Characterizing a Local Sewage Sludge Ash for Developing Alkali-Activated Geopolymer Mortar

Abstract

Production of ordinary Portland cement (OPC) has significant adverse effects on the environment and accounts for 5%–7% of CO₂ emissions globally. Geopolymer cement is potentially a sustainable alternative to OPC and is created by the activation of aluminosilicate materials with an alkaline solution that induces geopolymerization, causing gelation of the materials. Sewage sludge ash (SSA) contains relatively high contents of aluminosilicate materials and can be a precursor for geopolymers. This study aims to characterize a local SSA as a precursor to develop geopolymer mortar. Chemical and leaching analysis found no significant adverse environmental implications of the SSA. Strength activity index tests showed that the SSA has a moderate pozzolanic activity. The SSA was then utilized to develop a geopolymer mortar in conjunction with an alkaline solution consisting of sodium hydroxide and sodium silicate. Factors that influence the geopolymerization and strength gain of geopolymers were examined, including the molar ratio between silicon dioxide and sodium oxide and the activator/binder ratio. An optimal activator/binder ratio of 0.22 and a SiO₂/Na₂O molar ratio of 0.80 were determined to yield the highest compressive strength of the geopolymer mortar.