13

The properties and durability of mine tailings-based geopolymeric masonry blocks

S. Ahmari1,  and L. Zhang2     1Cornerstone Engineering Inc., Louisville, KY, USA     2University of Arizona, Tucson, AZ, USA

Abstract

The growing concerns about global warming and diminishing natural resources have led to research on developing innovative methods for producing green and sustainable construction materials. Utilization of mine tailings (MT) to produce geopolymer masonry blocks is a novel way to meet the emerging needs. The major challenge with the utilization of MT as construction material is their reactivity and phase composition, which can be addressed by improving geopolymerization through different methods such as preactivation by heating, addition of highly reactive materials, utilization of appropriate activator and curing temperature, and addition of calcium to the mixture. This chapter describes the physical and mechanical properties of MT-based geopolymer masonry blocks. The durability of MT-based geopolymer in aggressive environments and the effectiveness of immobilization of contaminants in the original MT after geopolymerization are also discussed.

Keywords

Durability; Geopolymer; Heavy metals; Masonry blocks; Mechanical properties; Microstructure; Mine tailings

13.1. Introduction

Mine tailings (MT) are pulverized rock that remains after the valuable metal-bearing minerals have been extracted in physical separation processes. MT are generally uniform in character and size and usually consist of hard, angular particles with a high percentage of fines. The major constituents of MT are silica and alumina although in many cases they also contain traces of heavy metals. Disposal of MT causes major concerns in the mining industry. Although there are alternative ways such as underwater storage for disposal of MT, they are commonly disposed of in large tailings impoundments, which are costly and occupy large areas of land. Disposal of MT in impoundments may also cause environmental and safety problems, including contamination of surface water, groundwater and soils, and collapse of tailings dams. In addition, tailings are also susceptible to erosion and dust generation due to wind.
Researchers have been working on the recycling and utilization of MT to produce construction materials, which not only addresses the problems related to MT disposal but also reduces the demand for natural materials that would need quarrying. To use MT as construction material, they need to be stabilized, not only to improve their engineering properties but also to encapsulate and immobilize the potential contaminants. Research has been conducted on stabilization of MT using calcium-based products such as ordinary Portland cement (OPC) (Sultan, 1978, 1979). Stabilization of MT with other type of additives such as asphalt, fly ash, and cement kiln dust (CKD) has also been reported by researchers (Teredesai, 2005). The results show enhanced mechanical properties of MT after stabilization with the mentioned additives. However, the stabilization of MT based on reaction with calcium is associated with several disadvantages including low acid resistance, poor immobilization of contaminants, and high energy consumption and greenhouse gas emissions related to production of OPC. A sustainable way to recycle and reuse MT is based on the geopolymerization technology (Ahmari, Chen, & Zhang, 2012; Ahmari, Zhang, & Zhang, 2012; Ahmari & Zhang, 2012; Giannopoulou & Panias, 2006; He, Jie, Zhang, Yu, & Zhang, 2013; Pacheco-Torgal, Castro-Gomes, & Jalali, 2010; Pacheco-Torgal, Castro-Gomes, Jalali, 2008a; Southam et al., 2007). In this method, MT are mixed with an alkaline solution and cured at ambient or slightly elevated temperature depending on the MT chemical properties and the synthesis condition. Different applications of MT-based geopolymer have been suggested, including road base material, cover material for tailings dam, masonry blocks, and construction concrete. This chapter discusses MT-based geopolymer masonry blocks.
Masonry blocks are a widely used construction material. They are mainly used to construct walls and partitions; however, they can also be used as load bearing components. Production of traditional masonry blocks is energy intensive and consumes a significant amount of natural materials, mainly clay. Quarrying for natural materials adversely affects the natural landscape and can release high level of waste materials. In order to reduce the consumption of natural materials, much research has been done on the utilization of waste materials to replace natural materials in production of masonry blocks (Zhang, 2013). For example, Chen, Zhang, Chen, Zhao, & Bao (2011) mixed hematite MT with fly ash and clay to produce masonry blocks and concluded that up to 84% of clay can be replaced with MT. However, the production process required firing at 980–1030 °C and thus was very energy intensive. Other researchers also showed that clay can be partially or fully replaced with waste materials such as fly ash and mixture of MT and Portland cement to produce masonry blocks (Freidin, 2007; Liu et al., 2011; Morchhale, Ramakrishnan, & Dindorkar, 2006; Roy, Adhikari, & Gupta, 2007). Of all the different methods for utilizing waste materials to produce masonry blocks, geopolymerization is a promising one (Freidin, 2007; Zhang, 2013).

