Ultrasonic characterization of the curing process of PCC fly ash–cement composites

The effects of the type of fly ash, mix proportion, and curing process on the pozzolanic reaction of fly ash–cement paste were investigated by ultrasonic techniques. Specifically, the speed of sound (SOS) and broadband ultrasonic attenuation (BUA) were used to investigate hydration activities of the fly ash–cement composite. SOS provided direct evidence of the delay in the hydration activity caused by mixing fly ash to the cement. The rapid heat of evolution during hydration activity, as indicated by a rapid increase in SOS, resulted in early stiffening of the Class C fly ash–cement composite. However, Class C fly ash–cement composite achieved a lower elastic modulus compared with Class F fly ash–cement composite. The hydration activity is observed to be highly dependent on the type of fly ash substituted for cement in the composite. The BUA provided the indirect evidence of ionic activities occurring during the hydration period and viscoelastic properties of the material.