps. matrix of pore size, the slag mixed fly ash-based 2) The carbonization proof property of concrete is not geopolymer showed good resistance to permeability, poorer than that of concrete of geopolymer. Usually, which might progressively be improved by the rising the higher the GPC strength, the better the resistance temperature curing. The microstructure of the matrix of the carbonation. Nevertheless, an alkaline acti- of geopolymer was densified by metakaolin particles, vator also affects the resistance of the carbonation this might lower the porosity. Geopolymers with in- of GPC. BFS-based and FA-based GPC resistance of creased Si/Al ratios generally present an increased carbonation is influenced by BFS ratio in the active permeability, and geopolymers with increased Na/Al fillers, NaOH content in the active activator solution ratios present a decreased permeability. The survey and BFS fineness. The effect of metakaolin-based and of the long time chloride resistance of GPC utilizing slag-based GPC is the gel chemistry of binders. It sug- a fast chloride permeability testing is opposite to the gested that is not just controlled by one parameter. law of Ohm and showed that RCPT is not an appro- Besides, due to the microstructure of OPC is diverse priate test means to assess the resistance of chloride from that of GPC, it is impossible to utilize the same of concrete of geopolymer. experiment to analyze the carbonation of concrete. 6) The freezing-thawing failure mechanism of geopoly- 3) There is a non-organic structure in the materials of mer concretes is mainly based on hydrostatic and os- geopolymer and indicate fixed higher anti-fire to that motic pressure theory. Geopolymers have a decreased of Portland cement binder. Elements as the alkali freeze-thaw resistance when they have an elevated content, alkali cation, and Si/Ai ratio are imperative Si/Al ratio and an increased freeze-thaw resistance and decide the chemical structure of geopolymers with an elevated Na/Al ratio because geopolymers when exposed to the rising temperatures. Those el- with increased Si/Al ratios present a generally in- ements shall be tailored to accomplish a suitable creased permeability, and geopolymers with high geopolymer mix for the application in a structural Na/Al ratios present a decreased permeability. The fire. Because FA-based GPC requires less water than frost resistance of geopolymer concretes improves metakaolin-based GPC during processing, FA-based with increasing slag content. Besides, the Na2O equiv- GPC is easier to adjust for thermal deformation. Com- alence content and activator modulus have a great paring to using sodium as an alkali cation, thermal effect on the freeze-thaw performance and mechan- shrinkage could be better reduced by using potas- ical properties of GPC. The degree of saturation is sium as an alkali cation. Besides, low-strength GPC the most important factor that influences the frost thermal performance is better than high-strength resistance of geopolymer concretes and causes the GPC. Due to macro-cracks and dehydration damage, freezing pressure and osmotic pressure to increase high-strength geopolymers have increased strength during the freeze-thaw process, and even salt precip- losses and increased dimensional instability. Low- itation produced a crystallization pressure. strength geopolymers will be slightly damaged because of dehydration, which can better adapt to vol- Acknowledgement: The research in this paper has been ume changes, and its strength increases when ex- supported by the National Natural Science Foundation of posed to heat. China (Grant No. 51708092) and China Postdoctoral Science 4) There are a large number of Al-O and Si-O structures Fund Project (Grant No. 2018M631894). in geopolymers. Geopolymers do not react with acids at room temperature and can be used to make acid- Conflict of Interests: The authors declared that they have resistant materials. The durability of fly ash-based no conflicts of interest to this work. 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 References their mineral composition to form an aluminosilicate network (N-A-S-H). What’s more, the degradation of [1] Lloyd, N., and B.V. Rangan. Proceedings of 35th Conference on the geopolymer matrix is the main cause of the loss Our World in Concrete and Structures, Singapore, August 25-27, of concrete strength in acid solution, not the degrada- 2010, Singapore Concrete Institute, 2010, pp. 25-27. tion of the aggregate. And Na2O dosage plays crucial [2] Mehta, P.K. Reducing the environmental impact of concrete. Con- parameters for the GPC. crete internatioal, Vol. 23, No. 10, 2001, pp. 61-66. 12 Ë Tian L. et al.

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