The effect of wetting and drying cycles and carbonation on thaumasite formation.

Thaumasite form of sulfate attack is having great attention since its discovering in series of foundations supporting motorway bridges in the UK in the late nineties of the last century. This is mainly due to its destructive effect on concrete structures, and the lack of information about its formation mechanisms. This research conducted a study on the effect of wetting and drying, carbonation and the effect of water to cement ratio on the thaumasite formation, and whether these effects are linked with other parameters such as cement type and sulfate concentration or not. 10 different mixes were produced based on four binder types in this study namely 100% CEM I, 90% CEM I + 10% Limestone filler, 50% CEM I + 50 %PFA and 30% CEM I +70%GGBS. A series of mortar samples of two types were prepared 50 mm cubes and 40 × 40 × 160 mm prisms. The samples were kept in three different solutions contain BRE DS3,DS4 based on magnesium sulfate MgSO4.7H2O in addition to deionised water. Two different temperatures 5°C and 20°C were also used to confirm the formation of thaumasite at ambient temperatures (20°C) and to accelerate its formation at 5°C. The effect of wetting and drying cycles on thaumasite formation was studied and compared with samples immersed continuously in the same solutions for 12 months. Powder-sulfate interaction and its effect on thaumasite formation was studied by grinding mortar samples to a fine powder, thus eliminating the permeability effect and enabling physical factors that affect the rate at which solutions can be transferred through the mortar to be separated from chemical factors that affect the rate at which the chemical reactions take place. Visual observations, mass and length changes were used to assess the mortar deterioration, along with X-ray diffraction, infra-red spectroscopy and SEM that were used to determine the mineralogy of deterioration products. For the cyclic wetting and drying exposure regime the results showed that wetting and drying cycles significantly delayed thaumasite formation compared with control specimens. For powder samples, it is found that thaumasite is readily formed in these powders after 3 months of exposure to sulfate solutions including GGBS and PFA samples, on the other hand cubes and prisms, exposed to the same solutions for 24 months showed no signs of deterioration. Thermodynamic modelling was used to predict the deterioration products for powders samples and good agreement between predicted and observed results was found.