Effect of admixtures on properties of alkali-activated slag concrete

This paper reports the results of an investigation on concrete that incorporated alkali-activated slag (AAS) as the only binder. The activators were liquid sodium silicates (4-7% Na, mass of slag) and a multi-compound activator (NaOH + Na2CO3) (8% Na, mass of slag). AAS utilizes industrial by-products and develops high early strength. However, some of its properties such as high shrinkage and poor workability impede its practical application. Admixtures used for ordinary portland cement (OPC) were tested to improve these properties of AAS concrete. Superplasticiser based on modified naphthalene formaldehyde polymers (S), air-entraining agent (AEA), water-reducing (WRRe), shrinkage-reducing (SHR) admixtures at dosages of 6-10 ml/kg, and gypsum (G) (6% of slag weight) were used. The paper presents the study of workability in the fresh state, shrinkage and compressive strength of AAS concrete, and the effect of admixtures and type of activator on these properties. Concrete activated by liquid sodium silicate had the best mechanical properties. AEA, SHR, and G significantly reduced its shrinkage. AEA also improved workability and had no negative effect on compressive strength. On the basis of this investigation, AEA was recommended for use in AAS concrete. (C) 2000 Elsevier Science Ltd. All rights reserved.