Durability of slag-blended cements in composite chloride-sulphate environments

Field: Engineering (Physical Sciences)

The problem of concrete durability in marine environments remains a major challenge for the construction industry. Chlorides and sulphates from sea water attack both the steel reinforcing bars and the concrete binder respectively. Chloride attack leads to steel corrosion, while sulphate attack leads to the formation of expansive ettringite. These challenges, combined with pressures to reduce CO2 emissions associated with conventional Portland cement production, have encouraged the increasing use of supplementary cementitious materials (SCMs). Ground granulated blast-furnace slag is one of the most widely used SCMs, since it offers the potential for the greatest replacement in cement clinker. However, the effects of chemical composition, temperature, slag loading, curing and exposure conditions, concerning changes to microstructure, mechanical strength and durability performance of slag-blended cements are yet to be fully understood. This situation is worsened in marine environments, by the limited information on the combined attack of concrete by chloride and sulphate. This is important, since these ions co-exist in real marine conditions. \nThe present study combines different experimental techniques to investigate the above stated effects on hydration, microstructure, mechanical and transport properties, including chloride binding, to provide improved understanding to the existing literature. The relationships between hydration, microstructure and durability performance have been highlighted, along with chloride binding. \nTwo slags of different chemical compositions (CaO/SiO2 ratios = 1.05 and 0.94), designated as slags 1 and 2, were each blended with CEM I 52.5R at 30 and 70 wt.% replacements to produce 4 blends. Paste and mortar samples were prepared at a constant w/b ratio of 0.5. Reference samples were prepared at w/c of 0.5 using CEM I 42.5R. The pastes were characterised for chemical and microstructural properties, while mortars were used for investigating mechanical and transport properties. Tests were performed under parallel temperatures of 20 and 38°C to reflect temperate and warm tropical climates. The samples were exposed to combined sodium chloride and sulphate, after curing in water for 7 or 28 days. \nHydration kinetics were investigated in paste systems using isothermal conduction calorimetry. Crystalline hydration products and phase assemblages were followed by x-ray diffraction (XRD), complemented with simultaneous thermal analysis (STA), to confirm and quantify the phases formed, including chemically bound water. The degrees of slag and clinker hydration were quantified using scanning electron microscope (SEM), coupled with energy dispersive x-ray (EDX) analysis. SEM-EDX spot analysis was also used to characterise poorly crystalline, calcium silicate hydrate (C-S-H). \nMicrostructural development was followed using SEM backscattered electron (BSE) image analysis. This was also used to quantify the paste porosity, which was then complemented with mercury intrusion porosimetry (MIP). Mechanical properties of mortar samples were investigated using compressive and flexural strengths. \nTransport properties were investigated using water sorptivity and gas permeability in mortar samples. Chloride penetration profiles and non-steady state diffusivity were investigated in mortar prisms, including free chloride penetration depths, using colorimetric approach. Also, chloride and sulphate penetration profiles were investigated in polished paste samples, using SEM-EDX spot analysis. This included analysis of atomic ratios to identify the phases binding chloride and sulphate, and their intermixing with the C-S-H. Chloride binding with and without the presence of sulphate, were investigated in paste samples. \nLength and mass change due to sulphate attack were investigated in mortar prisms and cubes respectively. Samples were exposed in combined chloride-sulphate solution by submersion or repeated wetting/drying cycles, for a period of 664 days. \nThe results show a positive influence of elevated temperature for the slag blends, leading to a refined microstructure, improved early age strengths and improved resistance to the transport of fluids, including chloride and sulphate. The presence of the combined salt solution led to increased flexural strength. Transport properties were improved during early stages of exposure to salt solution but worsened over longer periods. The developed multiple regression models reasonably predicted changes in mechanical and transport properties, considering the effects of temperature and slag loading. Length change and mass change reduced significantly at elevated temperature. Also, chloride binding was improved at elevated temperature but decreased in the presence of sulphate. The main phases binding chloride include Friedel’s salt, Kuzel’s salt and C-S-H, while sulphate was bound in ettringite, AFm and C-S-H. Generally, within the period of this study, there was a synergy between chloride and sulphate, as sulphate expansion was reduced, while chloride diffusivity was also reduced at the same time. The greatly improved durability properties of the slag blends at 38°C is significant for their application in warm climates. \n