Development of reactive carbonate-calcined clay-cement (C4) composites through synchronizing aluminate-carbonate reaction: Toward high compressive strength and low carbon emission

Abstract

Limestone calcined clay cement (LC3) offers a promising strategy for producing cement with significantly reduced CO₂ emissions. However, it generally exhibits limited early-age compressive strength due to the uncoordinated reaction between limestone and aluminates, which is a consequence of the low reactivity of limestone. Moreover, further reducing the CO₂ emissions of LC3 is a challenges. This study presents an innovative method for producing carbon-fixing calcium carbonates (Cc) with varied reactivities and highly reactive Si-Al gels via the mechano-carbonation of recycled concrete fines (CRCF), aimed at replacing limestone in LC3 production. This approach seeks to develop a high compressive strength and low-carbon cement composite, designated as Reactive Carbonate-Calcined Clay-Cement (C4). The results demonstrate that C4 shows a sustained improvement in compressive strength, with increases of over 30 % and 17 % at 1 d and 28 d, respectively, compared to the LC3. The superior early-age performance of C4 is attributed to a pivotal "reaction window" between the highly reactive Cc and Si-Al gels, which facilitates the generation of monocarbonate (Mc) due to the synchronized aluminate-carbonate reaction. The early precipitated Mc and calcium-silicate-hydrate (C-S-H) gels from the pozzolanic reaction of Si-Al gels serve as seedings, facilitating a higher cement reaction degree and the early strength development. Moreover, the moderately reactive Cc in CRCF still exhibits higher reactivity than limestone, enabling a more synchronized reaction with calcined clay at the later stages. This contributes to the formation of a more compact microstructure, thereby favoring the continued strength development. Moreover, C4 offers an additional advantage of reducing CO₂ emissions by over 50 % in comparison to LC3, thus contributing significantly to environmental sustainability.