Properties of High Performance Concrete with Supplementary Cementitious Materials

Abstract

The aim of this research was to evaluate the performance of high performance concretes (HPC) containing supplementary cementitious materials in both binary and ternary systems. Concretes were prepared to have a slump between 60-90 mm at a constant water-binder ratio of 0.30. The test variables included the type and the amount of supplementary cementitious materials (SCMs) such as silica fume (SF), class F fly ash (FA) and ground granulated blast-furnace slag (GGBS). Portland cement was replaced with FA up to 40%, silica fume up to 15% and GGBS up to a level of 70%. The physical properties were assessed from the compressive strength and transport properties (air permeability and sorptivity), whilst the durability characteristics were investigated in terms of carbonation, chloride diffusion, chloride migration, electrical resistivity and salt- scaling resistance. The hydration properties of cement pastes containing SCMs have also been investigated in order to understand the behaviour of HPC. For the hydration properties, cement pastes were prepared in two series (A and B). Mixes of series A had a fixed water-binder ratio of 0.30 with superplasticiser, whereas the water-binder ratios of mixes in series B were equivalent to the standard consistencies. The results confirmed that silica fume performed better than other SCMs for the strength development and bulk resistivity. In terms of the resistivity of concrete, the best performance was obtained for 15% SF concrete. When 7.5% SF was used in 40% FA and 50% GGBS concrete, the resistivity was less than that of the 15% SF concrete. However, in both cases, this was greater than the control concrete. The ternary mixes containing GGBS/FA and SF performed the best amongst all the mixes to resist the chloride diffusion. The mix containing FA showed favourable permeation results. All the ternary combinations can be considered to have resulted in high performance concretes with excellent durability properties. The maximum carbonation depth observed for concrete containing 40% FA and 7.5% SF was about 5 mm, which is less than the cover of reinforcing steel bars to cause corrosion. The non-steady state migration coefficients of concretes containing different types of ivSCMs were significantly lower than that of the control concrete. The coefficient was minimum for concrete containing 15% SF. The scaled mass for 40% FA concrete was the maximum among all the mixes. For concrete containing GGBS, there was no significant change in the scaled mass, when compared to the control concrete. However, the use of 15% SF was found to reduce the scaled mass. The use of 7.5% SF in 40% FA and 50% GGBS concrete was found to reduce the scaled mass. The X-ray diffraction (XRD) data showed that with the addition of high volumes of FA, GGBS and SF the peak intensity of Ca(OH) 2 was retarded and quartz peaks appeared in paste containing FA denoting a certain amount of crystalline silica in FA. This was more pronounced at later age. The combination of 40% FA and 7.5% SF showed the highest consumption of Ca(OH) 2 . Thermo-gravimetry (TG) data confirms that the addition of SCMs in cement resulted in the formation of decreased amount of Ca(OH) 2 in the hydration products considerably at later age when compared to the early age. Although the addition of high volumes of SCMs decreased the compressive strength of cement pastes particularly at the early age, the use of 15% SF improved the strength. However, from the cost point of view, t is suggested that a combination of 40% FA and 7.5% SF or 50% GGBS and 7.5% SF can be beneficially used to improve the hydration properties and compressive strength of cement paste.