Usage of blended cement and blended cement concrete has established their dominance and acceptability in the market thus establishing that in addition to strength, other properties such as impermeability, elastic modulus and resistance to corrosion, are critical parameters for long term durability and low life cycle cost.
Use of Mineral Admixtures such as fly ash and metakaolin as a blending component in OPC alters the hydration kinetics of the resultant cement with a decreased availability of the calcium hydroxide due to the secondary hydration reactions forming a much compacted, less pervious hydrated cement paste matrix. However, these aspects are a function of nature, fineness and pozzolanicity of mineral admixtures
At the authors laboratory, substantial work has been carried out to assess the efficacy of the different blended components through evaluation of the hydrated OPC and the blended cement pastes at different ages of hydration. The evaluations include analysis of free calcium hydroxide, mineralogy of the hydrated products, analysis of the unhydrated blending materials contents of the hydrates cement pastes at different ages of hydration viz. 1,3, 7, 28,60,90,180 days of hydration.
An attempt has been made to correlate the chemical, mineralogical and microstructural characteristics of the Low Lime Class-F fly ashes of varying amorphous content from different sources & Metakaolin with their pozzolanic reactivity.
Hydration studies have been carried out to assess the pozzolanicity of the Low Lime Class – F fly ashes of different sources & Metakaolin using a method evolved at the authors laboratory which monitors the kinetics of hydration of the blended cement pastes (made with 25% Fly ash under study ) and neat OPC pastes at different ages of hydration .
Based on the studies the authors rate the pozzolanicity of the different mineral admixtures and conclude that the differences in the amorphous contents of mineral admixtures result in a variability in the nature and content of the combined water (TGA/DTA), variability in Mineralogy (XRD) and Microstructure of the hydrated blended cement pastes indicating a difference in the stoichiometry of the CSH phases formed by the different mineral admixtures.