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

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.