Evaluation of coal combustion residuals in cement systems

Landfilled coal combustion residuals (CCRs) from a single coal-fired power plant in Colorado, USA, raised concerns over leaching trace metals into groundwater. This study investigated the stabilization and beneficial use of these CCRs, specifically fly ash (CLFA) and bottom ash (CLBA), in portland cement concrete. CLFA was evaluated as a supplementary cementitious material (SCM) at a 20% cement mass replacement rate, while CLBA was evaluated as a 20% partial fine aggregate replacement. Stabilization was assessed by relating trace metal concentrations in CLFA and CLBA to early-age cement hydration kinetics and 63-day monolith leaching behavior of CCR-amended concrete. Lithium concentrations were low in both CLFA and CLBA (17.3 and 8.65 mg/kg, respectively), whereas arsenic, vanadium, and barium exceeded risk-based residential screening levels. Isothermal calorimetry indicated that trace constituents did not significantly alter early-age cement hydration kinetics under the test conditions. Moreover, CCR-amended concrete mixtures did not exhibit higher cumulative trace metals release than control mixtures without CLFA or CLBA over the 63-day monolith leaching test period. To assess the beneficial use, CLFA was evaluated for pozzolanic reactivity, while concrete mixtures containing CLFA and CLBA were assessed for compressive strength and electrical resistivity. CLFA was found to be pozzolanic, though less so than conventional fly ash. Concrete made with CLFA and CLBA achieved 28-day strengths of 5400-7150 PSI and 56-day strengths of 6160-7660 psi. Overall, these results suggest acceptable performance and environmental stability at the tested replacement rate, although longer-term testing is still needed.