Quantification of hydrated cement products of blended cements in low and medium strength concrete using TG and DTA technique

Abstract

Pozzolanic reaction of blended cements cause pore refinement of microstructure of concrete and reduces the permeability. Different factors can influence the extent of reaction in concrete, such as type of cement, water–cement ratio, cement content, hydration time and type and replacement level of mineral admixture. Among them, the hydration time is the most important factor on the stability and the transformation of hydrated phases. In the present study using thermogravimetric analysis (TG/DTA), the extent of pozzolanic reaction of two blended cements such as Portland pozzolana cement (PPC), Portland slag cement (PSC) has been studied in low (20 MPa) and medium strength concrete (30 and 40 MPa). Three hydration times namely 28, 90 and 365 days were studied. Ca(OH) 2 and other calcium hydrates content were estimated. The study reveals that in addition to hydration time, cement content and water–cement ratio in proportioning the concrete mixes also play an important role in determining the pozzolanic reaction. Reduction in calcium hydroxide, Ca(OH) 2, content in PPC and PSC concrete confirms the occurrence of pozzolanic reaction. In 20 MPa concrete which is having low cement content has maximum calcium hydrate content than 30 and 40 MPa concrete. This is because of its permeable pore structure which facilitates more pozzolanic reaction to occur whereas in the case of 30 and 40 MPa concrete, the higher cement content and more dense pore structure restrict further reaction to proceed and thus reduce the formation of calcium hydrates in the pore structure. The water permeability studies at three curing periods viz. 7, 28, and 90 days also reveals that, in PPC and PSC concrete, the co-efficient of water absorption is less than OPC concrete in all the concretes studied and thus confirms that the reduced permeability of PPC and PSC concretes is mainly because of densification of pore structure by forming additional calcium hydrates.