The effect of pH and carbonation on the partially immersed mortar exposed to physical salt attack

In saline-alkali or salt-lake areas, the influence of pH value of underground aqueous solution and carbonation on the deterioration of cement-based materials remains an open question. This study utilizes various techniques to investigate the physical properties and degradation mechanism of partially immersed mortar incorporated with different dosages of fly ash exposed to combined action of salt solutions (10 wt% Na 2 SO 4 , 3.5 wt% NaCl + 10 wt% Na 2 SO 4 , and tap water) at different pH values (pH 5, 7, and 9) and carbonation (accelerated carbonation: 20 vol% CO 2 and natural carbonation: 0.035 vol% CO 2 ). The results indicate that the degree of deterioration of mortar increases with dosage of fly ash and pH value of salt solution, with the maximum mass loss exceeding 25 %. Physical sulfate attack (PSA) is responsible for the partially immersed mortar exposed to combined action of salt attack at various pH values and natural carbonation. However, when carbonation is accelerated, a more severe deterioration occurs, with the durability time of mortar reduced by nearly 60 %. PSA and decomposition of C-S-H are mainly responsible for the deterioration of mortar, while the formation of thaumasite and natrite is partially responsible for the deterioration of mortar. The presence of NaCl in mixed salt solution inhibits the precipitation of Na 2 SO 4 crystals; the precipitation of NaCl clogs the pore network of mortar, which impedes the progressive penetration of CO 2 . The presence of NaCl in mixed salt solution is beneficial to salt resistance and carbonation resistance of mortar. • The impact of pH of salt solution and carbonation on physical sulfate attack (PSA) of mortar is carried out in this study. • Degree of deterioration of partially immersed mortar increases with dosage of fly ash and pH value of salt solution. • The deterioration of partially immersed mortar exposed to salt solution at various pH and natural carbonation is PSA. • Under accelerated carbonation , PSA, decomposition of C-S-H, formation of thaumasite are all responsible for deterioration.