Influence of sulfur trioxide on volume change and compressive strength of eco-mortar

• When fly ash from a CFB co-fired boiler is mixed with water, it has a highly exothermic reaction, expansion of the particulate matter and a high pH value.
• Some producers of concrete use co-fired fly ash as a controlled low strength material (CLSM).
• The SO3 content might have been a factor that induced the volumetric expansion of the composite cement mortar.
• The volumetric expansion of the composite cement mortar might have been primarily induced by C-A-S‾-H and CaSO4·(H2O)8.

GGBS was mixed with circulating fluidized bed boiler (CFBB) co-firing fly ash (CFFA) to produce mortar, which was then subjected to compressive strength and volume stability testing. The test results revealed that the mortar composed of CFBB CFFA, and GGBS demonstrated improved strength; specifically, mixture ratio of 30%:70% by weight yielded the most optimal compressive strength (65%). When the CFBB CFFA exceeded 40%, apparent volume expansion and unstable compressive strength performance were observed. The SEM and XRD analysis revealed that C-A-S‾-H and CaSO4·(H2O)8 were generated; this result was attributed to the presence of excessive sulfur trioxide, causing volume expansion.