Acid Corrosion in Waste Based Geopolymer Binders

Acid Corrosion in Waste Based Geopolymer Binders

Prepared by: Gabrielle Davis

Acid Corrosion in Geopolymer Binders

Table of Contents 1.

Geopolymer Binders.................................................................................................................3 1.1.

3.

Using Geopolymer Over Portland Cement.......................................................................4

Geopolymer Reactions with Acids............................................................................................4 3.1

Sulfuric Acid.......................................................................................................................4

Blended Ash and Ash Replacements.......................................................................................8 Reinforced Geopolymer.........................................................................................................10 3.2

Nitric Acid........................................................................................................................11

Blended Ash and Ash Replacements.....................................................................................11 3.3

Hydrochloric Acid............................................................................................................12

Blended Ash and Ash Replacements.....................................................................................13 3.4

Acetic Acid.......................................................................................................................13

5.

Comparison With Portland Cement.......................................................................................14

6.

Summary and conclusions.....................................................................................................15

7.

References.............................................................................................................................16

2

Acid Corrosion in Geopolymer Binders

1. Geopolymer Binders Geopolymers are cementitious materials that utilize a solid aluminosilicate precursor, like coal fly ash or clay, which is activated with alkali solutions to make a binder. This binder is a disordered solid that contains unreacted particles and pore networks made by unused water from the alkali solution—water goes chemically unused during geopolymer gel formation, unlike in Portland cement’s calcium silica hydrate (CASH) gel formation 1. Alkali earth metals that were present in the activating solutions (likely potassium or sodium) ionize and play a counterbalancing role in keeping the repeating network of aluminum, silicon, and oxygen in place1. This three dimensional network will have aluminum in a tetrahedral coordination, with repetitions that take the form of poly(sialate) (-Si-O-Al-O-), poly(sialate-siloxo) (-Si-O-Al-O-SiO-), and poly(sialate-disiloxo) (-Si-O-Al-O-Si-O-Si-O-). The type of specific geopolymers looked at in this review are based on waste products like fly ash or blast furnace slag from combustion reactions, rather than metakaolin clay. The term “geopolymer” was thought up by J. Davidovits 2 and is currently one of the more popular terms used to describe this material, but it also has a host of other names as well. Other names used in academic literature to describe geopolymer-type systems, as recorded by Provis and Deventer, are “mineral polymers”, “inorganic polymers”, “inorganic polymer glasses”, “alkali-bonded ceramics”, “alkali ash material”, “soil cements”, “hydroceramics”, “aluminosilicate glass”, among others with “inorganic polymers” and “geopolymers” being the most popular 1. Geopolymers were born in the aftermath of a series of fires in France between 1971 and 1973. J. Davidovits startup company, now called CORDI-GÉOPOLYMÈRE, was the first to synthesize geopolymers after researching fire-resistant materials and turning to inorganic compounds for inspiration3. The first application of geopolymers was fire-resistant materials like a flameproof board made of geopolymer binder and wood chips, and has since branched off into uses in construction, waste encapsulation, and ceramics.

1.1. Using Geopolymer Over Portland Cement Geopolymers have gained traction as an Ordinary Portland Cement (OPC) replacement as OPC manufacturing lets off harmful emissions. Globally, the cement manufacturing industry alone accounts for around 5% of CO2 emissions4. Geopolymers utilize recycled materials and materials that don’t require much processing, like fly ash from coal power plants or mined clay, making the process less harmful for the environment. Some geopolymer properties, like strength, are highly dependent upon the starting materials and activating solution used making it easy to pick and choose what specific properties you need for your project 5. Compared with OPC, the geopolymer manufacturing process is less labor, environment, and energy intensive, and has a more finely-tuned final product—one could see why a company would utilize geopolymers in favor of OPC.

