Effect of fly ash blending in internally cured mortar under air-drying curing conditions

Abstract

[Abstract]: A proper humid curing of concrete structures is commonly considered as vital to guarantee their strength development and durability. However, undesired periods of air-drying curing conditions can take place. Therefore, the performance of concrete in this situation should be studied. This research work aims to better understand how internally cured fly ash blended cement mortars are affected by air-drying curing conditions. Coal bottom ash (CBA) and wood bottom ash (WA) were used as internal curing water reservoirs. Results show a poor strength development after the age of one week. Moreover, mortar specimens lose a considerable amount of weight during their first days of curing due to water evaporation. This severe drying, together with a long-term carbonation (depth, weight gain and shrinkage), affects the fly ash (FA) blended cement mortars more than the ordinary Portland cement (OPC) mortars. Furthermore, a pore structure analysis was carried out, showing that FA-blended mixtures present a higher total porosity and internal curing cannot effectively refine the pores in the matrix when OPC is the only binder. Self-desiccation was detected in combination with external drying thanks to a comparative study between weight changes and shrinkage. Internal curing can mitigate the self-desiccation shrinkage but increases the drying shrinkage. Moreover, the presence of the waste porous aggregates counteracts the internal curing effect decreasing the strength, and barely affects the carbonation process. As a general conclusion, it can be stated that air-drying curing conditions should be avoided and external wet or sealed curing should always complement internal curing, especially if blended cements are used.