13.2. Mine tailings (MT)-based geopolymer

Geopolymer is a family of aluminosilicates synthesized by alkali activating Si- and Al-rich materials and polycondensing tetrahedral silica (SiO4) and alumina (AlO4). The molecular expression of geopolymer gel can be represented by the following formula:

Mn[−(SiO2)z−AlO2−]n

image (13.1)

where M is the cation (sodium or potassium), n the degree of polymerization, z the Si/Al molar ratio that is normally in the range of 1–15 and can be up to 300 (Davidovits, 1982, 1994). The geopolymerization process can be described as a series of reactions that interact with each other: (1) dissolution of amorphous aluminosilicate raw materials in an alkaline solution, (2) formation of Si and/or Si–Al oligomers in the aqueous phase, and (3) polycondensation of the oligomers to form a three-dimensional aluminosilicate framework. The alkaline solution has two functions. First, it breaks the silica and alumina bonds in the initial material and dissolves them. Second, the cation in the alkali solution acts as a charge balancing agent for aluminum in the geopolymer structure. In principle, a wide range of materials, both natural minerals and industrial by-products, provided that they contain sufficient amorphous silica and alumina, can be used to synthesize geopolymer. However, the phase composition of the initial material determines its reactivity and thus the degree of geopolymerization. It is known that metakaolin is one of the most reactive materials for geopolymerization owing to its amorphous structure resulting from the high temperature it has experienced. MT are typically crystalline materials depending on their origin and the mineral operation they underwent in the past. They contain a substantial amount of silica as quartz. Quartz is one of the most stable materials and does not easily dissolve in alkaline solution. It makes MT less reactive in ambient temperature compared to other geopolymer source materials. Figure 13.1 shows the XRD pattern of a copper MT sample. It can be clearly seen that the major phase is crystalline, containing mainly quartz. The amorphous phase which emerges as a broad hump on the spectrum stretching from 18 to 34° is very minor. Therefore, MT without pretreatment can only be partially geopolymerized.
Another distinct characteristic of MT is their variability in terms of chemical composition, particularly the silica to alumina ratio (Si/Al), which is also an important factor that affects the geopolymerization process and the properties of the final geopolymer product. Table 13.1 summarizes the information on different types of MT used by researchers for geopolymer synthesis. High variations in the Si/Al ratio and its dependence on the source material can be clearly seen. In most of the reported cases, MT were cured at slightly elevated temperature, which is consistent with the above discussion about the low reactivity of MT. Room cured MT-based geopolymer products may be used as low specification construction materials such as the material for capping tailings dam (Van Jaarsveld, Lukey, van Deventer, & Graham, 2000). The reactivity of MT can be improved using different methods such as heating to high temperature and then rapid cooling to break down the crystalline structure. The crystalline nature of silica in MT can also be compensated by using sodium silicate solution as an activator to provide external soluble silica. In this case, the sodium silicate solution not only acts as the activating agent but also provides soluble silica monomers, which readily take part in the reaction.
image
Figure 13.1 XRD pattern of MT (A: Anorthite, C: Cuprite, G: Gypsum, L: Labradorite, R: Calcium Carbonate, S: SiO2). Zhang et al. (2011).

Table 13.1

Summary of information on different types of MT used by different researchers for geopolymer synthesis

Source of MTInitial Si/AlProduct applicationCuring temperature (°C)Improvement methodReferences
Copper mine7.8Undefined60Addition of fly ashZhang et al. (2011)
Copper mine7.8Masonry block90Elevated curing temperature/precompressionAhmari and Zhang (2012)
Tungsten mine2.73UndefinedRoomDehydroxylationPacheco-Torgal et al. (2008a, 2010)
Copper and gold mine2.69–14.4Mining backfill70Addition of fly ash or slagSoutham et al. (2007)
Red mud0.3Building materials60Addition of ferronickel slagGiannopoulou et al. (2009)
Gold mine3.5–4.5Capping of hypersaline MTRoomAddition of fly ashVan Jaarsveld et al. (2000)
Copper mine6.7Stabilization of tailings60Addition of fly ashGiannopoulou and Panias (2006)

image

13.2.1. Dehydroxylation of MT

Dehydroxylation involves the heating process through which the hydroxyl group (OH) is released by forming a water molecule (Frost & Vassallo, 1996). The generally accepted mechanism of dehydroxylation of kaolinite is the interaction between two liberated hydroxyl as below:

OH−↔H++O2−H++OH−↔H2O

image (13.2)

