Waste into Construction Materials: Geopolymers from Recycled Sources

原始链接: https://www.mdpi.com/2313-4321/10/3/118

This study investigated the chemical resistance and microstructural properties of geopolymers made from brick and cement waste activated with waste glass-derived silicate (SG). Sample 6 (50% brick, 20% cement) exhibited the best resistance to water, acid (HCl), and alkali (NaOH) exposure compared to other geopolymer formulations and Portland cement. Its superior performance is attributed to a balanced combination of Si/Al, Si/Na, Si/Ca, Si/K, Si/Mg, and Si/Fe ratios within the geopolymer matrix, leading to a well-polymerized, dense network. FTIR analysis confirmed the presence of a well-formed aluminosilicate network in all geopolymer samples, with characteristic bands indicating O-H stretching, O-C-O stretching, and Si-O-Si/Si-O-Al stretching vibrations. XRD analysis revealed the presence of quartz, calcite, portlandite, and wollastonite phases, the latter two forming during geopolymerization. SEM micrographs showed a compact and flaky microstructure in the geopolymer samples, with some unreacted glass particles. EDX analysis confirmed an increased presence of silicon, aluminum, and calcium, validating the formation of the aluminosilicate gel and calcium-containing phases, contributing to the enhanced chemical resistance of Sample 6.

A Hacker News discussion revolves around using recycled waste to create geopolymer construction materials. The original post links to an article about making geopolymers from recycled sources. One commenter, tastyfreeze, mentions using silica gel cat litter to make sodium silicate and plans to experiment with crushed glass. Another commenter, busssard, asks about the recyclability of the new geopolymer. PaulHoule responds that "valorization" is crucial for circular economy research, but recycling can be challenging due to the low cost of the resulting products compared to the initial effort. He points out that geopolymer concrete has the potential for a long lifespan and can be recycled, citing a scientific article. Tastyfreeze simplifies the recycling aspect, stating that geopolymer is essentially stone and can be treated as such for recycling purposes.
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原文

2.1. Chemical Resistance Tests

Water, acid, and alkali resistance are important durability properties in construction materials [9,10], and their effects were investigated in this research work. Table 1 presents the weight loss for each sample, including commercial Portland cement, following immersion in tap water, 0.1 M HCl, and 0.1 M NaOH. At the same time, Figure 1 illustrates the average sample mass after immersion, normalized to the average initial mass (see Figure S1 for the average absolute masses).

Sample 6, comprising 50 wt% of brick waste (B) and 20 wt% cement waste (C) geopolymerized with glass-derived silicate, exhibits the lowest weight loss (0.12%) in tap water immersion, indicating the highest resistance among the tested samples. In contrast, Sample 7, which has the same composition but is geopolymerized with commercial silica, shows the highest weight loss (0.30%) in water, indicating the lowest resistance. Portland cement displays a weight loss of 0.26%, comparable to most geopolymeric samples, except Sample 6, which outperforms it.

In HCl, Sample 6 demonstrates the highest resistance, with a weight loss of 1.18%. Sample 3, produced from brick waste geopolymerized with commercial silica, exhibits the highest weight loss (2.96%) in this environment, indicating the greatest vulnerability to acidic conditions. Portland cement showed a weight loss of 2.66%, higher than most geopolymer samples except Sample 3, suggesting greater susceptibility to acid attack than the other geopolymeric materials. In fact, the orange coloring in Figure 2 indicates the formation of iron(III) oxide at the specimen surface, which may be attributed to the surface migration of Fe(III) ions from the tetracalcium aluminoferrite within the Portland cement clinker.

Sample 6 again exhibits the lowest weight loss (1.11%) in NaOH, indicating strong resistance to alkali exposure. Conversely, Sample 3 shows the highest weight loss in NaOH (1.92%), consistent with its behavior in HCl, suggesting a generally lower resistance to harsh environments. Portland cement displays a weight loss of 1.97, exceeding most geopolymer samples, excluding Sample 3.

