New insights into the interaction between seawater and CO2-activated calcium silicate composites

This article presents the investigation findings on the combined effect of seawater and carbonation curing on two types of binders – blended binder containing blast furnace slag (BFS) and laboratory synthesized pure β-C₂S. Samples were prepared using freshwater and seawater as mixing water. After casting, the samples were exposed to accelerated CO₂ curing for 7 days and then exposed to seawater for up to 90 days. The results revealed that the use of seawater as mixing water has substantially different effects on the performances of β-C₂S compared to blended cement. Specifically, the use of seawater as the mixing water resulted in a threefold increase in the amount of carbonates formation in β-C₂S paste compared to the samples prepared by mixing with fresh water. The seawater mixed and CO₂ cured β-C₂S paste samples showed continuous increase in strength even after extended exposure to seawater and reached around 75 MPa strength, which is nearly 100 % increase compared to the samples prepared with freshwater mixing. For β-C₂S samples, the presence of Mg ions along with slightly higher pH resulted in the formation of vaterite and Mg-calcite contributing to superior performances. Additionally, after exposure to seawater, the silica gel phase captured Mg from seawater to form M-S-H. However, such drastic benefits of using seawater were not observed in the case of blended binders. The presence of Al in blended cement led to the formation of layered double hydroxides, including hydrotalcite and hydrocalumite, which limited the benefits of using seawater. Additionally, the presence of Al also resulted in the formation of ettringite when exposed to seawater. Because of these effects, a slight reduction in strength was observed in case of carbonation cured blended cement after their exposure to seawater.