2. Geopolymer Reactions with Acids From animal husbandry runoff, sewer systems, acid rain, and natural soil content, concrete may find itself in contact with acids more often than you would think 6. The most common is sulfuric acid, which is especially present in sewer systems due to sulfur oxidizing bacteria like Thiobacillus6.

2.1 Sulfuric Acid A recent study done by Chindaprasirt, Paisitsrisawat, and Rattanasak in 2014 detailed highcalcium fly ash from a new process—fluidized bed combustion, which is a low-temperature process—ground with silica fume. A 15 M sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) mix was used as an activator. River sand was used as non-reacting aggregate. Samples with 1.5%, 3.75%, and 5% added silica fume (by weight) were prepared and immersed in 3% sulfuric acid (H2SO4) for ASTM C109-compliant lengths of 7, 28, and 90 days. The control (0% silica fume, SF) compressive strengths were 10.5, 10.8, and 12.0 MPa, respectively. While all samples showed the same trend in which the compressive strength improved over time, the 3.75% SF sample had the best results with strengths of 13.0, 15.5, and 22.0 MPa—the 90-day compressive strength being almost double that of the control sample 7. It was noted that the 4

differing SF percentage samples may have had different hardening reactions—the 5% SF sample had better results over time due to a pozzolanic reaction, while the 3.75% SF sample formed CASH gel along with its geopolymer matrix as confirmed by XRD, which helped the compressive strength results7. After 3 months of exposure to H2SO4, all samples’ compressive strengths reduced to ~5 MPa, which the authors explained was due to sulfur attack on the high calcium content7. Lloyd et al. released a rather comprehensive study in 2012 detailing acid corrosion in geopolymer concrete (“inorganic polymer binders”, as they called it). This study involved exposing specimens to both nitric and H2SO4 with pH values between 1 and 3. Lloyd et al. took three different fly ashes and mixed, one of them mixed with differing proportions of ground granulated blast furnace slag (GGBS). This study used 2 type F ashes and 1 type C ash from Australia and New Zealand. One of the type F fly ashes was also replaced 10% 25%, 50%, and 100% by the GGBS (GGBS will be detailed further below). Alkali concentrations were at 3%, 7%, 11%, and 15%, relative to the binder mass. Dissolved silica in activating solutions were at 4%, 7%, 10%, and 13% as relative to binder mass. Activating solutions containing sodium, potassium, and a mix of the two were used. Water/binder ratios were 0.275, 0.300, 0.325, and 0.375. The study mentioned that the depth of corrosion was a better way of discerning acid damage for this test as compressive strength loss was not measured and mass loss may be misleading due to different mechanisms used between acids and binder types 6. It was found that pH strongly influences the rate of corrosion, and that H2SO4 will cause more damage than nitric acid—this can be explained by H2SO4 being categorized as “strong” and nitric acid ( HNO3) as “weak”. The authors explained the mechanism: After the H2SO4 has dissociated to HSO4- and H3O+, it will further break down into more H 3O+ and SO4- . The additional H3O+ ions will, in turn, attack the binder. It was also said that this same mechanism is the reason that weak acids will do more against OPC binders6. This study also contradicted the belief that geopolymer binders were highly impervious to acid corrosion after initial damage, as explained in a waste management conference in 19938. The authors found that the corrosion at low pH levels (1 and 2) was limited by diffusion speed rather than chemical reaction time.

