**4. Discussion**

The geopolymerization process of geopolymers with SiC sludge is illustrated in Figure 7. In steps 1 and 2, the active components of metakaolin and SiC sludge particles were dissolved to form monosilicates (Q0) that were linked to the end groups (Q1), middle chain groups (Q2), layers and branching sites (Q3), and [SiO4] and [AlO4] tetrahedrons in the AASs. In step 3, primary particles of oligomeric gel were formed by the oligomers through the polycondensation reaction in the AASs. In step 4, the oligomers

were polycondensed to form geopolymerization gels. In step 5, the polymers were polycondensed and accumulated around the unreacted SiC sludge surface to form 3D networks (Q4).

**Figure 7.** The geopolymerization process of geopolymers with SiC sludge.

At an early curing time, for 0.8NS0SCS, the sum of the silicate derivatives (Q1, Q2, and Q3) was 13.59% (Table 4). After 28 days of curing, the percentage of silicate derivatives increased to 19.23%, as shown in Table 4. The results revealed that when the NS/SS ratio was relatively low, an increase in the NaOH content caused the generation of sufficient OH− in the system to increase the solubility. This solubility increase caused hindrance in polycondensation, as indicated by the heat evolution rate and DTA/TG results (Figure 1 and Table 3). The inhibition of polycondensation reduced the mechanical strength (Figure 2). An increase in the NS/SS ratio promoted the dissolution of the initial solid, thus accelerating the geopolymerization reaction and leading to the rapid formation of geopolymer network structures. During the geopolymerization reaction, silicate derivatives were transformed into geopolymer gels. Therefore, the weight percentage of the silicate derivatives of 1.6NS0SCS decreased to 10.06% after 28 days of curing. Furthermore, the synergistic effect between the SCS and the MK was assigned to the promotion of the geopolymerization reaction, which caused the flexural strength of 1.6NS10SCS to increase to 6.42 MPa after 28 days of curing. First, the silica and alumina of metakaolin were dominant in the geopolymerization reaction, and the addition of silicon carbide sludge, which had silica, was provided more reaction paths. In addition, Rahman et al. [14] also noted that given the increase in flexural strength due to simultaneous inclusion of silica and silicon carbide whiskers, it is possible that silica particles are compatible with the metakaolin-based geopolymers, which is effective in consolidation [14]. According to the DTA/TG results, the weight loss percentage of 1.6NS10SCS increased to 14.62% from 400 to 750 ◦C, as shown in Figure 3b. This fact implies that the synergistic effect promoted the oligomers to combine and form integral geopolymer gels [20].

However, geopolymers with high replacement levels of SiC sludge generated an inadequate amount of leading precursors, thus limiting the polycondensation process. For example, for 1.6NS30SCS and 1.6NS40SCS, the weight percentages of silicate derivatives (Q1, Q2, and Q3) were 9.81% and 9.13%, respectively, as presented in Table 4. In steps 1 and 2, SiC sludge dissolves to release Al3+ and Si4+, which are hydrolyzed into [AlO4] and [SiO4] tetrahedrons, respectively, immediately after contact with the AASs. The amounts of dissolved silica and alumina from MK are limited due to precipitations of geopolymer gels around the surface of SCS particles, which caused the number of [SiO4] and [AlO4] tetrahedrons in the system to be low. Thus, the amount of oligomeric gel was low. Because the N-A-S-H gels activated by the sodium silicate solution required an increased amount of oligomeric gel, the rate of the geopolymerization reaction decreased. Therefore, the heat evolution duration of the second exothermic peak increased from 26.15 to 33.32 h as the replacement levels of SiC sludge increased from 30% to 40% (1.6NS30SCS and 1.6NS40SCS), as shown in Figure 1c. Additionally, the flexural strengths of 1.6NS30SCS and 1.6NS40SCS were 5.31 MPa and 2.73 MPa, respectively (Figure 2c). This result indicated that a relatively high SiC sludge content (more than 20%) might block the synergistic effect between SCS and MK, which is confirmed by the heat evolution rate, DTA/TG results, and 29Si MAS NMR analyses. This is consistent with the SEM images results of previous studies [29] that the

amorphous gel products had gradually filled the pores of the SiC sludge-based geopolymer, resulting in an increase in density and compressive strength of the geopolymer structure [29]. The synergistic effect of the SiC sludge and MK promoted the reaction progression, which caused increasing amounts of amorphously structured geopolymeric gels in the geopolymerized system [29]. Then, the unreacted SiC sludge was in the form of uniform plate particles [29]. In summary, this study demonstrated that this renders SiC sludge as promising additives for the production of metakaolin-based geopolymers. As a structural material, the SiC sludge-based geopolymer is a potential replacement material for OPC.


