Microstructure of Portland cement blended with high dosage of sewage sludge ash activated by Na2SO4

Abstract

Sewage sludge ash (SSA) used as a supplementary cementitious material is one of the effective methods for sewage sludge disposal. Because of the weak pozzolanic reactivity of SSA, the incorporation of high dosage of SSA results in a significant degradation for the properties of Portland cement (OPC). This greatly limits the utilization of SSA. In this study, Na2SO4 was used as an activator and its effect on the hydration and microstructure of Portland cement blended with high dosage of sewage sludge ash (50% and 60%) was investigated. The SSA-OPC blended mortars exhibit extremely low compressive strengths due to the poor pozzolanic activity of SSA and its inhibition effect on the hydration of the blended systems. The compressive strength of the blended mortars is significantly improved with the addition of Na2SO4. At early age, the hydration of C3S is accelerated by Na2SO4. Thus, the generation of C-S-H gels and Ca(OH)2 are promoted to strengthen the matrix. Besides, Na2SO4 enhances the reaction of C3A with SO42− and thus promotes the formation of AFt in the blended systems. At later age, the pozzolanic reaction of SSA with Ca(OH)2 in the blended systems is enhanced because the concentration of alkali of the pore solution is increased with the incorporation of Na2SO4. This promotes the precipitation of additional C-S-H gels to fill the space around SSA particles and results in a dense microstructure of the matrix. Moreover, the addition of Na2SO4 avoids the undersulfation of the blended systems, leading to a transformation of the aluminate products from hexagonal plate-shaped AFm crystals and short needle-like AFt crystals with high C/S ratios to long needle-like AFt crystals with extremely low C/S ratios. The formation and growth of the long needle-like AFt crystals effectively refine the pore structure and increase the compressive strength of the blended systems.