Development of low thermal conductivity UHPC and characterization of its mechanical properties and microstructure

초록

Improving the insulation performance of cement-based construction materials (CCM) is crucial for enhancing the energy efficiency of buildings and reducing CO2 emissions in the construction sector. This study aimed to develop ultra-high-performance concrete (UHPC) with low thermal conductivity and excellent mechanical properties by using coal bottom ash (CBA), a by-product of coal-fired power generation, as fine aggregates. The effects of replacing silica sand with CBA on the density, compression behavior, tensile behavior, fiber bond behavior, and thermal properties of UHPC were evaluated. Additionally, TG/DTG, MIP, and SEM analyses were conducted to investigate changes in the microstructure of UHPC. Experimental variables included CBA fine aggregate substitution rates of 0, 25, 50, 75, and 100 %, and fiber incorporation at 0 and 1.5 % by volume. Results showed that UHPC density decreased by up to 11.38 %, while compressive strength decreased by up to 11.41 % with increasing CBA substitution. The incorporation of 1.5 % steel fiber increased compressive strength by about 8.9 %, maintaining a high compressive strength of 143-153.6 MPa even with 100 % CBA substitution. Although CBA substitution reduced the precision of elastic modulus predictions, the introduction of the aggregate coefficient improved prediction accuracy. For tensile behavior, CBA substitution significantly reduced tensile strain capacity and strain energy density rather than tensile strength. Thermal diffusivity and conductivity decreased significantly with higher CBA substitution rates, showing a 48-52 % reduction for 100 % CBA substitution. CBA substitution increased micropores, mesopores, and entrained air but reduced macropores above 50 % substitution due to additional hydrate production from CBA's pozzolanic reaction.

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