Influence of Portland cement and sewage sludge ash additions on metakaolin-based geopolymers

This study evaluates the impact of Portland cement (PC) and sewage sludge ash (SSA) on zeolite formation in metakaolin (MK)-based geopolymer mortars. Various parameters, including NaOH concentration and PC/SSA content, were analyzed. The mortars were cured in a thermal bath at 65 °C and evaluated using compressive strength testing, thermogravimetry (TG/DTG), X-ray diffraction (XRD), and scanning electron microscopy (FESEM-EDX). The results emphasize the crucial role of NaOH concentration in zeolite formation. Mortars with 8 mol kg⁻¹ NaOH exhibited slower crystallization, leading to greater strength stability. Specimens containing 10 % PC maintained their strength over 56 days, reaching 47.2 MPa. While SSA initially reduced compressive strength, it improved long-term performance. After 56 days, mortars with 9.5 % and 19 % SSA outperformed SSA-free mixes, which stabilized at 24.3 MPa, maintaining strengths of 42.5 MPa and 33.7 MPa, respectively. These findings highlight the synergistic effect of incorporating PC and SSA into MK-based geopolymer mortars, particularly for longer curing periods. Physicochemical analyses revealed that the primary binding phase, N-A-S-H, gradually transformed into zeolite Na-P1, causing strength degradation. However, the presence of calcium (from PC), iron, and sulfur (from SSA) altered the geopolymer matrix into N-(C, F, $\bar{S}$)-A-S-H, effectively delaying zeolite formation. This study demonstrates the complex interaction between PC, SSA, and zeolite formation in geopolymer mortars. By adjusting these components, it presents a viable approach to enhancing the durability and mechanical performance of MK-based geopolymers.