Effects of accelerated carbonation on the chemical and microstructural evolution of recycled different SCMs blended cement pastes

Abstract

To mitigate the environmental impacts of cement production, which contributes to more than 7 % of global CO₂ emissions, strategies such as the use of supplementary cementitious materials (SCMs), the development of new low-carbon cementitious materials, and the incorporation of recycled waste materials have been implemented. This study comprehensively investigates the chemical changes and microstructural evolution of CO₂-cured recycled cement pastes from different waste SCMs blends, including silica fume (SF), fly ash (FA), ground-granulated blast-furnace slag (GGBS), and limestone (LS). Using ²⁹Si NMR, TG, QXRD, SEM, TEM, MIP, and mechanical tests, we identify distinct carbonation patterns between different recycled SCMs-blended cement (RBC) pastes. These differences are attributed to variations in β-C₂S, calcium hydroxide, and C₂AS (gehlenite) content. The thermal activation temperature plays a significant role in the carbonation behavior of RBC pastes, with higher temperatures leading to increased crystallization of β-C₂S and a reduction in crystalline defects. The enhanced compressive strength of RBC pastes under CO₂ curing is linked to the carbonation of portlandite and belite at the surface, resulting a "C-S-H and silica gel matrix inlaid with a polycrystalline CaCO₃ hoop layer" that markedly improves microstructural densification and decreases total porosity. With a low-carbon emission of 0.237–0.316 tCO₂/t during RBC paste production and its potential for CO₂ capture, RBC presents an obvious potential as a negative-carbon cementitious material.