5

The above authors carried out a series of tests at pH 1 due to how accelerated the attack is at that level. All variables detailed above, as well as a few others, were examined. Increased dissolved silica content raised the acid resistance of the binder. Increased alkali content dropped the corrosion rate—raising the alkali content from raising it from control (7%) to 11% cuts it down over a third, and raising from control to 15% cut the corrosion rate in half. Unfortunately, higher alkali content also making the corroded layer more prone to cracking due to shrinkage, leading to a higher weight loss over time 6. Oddly enough, it was also found that using a high alkali content and fly ash sources with a high amount of acid-soluble materials like aluminum, calcium, and sodium resulted in a binder with higher acid resistance—this is likely due to decreased pore size in high alkali materials. Higher water content correlated with a higher corrosion depth, also due to porosity—since water doesn’t interact in geopolymer binder formation, any leftover water creates pore networks. Fewer pore networks lead to lower porosity, which hinders acid attack penetration. The effect of different alkali metals was examined by using a potassium or sodium activating solution, and an equimolar mix of the two. It was found that alkali type had little effect on the acid corrosion rate6. When activating a geopolymer reaction, using only hydroxides (i.e. no soluble silicate) will form crystalline structures at the expense of geopolymer gel 9 and it has been proposed that these crystals, mostly zeolites, are more acid resistant than the gel 10. The authors found that samples with lower amounts of soluble silicate were, in fact, slightly more corrosion resistant than their higher silicate percentage counterparts. However, the presence (or absence) of the soluble silicate had little effect on the corrosion rate 6. The authors explained that this may be due to curing conditions of the hydroxide-only samples, which were different compared to the ones containing soluble silica, so this claim may need further exploration. To explore the effect of the fly ash source on the finished product, a solution with 7% Na 2O, 7% SiO2, and a water/binder ratio of 0.325 were used on all samples. The two class F fly ash samples behaved similarly, while the class C fly ash had a significantly higher acid attack resistance, likely due to the higher calcium content. The effect of the aggregate was also explored, adding 25 and 50% impermeable 100 µm crushed quartz aggregate to the mix. Aggregate fineness appeared to have no effect on the acid corrosion rate6. 6

In 2009, Thokchom, P. Ghosh, and S. Ghosh prepared a fly ash mortar using Class F fly ash from a power plant near Kolkata, India, river sand, and a NaOH/Na 2SiO3 solution11. Three specimens were made using 5%, 6.5%, and 8% sodium oxide (Na 2O) content, which creates NaOH when added to water, and a flat water/fly ash ratio of 0.33. The fly ash and activator solution were mixed for 5 minutes before sand was added, then mixed another 5 minutes before transferring the mixture into cube moulds and vibrated via a vibrating table for 2 minutes. The samples were heat cured at 85 °C for 48 hours and cooled to room temperature and let sit in room temperature for 28 days before testing. The samples were left in 10% H 2SO4 solution for a period of 24 weeks, with testing occurring at regular intervals. It was noted that Na 2O content was linked to the final properties of the samples—a higher Na 2O content was correlated with a slow dealkalization rate, higher weight loss, and lower overall strength loss 11. The 8% Na2O specimen retained 54.8% of its original strength of 40 MPa, the 6.5% Na 2O specimen retained 42.9% of its original 37 MPa, and the 5% Na2O specimen retained only 29.4% of the original 22 MPa11. Sreevidya, et al. repeated the above experiment in 2012 using a 5% H 2SO4 solution, class F fly ash from near Metthur, India, and river sand 12. 50 mm cubes of mortar were made as above, using two different curing methods: curing at 60 °C for 24 hours and then letting it sit at room temperature for 28 days, and curing at ambient temperatures with no heat for 28 days. Samples were immersed in 5% H2SO4 for 14 weeks. In this experiment, the water/binder ratio was used as a comparison between the samples. Water/binder ratios of 0.376, 0.386, 0.396, and 0.416 were used. A higher water/binder ratio correlated with lower weight loss percentages and higher compressive strengths. The trends were similar between both heat and ambient curing methods. There was a low strength percentage loss after acid submersion in both curing methods12. Wallah and Rangan released a study of the effects of year-long H2SO4 exposure to geopolymer products made from class F fly ash. Both mortar and cement product specimens were exposed to 0.25%, 0.5%, 1% and 0.5%, 1%, and 2% H 2SO4 solutions (respectively) for periods of up to a year. Results were as expected from papers described previously. The concrete specimens exposed to 2% H2SO4 solution had a compressive strength loss of 65%, from ~55 MPa to ~19 MPa13. The geopolymer mortar was exposed to a maximum of 1% H 2SO4 solution and had about 7

