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% Na2SO4, 3.5 wt% NaCl + 10 wt % Na2SO4, and tap water) at different pH values (pH 5, 7, and 9) and carbonation (accelerated carbonation: 20 vol% CO2 and natural carbonation: 0.035 vol% CO2). 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 Na2SO4 crystals; the precipitation of NaCl clogs the pore network of mortar, which impedes the progressive penetration of CO2. The presence of NaCl in mixed salt solution is beneficial to salt resistance and carbonation resistance of mortar.