Effect of geothermal waste on strength and microstructure of alkali-activated slag cement mortars J.I. Escalante-Garcı ´a * , A.V. Gorokhovsky, G. Mendoza, A.F. Fuentes Centro de Investigacio ´n y de Estudios Avanzados (Cinvestav-Saltillo), PO BOX 663, Saltillo, Coahuila, CP 25000, Mexico Received 10 October 2001; accepted 26 March 2003 Abstract Mortars of blast furnace slag replaced with 10% of a geothermal silica waste were cured for 90 days. The binder was activated by 6 wt.% Na 2 O equivalent of NaOH and water glass. The presence of the silica enhanced the formation of hydration products as shown by nonevaporable water (NEW) results. Backscattered electron images indicated that the microstructures of blended slag had less porosity than those of neat slag mortars and the interfacial zone between aggregate and hydration products was dense and of homogeneous composition similar to the matrix of hydration products. The main hydration products were C-S-H and for NaOH a hydrotalcite type phase was found as finely intermixed with the C-S-H. D 2003 Elsevier Ltd. All rights reserved. Keywords: Microstructure; EDX; Mechanical properties; Alkali-activated cement; Geothermal silica waste 1. Introduction Blast furnace slag has been used in combination with Portland cement, as aggregate or cementive fraction; improv- ing the characteristics of Portland cement [1]. Binders based on alkali activated are of interest due to their properties in comparison to Portland cement—fast development and higher levels of mechanical strength, better durability, etc. [2–5]. Many papers concentrate on different variables that affect the properties of activated slags, e.g., the effect of the activating agents on the hydration products and mechanical properties [6–8] and microstructures [2,9,10]. However, there is a reduced number of reports on the use of other by- products in combination with activated blast furnace slags [11–14] or with Portland cement [15,16]. One by-product with potential as a cementitious additive is the geothermal silica waste (GSW) that originates from the electricity gen- eration from geothermal resources, which are available worldwide. GSW is similar to microsilica: it has high SiO 2 content and very high surface area. Detailed information regarding the origin and treatment of the GSW was provided and discussed elsewhere [17,18]; additionally, its pozzolanic behavior is under study in blends with Portland cement [19]. Microsilica and other fine-sized materials are added to Portland cement to increase strength by means of the micro- filler effect and its pozzolanic behavior, to improve durabil- ity [20] and mechanical strength by reducing the porosity of the hydration products formed in the interfacial zone hcp- aggregate [8,21,22]. Inert and hydraulic admixtures can accelerate the hydration reactions in Portland cement [23– 26] as they are potential sites for heterogeneous nucleation of hydration products, improving mechanical strength. Ultra- fine materials have also been used in the activation of slags to improve mechanical properties [12,13]. This work concentrated on mortars with a binder, activated by sodium hydroxide and waterglass, formed by blast furnace slag partially replaced by a GSW. The report includes results from the reactivity of the systems, elemental distribution in the hydration products and interfacial characteristics; for the activation with NaOH, some results are presented for the composition of the hydration products formed. 2. Experimentation 2.1. Materials and mortars preparation The granulated slag was supplied and sampled at the Cementos Apasco Plant (Ramos Arizpe, Mexico). A pilot ball mill was used to prepare the slag to Blaine surface areas 0008-8846/03/$ – see front matter D 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0008-8846(03)00133-9 * Corresponding author. E-mail address: jieg@saltillo.cinvestav.mx (J.I. Escalante-Garcı ´a). Cement and Concrete Research 33 (2003) 1567 – 1574