High temperature and sulphate effect in cement-based composites

In this thesis, the mechanical and microstructural behavior of fiber-reinforced cementbased composites exposed to elevated temperatures up to 600 °C and sulfate attack was comprehensively investigated. The experimental study considered various fiber types (steel and polypropylene), fiber slenderness ratios, the use of silica fume as a mineral admixture, and three different curing regimes (standard water, sodium sulfate, and potassium sulfate solutions). After being subjected to high temperature, the composite specimens were tested at 7, 28, and 90 days of curing to evaluate their performance. Compressive strength tests revealed that mixtures incorporating steel fibers, particularly those with high slenderness and 16-micron diameter (BF16), demonstrated the best mechanical performance after high-temperature exposure. In contrast, polypropylene fiber-reinforced composites showed significant strength loss due to their low melting point, which led to degradation under thermal stress. Silica fume contributed positively to strength development under all environmental conditions by enhancing pozzolanic activity and reducing porosity, thus promoting a denser microstructure. Notably, specimens cured in potassium sulfate solution exhibited better mechanical performance compared to those in other sulfate environments. Scanning electron microscopy (SEM) analyses showed that in sulfate-rich conditions, especially in Na₂SO₄ solution, needle-like ettringite formations and microcrack development were more prominent. The presence of silica fume was found to mitigate these effects by reducing portlandite content, thereby limiting harmful reactions with sulfate ions and preserving microstructural integrity. In silica fume-modified mixtures such as BF16-SF and PP30-SF, a more uniform C-S-H matrix and fewer cracks were observed. Ultrasonic pulse velocity (UPV) measurements indicated that internal structural characteristics of the material evolved over time and were influenced by environmental factors. In composites with polypropylene fibers, the melting of fibers under high temperature led to void formation, resulting in the highest levels of UPV variation. Conversely, in mixtures containing silica fume, the densifying effect of ongoing pozzolanic reactions improved UPV outcomes over time. Sulfate curing conditions also enhanced UPV changes due to the formation of secondary hydration products. Overall, the findings of this study provide valuable insights for the development of durable and high-performance cement-based systems capable of maintaining structural integrity under combined exposure to fire and chemical attack. Keywords: Cement-based fiber-reinforced composites, high temperature, sulfate resistance. UPV