Cementos

A Study of Rheological and Mechanical Properties of Mixed Alkali Activated Slag Pastes
Cheng Qing-Hua and ShondeepL. Sarkar
Department of Civil Engineering, Faculty of Applied Sciences, Universit6 de Sherbrooke, Sherbrooke, Qu6bec, Canada

Slag, a by-product of the iron/metaUiferrous industry, is known for its latent hydraulicity. This research is directed to a study of alkali activation ofslag, using a combination of sodium silicate or water glass (Na2SiO 3) and slaked lime. The mechanical and rheological properties of these activated slag pastes are reported. The mechanism of mixed activation is described, and the factors responsible for enhanced reactivity, high strength, and unusual rheology are discussed. ADVANCEDCEMENTBASEDMATERIALS1994, 1, 178184 KEY WORDS: Alkali activation,Compressive strength, Curing, Hydration, Lime, Pat area, Setting time, Slag, Waterglass

lag, a nonmetallic by-product of the iron/ metalliferrous industry, is basically composed of calcium and magnesium (alumino) silicates and glass. Although it is extensively used in blended cement and concrete, the strength development of slag by itself is relatively slow. Despite this inadequacy, severalother intrinsic advantages of using slag either as a blending component in cement or as a mineral admixture in concrete are well documented [1,2]. Past and ongoing research has shown that the potential hydraulicity of slag can be considerably enhanced by alkali activation [3]. Increased energy costs call for a reevaluation of many existing technologies insofar as utilization of industrial by-productsand wastes is concerned, not to mention the search for less expensive types of binders, which has evoked a new interest in this type of material [3]. A binder is needed that can partially or totally replace an energy-intensive material such as Portland cement. Alkali activation of slag has been in practice for over three decades [4]. The first reported practical application was in Ukraine in 1958[5]. Recently it was demonstrated [6] that alkali activated slag concrete (AASC)

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is better than Portland and high alumina cement for immobilization of radioactive wastes; AASC-based waste form exhibits lower Cs + leachability. According to Metso and Kajaus [7], slag can be readily activated by different inorganic compounds, such as Portland cement clinker, lime, sodium carbonate, gypsum,and water glass. Of these, alkalies still hold the greatest potential as activation agents [3], a view recently endorsed by Wu et al. [8]. Nevertheless, the properties and mechanism of these products are still not fully understood. It has been suggested that alkali activation contributes to the production of more C-S-H and other hydrates, zeolites for example, that promote increased bonding andcompactness in the microstructure [9]. Gehlenite hydrate is reportedly formed if sodium hydroxide is used, whereas lime activated slag produces several calcium aluminohydrates. Mixed activation using gypsum and lime or gypsum and soda has been theorized [10], and the results have been summarized by Regourd [9]. However, a combination of water glass and lime is less reported [11], although lime playsa dual role: it improves the mix homogenization and thus its workability [12], in addition to being an auxiliary activator in the presence of water glass [13]. In this context, the present investigation reports the results of the influence of different proportions of water glass and lime (as activators) on the mechanical and rheological properties of slag pastes. Based on the findings, furtherinsight on the activation mechanism is also offered.

Experimental Methods
Although the chemical composition of slag depends on the materials introduced into the blast furnace, its compositional range of variation from an individual plant is far less than that from one source to another; modern technology produces very low variability in composition from a single source.
Received August I,...