Mechanical properties and microstructure of ultra-lightweight cement composites with fly ash cenospheres after exposure to high temperatures

Abstract

This paper investigates the mechanical behaviour and micro-structure of a new type of ultra-lightweight cement composite (ULCC) using cenospheres as lightweight aggregates exposed to high temperature up to 900 °C. This type of ULCC material has a density less than 1400 kg/m3 and compressive strength up to 60 MPa and thus it has high compressive strength to weight ratio compared to other types of concrete materials. To prevent the spalling of ULCC material when exposed to high temperature, synthetic fibres are needed. In this paper, ULCC materials comprising eight different mixtures considering different contents of polypropylene (PP) fibres, steel fibres, hybrid fibres and fly ash replacement for cement are examined. The effect of fibre content, fibre types, fly ash replacement for cement are quantified in terms of their compressive strength, flexural strength, elastic modulus after exposed to elevated temperature. In addition, weight loss, failure modes, load-deflection and stress–strain curves are reported. Macro scale examination of the specimens was carried out to investigate the modification in the physical behaviour i.e. color changes, cracking and spalling of ULCC at various temperatures. Microstructural characterization of specimens was examined before and after exposure to temperature deterioration by using scanning electron microscopy (SEM). Results indicates that ULCC containing small amount of PP fibre can improve the fire resistance of ULCC and eliminate the explosive spalling behaviour of ULCC for temperature up to 900 °C. Hybrid fibres improves both fire resistance and ductility after elevated temperature exposure. Finally, recommendations are made in terms of the use of fibre mixes and fly ash replacement amount to achieve the desired structural performance of ULCC materials when exposed to fire.