an 88% strength loss after a year13. The coarse aggregate used in the concrete product helps retain compressive strength as the degradation is done through the binder itself—the concrete product with coarse aggregate has less binder in the product overall when compared to the mortar so the acid attack affects the mortar more than the concrete13. Blended Ash and Ash Replacements Ariffin, Bhutta, Hussin, Mohd Tahir, and Aziah’s experiment detailed the acid resistance of a geopolymer concrete made from a 70/30 mix of low-calcium fly ash and palm oil fuel ash, both from Malaysia. Tests were also done on OPC, which will be detailed in the appropriate section. Paste was prepared using a 0.4 alkali solution/ash ratio. 100 mm cube molds were prepared and vibrated on a vibrating table for 10 s, and cured at 28 °C for 28 days, with the 28-day strength of the ash mix mortar being 28 MPa. Specimens were exposed to 2% H 2SO4 solution (pH 1) for up to 18 months. After 18 months of exposure, the geopolymer samples were visually unchanged, despite a little “softening” of the surface and had an 8% weight loss. The overall 18-month strength loss was only 35%, and

XRD and FTIR analysis showed no degradation of the

geopolymer matrix or sulfate precipitation after exposure 14. Bhutta, Ariffin, Hussin, and Lim repeated the this using 70 mm cube samples that were cured for 24 hours at 90 °C and subjected to 2% H2SO4 for periods of 28, 56, 90, 180, and 360 days. The study also included room temperature cured samples, which are already detailed in the prior study with similar results. The results were compared to a control OPC sample. The same visual effects were observed. Geopolymer samples lost less than 3% of their original mass. After a period of 360 days, the heat cured sample lost 40% of its original 28-day strength 15. The authors stated the loss in compressive strength is due to the acid attack mechanism detailed in Bakharev’s 2005 study, in which sodium or potassium is replaced with hydrogen or hydronium 10. Bakharev’s results regarding acetic acid will be detailed in the appropriate section—his results regarding H2SO4 agree with the rest of the applicable studies outlined in this review. In 2013, Sun and Wu released a study on the freeze-thaw and chemical resistance of fly ash based geopolymers. This study included blends of class F fly ash with metakaolin, and an OPC control. Samples were exposed to water as a control, along with 0.05% and 3% H 2SO4 solutions for up to 24 weeks. The OPC control sample will be detailed in the appropriate section. 8

Geopolymer samples showed little surface damage after the 24 week period for the 3% H 2SO4 solution, but damage was accelerated appearing after only 1 week for the 3% samples 16. In the 0.05% solution, the fly ash samples lost only 0.05% of its initial mass after the 24 week period, while the samples exposed to 3% H2SO4 lost between 20-30% of their initial mass by the 6 week mark16. In Lloyd et al.’s comprehensive 2012 paper, they also tested corrosion properties by replacing 10%, 25%, 50%, and 100% of one of their class F fly ashes with GBBS. The solution had 7% Na 2O and SiO2, and had a water/binder ratio of 0.350. The addition of GBBS showed a large effect on corrosion rate—the 100% GGBS sample had a very low corrosion depth when compared to the control sample after 28 days (~2 mm compared with ~6 mm) 6. This also contradicts previouslyheld beliefs on geopolymer corrosion, and indicates that the formation of CASH gel is not, in fact, detrimental to geopolymer acid resistance, and it appears as if CASH gel actually enhances acid resistance6,17. In the 100% GGBS sample, gypsum was formed which lead to cracks in the corroded layer—despite this, the 100% GGBS sample was more acid resistant than the fly ash based samples. This could be explained by GGBS’ reactivity—there is more gel formed per volume of paste. Permeability and pore size are also decreased with the addition of GGBS, which plays a large role in acid resistance 18. The GGBS/fly ash mixes had diffusion-limited acid resistance, which hints at gypsum helping with acid diffusion resistance. Earlier tests in 2005 and 2006 done by Allahverdi and Skvara agree with the above results from Lloyd et al. A 50/50 mix of fly ash and GBBS were mixed to create samples that were immersed in H2SO4 solutions of pH 1, 2, and 3. After 60 days of exposure, gypsum crystals were present in the cracks of the samples immersed in pH 1 solution. The mechanism for attack at pH 1 was described as having 2 steps: ion exchange, followed by calcium diffusion. The leached calcium reacts with the H2SO4 to create gypsum crystals inside the geopolymer matrix, which seems to have a corrosion deterring effect and acts as a kind of barrier 19. However, the attack changes at pH 2 and 3. At pH 2, gypsum crystals form in the cracks of the corroded layer rather than in the geopolymer matrix. The gypsum crystals in this region can actually have a harmful effect due to expansion20, even though the protective effect is still there. Lack of gypsum in the geopolymer matrix shows that sulfate ions do not diffuse into the matrix at pH 2. At pH 3, there were no 9

