Influence of Fly Ash as Cement Substitution on Accelerated and Natural Carbonation of Concrete

The use of fly ash (FA) for the production of new concrete seems to be a promising solution for “greening” of construction industry. However, replacement of cement with supplementary cementitious materials such as fly ash influences concrete performance especially in terms of durability. One of major durability problems worldwide is carbonation-induced corrosion, given that a large number of infrastructural objects are exposed to a CO2-rich environment. The objective of this research was to analyse the influence of cement substitution level on accelerated and natural carbonation of concrete. Experimental program considered testing of 10 concrete mixtures selected in two groups – with water to binder ratio of 0,5 (400 kg/m3 of binders) and 0,6 (300 kg/m3 of binders), while in each group the cement substitution ratio was varied from 0% to 50%. Carbonation depth was measured after 14, 21 and 28 days of exposure to 2% CO2 in carbonation chamber, while the twin samples were exposed to natural carbonation. It was shown that accelerated carbonation depths were similar in both groups for mixtures up to 30% of FA, but they were doubled and tripled for larger replacement levels. Using the previously modified fib carbonation model for service life design, the prediction of natural carbonation was made. A reliability of proposed modification was assessed by comparison between predicted and measured values of natural carbonation after 19 and 34 months of exposure.