The process requires proton delocalization at specific hydroxyl sites. The consequence of the release of structural water through dehydroxylation is distortion and buckling of polymeric structure of aluminosilicates, resulting in disordered structure (Sperinck et al., 2011).
Pacheco-Torgal & Jalali (2010) studied dehydroxylation of tungsten mine waste with and without adding sodium carbonate. They heated the mine waste and cooled it down rapidly to assure that the crystalline structure is transformed to an amorphous one. The reason for the addition of sodium carbonate was to reach the criteria Na2O/SiO2 > 0.2 given by Davidovits (Davidovits, 1999). The study indicated that with no addition of sodium carbonate, dehydroxylation at 960 °C resulted in the highest compressive strength of the geopolymer product. The addition of sodium carbonate did not lead to an optimal temperature (dehydroxylation state) although the compressive strength of sodium carbonate-added specimens was higher.
The dehydroxylated tungsten MT-based geopolymer concrete exhibited good physical and mechanical properties (Pacheco-Torgal et al., 2008a). The water absorption by immersion (WAI) after 24 h immersion in water was on average below 3.4%. The capillarity water absorption coefficient (CWAC) was below 0.015 g/cm2. Comparison between the CWAC of the geopolymer concrete and that of the aggregates indicated that the measured values (0.0007–0.005 g/cm2) primarily represent the water absorption of the geopolymer paste. The measured static elastic modulus (29.7–34.9 GPa) showed dependence on the type and size of the aggregates used. The compressive strength after 50 days of curing was reported to increase to about 40 MPa for hardened geopolymer paste and more than 90 MPa for geopolymer concrete. This indicates the effectiveness of dehydroxylation for pretreatment of MT. However, since the main goal for using MT-based geopolymer is to develop sustainable materials and considering the energy intensiveness of the dehydroxylation process, commercialization of this process is still questionable.

13.2.2. Addition of other aluminosilicate source material

Production of hybrid MT-based geopolymer by adding other source material containing reactive aluminosilicates is another option to improve the properties of MT-based geopolymer products. Dissolution of alumina tends to be easier than silica as it takes less energy to dissolve it. The major problem with MT is the high stability of silica. Addition of a more reactive source material such as fly ash or slag will mainly help provide an additional source of silica although it may be a source of alumina as well. Mixture of MT with fly ash or slag creates a new system with a different Si/Al ratio from the initial one. Therefore, it is important to proportion the mixture in a way to achieve the optimal Si/Al ratio. Addition of fly ash to MT helps produce a geopolymer with better properties not only due to the increased reactivity and adjustment of the Si/Al ratio, but also due to the filling effect of fly ash particles, which are much smaller than MT particles. Fly ash particles fill the voids between MT particles and form a denser microstructure, which eventually leads to higher compressive strength. Zhang, Ahmari, & Zhang (2011) showed the increase of compressive strength from approximately 3–20 MPa for copper MT-based geopolymer paste after curing at 60 °C for seven days due to the addition of fly ash. Their study indicated that the amount of added fly ash did not have an effect on the curing time required to reach the ultimate strength, and all specimens gained their ultimate strength within seven days. The addition of fly ash also led to an obvious change in the mode of failure and the ductility of the geopolymer paste specimens. It was shown that the pure MT-based geopolymer specimen failed with no distinct peak stress while the fly ash-added MT-based geopolymer specimen failed with a sharp peak and at larger strains.
Giannopoulou, Dimas, Maragkos, & Panias (2009) studied the production of geopolymer using a mixture of metakaolin and red mud. The mixture by weight comprised 15% metakaolin. In the optimum condition corresponding to 8 M NaOH concentration, the compressive strength up to about 20 MPa and the water absorption of 1.9% were obtained. It is noted that for any geopolymer system, there is an optimum NaOH concentration depending on the type of source materials.

13.2.3. Utilization of soluble silica in activation solution

Additional silica can also be provided by adding a soluble silica source such as sodium silicate to the activation solution. Mixed NaOH and sodium silicate solution is commonly used in the synthesis of geopolymers. This makes geopolymerization faster because the silica species are already available in the activation solution. Addition of sodium silicate up to a certain level results in higher strength, and beyond that, the strength declines. Ahmari et al. (2012a) found that for copper MT, the amount of sodium silicate corresponding to SiO2/Na2O = 1.0–1.26 is the optimum one and further addition of sodium silicate results in strength loss. This is possibly because too much sodium silicate hinders evaporation of water and formation of polymeric structure by preventing the contact between the solid material and the activation solution through precipitation of the Si–Al phase (Cheng & Chiu, 2003; Lee & Van Deventer, 2002). The study also indicated that despite the high initial Si/Al ratio, addition of sodium silicate still improved the mechanical properties of the geopolymer binder. The reason is because although the initial Si/Al ratio is high, only a small amount of silica is available for geopolymerization. The initial or nominal Si/Al accounts for both the crystalline and amorphous phases of silica, but the amorphous phase is the major part participating in geopolymerization. That is why addition of sodium aluminate to the NaOH solution did not result in improvement of strength.

