Study of thaumasite and ettringite phases formed in sulfate/blast furnace slag slurries using XRD full pattern fitting

The thaumasite form of sulfate attack (TSA) has been investigated using a method known to accelerate the formation of the sulfate minerals thaumasite, ettringite and gypsum. Mixes containing different cements and aggregates in magnesium sulfate solution were prepared at different water:solid ratios. The work concentrated, in particular, on the role of blast furnace slag as a cementitious material in preventing the formation of thaumasite. The formation of sulfate minerals in the mixes was followed for a period of two years by X-ray powder diffraction. Full pattern fitting, a computer-based XRD data analysis technique, was used to identify the nature of the thaumasite/ettringite solid solutions produced. Thaumasite was formed only in mixes containing carbonate-bearing aggregates at higher water:solid ratios. In most of the mixtures containing blast furnace slag cement (70% blast furnace slag cement, 30% Portland cement), an ettringite-based solid solution was the main sulfate-bearing phase produced. Only one of the mixes containing blast furnace slag cement was found to produce thaumasite. Some samples, containing only blast furnace slag as the cementitious component, produced gypsum and no thaumasite/ettringite. The variation in the exact nature of thaumasite/ettringite produced with different aggregates, cements and water:solid ratios is discussed.