Ettringite dependace in thaumasite formation

Internal Sulphate Attack in Masonry Mortars with Thaumasite Formation

The present paper focuses on the study of mortar samples where expansions with thaumasite formation occur as a consequence of sulphate attack. The samples correspond to a masonry mortar used in a rural construction located in the Spanish province of Toledo made of cement with limestone filler addition CEM II/AL. Composition and microstructure of the mortars have been analysed by means of scanning electron microscopy (SEM) using secondary and backscattered electrons (BSE) and X-ray diffraction (XRD). The results show that aggregates are contaminated with gypsum, which is the source of the sulphates for the internal attack. It seems that thaumasite is formed through an ettringite transformation where aluminium atoms are replaced with silicon atoms by means of a solid solution. The study highlights that thaumasite can be formed in warm weather through an internal sulphate attack due to gypsum contamination of aggregates.

The Mechanical Properties of Early Aged Shotcrete under Internal Sulfate Attack

Shotcrete is the primary material for tunnel support due to its early rapid hardening characteristics. During tunnel construction in a sulfate environment, the hardening law of concrete will be affected. In this study, samples were prepared at six different curing times and immersed in four different concentrations of sulfate solutions. A uniaxial test was conducted and analyzed to investigate the effect of sulfate attack on the mechanical properties of early aged shotcrete materials. Results indicated that waterlogged shotcrete does not have apparent cracks on the outside. The stress–strain curve or ultimate compressive strength of the samples showed that the effect of sulfate on shotcrete should be differentiated into chemical and physical sulfate attacks, according to the concentration of sulfate ions. The two parameters in the equation of the hardening behaviors of sulfate attack samples, ultimate compressive strength, and time constant, are related to sulfate concentration. The crack damage stress threshold of samples demonstrates that high-concentration sulfate corrosion leads to an impact on the durability of shotcrete.