Carbonation and its effect on microstructure of concrete with fly ASH and ground granulated blast furnace slag

Carbonation induced corrosion of the embedded stee reinforcement is a major deterioration mechanism of reinforced concrete structures, mainly in inland / highway environments of tropical countries. Carbonation leads to uniform reduction in the cross sectional area of steel reinforcement. In general, Supplementary Cementitious Materials (SCMs) can increase durability of concrete. The current study evaluates the influence of three SCMs such as Ground Granulated Blast Furnace Slag (Slag), Class F fly ash and Class C fly ash on the carbonation of concrete, assessed by both macroscopic and micro-analytica investigations. The macroscopic behaviour of concrete was assessed using accelerated carbonation and natura carbonation tests. The micro-analytical studies were conducted to understand the alteration in the micro structure of concrete with SCMs under carbonation exposure. These tests include Scanning Electron Microscopy (SEM), X - Ray Diffraction (XRD) and Thermo Gravimetric / Differential Scanning Calorimetry (TG/ DSC). The results showed that depth of carbonation is more for concretes with SCMs. However, at lower replacement levels, the difference as compared to OPC is not much significant in the case of Slag and Class C fly ash. A relationship between natural carbonation and accelerated carbonation depth was proposed based on CO2 concentration and climatic influence (tropical regions). The microstructural modifications of the paste matrix due to carbonation are explained in terms of calcite formation, decalcification of CSH and change in porosity level.