Hydration of Blended Cement with Halloysite Calcined Clay

The effects of calcined kaolinitic clays as supplementary cementitious materials (SCMs) on the performance of pastes and mortars have been well studied. Less attention has been paid to the thermal transformation of halloysite than that of kaolinite and its possibility to be used as SCMs. Halloysite and kaolinite have identical chemical composition, except that halloysite may have two molecules of H2O, as interlayer water. The content of additional water in the interlayers of halloysite has a decisive influence on the crystal morphology, which is generally curled rather than platy as in kaolinite. Common forms are elongated tubes and spheroids. The aim of this investigation is to study the hydration of blended cements with 25% of different calcined clays to evaluate the influence of the content and the morphology of halloysite in the development of the hydratation compounds, and compressive strength of mortars. Three clays with different halloysite/kaolinite content, and different morphology were analyzed. The hydrated phases present in pastes at 2, 7, and 28 days were identified by X-ray diffraction (XRD), and the content of CH by differential thermal analysis (DTA/TG). The compressive strength of mortars was tested at 2, 7, and 28 days. The pozzolanic reactivity of the calcined clays was influence by the kaolinite content and morphology of halloysite in natural clays. This results in different crystalline and amorphous aluminic phases obtained at different ages, and that the ensemble results differ, this affects the porosity and the compressive strength.