13.2.4. Use of appropriate curing temperature

Curing temperature is a crucial factor in geopolymerization of MT due to its lower reactivity compared to other source materials. By increasing curing temperature, more energy is delivered to dissolve the silica and alumina species from the surface of MT particles. It is known that geopolymerization of an aluminosilicate-containing material leads to the highest strength at a certain temperature. This optimum temperature was found to be 90 °C for copper MT (Ahmari et al., 2012a). A summary of the optimum curing temperatures for different source materials and NaOH concentrations is shown in Table 13.2. The optimum curing temperature decreases when the source material is more reactive or the NaOH concentration is lower. The MT-based geopolymer binder reaches the highest strength at 75 °C for 5 or 10 M NaOH and 90 °C for 15 M NaOH, respectively. Ahmari et al. (2012a) conducted leaching tests on MT powder to better understand the relationship between dissolution of silica and alumina species and temperature (see Table 13.3). From 60 °C to 90 °C, there is a significant increase in dissolution of Si and Al. Elevated NaOH concentration has a similar effect on dissolution of Si and Al but the effect of temperature is prevalent. The increased dissolution of Si and Al explains why the compressive strength of MT-based geopolymer binder increased from about 3 to 20 MPa when the curing temperature was increased from 60 to 90 °C. The kinetics of dissolution of Si and Al also explains why sodium aluminate is an effective activator at 90 °C but not at 60 °C. The reason is that the Si/Al ratio is very high at 90 °C and the addition of sodium aluminate helps decrease this ratio in the aqueous phase to be closer to the optimum one.

Table 13.2

Optimum curing temperatures from different researchers for different source materials

Source materialOptimum curing temperature (°C)NaOH concentration (M)References
Natural zeolite407Villa, Pecina, Torres, and Gómez (2010)
Glass cullet405–10Cyr, Idir, and Poinot (2012)
Class C fly ash608.1Guo, Shi, and Dick (2010)
Class F fly ash757.5Van Deventer et al. (2006)
Class C fly ash758.1Guo et al. (2010)
Class F fly ash807Yunfen, Dongmin, Wenjuan, Hongbo, and Lin (2009)
Metakaolin8513.6Yao, Zhang, Zhua, and Chen (2009)
MT755 and 10Ahmari et al. (2012a)
MT9015Ahmari et al. (2012a)

image

Ahmari et al. (2012a).

Table 13.3

Leaching test results on Si and Al concentration of MT powder soaked in NaOH of different concentrations at two different temperatures

Temperature (°C)6090
NaOH (M)5101551015
Si (ppm)71171233184639704570
Al (ppm)2876121299319550
Si/Al2.442.161.855.9311.97.98

image

13.2.5. Addition of calcium

Other additives may also be used to enhance the mechanical properties of MT-based geopolymer. However, the improvement might not be necessarily due to the enhanced geopolymerization process. For example, calcium-containing material such as calcium hydroxide can be added to MT. Pacheco-Torgal, Castro-Gomes, & Jalali (2008b) studied the effect of added calcium hydroxide on geopolymerization of tungsten mine waste and noted that addition of more than 10% calcium hydroxide leads to strength decrease after 14 days. This is possibly due to the formation of CSH gel and the competition between geopolymer and CSH gel for silicates. Other possible reasons are the occurrence of shrinkage cracks at the interface of binder and aggregates and the formation of ettringate.
Ahmari and Zhang (2013b) also studied calcium added MT-based geopolymer. They added CKD to improve the durability of copper MT-based geopolymer bricks. The study indicated that the compressive strength and durability of the geopolymer binder improved significantly after the addition of CKD.
The effect of calcium on the geopolymerization process is still not completely known. Some researchers believe that calcium also takes place in geopolymerization and serves as a charge balancing agent. The other possible effect of calcium on geopolymerization is the increase of alkalinity of the activation solution and consequently the improvement of dissolution of silica and alumina species.