gypsum crystals present, though there was a thin layer of corrosion with micro cracks due to shrinkage20. It was noted that the mechanism at pH 3 H2SO4 was similar to the attack mechanism of pH 3 HNO3 as described in 2001 by the same author—the charge balancing alkali ions are leached out and the tetrahedral alumina is ejected20,21. In 2011, Fernando and Said made geopolymer binders using mine waste. 50 mm cubes were immersed in 5% sulfuric, nitric, and hydrochloric acid for 20 days and then oven dried. The H2SO4 results were similar to others described here, listing the voluminous crystal formation as a main source of degradation in geopolymer binders exposed to H2SO422. Reinforced Geopolymer Kannapiran, Sujatha, and Nagan (2013) performed a H 2SO4 test on geopolymer samples that were reinforced with 6 mm and 8 mm steel rods placed at the corners of a 100 mm by 100 mm width and 500 mm long geopolymer sample, and then cured at 70 °C for 24 hours and left to cool to room temperature. Reinforced geopolymer sample was exposed to 10% H 2SO4 for up to 120 days. Samples were also exposed to a 5% HCl and 5% H 2SO4 mixture that will be detailed in the “other acids” section. At 180 days, the samples exposed to the 10% H 2SO4 exhibited the usual greenish discoloration, pores, and slight weight loss. XRD confirmed the loss of Zeolites and the formation of Mullite, a silicate mineral. The ultimate load of the control sample was 47.00 kN, and the 180-day ultimate load of the 10% H 2SO4 sample was 42 kN, resulting in only a 10% loss of strength23.

2.2 Nitric Acid Thokchom, S. Ghosh, and P. Ghosh performed an experiment in 2011 that involved placing 3 different specimens in 10% HNO3 for up to 24 weeks. Specimens were made with fine river sand, local class F fly ash, and an activating solution that was a mix of NaOH and Na 2SiO3. The HNO3 has different results—weight loss and Na2O had a negative correlation, but the negative strength trend stayed the same, noting that there was less strength loss overall when using HNO3

as opposed to H2SO4. Weight loss may be explained through EDX analysis of samples,

which showed that The 5% Na2O sample kept only 30.6% of its original strength of 22 MPa, the 6.5% sample kept 37.5% of the original 37 MPa, and the 8% sample kept 60.3% of its original 40 10

MPa, which were all 28 day compressive strength values 24. It was noted that samples with lower alkali content had a higher porosity after the testing, which may account for the strength loss. Blended Ash and Ash Replacements In 2001, Allahverdi et al. exposed geopolymer samples made from a 50/50 mixture of fly ash and GGBS to HNO3 at pH 1, 2, and 3. The acid attack mechanism was shown to be the same across all 3 pH levels: leaching of the counterbalancing sodium or potassium ions along with the ejection of the four-fold alumina17. This is the same mechanism found in acid attacks done by pH 3 H2SO420. Fernando and Said’s study on mine waste based geopolymers activated with NaOH/Na 2SiO3 solutions saw results that agreed with the above information, adding that limestone aggregate seemed to worsen the effects of acid attack in HNO322.

