Resistance of alkali-activated slag concrete to acid attack

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Abstract

This paper presents an investigation into the durability of alkali-activated slag (AAS) concrete exposed to acid attack. To study resistance of AAS concrete in acid environments, AAS concrete was immersed in an acetic acid solution of pH=4. The main parameters studied were the evolution of compressive strength, products of degradation, and microstructural changes. It was found that AAS concrete of Grade 40 had a high resistance in acid environment, superior to the durability of OPC concrete of similar grade.

Introduction

There is a need in the development of chemically resistant materials for the use in the aggressive environment of food manufacturing, chemical, mining, mineral processing, and other industries. Acid resistance is one of the required properties for structural materials used in these applications. It is known that Portland cement concrete does not posses a high acid resistance due to its chemical composition that includes high-calcium compounds.

Durability of alkali-activated slag (AAS) concrete is being investigated because it was reported to have a superior durability in aggressive environments as compared to OPC [1], [2], [3], [4]. It contains significantly less Ca in its composition and has a very low permeability, and thus is expected to be more durable than OPC in an acid environment. In the previous papers, the durability of AAS concrete when exposed to a sulfate environment and carbonation was studied [5], [6]. AAS concrete had a superior durability to OPC in sulphate attack but was more exposed to carbonation. This paper presents the study of the durability of AAS concrete produced using Australian slag when exposed to low pH environment of acetic acid.

Section snippets

Materials

The chemical composition and properties of slag are summarised in Table 1. The blast furnace slag is a granulated product ground to fineness of about 460 m2/kg, with the particle size range of 1 to 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. The chemical composition and properties of ordinary Portland cement used in OPC concrete preparations are also detailed in Table 1.

AAS concrete was prepared using

Results

The photographs of the AAS and OPC concrete cylinders exposed to the acid solution for 2 months are found in Fig. 1a and b. The AAS specimens had no change of their appearance and had a small increase in mass. The OPC samples, on the contrary, had soft white depositions on the surface, softening of concrete, a significant increase in the mass of samples if measured with the white deposits (up to 20 g/kg), and a loss of mass (about 8 g/kg) if measured with the deposits removed.

The compressive

Discussion

AAS concrete samples had some softening of the surface layer when exposed to the acid solution, but the damage was significantly less than in OPC concrete. AAS concrete had less strength loss and depth of concrete layer with the pH reduction and also less visible signs of deterioration than OPC concrete. Therefore, AAS concrete performed better than OPC concrete when exposed to acid solution.

The action of acids on cement paste consists in an attack on the components of the hardened cement

Conclusions

The investigation of durability of AAS concrete examined the performance of AAS concrete exposed to acetic acid solution. It was found that AAS concrete of Grade 40 has a superior durability to OPC concrete of similar grade under acid attack. The mechanism of deterioration involves decalcification of C-S-H and formation of soluble salt calcium acetate. Slag paste was found more resistant to deterioration in acid solution than OPC paste.

Acknowledgements

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

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