13.3. Synthesis and physical and mechanical properties of MT-based geopolymer masonry blocks

13.3.1. Synthesis of MT-based geopolymer masonry blocks

The conventional process (Figure 13.2(a)) for producing masonry blocks is energy intensive and consumes natural materials (mainly clay). The geopolymerization method for producing masonry blocks from MT (Figure 13.2(b)) includes fewer steps and consumes much less energy than the conventional method.
image
Figure 13.2 Schematic demonstration of production of masonry blocks: (a) using conventional method and (b) using geopolymerization method.
To produce MT-based geopolymer masonry blocks, first MT is mixed with an alkaline solution prepared in advance at a specified water to solid ratio. Ahmari and Zhang (2012) used only NaOH to prepare the activation solution. MT were used as received from the tailings dam except that the agglomerated particles were crushed by hand. The resulant mixture can vary from semidry to semipaste. Then the mixture is compressed with a forming machine to a certain pressure (usually no water should be squeezed out of the mold). After the forming pressure is released, there might be a slight rebound due to the elastic deformation; however, the major portion of the deformation is plastic and related to the elimination of air voids inside the mixture. The relative amount of elastic deformation decreases with a slower loading rate. The study by Ahmari & Zhang (2012) indicated that 10 min is an appropriate forming time. After the blocks are formed, they are placed in an oven for curing at a specified temperature. Ahmari & Zhang (2012) showed that for copper MT-based geopolymer masonry blocks using an NaOH activation solution, 90 °C is the optimum temperature. It is noted that the optimum curing temperature may change for different types of MT or different types of activation solutions. As discussed earlier, the optimum curing temperature decreases when the reactivity of the source material is higher. Figure 13.3 shows some of the MT-based geopolymer blocks produced in the laboratory following the above procedure. For commercial production, the cured blocks will then be packed and delivered.
image
Figure 13.3 MT-based geopolymer blocks produced in laboratory.

13.3.2. Physical and mechanical properties of MT-based geopolymer masonry blocks

The physical and mechanical properties of MT-based geopolymer masonry blocks are affected by several factors. Due to the crystalline characteristics of MT, curing temperature has an especially important effect. Increasing curing temperature up to the optimum level results in dissolution of more amorphous silica and alumina species from the MT particles and thus higher compressive strength of the geopolymer product. However, further increase in curing temperature above the optimum level results in a decrease of the compressive strength. Too high a curing temperature results in not only rapid dissolution of silica and alumina but also accelerated polycondensation. Rapid polycondensation causes fast growth of geopolymer gel and inhibits further dissolution of silica and alumina species. Too high a curing temperature also causes fast dehydration of the blocks, especially when the blocks are uncovered, and leads to incomplete geopolymerization.
Initial water content and forming pressure are other important factors affecting the properties of MT-based geopolymer blocks. Sufficient water is required for dissolution of silica and alumina species and initiation of polycondensation of the dissolved species. The forming pressure needs to be selected based on the initial water content so that the mixture is compressed to the highest degree without squeezing out the activation solution during the forming process. This means for an initial water content, there is a corresponding optimum forming pressure. The study by Ahmari & Zhang (2012) indicated that the increase of initial water content from semidry to semipaste consistency results in higher compressive strength of the MT-based geopolymer blocks. This is because, first, as mentioned earlier, water serves as a medium for geopolymerization and lack of water results in incomplete geopolymerization. Second, higher initial water content means more NaOH at a constant NaOH concentration. NaOH has two important roles in geopolymerization: dissolution of Si and Al and charge balancing of alumina tetrahedral in geopolymer structure. Figure 13.4 shows the effect of initial water content and forming pressure on the compressive strength of MT-based geopolymer blocks synthesized with 15 M NaOH solution and cured at 90 °C for seven days. It can be seen from the figure that there are two ways to increase the compressive strength of the blocks: using lower initial water content and higher forming pressures or using higher initial water content and lower forming pressures. Increasing the amount of NaOH solution is much more effective than increasing the forming pressure for improving the compressive strength of the MT-based geopolymer blocks. The NaOH solution affects the chemical reaction and thus the amount of generated geopolymer gels, but the forming pressure influences the physical structure by compacting the MT matrix. Figure 13.5 compares the micrographs of two blocks prepared respectively at 12% and 16% initial water contents and the corresponding 25 and 0.5 MPa forming pressures. The 16%/0.5 MPa block has a denser microstructure than the 12%/25 MPa one.
image
Figure 13.4 Unconfined compressive strength (UCS) versus forming pressure for MT-based geopolymer blocks prepared at different initial water contents and 15 M NaOH concentration and cured for seven days at 90 °C. Ahmari & Zhang (2012).
Table 13.4 shows the compressive strength, unit weight, water absorption, and abrasion index (resistance) of MT-based geopolymer blocks prepared with 16% initial water content and different forming pressures and cured at 90 °C for seven days. The compressive strength was measured using unconfined compression tests. The unit weight was measured by simply using the compressive strength samples. The water absorption was determined by soaking the blocks in water for a certain period of time and then measuring the weight increase of the blocks. And the abrasion index was determined from the unconfined compressive strength (UCS) and water absorption using the following expression:
image
Figure 13.5 SEM micrographs at initial water content/forming pressure combinations of 12%/25 MPa (a) and 16%/0.5 MPa (b) for the specimens cured at 90 °C for seven days (GP: geopolymer, MT: mine tailings particle). Ahmari & Zhang (2012).

