Reactivity of dealuminated kaolin and burnt kaolin using cement kiln dust or hydrated lime as activators

Abstract

Pozzolanic activity of dealuminated kaolin (DK) and burnt kaolinite clay (BK) was studied using cement kiln dust (CKD) or hydrated lime (CH) as activators. The different solid dry mixtures made of DK–CKD, BK–CKD, DK–CH, BK–CH, SF–CKD and SF–CH were hydrated in the suspension form, while paste hydration was done using DK–BK–CKD mix. Chemically combined water and free lime contents were determined at various ages of hydration and the hydrated samples were characterized using DSC, XRD and FTIR techniques. Combined water contents of the blends containing DK activated with CKD or CH are higher than those of mixes containing BK and SF activated with CKD or CH. Evidently, the suspensions containing DK possess relatively lower values of free lime contents than those of mixes containing BK and SF; this is due to the higher pozzolanic activity of DK as compared to BK and SF. The results of DSC and XRD analysis of suspension hydrated samples indicated the formation and later stabilization of calcium silicates hydrates (CSH) and calcium aluminosilicate hydrates (C3ASH4 and C2ASH8) as the main hydration products in addition to free calcium hydroxide (CH), which is completely consumed at 12h for mixes containing DK. Compressive strength and SEM examination were done for paste hydrated specimens made of mix IV (35% DK, 35% BK and 30% CKD) as an application. The microstructure of the hardened paste after 7days of hydration displayed the formation of nearly amorphous and microcrystalline CSH products as well as rod-like crystals of ettringite with a partial filling of the available pore spaces of the hardened paste. On prolonged hydration the microstructure displayed crumpled foils and dense structure of tobermorite-like CSH as the main hydration product which deposited in the originally water-filled space; this kind of CSH product is mainly formed as a result of interaction of active DK and BK with CH of CKD.