The thaumasite form of sulfate attack in Portland-limestone cement mortars stored in magnesium sulfate solution

Abstract

Mortar prisms (40×40×160 mm 3) made at a water:solids ratio of 0.5 and a cement:aggregate ratio of 1:2.5 from Portland cement with 0%, 5%, 15% and 35% limestone additions were cured in water at 20 °C for 28 days. They were subsequently stored in air and submerged in 1.8% magnesium sulfate solution at 5 and 20 °C for a year. The prisms were inspected visually every 28 days, the solution was changed every 84 days, and selected samples were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) after specified intervals. The thaumasite form of sulfate attack was readily identified in the ordinary Portland cement (OPC) – 35% limestone mortar after 126 days storage in magnesium sulfate solution. The surface layer of the prism had spalled and was mushy, while the core was still solid and sound. Gypsum, thaumasite and brucite were identified in the surface layer. Secondary electron images of polished sections of deteriorated Portland-limestone cement mortars revealed the microstructure of the cement to be suffering from the thaumasite form of sulfate attack. The extent of this attack was greater at 5 °C than at 20 °C, although some thaumasite was formed even at the higher temperature.