Table 13.4

Physical and mechanical properties of MT-based geopolymer blocks prepared at 16% initial content and cured at 90 °C for seven days

Forming pressure (MPa)UCS (MPa)/(psi)Unit weight (kN/m3)24 h water absorption (%)Abrasion index
0.528/404019.660.930.02
1.525/359119.702.180.06
3.022/325019.892.920.09
5.021/308619.933.450.11
15.021/305919.963.150.10

image

AbrasionIndex=100×absorption(%)UCS(psi)

image (13.3)

The American Society for Testing and Materials (ASTM) has different requirements for the different applications of bricks and blocks (see Table 13.5). The minimum required UCS varies between 4.8 MPa for load bearing wall tiles and 55.2 MPa for severe exposure (SX) grade pedestrian and light traffic paving bricks. The maximum water absorption varies between 8% for SX grade pedestrian and light traffic paving bricks to 25% for load bearing nonexposed wall tiles. The requirements for abrasion resistance only apply to pedestrian and light traffic paving bricks and the maximum abrasion index values are between 0.11 and 0.5. The MT-based geopolymer blocks prepared with 16% initial water content and different forming pressures and cured at 90 °C for seven days (see Table 13.4) meet all the ASTM requirements for different types of applications except for the SX grade pedestrian and light traffic paving bricks.

Table 13.5

ASTM specifications for different applications of bricks and blocks

Title of specificationASTM designationType/gradeMinimum UCS (MPa)Maximum water absorption (%)Abrasion index
Structural clay load bearing wall tileC34-03LBX9.616NA
LBX4.816NA
LB6.825NA
LB4.825NA
Building brickC62-10SW20.717NA
MW17.222NA
NW10.3No limitNA
Solid masonry unitC126-99Vertical coring20.7NANA
Horizontal coring13.8NANA
Facing brickC216-07aSW20.717NA
MW17.222NA
Pedestrian and light traffic paving brickC902-07SX55.28Type I0.11
MX20.714Type II0.25
NX20.7No limitType III0.50

image

Notes: LBX = load bearing exposed; LB = load bearing nonexposed; SW = severe weathering; MW = moderate weathering; NW = negligible weathering; SX = severe exposure; MX = moderate exposure; NX = negligible exposure; and Type I, II, and III = subjected to extensive, intermediate, and low abrasion, respectively.

The mechanical properties of MT-based geopolymer blocks can be improved by adding other higher reactivity source materials. Figure 13.6 shows the UCS of MT-based geopolymer blocks containing different amounts of ground furnace slag (GFS). Addition of GFS results in a significant increase of UCS. The improving effect of GFS can be attributed to a number of factors such as the favorable physical and chemical properties and phase composition of the GFS. GFS is much more amorphous than MT due to the extremely high temperature and subsequent rapid cooling it has experienced. This contributes to easier dissolution of silica as a major component of GFS in alkaline solution. GFS may contain crystalline minerals of Si such as fayalite (Fe2SiO4), but the silica in fayalite is easier to liberate and dissolve in alkaline solution than that in quartz. Another advantage of adding GFS to MT is that GFS improves the workability of the mixture and thus less water is required for constant consistency. For example, at 10% water content, GFS-added MT shows the same consistency as the pure MT at 16% water content. Ahmari & Zhang (2013b) also investigated the improvement of properties of MT-based geopolymer blocks by adding CKD. CKD is a by-product of the Portland cement production process, which is collected from cement kiln exhaust gases. CKD mainly contains Ca and Si, which are beneficial for geopolymerization. The addition of CKD reduced the workability of the mixture mainly due to the fast dissolution of calcium and the very fine size of the CKD particles. However, as discussed in the next section (see Figure 13.7), the addition of CKD significantly increased the strength of the MT-based geopolymer blocks.
image
Figure 13.6 UCS versus slag content for geopolymer blocks prepared at 15 M NaOH and cured at 90 °C for seven days.
image
Figure 13.7 Dry and wet UCS (before and after immersion in water) versus CKD content for geopolymer block specimens prepared with 16% initial water content, 15 M NaOH, and cured at 90 °C for seven days. (The percentage numbers show the percentage loss of strength after immersion in water for seven days.)

