Thermal and carbonation activation of recycled cement paste: A comparative data analysis and future research directions

Concrete is the predominant construction material worldwide, but cement production generates
significant CO2 emissions. Furthermore, large amounts of concrete waste are difficult to recycle
and are therefore mostly landfilled. Using recycled concrete fines (RCF) as a supplementary
cementitious material (SCM) through activation offers a promising strategy to reduce clinker
consumption while valorizing this waste stream. This study provides comprehensive insights
into the effectiveness of two activation methods, thermal activation and carbonation activation,
through systematic quantitative evaluation based on published literature. Thermal activation
dehydrates the material, forming a modified calcium silicate phase (C2S) with hydraulic
reactivity. Carbonation activation produces CaCO3 and an amorphous alumina–silica gel with
pozzolanic activity. Both methods substantially improve the performance of recycled cement
paste (RCP) relative to untreated RCP, although neither matches OPC performance. Thermal
activation demonstrates greater potential than carbonation activation. However, limited data
and substantial variations in starting material, replacement level, particle size, and activation
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conditions impede direct comparison and prevent their individual effects from being reliably
isolated.