Influence of Slag Composition and Temperature on the Hydration and Performance of Slag Blends in Chloride Environments

The use of GGBS as supplement for cements has been shown to improve the long-term strength and durability properties of concrete. In practice, while the chemical composition of GGBS from a single plant may be constant, due to the varying sources from which GGBS is obtained the chemical composition from plant to plant may vary. The wide variability in the use of GGBS as a SCM in different climates, coupled with differences in chemical composition, is bound to have impact on the performance of slag blends. This study investigated the combined influence of difference in slag composition and temperature on the performance of slag blends. Performance was evaluated in terms of strength and transport properties. Paste samples were characterised by calorimetry, TGA, XRD and SEM to follow hydration and microstructural development. Mortar samples were used to follow strength development and transport properties. All tests were carried out at temperatures of 20 and 38°C. Curing at 38°C accelerated the early hydration, but not the later hydration. This led to higher early strengths and lower later strengths, and was attributed to the coarsening of the pore structure caused by the high temperature curing. Except at the early ages at 20°C, both slag blends showed better strength performance than the reference cement. The slag blends also showed better transport properties than the reference cement, especially at 38°C, and this was attributed to their finer pore structure and higher chloride binding capacity. Of the two slags studied, the more reactive slag (slag 1) had better performance, especially at 38°C. Performance of the slag blends at 20°C was influenced mainly by the length of curing, and by the difference in chemical composition at 38°C

The effect of slag composition and curing duration on the chloride ingress resistance of slag-blended cements

This paper reports the influence of varying curing durations on the chloride ingress resistance of slag blends. Samples were prepared by combining two slags (S1 and S2) with CEM I 52·5R cement at 30% by weight of slag. Mortar samples were cured for either 7 or 28 d, before being exposed to a 3% sodium chloride solution. Samples were subjected to two chloride exposure conditions (either complete submersion or a cyclic 6 h wetting and drying cycle). Depths of free chloride penetration as well as total and water-soluble chloride contents were measured for the exposed samples. Samples cured for 28 d before exposure showed far better resistance to chloride ingress than those cured for 7 d. The cement–slag blend with S1, having an alumina content of 12·23%, performed better than the blend with S2 (alumina content of 7·77%), especially for samples cured for 7 d. This was attributed to the higher basicity of S1. However, when the samples were cured for 28 d, the difference between the performances of the slag blends became minimal. Despite this, the overall performance of the slag blends, which was compared against a CEM I 42·5R-type cement, was found to be better at both curing durations

Performance of plain and slag-blended cements and mortars exposed to combined chloride-sulphate solution

The durability of reinforced concrete structures exposed to aggressive environments remains a challenge to both researchers and the construction industry. This study investigates the hydration, mechanical properties and durability characteristics of ground granulated blast-furnace slag (GGBS) - blended cements and mortars exposed to a combined sodium chloride - sulphate environment, at temperatures of 20°C and 38°C. The conditions were chosen so as to assess the performance of slag blends under typical temperate and warm tropical marine climatic conditions. Slags, having CaO/SiO2 ratios of 1.05 and 0.94, were blended with CEM I 52.5R at 30% replacement level to study the influence of slag composition and temperature. Parallel control tests were carried out with CEM I 42.5R. Pastes and mortar samples were cast using 0.5 water to binder ratio, pre-cured for 7 days in water before exposure. Flexural strengths were determined once the samples were 7, 28 or 90 days old. Hydration was followed using x-ray diffraction (XRD), thermal analysis, and calorimetry. Also, sorptivity, gas permeability and chloride diffusion tests were carried out on mortar samples to measure transport and durability characteristics. The results show improved mechanical and transport properties for slag blended cements exposed to environments rich in sodium chloride and sulphate

Influence of slag composition and curing duration on the performance of slag blends in chloride environments

This study investigated the influence of chemical composition of Ground Granulated Blast Furnace Slag (GGBS) and curing duration on the performance of slag-cement blends; specifically strength and transport properties. Two slags (1 and 2) with Ca/Si ratios of 1.05 and 0.94 respectively, were used to partially replace 30% of a CEM I 52.5R cement. Various tests, including compressive strength, isothermal calorimetry, water absorption, sorptivity, and chloride ingress were conducted on mortar specimens to measure the performance of the slag blends against a CEM I 42.5R cement. The mortar specimens were pre-cured for 7 and 28 days before exposure to a 3% sodium chloride solution. Two different exposure conditions were studied (one in which the samples were submerged completely in the salt solution, and the other in which the samples were subjected to a 6 hr wetting and drying cycle). The results obtained showed similar performance for both slags at longer curing durations; with the more basic slag 1 exhibiting better transport properties than slag 2 for both exposure conditions at shorter curing durations. This suggests that curing duration as well as variation in chemical composition of slags affects the performance of slag blends in chloride environments

