Partial substitution of Portland cement (PC) with fly ash (FA) offers the practitioner expanded scope for optimal utilisation of materials. This method requires careful evaluation of the resulting blended cement beyond the basic age-strength relationship. For the construction practitioner, workability, rate of strength gain, and inherent variability are major factors in meeting strength requirements and determining costs. A blend may be characterised by a mass efficiency factor which is the mass ratio of PC to FAwhich will yield the same target strength at fixed maturity and workability. Alternatively, the maturity efficiency factor may be determined as the ratio of log maturity values for the cements to reach a given target strength. Both efficiency factors incorporate important economic parameters of interest to the practitioner.
Because of its glassy character, FA attracts calcium hydroxide as well as sodium and potassium ions released into solution by the PC component. The risk of alkali-aggregate reaction may therefore be reduced. This characteristic may prove to be of such over-riding importance that a new approach to mix design may become necessary. The essence of such an approach would be the provision of sufficient FA to absorb reactive species released by the PC. This might involve increased PC+FA and water, with concomitant reduction in aggregate and in fineness modulus.
Dimensional changes such as shrinkage and creep affect the distribution of load between steel and concrete in columns, the deflection of beams and slabs, and cracking where displacements are restrained. In the case of PC concrete, both creep and shrinkage depend on the volume ratio of hydration products and the movement of pore water caused by drying and by mechanical load. In general the magnitude of creep and shrinkage varies with Vp, the volume proportion of cement paste, and reduces with wo*. Part substitution of PC with FA generally results in increase of Vp, but this is compensated by decrease in wo*. The nature of the hydration products varies with PCiFA ratio, and thus quantities such as the internal surface area, the hydraulic mean radius of pores and the effective modulus—the ratio of stress to total strain—must be evaluated.
In general the data required by the practitioner is not available in the literature and he must have recourse to ad hoc tests in order to establish basic workability-time-temperature-strength data. This is, potentially, a fruitful area of research.