Evaluation of Mechanical and Thermal Properties of UHPC with Coal-Fired Bottom Ash Fine Aggregate

Enhancing the insulation performance of construction materials for improving building energy efficiency and addressing pollution caused by coal bottom ash (CBA), of which approximately 1.5 million tons is generated annually in Korea, making it a significant environmental issue. Although there are studies on using CBA to reduce the thermal conductivity of cement-based construction materials (CCMs) such as concrete and to promote sustainability in the construction industry, the resulting strength levels remain low. To address these challenges, this study experimentally evaluated the effects of replacing silica sand with CBA on the mechanical and thermal properties, as well as the microstructure, of ultra-high performance concrete (UHPC). The evaluation included measurements of density, compressive behavior, direct tensile behavior, and thermal properties, along with TG/DTG and MIP analyses. According to the experimental results, the use of CBA fine aggregate decreased the density, compressive strength, and direct tensile strength of UHPC. However, even when 100 % CBA fine aggregate was used, the resulting UHPC still exhibited a high compressive strength of 143–153.6 MPa. Additionally, the use of CBA significantly reduced the thermal conductivity of UHPC, and the 100 % CBA replacement variant showed a 49 % decrease in thermal conductivity compared to UHPC without CBA. Microstructural analysis further revealed that the substitution of silica sand with CBA increased the porosity of UHPC, but when the substitution rate exceeded 50 %, additional hydrates generated due to the pozzolanic reactivity of CBA contributed to a reduction in UHPC’s macro pores.