Improvement of deltaic lateritic soil using river sand and cement for use as pavement construction material

This study examined the effect of mechanical and chemical improvement on deltaic lateritic soils in Warri East in Delta State, Nigeria. Mechanical stabilization was carried out by adding river sand to the natural soil in various proportions, while chemical stabilization was carried out using cement and a mixture of cement and sand. Compaction and CBR tests were conducted on the natural soil before and after stabilization. From the results obtained, it was seen that the mechanical stabilization method improved the strength properties of the soil making it suitable for use as subbase materials, though not as much as the chemical stabilization method or the mixed method of stabilization. It was concluded that using a combination of cement and sand as a stabilizing agent for deltaic lateritic soils can lead to significant reduction in the amount of cement required for soil stabilization thus saving costs

Chloride binding and diffusion in slag blends: Influence of slag composition and temperature

This study has investigated the impact of a change in GGBS chemical composition on the chloride ingress resistance of slag blended cements under different temperature regimes. Two slags, having alumina contents of 12.23 and 7.77% respectively, were combined with a CEM I 52.5 R at 30 wt% replacement. Chloride binding and diffusion tests were conducted on paste and mortar samples respectively. All tests were carried out at temperatures of 20 °C and 38 °C. The higher temperature resulted in an increase in chloride binding; attributed to greater degrees of slag hydration. Despite this, chloride ingress was greater at 38 °C; attributed to changes in the pore structure and the chloride binding capacities of the slag blends. The more reactive, aluminium-rich slag performed better in terms of chloride binding and resistance to chloride penetration, especially at the high temperature and this was attributed to its higher alumina content and greater degree of reaction at 38 °

Resistance of Concretes to External Chlorides in the Presence and Absence of Sulphates: A Review

Corrosion of steel reinforcement due to chloride attack remains a major reinforced concrete durability concern. The problem is prevalent for concrete structures located within marine environments or frost-prone locations where chlorides containing de-icing salts are used. This paper is a state-of-the-art review into chloride binding in Portland cement concrete, with consideration of the differences induced by the presence of sulphates, such as found in seawater. The review also considers the use of supplementary cementitious materials (SCMs), the use of which has increased because of their potential to enhance durability and reduce the carbon footprint of concrete production. Such materials impact on phase assemblage and microstructure, affecting chloride binding and transport properties. Therefore, field and laboratory studies are critically reviewed to understand how these could help in the design of more durable concretes. The contributions of chloride binding, hydrate compositions and microstructures of the binding materials affecting chloride transport in concretes are also evaluated to suggest a more robust approach for controlling the problem of chloride attack

Expansion of CEM I and slag-blended cement mortars exposed to combined chloride-sulphate environments

This study investigates the effects of specimen curing duration, temperature, and slag composition on expansion of CEM I and composite slag-cement mortars exposed to a combined NaCl and Na2SO4 solution for up to 664 days. Test prisms prepared at 0.5 w/b ratio, were wet-cured for either 7 or 28 days prior to submersion in a combined salt solution at temperatures of 20 or 38 °C, to simulate temperate or warm tropical climates respectively. Equivalent reference specimens were stored in saturated limewater at 20 °C and tested in parallel. Mortar samples were used to investigate expansion and sorptivity, while corresponding paste specimens were prepared, cured and exposed under similar conditions for chemical and microstructural investigation. Such characterisation was performed on specimens immediately prior to exposure to salt solution and after the onset of expansion. The results show significant resistance to sulphate-induced expansion for specimens cured and exposed at 38 °C. For slag blends, the influence of exposure temperature was found to be more pronounced than curing duration. Differences in slag composition and curing duration also played key roles on the expansion resistance of mortar specimens. Expansion was attributed to the formation of ettringite crystals due to the reaction of aluminate phases of the binders with sulphate ions, although Friedel's salt and Kuzel's salt were also formed. The presence of chloride mitigated sulphate expansion of CEM I. For slag blends, it was shown that sulphate expansion was significantly reduced with increasing slag contents