Thermal analysis to assess pozzolanic activity of calcined kaolinitic clays

Abstract

The use of calcined clays as partial replacement of cement is encouraged since it promotes the reduction of the green-house gas emission and the energy requirement of cement-based material, maintaining or enhancing the mechanical properties and the durable performance of these materials. In this paper, the use of thermal methods—DTA/TG and calorimetry—to select and to evaluate two kaolinitic clays for their use as pozzolanic materials was explored. The content and crystallinity of kaolinite in clays can be determined by DTA/TG analysis, and this technique is also suitable to select the calcination temperature for complete kaolinite dehydroxylation. Calorimetric analysis on blended cements (30 % by mass of replacement) can differentiate the reactivity of calcined kaolinitic clays. Results show that more reactive calcined kaolinitic clay develops the second and third peaks earlier than those of PC with great intensity and high acceleration. The reactivity of calcined clays is associated to raw materials containing kaolinite with high structural disorder that determines calcined clays with large specific surface area, high grindability, and small mean particles size (d 50) for the same grinding objective. Finally, the DTA/TG analysis can determine the type and the amount of hydrated phases obtained at different ages to evaluate the pozzolanic reaction of calcined clay in accordance with the standardized pozzolanic activity index.