Internal Sulphate Attack in Masonry Mortars with Thaumasite Formation

Internal sulphate attack in construction materials remains a poorly understood degradation mechanism. Prior research has shown that external sulphate exposure can lead to expansive reactions in concrete, but internal sources are less studied. It was already known that gypsum contamination in aggregates can contribute to durability issues. However, the specific role of internal sulphate sources in thaumasite formation is unclear. No prior work had resolved how internal gypsum contamination interacts with cementitious systems. This gap motivated a closer examination of masonry mortars affected by internal sulphate attack. The study of thaumasite formation in warm climates is particularly underexplored. Understanding the transformation from ettringite to thaumasite is critical for durability assessments. This paper addresses these uncertainties through detailed mineralogical analysis.

The aim of the study is to investigate the mechanisms of internal sulphate attack in masonry mortars. The researchers focus on a specific case of a rural construction in Toledo, Spain. The mortar contains CEM II/AL cement with limestone filler. The goal is to determine the source of internal sulphates and their role in thaumasite formation. By analyzing the microstructure and composition, the study seeks to clarify the transformation process. The researchers aim to identify whether gypsum contamination in aggregates is the primary cause. Understanding the ettringite-to-thaumasite transition is central to the study. This work provides insights into how internal sulphates affect cementitious materials in warm climates.

The study employs scanning electron microscopy (SEM) with secondary and backscattered electrons (BSE) to examine the microstructure of the mortar samples. X-ray diffraction (XRD) is used to identify the mineralogical composition of the samples. The researchers analyze the distribution of aggregates and their contamination levels. They investigate the presence of gypsum as a potential internal sulphate source. The transformation process from ettringite to thaumasite is studied using these techniques. The analysis includes both qualitative and quantitative assessments of mineral phases. The experimental approach focuses on the spatial and chemical characteristics of the reaction products. The results are interpreted in the context of internal sulphate attack mechanisms.

The analysis reveals that aggregates in the mortar are contaminated with gypsum, which serves as the internal sulphate source. Thaumasite is identified as a reaction product formed through an ettringite transformation. The transformation involves the substitution of aluminium atoms with silicon atoms in a solid solution. This process occurs under warm weather conditions, indicating a temperature-dependent reaction. The presence of thaumasite suggests an internal sulphate attack rather than external exposure. The mineralogical analysis confirms the role of gypsum in the degradation process. The study shows that internal sulphates can lead to expansive reactions in cementitious systems. These findings highlight the importance of aggregate quality in masonry mortars.

The study concludes that internal sulphate attack in masonry mortars can lead to thaumasite formation. The source of sulphates is identified as gypsum contamination in the aggregates. The transformation from ettringite to thaumasite is confirmed through mineralogical analysis. The process involves a solid solution where silicon replaces aluminium atoms. This reaction is facilitated by warm weather conditions, as observed in the study location. The findings suggest that internal sulphate sources can cause significant degradation in cementitious materials. The role of aggregate contamination in durability issues is emphasized. These results provide a basis for further investigation into internal sulphate attack mechanisms.