Recycling sewage sludge ash and limestone for sustainable cementitious material production

Abstract

Supplementary cementitious materials have significant potential to reduce greenhouse gas emissions in the production of construction materials. This work investigated the synergistic effect of partially replacing cement by sewage sludge ash and limestone for sustainable cementitious material production. The hydration phases and pore structure characteristics were determined by X-ray diffraction and the BET nitrogen sorption method, respectively. A central composite rotational design (CCRD) was used to study the effect of the water binder ratio (w/b), sludge ash and limestone content on the compressive strength. The results of microstructure tests showed that the addition of limestone enhanced the formation of carboaluminate hydrates. The additional hydration products filled in large pores of paste, resulting in a well-refined microstructure of the ternary mixture. Thus, the 90-day strength activity index (SAI) of mortar with 15% sludge ash and 7.5% limestone was 100.6% compared to the reference. Despite the adverse effect of limestone on the compressive strength, the synergistic effect of sludge ash and limestone contributed to the reduction of economic cost and greenhouse gas emission in the production of sustainable cementitious materials. For the same compressive strength level, the ternary mixture composed of 15% sludge ash and 7.5% ground limestone reduced Portland cement consumption by 23.13% and CO2-eq emission intensity by 13.52%.