On improved microstructure properties of slag-based UHPC incorporating calcium formate and calcium chloride

The study aimed to achieve high strength in slag-based cementless ultra-high performance concrete (UHPC) while investigating the role of calcium formate and calcium chloride as accelerators using advanced characterization and microstructural analysis. The UHPC specimens were evaluated using various techniques, including heat of hydration, thermogravimetric analysis, X-ray diffraction, nuclear magnetic resonance, scanning electron microscopy, and porosity measurements. The results showed that both accelerators improved the UHPC properties, with calcium formate being more effective than calcium chloride, as evidenced by higher heat of hydration, improved microstructure, and lower porosity. Furthermore, the thermodynamic modeling revealed insightful information on the reaction mechanisms with C-(N-)A-S-H and hydrotalcite as the primary hydration products formed in the matrix. Notably, the UHPC specimens with calcium chloride exhibited a very low pH value, which could increase the risk of corrosion in reinforced structures. Therefore, the use of calcium formate as an accelerator can lead to the development of cementless UHPC with superior properties.