Chemistry of Composite Cements; Interaction between Blast Furnace Slag and ‎Natural Pozzolan in Portland Cement Mixtures

Composite cement has several technical, environmental, and economic advantages. Thus, we investigated the properties of a new composite cement composed of four components, including ground granulated blast-furnace slag from Isfahan ‎Steel Co., pozzolan from Rafsanjan Mine, Portland cement from Sepahan Cement Co., and ‎limestone. The chemical and physical properties and activity of the above components ‎were determined. Eight composite cements were prepared and their physical and chemical ‎properties were characterized. After compressive strength test, measurement of flowability and setting time, a composite cement made of 20% slag, 10% pozzolan‎, and 2.5% limestone was selected for additional investigations. After 120 days of curing, SEM, XRD, and TGA testing on cement pastes made of the proposed blend, the control cement, ‎Portland-blast-furnace slag cement (30% slag), and Portland-pozzolan cement (30% pozzolan) were investigated. The blast-furnace slag improved ‎the long-term compressive strength of the composite cement, provided a greater flow to the mortar, ‎and displayed less permeability. In contrast, the pozzolan offered essential benefits, including proper initial strength, suitable setting time, and good grind-‎ability. The standard quality tests do not evaluate the durability of concrete containing a composite cement, while our study is based on the hydration reactions that occurred in the composite cement for four months. The XRD results indicated that the slag had few ‎crystals, whereas numerous crystals were observed in the pozzolan, potentially reducing the ‎compressive strength in the composite cement. The experimental results revealed that the slag and pozzolan positively affect the properties of composite cement and improve its capabilities.‎