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 an d 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. rn particular non-destructive and d estructive tests were used to identify and examine the interaction of the c hanges in mechanical properties with the onset of sulphate attack. To achie ve these objectives, 40 x 40 x 160 mm mortar specimens were subjected to ai r curing at 5 degreesC and 20 degreesC, and also immersed in 1.8% MgSO4 sol ution, again at 5 degreesC and 20 degreesC. The limestone content in the te st 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 nondest ructive tests used in the study included pulse velocity, dynamic modulus, s hrinkage, expansion and changes in mass. In addition, flexural and compress ive strengths were determined. All the tests were carried out up to one yea r: The results obtained from the tests have then been critically analysed. It is shown that mortar prisms containing 35% limestone and exposed to magn esium sulphate solution at 5 degreesC suffered extensive damage and deterio ration within one year. In addition, prisms containing 15% limestone gave s trong signs of impending damage due to sulphate attack. The results of the non-destructive and destructive tests were highly complementary, and confir med each other. The major mechanical deterioration occurred through the los s of compressive strength of almost 75% of that of the original unaffected mortar, whilst the loss in flexural strength was relatively small. The resu lts strongly indicate that compression members, containing 35% limestone an d exposed to aggressive sulphate environment at temperatures of about 5 deg reesC, can be seriously at risk in their ability to carry loads. On the oth er hand, it is unlikely that flexural members under these conditions would suffer the same degree of serious structural damage.