Wood bottom ash as sand replacement in concrete: Mechanical properties and prediction of durability

The global issue of sand shortages require research in alternatives to natural sand as fine aggregate in concrete. Bottom ash from fluidized bed boilers are promising candidates for application as a partial substitution. This study investigates the mechanical properties of concrete with 0 %, 25 % and 50 % wood bottom ash substituting natural sand. Three wood bottom ashes from Denmark were tested: two from a circulating fluidized bed (CFB) boiler, and one from a bubbling fluidized bed (BFB) boiler. Characterization of the ashes was done by XRF, XRD and X-ray tomography, as well as measurements of density and absorption. Concrete cylinders are tested for their compressive strength and E-modulus at 14, 28 and 180 maturity days. To make an estimation of the durability, the electrical resistivity was measured on the specimens before each mechanical test. Characterization of the ash indicated limited reactivity, despite a high content of SiO 2 . X-ray analysis showed that many of the particles had developed cracks and internal pores during the combustion process. The effect on the mechanical properties of the concrete appeared to be minimal, as the compressive strength of the specimens with CFB and BFB was increased compared to the reference mix with 100 % natural sand. The exception was the mixes with 25 % and 50 % BFB ash, which developed cracks between the tests at 28 days and 180 maturity days. The E-modulus of concrete decreased with higher ash contents.