2.3 Hydrochloric Acid Sreevidya, et al. also conducted their 2012 experiment using hydrochloric acid (HCl). Compressive strength values for both heat and ambient curing methods were slightly lower than the H2SO4 results, with the exception of ambient cured mortar with a 0.416 water/binder ratio, where the final compressive strength turned out to be higher than the H 2SO4 sample, yet still lower than the unaltered 28 day strength12. A 2009 study looked at a specific application of geopolymer cement that involved high molarity HCl. Five geopolymer and 4 OPC samples were made to compare acid corrosion resistances. Alkali activation was done using a mix of 14 M NaOH and Na 2SiO3. Samples were subject to an accelerated test using two different phases with different heat and acidity conditions. Phase 1 went on for 45 days and tested samples with water at 90 °C, 12% HCl at 45 °C, and 30% HCl at 25 °C. A control geopolymer sample was exposed to water and ambient temperatures. The second phase had 3 geopolymer samples (2 unused in phase 1 and 1 used) exposed to 22% HCl solution at 95 °C for 60 days. The main focus of the study was on mass loss and surface changes as the study was meant to simulate conditions in a pickling tank where only one side is exposed to the acid. The authors found that despite the surface pores and color change, the geopolymer samples were still considered acid resistant 25. The furthest the acid penetrated into a 11

geopolymer samples was 10.6 mm, much better than the OPC samples. The authors noted a positive correlation between penetration depth and temperature, as well as a positive correlation between weight loss and temperature when acids are present—the sample with 12% HCl at 45 °C had a higher weight loss than the room temperature sample in 30% HCl 25. The no HCl sample at 95 °C retained more weight than the samples exposed to 12% and 30% HCl solution. It was also found that the phase 2 weight loss in the geopolymer samples seemed to level off after an initial loss. After acid exposure (110 days total), geopolymer samples were subject to ASTM 642 compressive strength tests that had the expected result of geopolymer having a higher compressive strength than OPC after acid exposure. The authors believe this is due to microsctructure formation during curing, and note that curing conditions may be incredibly important to microstructure formation and taking the time to optimize these conditions in your process may be worth it. Kannapiran, Sujatha, and Nagan also tested a 5% HCl and 5% H 2SO4 mixture. There was a little surface corrosion with a penetration depth of 2 mm and little change in the geopolymer microstructure after 180 days. The ultimate load of the sample was 44.98 kN, a 4% loss from the control sample’s 47 kN23. Blended Ash and Ash Replacements Fernando and Said’s mine waste sodium alkali activated geopolymer study saw similar results in 5% HCl as described above. The weight loss mechanism for HCl (as well as other acids) was explained as calcium compounds in the binder leaching out and creating soluble byproducts with the HCl22.

2.4 Acetic Acid Bahkarev’s 2005 study looked at how geopolymer binders performed in a pH 2.4 solution of acetic acid. Properties were measured at up to 180 days of exposure to acid. Weight loss in acetic acid samples (as opposed to pH 0.8 H 2SO4) was much smaller over a period of 180 days. Compressive strength in fly ash samples exposed to acetic acid and activated with a sodium based solution had an odd behavior—there was actually a positive trend in compressive strength peaking at around 100 days after the initial strength loss before continuing the 12

negative trend. These fluctuations are explained by the possible breakdown of geopolymer material and alkali migration during acid attack 10. While there was some crystalline zeolite growth, there wasn’t as much as there were present in the samples exposed to H 2SO4.

