Deterioration mechanism of OPC mortar under combined chemical and physical sulfate attack in partial immersion

This investigation examines the deterioration mechanisms of Ordinary Portland Cement (OPC) mortars, affected by the dual impact of Physical Salt Attack (PSA) and Chemical Sulfate Attack (CSA). The study utilizes an accelerated partial-immersion sulfate exposure protocol with characterization through visual inspections, SEM, EDX, XRD, and MIP. The results reveal distinct ion migration patterns, highlighting capillary action drives sulfate ions penetrating the core of the immersed portion, while surface evaporation promotes thenardite sub-efflorescence and intensifies degradation in the dry portion. OPC mortars show corner scaling and core microcracking in the immersed portion due to the formation of expansive minerals like ettringite and gypsum. The extent of scaling in dry portion is primarily governed by the W/C ratio, which influences the balance between effective crystallization pressures—determined by pore size filled—and the mechanical properties of the microstructure. OPC mortar with a high W/C ratio experienced progressive delamination in the dry portion due to their smaller size range of pores where crystals grew and higher porosity, which caused greater effective crystallization pressures from the combined effects of CSA and PSA.