Carbonation of fly ash concrete

Abstract

The carbonation of fly ash concrete is reported, with particular emphasis on the role of curing. Concretes with nominal strength grades C25, C35 and C45 and a range of fly ash levels (0–50%) were exposed to various treatments during the first 28 days; i.e. different moist curing periods anda range of ambient temperatures and relative humidities after curing. After28 days the concretes were stored either internally or externally (sheltered) and the rate of carbonation was monitored. The results emphasize the importance of adequate curing for concrete durability, irrespective of the presence of fly ash. In some cases increasing the initial curing period from 1to 7 days had the effect of reducing carbonation by 50%. Concretes with up to 30% fly ash carbonated to a similar or slightly greater degree compared with OPC concretes of the same strength grade. However, concretes containing 50% fly ash carbonated at significantly greater rates. A graphical model is presented which allows prediction of the carbonation rate to be made from a knowledge of the concrete mix (strength grade and fly ash content), degree of curing, ambient conditions during and after casting, and type of exposure.