Engineering properties and structural implications of Portland limestone cement mortar exposed to magnesium sulphate attack

The overall aim of this paper is to establish the engineering properties and material durability of Portland limestone cements, particularly when they are exposed continually to an environment rich in magnesium sulphate ions, and conducive to thaumasite formation. In particular, non-destructive and destructive tests were used to identify and examine the interaction of the changes in mechanical properties with the onset of sulphate attack. To achieve these objectives, 40 × 40 × 160 mm mortar specimens were subjected to air curing at 5°C and 20°C, and also immersed in 1·8% MgSO₄ solution, again at 5°C and 20°C. The limestone content in the test specimens was kept at 0%, 5%, 15% and 35% replacement of cement by mass. The formation of thaumasite was checked and confirmed by x-ray diffraction supported by thermal analysis and scanning electron microscopy. The non-destructive tests used in the study included pulse velocity, dynamic modulus, shrinkage, expansion and changes in mass. In addition, flexural and compressive strengths were determined. All the tests were carried out up to one year. The results obtained from the tests have then been critically analysed. It is shown that mortar prisms containing 35% limestone and exposed to magnesium sulphate solution at 5°C suffered extensive damage and deterioration within one year. In addition, prisms containing 15% limestone gave strong signs of impending damage due to sulphate attack. The results of the non-destructive and destructive tests were highly complementary, and confirmed each other. The major mechanical deterioration occurred through the loss of compressive strength of almost 75% of that of the original unaffected mortar, whilst the loss in flexural strength was relatively small. The results strongly indicate that compression members, containing 35% limestone and exposed to aggressive sulphate environment at temperatures of about 5°C, can be seriously at risk in their ability to carry loads. On the other hand, it is unlikely that flexural members under these conditions would suffer the same degree of serious structural damage.