Strength and Microstructural Properties of Water Glass Activated Slag
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STRENGTH AND MICROSTRUCTURAL PROPERTIES OF WATER GLASS ACTIVATED SLAG CHENG QING-HUA, AREZKI TAGNIT-HAMOU AND SHONDEEP L. SARKAR, Faculty of Applied Sciences, Universit6 de Sherbrooke, Quebec, Canada, JiK 2R 1. ABSTRACT Slag as a supplementary cementitious material is known for its latent hydraulic properties. Thus, constant efforts are being directed towards improving its reactivity. This mostly involves the use of alkali salts. The authors present the results of alkali activation of a slag from Hamilton, Ontario, Canada, by means of water glass (Na2SiO3). Distinct improvement in early and late age strength was noted when water glass was used in combination with CH. The microstructure of this activated slag paste at 28 days is described. The microanalytical techniques used included SEM/EDXA, and XRDA. INTRODUTION Slag, as a cementious material, has an inherent drawback - its latent hydraulicity. The cementitious action of slag depends on its glass content [1]. Regourd [2] showed that only a very small amount of immediate reaction takes place when slag is mixed with water. To solve this problem, an activator is usually used. Initially, the activator was CH, either directly from the powder, or derived from portland cement hydration [3]. In some cases gypsum has also been used [4]. During the 1970s, the use of alkali activators was developed [5-7]. Alkali activators constitute alkali hydroxide, ROH, non-silicic salts of weak acids, R2CO3, R2SO3, R2S, RF, and silicic salts of R20(n)SiO2, where R represents an alkali metal ion (Na+,K+). It is said that when slag is alkali activated, the principal hydrates are C-S-H and zeolite [8,9]. Researchers in the USSR [101, however, report that different activators produce different hydrates, though C-S-H gel is always present. Alkali-activated slag cement can be used to achieve concrete strengths greater than 70MPa without any chemical admixture [8,111. In the present study water glass with and without lime was used as the activator of a Canadian slag. EXPERIMENTAL Raw Materials Slag: The chemical composition of the slag used is given in Table 1. Besides the glass phase identified from the hump in the XRD pattern, XRD analysis indicated small amounts of crystalline phases such as gehlenite and merwinite with their principal peaks at 2.87A and 2.67A respectively. From the secondary electron image (SEI) of this slag, shown in Figure 1, it is evident that the particles contain fairly high amount of glass. Water Glass: The soluble sodium silicate used had a composition of 10.8%Na2O, 29.7%SiO2 and 59.5%H20. The silica modulus(Ms) was 2.85. Slag Paste Preparation: The pastes were prepared by mixing water with slag containing different amounts of water glass and lime, the latter added in the slurry form. The Ms was regulated to 1.5
Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society
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Figure 1. SEI of slag. C = crystalline phases, G = glass by adding NaOH solution. The compositions of the pastes are listed in Table 2. The water to binder (W/B) ratio of all the pa
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