13.4. Durability of MT-based geopolymer masonry blocks

The durability of geopolymer in an aggressive environment depends on a variety of factors such as the source material of geopolymer, the type of activator, and the type of aggressive ions. The more reactive source materials result in a higher degree of geopolymerization and denser microstructure and thus higher resistance to acid attack. Ahmari & Zhang (2013a) studied the durability of MT-based geopolymer blocks in water and nitric acid with a pH of respectively 7 and 4. They reported a significant strength loss after immersion for about four months. They accounted partial geopolymerization of the MT as one of the reasons for the strength loss. Similarly, Silva, Castro-Gomes, & Albuquerque (2010) reported significant strength loss of MT-based geopolymer used as artificial aggregates for wastewater treatment plant after immersion in water. The strength loss was attributed to the dissolution of unreacted NaOH particles that fill the voids, which leads to increase of porosity of the geopolymer matrix and migration of the aggressive solution into the micropores. Migration of reacted cations into the solution can also account for the strength loss. It is likely that the cations in the geopolymer structure are replaced with the cations in the solution if geopolymer binder is immersed in salt such as sodium sulfate (Bakharev, 2005). In this case, the cation exchange rate depends on the pore size. Bakharev (2005) showed that the exchange rate is higher when potassium hydroxide (KOH) instead of NaOH is used as the activation solution, due to larger pore size. That is why larger cracks were observed in the specimens activated with KOH.
Pacheco-Torgal, Castro-Gomes, & Jalali (2010) studied the durability of tungsten MT-based geopolymer concrete with different types of aggregates including schist, limestone, and granite and immersed in 5% sulfuric, hydrochloric, and nitric acid solutions for 28 days. Overall, the geopolymer concrete blocks were more durable than the Portland cement concrete blocks. The calcium in Portland cement reacts to the acid anions and forms soluble salts, resulting in more porous matrix and further invasion of acid into the matrix. The geopolymer concrete specimens exhibited variable durability depending on the aggregate type and the acid type. All the geopolymer concrete specimens immersed in the hydrochloric acid solution exhibited low weight loss regardless of the aggregate type. The specimens made with schist aggregate exhibited good durability in most of the acids. The specimens made with limestone aggregates showed expansion and cracks in sulfuric acid. This was due to the reaction of sulfate in the solution with limestone and the formation of expanding minerals (Pacheco-Torgal et al., 2010). It needs to be noted that because it was calcined, the used tungsten MT was highly reactive. In addition, they added calcium hydroxide to improve the mechanical properties of the tungsten MT-based geopolymer concrete. This can be the reason why the tungsten MT-based geopolymer concrete exhibited better durability than the pure MT-based geopolymer blocks studied by Ahmari & Zhang (2013a) and Silva, Castro-Gomes, & Albuquerque (2010).
As discussed in Section 12.3, lower forming pressure associated with higher water content (which means more NaOH at the same NaOH concentration) results in stronger MT-based geopolymer blocks. The UCS of 16%/0.5 MPa blocks (prepared at 16% initial water content and 0.5 MPa forming pressure) was higher than that of 12%/25 MPa blocks. However, both types of blocks exhibited significant strength loss after immersion in water or nitric acid. Nevertheless, the durability of the 12%/25 MPa blocks was slightly better. This means the durability requirement favors the utilization of a higher forming pressure. The denser microstructure of the 16%/0.5 MPa blocks resulted due to the formation of larger amounts of geopolymer gels, but the dissolution finally resulted in a more porous microstructure. In contrast, the compact microstructure of the 12%/25 MPa specimens resulted due to the physical compaction of the MT and although smaller amounts of geopolymer gels were generated, a slightly more compact microstructure remained after immersion.
Ahmari & Zhang (2013b) added CKD to improve the durability of MT-based geopolymer blocks. Figure 13.7 shows the UCS of CKD-added MT-based geopolymer blocks prepared at 16% initial water content and formed at 0.5 MPa, before and after immersion in water, respectively. Addition of a small amount of CKD results in significant improvement of strength of the geopolymer blocks, due to several reasons. Dissolution of CKD in alkaline solution leads to release of a substantial amount of calcium cations, which further increases the alkalinity of the activation solution. As discussed earlier, higher alkalinity of the activation solution results in dissolution of larger amounts of silica and alumina from MT. Soluble calcium also leads to the formation of Ca(OH)2, which eventually yields CaCO3 due to a reaction to air. This acts like a cementing agent and decreases the porosity of the geopolymer matrix. CaCO3 has good stability in water and alkaline condition and thus further enhances the durability of the MT-based geopolymer blocks. Figure 13.8 shows the XRD spectra of MT and CKD and the 16%/0.5 MPa blocks before and after immersion in water. It can be seen that the calcium in CKD as CaO and Ca(OH)2 disappears after geopolymerization, but the peak corresponding to CaCO3 increases. CKD contains a substantial amount of silica, which after dissolution can take part in geopolymerization. These all indicate formation of a denser microstructure and consequently more durable geopolymer blocks. The released calcium can also serve as charge balancing agent for the geopolymer structure. One reason for the deterioration of geopolymer is cation exchange and release of alkali cations. Since calcium has a low tendency to cation exchange, the durability of the geopolymer blocks will increase.
image
Figure 13.8 XRD patterns of MT and CKD powders and geopolymer blocks prepared at 15 M NaOH, 16% initial water content, 10% CKD, and cured at 90 °C for seven days and before and after immersion in water (A: albite, G: gypsum, P: sanidine, S: quartz, C: CaO, O: CaCO3, S: SiO2, T: Ca(OH)2).

