Compressive strengths of mortar cubes from hydrated lime with cofired biomass fly ashes

•Cofired biomass fly ash has 2–3 times strength of C and F, and 3–6 times that of wood, in its lime mortar.•Wood fly ash’s poorer mechanical performances might come from its large particle size and high unburned carbon content.•Mortar strength increases with ash percentage increase from 60%, 70% to 80% of the binder mass.•Elevated temperature increases the earlier but decreases the later ash mortar’s strength (23–63 °C).•All fly ashes’ one month strength reaches 60–80% that of their one year.

This document comprehensively compares strength of 5 cm mortar cubes made from binders of hydrated lime and cofired biomass fly ashes. Samples of cement only, with the same binder mass, provide controls and comparisons. Compressive strengths are tested from one to 12 months and samples were cured under carbonation free conditions. Four cofired (coal and herbaceous/woody) biomass fly ashes consistently develop two to three times that of Class C and F, indicating that cofired biomass fly ashes need more reevaluation as an admixture in concrete rather than exclusion by ASTM C 618 because of its “non coal” origin. However, the investigation does suggest that pure woody biomass fly ash may represent unacceptable pozzolans for use in concrete.