New Insights into the Carbonation Behavior of C3S2: Elucidating the Role of Silica Gel in the Mechanical Strength of Carbonated Matrix

Accelerated carbonation is expected to be a potential technique for sustainable development of cement-based materials due to the advantages of rapid strength gain and CO2 sequestration. This work addresses the characteristics of C3S2(3CaO·2SiO2) under accelerated carbonation including the dissolution of ions, reaction degree, phase assemblage, microstructure, and mechanical strength. The compression-molded C3S2 specimens achieve a compressive strength of 108.4 MPa within 24 h. The reaction degree of C3S2 calculated based on 29Si MAS NMR spectra reaches 54.8% and 66.3% after 1 and 21 d of CO2 exposure, respectively, which are much higher than those of carbonation degree. Different from the previous studies, silica gel is found to be distributed evenly between calcium carbonate in C3S2 resulting from the slower dissolution rate of Ca2+. The interface between silica gel and calcium carbonate distracts crack advance and provides stress relief, which could be another contribution of silica gel on mechanical strength apart from filling of pores and densification of the matrix.