Calcined natural clays: Performance evaluation as cementitious material

Abstract

This study was done to evaluate technical feasibility of using unrefined, relatively low quality clay materials for the production of industrial pozzolan based on calcined clay. The ultimate objective is to provide a greater degree of flexibility regarding the range of cementitious materials that can be incorporated in cement. In the first part of the study, a wide variety of clay material samples containing different clay types and amounts have been received to evaluate their suitability for calcined clay production. These samples were examined visually and by X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD). The most interesting samples were calcined under controlled laboratory conditions and their performance evaluated in cement paste and mortar. The test results clearly showed that highly reactive pozzolan can be produced from unrefined clays and its performance is related to chemical and mineralogical composition of the raw material. The influence of calcination conditions on performance of the calcined natural clays has been investigated in the second part of the study. For this purpose, four previously investigated clay materials have been selected and calcined under different conditions (temperature, residential time). The calcined clay samples were examined by XRD and their performance again evaluated in cement paste and mortar. When mixture of clay minerals is present, the optimal calcination temperature can be sometimes hard to identify as the compromise has to be made between compressive strength development and water demand. This is especially true for raw materials containing swellable clays. The optimal conditions for calcining the mixtures of various clay minerals are discussed in the light of existing guidelines for calcination of pure clay minerals and results of our study.