3. Comparison With Portland Cement Ariffin, Bhutta, Hussin, Mohd Tahir, and Aziah’s paper tested the H2SO4 (pH 1) corrosion of OPC as well as fly ash. Despite having a very similar 28 day strength of 27 MPa (compared to 28 MPa), OPC performed poorly in all aspects compared to the geopolymer—visually, the OPC sample was severely deteriorated and had a 20% loss in mass after 18 months with a 68% loss in compressive strength, compared to the blended ash’s 8% and 35%, respectively. The calciumrich OPC binder caused the precipitation of gypsum, which led to instability 14. The results for Bhutta, Ariffin, and Hussin’s other 2013 study saw similar results for the geopolymer and OPC samples that were close to those detailed in the prior paper, with 12 month geopolymer and OPC strength loss at 44% and 70%, respectively15. Sun and Wu’s study included an OPC control sample in their basic acid corrosion test. The OPC underwent severe corrosion in the 3% H 2SO4 sample by the end of 1 week, including surface blistering and white deposits on the surface16. The OPC control also lost 33% of its initial mass within 6 weeks in the 3% H2SO4 sample16. In 2012, Lloyd, et al. noted that, in regards to corrosion depth at H 2SO4 with a pH of 1, geopolymer binders performed worse than OPC and other calcareous counterparts. This contradicts previous research done by Shi and Stegemann for paste samples in H 2SO4 at pH 3, in which they observed OPC paste was corroded more rapidly than GGBS paste 26. However, this research was done on uncured pastes rather than finished concrete, and it is expected that the lack of a porous interfacial transition zone, or ITZ—the narrow region around aggregate particles with fewer binder particles found in cements, which has been cited as a source of degradation in OPC—in geopolymers will provide an advantage. It is also worth noting that acid corrosion mechanisms are different between OPC and other binders used in Shi and

13

Stegemann’s study, which may mean that corrosion depth is not the best way to determine overall corrosion in those binders. Chaudhary and Liu’s study had the expected results of geopolymers performing better than OPC after acid corrosion. While the highest penetration depth for the geopolymer samples was 10.6 mm, the highest depth for the OPC samples was 16.9 mm. It was shown that OPC doesn’t behave like geopolymers do during acid exposure—geopolymer weight loss will level off after initial exposure while OPC weight loss was more severe and followed what appeared to be an exponential curve over time25. Previous acid exposure did not matter—the previously exposed sample had the same magnitude of weight loss as the samples that had no prior acid exposure25.

4. Summary and conclusions From the prior section about the comparison between OPC and geopolymers, it can safely be said that geopolymer is better at resisting acid attack than OPC. A big player in this is the calcium content—you will notice that geopolymers based on high calcium precursors like blast furnace slag lose more weight and compressive strength. This trend agrees with OPC being weaker in acids due to OPCs high calcium content. Using geopolymer precursors with the lowest amount of calcium possible, like class F fly ashes, will have a positive effect on acid resistance. The method of attack between OPC and geopolymers is different—geopolymers will suffer from microcracks, ion exchange between hydrogen/hydronium and sodium/potassium, and aluminum leaching while OPC’s high calcium content will lead to gypsum formation, which causes more cracking, as well as calcium precipitates being leached out of the binder. OPC also has what is called an interfacial transition zone, where there are gaps between smaller cement particles and bigger aggregates, which has been cited as a source of concrete degradation due to its relatively high porosity. While gypsum can form in both OPC and geopolymer cements after acid exposure, gypsum seems to have a protective property in geopolymers exposed to low concentrations of acid or soft acids, especially in microcracks, while gypsum formation will lead to more degradation in OPC. The severity of attack between sulfuric, hydrocholoric, and HNO3

can be explained by the difference between hard acids, like HCl, and soft acids like HNO3. 14

Sulfuric acid is classed as neither hard or soft, but the sulfur products created upon dissociation will attack calcium in the geopolymer matrix or aggregate.

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