Microstructural Analyses of High Strength Concretes Containing Metakaolin at High Temperatures

Abstract

In this study, the effects of high temperatures on the mechanical and microstructural properties of high strength concretes (HSCs) containing metakaolin (MK) are investigated. For this purpose, the concrete mixtures containing MK were produced with a water-binder ratio of 0.2. The mechanical properties of these concretes at 25, 250, 500 and 750 °C temperatures were determined. Besides, the effect of high temperature on the microstructural properties of cementitious matrix, interfacial transition zone between cement and aggregates, and aggregates of concretes were inspected by X-ray diffraction, scanning electron microscope and plane polarized transmitted light (PPTL) analyses. The results indicate that the ultrasound pulse velocity (U pv), compressive strength (f c), flexural strength (f fs) and splitting tensile strength (f sts) values of these concretes decrease with the increasing of the high temperature especially after 250 °C. The heated concrete specimens were also examined at both macro- and micro-scales to determine the discoloration, alteration and cracks of HSC at different temperatures. PPTL analyses show that increasing temperature causes impairing of interfaces between aggregate particles and cementitious materials. The results also show that the partial replacement of 10 % MK with cement has the best performance on the mechanical properties of HSC.