Given that Samples 2, 4, and 6 were produced using the same alkaline activator derived from glass waste, the differences in their chemical resistance could primarily stem from the variations in their precursor compositions (ratios of brick waste and cement waste) and the resulting Si/Al, Si/Na, Si/Ca, Si/K, Si/Mg, and Si/Fe mass ratios within the geopolymer matrix (Table 2).
Despite its Si/Al ratio of 1.8 falling within the range of Samples 2 (1.93) and 4 (1.75), Sample 6, composed of a specific blend of 50 wt% brick and 20 wt% cement, demonstrated the best performance in chemical resistance, highlighting the complex interplay of various elemental ratios. While not exhibiting the highest Si/Al ratio, the ratio in Sample 6 may represent a more optimal balance for the alkali activation conditions provided by the glass-derived activator, potentially leading to a well-polymerized network with a favorable balance of durability, unlike less stable or more soluble gel structures that can result from excessively high or low ratios [9,18]. Among the three samples activated with the same glass-derived activator, Sample 6 presented the lowest Si/Na ratio (3.48), which can contribute to a higher density of negatively charged aluminate and silicate species, resulting in a more tightly bound network due to enhanced electrostatic interactions with sodium ions, thus hindering the penetration of aggressive H+ and OH ions [19]. With a Si/Ca ratio of 3.13, intermediate to Samples 2 and 4, the balanced calcium content in Sample 6 likely contributed to a denser microstructure by filling pores and potentially forming bridging structures without compromising the aluminosilicate network’s integrity [20]. Similarly, the Si/K ratio of 20.89 in Sample 6, situated between the ratios of the other two samples, in conjunction with sodium, may have resulted in a more stable and less leachable alkali aluminosilicate gel, as the type and concentration of alkali cations influence the geopolymer matrix’s stability in aggressive environments [21]. The intermediate Si/Mg ratio of 26.85 in Sample 6 suggests a balanced presence of magnesium that might have contributed to denser phase formation or positively influenced the gel structure for chemical resistance [8]. Finally, the Si/Fe ratio of 7.23 in Sample 6, again intermediate to the other two samples, indicates a moderate iron content that might have positively influenced the polymerization process or surface properties, potentially enhancing resistance to aggressive solutions, unlike the iron-related degradation observed in Portland cement exposed to HCl [22]. Therefore, the superior chemical resistance of Sample 6 might be due to a synergistic effect of a more optimized balance across multiple elemental ratios resulting from its specific precursor blend and its interaction with the glass waste-derived activator.

Overall, samples produced using glass waste-derived silicate demonstrate greater immersion resistance than those fabricated with commercial sodium silicate or Portland cement under identical conditions.

2.2. Characterizations

The following studies focus on the characterization of samples produced by alkalinization with waste glass-derived silicate.

