Uptake of $^{14}$CO$_3$$^{2−}$ by HCP (CEM III/C and CEM III/C + CaCO$_3$) in the presence and absence of ISA and chloride

The retention of $^{14}$CO$_3$$^{2−}$ by hydrated cement pastes (CEM III/C; CEM III/C + CaCO$_3$) was investigated in the absence and presence of isosaccharinate (ISA, 10$^{−5}$–0.2 M) and NaCl (≈10$^{−3}$–2.0 M). The moderate retention observed for both materials at short contact times (t = 2 days) is primarily attributed to adsorption on C–S–H phases. The differential retention behaviour at longer contact times is tentatively explained by the different assembly of carbonate-bearing phases (calcite, monocarbonate, hydrotalcite) in both binders, which are responsible for isotopic exchange processes. ISA has a minor impact on $^{14}$CO$_3$$^{2−}$ retention except at high [ISA] and low S/L ratios, where partial cement dissolution is triggered by the formation of CaISA$_{-H}$(aq) and Ca(ISA)$_2$(cr). NaCl has a negligible impact on the retention of $^{14}$CO$_3$$^{2−}$, in spite of its influence on surface charge and phase distribution. This work provides an improved quantitative description and mechanistic understanding of $^{14}$CO$_3$$^{2−}$ retention in cementitious systems.