13.5. Environmental performance of MT-based geopolymer masonry blocks

MT are susceptible to acid mine drainage, meaning that the sulfide minerals such as pyrite in MT oxidize and result in generation of sulfuric acid. The generated sulfuric acid lowers the environment pH and consequently results in release of heavy metals. Therefore, it is important to ensure that MT-based geopolymer blocks are stable and will not release toxic substances such as heavy metals. The environmental safety of MT-based geopolymers has been studied by different researchers. Pacheco-Torgal et al. (2010) demonstrated that the concentration of released heavy metals from the tungsten MT-based geopolymer concrete is below the Portuguese standard limits for irrigation purposes but above those for potable water. There was a high concentration of Na in the leachate due to the dissolution of unreacted NaOH.
Giannopoulou & Panias (2006) performed leaching analysis on fly ash-added MT-based geopolymer and showed that the MT powder in acidic condition (1 M sulfuric acid) released a significant amount of Mn, Fe, Mg, Mn, K, Na, Ca, and Si. However, in a neutral condition Mg, Ca, Si, and Al exhibited the highest release rate. This is because of the dependence of solubility of heavy metals on pH. The concentration of released heavy metals from MT-based geopolymer was below the Greek standard thresholds and significantly lower than that from the MT powder.
Ahmari & Zhang (2013a) also evaluated the environmental performance of MT-based geopolymer blocks. They immersed the blocks in nitric acid (pH = 4) and water (pH = 7) for three months and monitored the concentration of different heavy metals in the liquid. The study showed that there was a significant amount of heavy metals released from the MT powder, especially at pH = 4. At neutral condition, Na, Mg, K, and Ca exhibited a high release rate while in acidic condition, besides them, Mn, Cu, and Zn also had a high concentration in the liquid. For geopolymer blocks, the concentration of all these released metals (except for Na, which was due to unreacted NaOH) dropped significantly. As and Mo exhibited higher concentrations in the case of geopolymer blocks due to their higher solubility in alkaline condition. Overall, for all geopolymer blocks, the leached heavy metals showed a concentration much lower than the USEPA standard limits.
From the discussion on durability and leaching analysis, it appears that higher forming pressures favor durability and immobilization efficiency although lower UCS is obtained. To avoid compromising strength with durability and immobilization efficiency, hybrid systems such as CKD or slag-added MT geopolymer blocks can be used.

13.6. Conclusions and future trends

MT are a potential source material for producing geopolymer masonry blocks despite their dominant crystalline phase. The low activity of MT can be addressed using different methods such as dehydroxylation, addition of other more reactive materials, utilization of soluble silica in the activation solution, optimization of the synthesis condition such as curing temperature, and addition of calcium. The research has indicated that the physical and mechanical properties of MT-based geopolymer blocks can meet the ASTM standard requirements for most applications of bricks. The research has also shown the durability and environmental safety of MT-based geopolymer blocks.
Since most of the research so far is in laboratory scale, further research in full-scale should be conducted in order to promote the production and utilization of MT-based geopolymer masonry blocks in practice. The full-scale study should investigate the different practical aspects such as the commercial production method, the costs, and the long-term performance.

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