Figure 3 displays ATR-FTIR spectra recorded for the geopolymeric Samples 2, 4, and 6, all made with SG, the noise observed in the spectral region between 2500 and 2000 cm−1 being probably due to instrumental interference. The characteristic bands are summarized in Table 3 and are in agreement with the literature on similar geopolymerized samples [23,24].
The spectra of Samples 2, 4, and 6 (Figure 3) show characteristic bands (Table 3) around 3160–3240 cm−1, 1408–1424 cm−1, and 968 cm−1. The bands around 3160–3240 cm−1 can be attributed to O-H stretching, indicating the presence of hydroxyl groups, likely from residual water or structural OH within the geopolymer matrix [25]. The bands in the 1408–1424 cm-1 range correspond to O-C-O stretching, suggesting the presence of carbonate species, which can arise from atmospheric carbon dioxide reacting with alkaline components in the geopolymer [11]. The prominent band at around 968 cm−1 can be assigned to Si-O-Si and Si-O-Al stretching vibrations, which are the primary indicators of the geopolymerization process, signifying the formation of the aluminosilicate gel network [26,27]. These FTIR results align with the observed chemical resistance. The presence of a well-formed aluminosilicate network contributes to the material’s ability to withstand aggressive chemical attack. Sample 6, which demonstrated the best chemical resistance, also exhibits these characteristic bands, suggesting a denser and more stable geopolymer structure.
Figure 4 shows the X-ray diffractograms of Samples 2, 4, and 6, which were geopolymerized using SG. These Diffractograms indicate the presence of quartz, calcite, portlandite, and wollastonite phases, consistent with literature data for similar geopolymers produced with commercial silicates [28]. Comparison with the diffractograms of the starting waste-derived matrices (Figure 5) reveals the formation of portlandite and wollastonite crystalline phases upon geopolymerization, as these phases are absent in brick and cement waste. While B (Figure 5, blue line) exhibits diopside and quartz phases, Sample 2, where B is the only aluminosilicate matrix, also shows portlandite and calcite [25]. Quartz is likely derived from the original waste materials, as it is a common mineral in both cement and brick. Calcite can form due to carbonation during the geopolymerization process or sample storage, while portlandite and wollastonite are products of the alkali activation and subsequent reactions. The formation of these crystalline phases provides further insight into the chemical resistance of the geopolymers. For instance, the presence of wollastonite, a calcium silicate mineral, can contribute to improved mechanical properties and chemical stability. As suggested by XRD, the absence of significant amounts of highly soluble phases in Sample 6 correlates with its superior performance in the chemical resistance tests.
Scanning Electron Microscopy (SEM) micrographs are recorded to examine the microstructural changes in waste-derived precursors following geopolymerization with glass-derived silicate (Figure 6). Sample 4 exhibits a compact, flaky structure, which is also observed in Samples 2 and 6. These latter samples also show that unreacted or partially reacted glass materials are incorporated into their structure. Pristine brick waste (Figure 7) presents characteristics of a vitreous morphology, probably forming during kiln firing and not due to geopolymerization, which occurs at room temperature. Table 4 summarizes the results of the EDX elemental analyses of the rectangular regions shown in the SEM micrographs presented in Figure 6 and 7. Table 4 summarizes the EDX elemental analyses of the rectangular regions in the SEM micrographs shown in Figure 6 and Figure 7. The Si/M ratios presented in Table 2 were calculated according to the results of the EDX elemental analysis. Microstructural and chemical composition changes were observed in Samples 2, 4, and 6 compared to the precursors. SEM images reveal a compact and flaky microstructure in all three samples, indicating the formation of a dense geopolymer matrix. The presence of unreacted or partially reacted glass particles suggests that the geopolymerization process may not have fully consumed all the precursor materials. However, providing a heterogeneous structure can also contribute to the material’s overall stability. The vitreous morphology of the pristine brick waste is a result of the high-temperature firing process used in brick production. This structure is distinct from the geopolymer matrix formed at room temperature, highlighting the microstructural transformation achieved through alkali activation.
As shown in Figure 6 and detailed in Table 4, the EDX spectra of the geopolymers revealed an increased percentage of silicon, aluminum, and calcium, confirming the presence of wollastonite and portlandite, which was previously established by XRD. Additional SEM micrographs of the samples, captured at varying magnifications, are presented in Figures S2–S4.
The EDX data presented in Table 4 were acquired from area scans corresponding to the SEM micrographs shown in Figure 6 and Figure 7. EDX analysis supports the XRD findings, showing an increase in silicon (Si), and aluminum (Al) content in Sample 6 compared to the starting materials. This increase confirms the formation of the aluminosilicate gel and calcium-containing phases like wollastonite and portlandite. The elemental composition, particularly the Si/Al and Si/Ca ratios, plays a crucial role in determining the geopolymer’s properties. Sample 6, with its specific elemental ratios (as discussed in Section 2.1), exhibits a microstructure that contributes to its enhanced chemical resistance.
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