Calcium carbide residue as auxiliary activator for one-part sodium carbonate-activated slag cements: compressive strength, phase assemblage and environmental benefits

Abstract

Sodium carbonate (Na2CO3) has the potential to become an efficient solid activator for preparing one-part alkali-activated ground granulated blast furnace slag cements. However, the Na2CO3-activated slag binders usually exhibit the delayed hardening time and slow early-age strength development. In this study, calcium carbide residue (CCR), an industrial waste mainly composed of Ca(OH)2, was applied as a type of auxiliary activator to accelerate the reaction kinetics of Na2CO3-activated slag binders. Effects of CCR additions on the compressive strength and phase assemblage of the binders were investigated, and the environmental benefits from the use of CCR were evaluated. The results showed that the initial reaction between CCR and Na2CO3 removed CO32- ions and meanwhile produced OH- ions in the aqueous phase to enhance the alkalinity. The Ca2+ ions released from the dissolved CCR particles promoted the polymerization of C-A-S-H type gel, associated with the formation of other calcium-containing products of calcite, calcium hemicarboaluminate and calcium monocarboaluminate. Hydrotalcite-like phase was identified in the inner product layer formed around the slag particles. The CCR addition promoted the hardening and consequently enabled the pastes to set within 7 h. As the optimum mixture in terms of strength development, the binder with 8% Na2CO3 and 2.5% CCR exhibited the maximum 1-day and 28-day compressive strengths of 21.8 and 37.7 MPa, respectively. When compared with the PC paste, production of this CCR-modified Na2CO3-activated slag cements had considerable savings of 94%, 87% and 20% in CO2 emissions, energy consumption and cost, respectively.