The deterioration mechanisms of hardened cement paste exposed to combined action of cyclic wetting-drying, salt attack and carbonation

In saline soil or salt lake regions, cement-based materials suffer from combined action of salt attack (SA), carbonation, and cyclic wetting-drying (CWD), and its deterioration mechanism remains an open question. Taking advantage of multiple techniques, this study is dedicated to unveiling the deterioration mechanism of hardened cement paste (HCP) exposed to combined action of SA (sulfate salt or sulfate-chloride salt), carbonation (natural carbonation, 0.035 vol.% CO2 or accelerated carbonation, 20 vol.% CO2) and CWD. The results show that, when exposed to combined action of SA, carbonation and CWD, HCP exhibits distinct deterioration mechanisms depending on the concentration of carbon dioxide: chemical sulfate attack (i.e., formation of ettringite and gypsum) is mainly responsible for the deterioration of HCP in the case of natural carbonation, whereas decomposition of C-S-H and formation of gypsum are both responsible for the deterioration of HCP in the case of accelerated carbonation. The presence of sodium chloride in sulfate-chloride solution inhibits the sulfate attack as well as carbonation attack of HCP.