Effects of superimposed carbonation synergy on BOFS cement-based materials

Carbonation curing has been widely investigated for sequestering CO2 and improving strength and durability of cement-based materials. Due to the low permeability, the carbonation reaction normally occurs in the outer layer of samples. To increase the overall CO2 capture rate, a superimposed carbonation method was proposed by coupling carbonation pretreatment of BOFS and carbonation curing of BOFS-cement paste. BOFS was carbonated at the pressure of 30 bar and then used as supplementary cementitious material. BOFS-cement paste was carbonated again under the same condition. The carbonation pretreatment of BOFS made the particle size larger, accompanied by the formation of calcium carbonate and amorphous silica gel. CO2 sequestration of specimens was more than twice by applying superimposed carbonation compared with using carbonation pretreatment or carbonation curing alone. Superimposed carbonation can make up for the strength reduction caused by the incorporation of low activity BOFS. Besides, superimposed carbonation significantly improved the pore structure of BOFS-cement paste by reducing the porosity and average pore diameter. During carbonation, much wellcrystallized calcite was formed, while C-S-H gel, calcium silicate, and portlandite were consumed. This study is promising in terms of the utilization of BOFS, and providing a new method for preparing low-carbon BOFS cement-based composite.