The formation of thaumasite in a cement:lime:sand mortar exposed to cold magnesium and potassium sulfate solutions

Abstract

This paper, which is presented in two parts, describes the performance of a laboratory-prepared masonry mortar after exposure to cold magnesium and potassium sulfate solutions. The objective of the study was to investigate the conditions under which the thaumasite form of sulfate attack can affect masonry mortars. This work was funded by the UK Department of the Environment, Transport and the Regions (DETR) and information contained in the following paper was included in the recently published DETR Report on the thaumasite form of sulfate attack (Department of the Environment, Transport and the Regions. The thaumasite form of sulfate attack: risks, diagnosis, remedial works and guidance on new construction. Report of the Thaumasite Expert Group, DETR, January 1999).In Part I of the investigation, mortar tablets (10×28×34 mm3) were prepared from a 1:1:5.5 cement:lime:sand, air entrained mortar to which powdered calcite had been added during mixing. Each mortar tablet was crushed before exposure in order to facilitate reaction and stored fully immersed in 200 ml of solution for several months. Two sets of storage conditions were used – one where the experiment was exposed to the atmosphere and the other where atmospheric exposure was prevented. The test material was sampled at intervals for analysis by X-ray diffraction to determine the nature of the products and the sequence of chemical events involved. Thaumasite was readily produced in both magnesium and potassium sulfate solution, following the prior formation of ettringite. It was not formed under conditions where ettringite was unstable, suggesting some involvement of the latter in the thaumasite forming process. It was also found that a rapid type of carbonation prevailed in potassium sulfate solution exposed to the atmosphere. This process, which has been called ‘alkali carbonation’, destroyed both ettringite and thaumasite.In Part II of this investigation, the same mortar tablets were used but this time the performance of the whole tablets of uncrushed mortar was tested in the same sulfate solutions so that the physical effects of sulfate attack on hardened mortars could be assessed. The test method used was similar in principle to the BRE mortar durability test which combines the cyclic administration of sulfate solution with intermediate drying to simulate the processes occurring in practice in brickwork mortar. X-ray diffraction analysis of the tablets was carried out when first signs of sulfate attack were observed and also after severe damage had occurred. These showed that early damage appeared to be mainly due to ettringite formation but both ettringite and thaumasite were involved at the severe damage stage. Some non-calcite containing mortars were also examined during this phase of the investigation and results have found them to be slightly less durable than their added-calcite counterparts, particularly in weak magnesium sulfate solution. This was attributed to the improved impermeability of added-calcite mortars rather than any inherent chemical resistance to sulfate attack.Even though Part I concentrated purely on the ‘chemical’ interactions between mortar and solution, the reaction products and sequences were found to be very similar to those discovered in Part II, where physical barriers to sulfate ingress had to be overcome prior to chemical attack. This provides confirmation that any masonry mortar can potentially deteriorate in the presence of excess sulfates providing the temperature is low, the mortar contains an available source of calcium carbonate, the brickwork is consistently wet and the pH of the reaction zone is maintained at 10.5 or above. Having said this, the extent of sulfate attack of brickwork in the field is small and should not become a major problem in practice, provided the current recommendations [Department of the Environment, Transport and the Regions. The thaumasite form of sulfate attack: risks, diagnosis, remedial works and guidance on new construction. Report of the Thaumasite Expert Group, DETR, January 1999] (especially the avoidance of using sulfate-bearing bricks in exposed situations) are adhered to.