Upcycling of waste hydrated cement paste containing high-volume supplementary cementitious materials via CO2 pre-treatment

Abstract

For past decades, supplementary cementitious materials (SCMs) have been widely used in the production of concrete for the purpose of reducing cement carbon footprint as well as enhancing the durability of concrete structures. Hence, most of the discarded concrete could contain a certain amount of SCM such as fly ash (FA) and ground granulated blast furnace slag (GGBS). This paper aims to improve the quality of waste hydrated cement paste powders containing 50% FA (FACP) and 50% GGBS (BSCP) with the aid of CO2 pre-treatment and study its feasible use as new potential SCM. The results show that carbonation changes the silicate network from Q2 units to Q3 units, and especially more obvious for the BSCP compared to FACP. The carbonation products of carbonated FACP are mainly calcite, whereas both calcite/aragonite co-existed in carbonated BSCP. Hence, the inclusion of 10%–15% of carbonated FACP and 10%–20% of carbonated BSCP in new cement paste can induce the formation of calcium aluminate mono-carbonate and improve the compressive strength. The 29Si NMR results also found that carbonated BSCP had a higher reactivity. Overall, both carbonated waste powders can be used as a new SCM up to 30% in the cement paste without compromising the 28-day strength.