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Effect of elevated temperature curing on properties of alkali-activated slag concrete

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Abstract

This investigation is focused on the effect of curing temperature on microstructure, shrinkage, and compressive strength of alkali-activated slag (AAS) concrete. Concrete prepared using sodium silicate and sodium hydroxide as the activator had greater early and flexural strength than ordinary Portland cement concrete of the same water/binder ratio, but it also had high autogenous and drying shrinkage. Heat treatment was found to be very effective in reducing drying shrinkage of AAS concrete and promoting high early strength. However, strength of AAS concrete at later ages was reduced. Microstructural study revealed an inhomogeneity in distribution of hydration product in AAS concrete that can be a cause of strength reduction. Pretreatment at room temperature before elevated temperature curing further improved early strength and considerably decreased shrinkage in AAS concrete.

Introduction

Use of alkali activated slag (AAS) in concrete manufacturing has environmental benefits because its production requires less energy than ordinary Portland cement (OPC) and utilises industrial by-products. AAS concrete has superior durability in aggressive environments compared to OPC 1, 2, 3, 4. However, it was found that AAS has considerably higher shrinkage than that of OPC concrete [5]. Previous investigation showed the effect of admixtures on workability, strength, and shrinkage of AAS concrete [6]. It was found that admixtures developed for OPC are not effective in the case of AAS and have some side effects such as reduced strength. The focus of the current investigation is on study of the effect of heat treatment. The aim is to determine how curing temperature influences shrinkage and compressive strength of AAS concrete. Compared with OPC, AAS has a benefit of early strength development. This property is important for precast concrete manufacturing because it permits a short cycle. Curing at elevated temperature may further accelerate strength development in AAS concrete. The study on OPC concrete showed that heat treatment provided significant improvement in volume stability [7]. Thus it can be expected that AAS will have a benefit of low shrinkage.

Section snippets

Slag

The chemical composition and properties of the slag used are summarised in Table 1. The blast furnace slag (SteelCement Ltd., Port Melbourne, Australia) is a granulated product ground to fineness of about 460 m2/kg, with the particle size range of 1–10 μm, and is neutral with the basicity coefficient Kb = (CaO + MgO)/(SiO2 + Al2O3) equal to 0.93. The slag is supplied with 2% blended gypsum.

Activators

The activators investigated were liquid sodium silicate (PQ Australia Ltd., Dandenong, Victoria,

Results

Table 4 shows the results of strength measurements of AAS and OPC concretes cured at room temperature. AAS exceeds OPC in flexural and compressive strength at early age; however, shrinkage of AAS exceeds by 2 to 2.2 times that in OPC concrete.

Strength

The observed initial rapid strength development of heat-cured AAS concrete and its slow later growth are similar to results obtained for OPC Fig. 1, Fig. 2 and are reported earlier [11]. For AAS the later strength was significantly reduced. It is correlated with an inhomogeneity of microstructure observed in heat-treated AAS. Similar to OPC, slow strength development in AAS at elevated temperatures was attributed to inhomogeneity of the microstructure, localisation of hydration product near

Conclusions

  • 1.

    Heat treatment considerably accelerates the strength development of AAS concrete, but at later ages compressive strength of the materials is reduced compared with concrete cured at room temperature.

  • 2.

    Heat curing considerably reduces shrinkage of AAS concrete, making it comparable with shrinkage of OPC concrete.

  • 3.

    Compared to OPC heat-treated AAS has a more open microstructure, with hydration products localised near slag grains and relatively open interstitial spaces.

  • 4.

    The heat treatment history has an

Acknowledgments

The financial support for this project was provided by Independent Cement and Lime Pty Ltd, Blue Circle Southern Cement Ltd, and Australian Steel Mill Services. The efforts and assistance with the laboratory work provided by Jeff Doddrell, Roger Doulis, and Peter Dunbar are also gratefully acknowledged.

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