Effect of colemanite and fiber reinforcement on the mechanical and high-temperature properties of mortars containing an air-entraining admixture

The manufacture of cement for concrete generates a substantial amount of carbon dioxide emissions. As environmental concerns grow, the use of alternative binder materials to replace cement is becoming increasingly significant. Turkey has abundant boron and mineral deposits, making colemanite a promising alternative binder material. In this study, the effects of colemanite and various fiber types (steel, basalt, carbon, and polypropylene) on the compressive strength, flexural strength, water absorption capacity, and high-temperature resistance of mortar mixtures containing air-entraining admixtures were investigated. For this purpose, 15 different mortar mixtures were prepared using CEM I 42.5R Portland cement, colemanite, standard sand, a superplasticizer, an air-entraining admixture, and four types of fibers (steel, basalt, carbon, and polypropylene). The air-entraining admixture was added to all mixtures at 0.1% by weight of cement. The water-to-binder ratio, sand-to-binder ratio, and flow values were kept constant at 0.485, 2.75, and 190±20 mm, respectively. Colemanite was replaced with cement at rates of 0%, 3%, and 5%, while fibers were added at a volume fraction of 0.5%. The workability of the fresh mixtures was examined. Compressive and flexural strengths, water absorption capacities, and high-temperature resistance of 28-day specimens were determined. Additionally, drying shrinkage behavior was investigated, and selected specimens were analyzed using SEM for microstructural examination. The results indicated that the inclusion of colemanite and fibers increased the demand for superplasticizer. Both compressive and flexural strengths improved with the addition of colemanite and fibers. Among the fiber-reinforced mixtures, steel fiber mixtures containing colemanite exhibited the highest strength gains. In mixtures without colemanite, basalt and carbon fibers were more effective in enhancing compressive strength. An inverse relationship was observed between water absorption and compressive and flexural strength. After exposure to high temperatures, the highest strength values were recorded in steel fiber mixtures containing 3% and 5% colemanite. However, increasing the colemanite replacement ratio led to greater strength losses. The drying shrinkage of specimens containing colemanite and fibers significantly decreased, with carbon fibers demonstrating the most effective shrinkage control among all fiber types.