Coarse blast furnace slag as a cementitious material, comparative study as a partial replacement of Portland cement and as an alkali activated cement J.I. Escalante-Garcia a, * , L.J. Espinoza-Perez a , A. Gorokhovsky a , L.Y. Gomez-Zamorano b a Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Unidad Saltillo, Ceramic Engineering Group, Carr Saltillo-Monterrey km13, Saltillo, Coahuila CP 25900, Mexico b FIME Universidad Autonoma de Nuevo León, Ing. de Materiales, Av Universidad s/n Ciudad Universitaria, San Nicolas de los Garza, Nuevo León, CP 66451, Mexico article info Article history: Received 14 July 2008 Received in revised form 17 January 2009 Accepted 8 February 2009 Available online 9 March 2009 Keywords: Concrete Blast furnace slag Blended cements Alkaline activation Strength Microstructure abstract The cementitious performance of a coarse granulated blast furnace slag, 2900 cm 2 /g, was investigated in concretes of 230, 280 and 330 kg binder/m 3 . First, the slag partially replaced 30%, 50% and 70% of Portland cement, the strength reduced as the amount of slag increased; however, for high binder contents, similar strengths were attained for lower Portland cement contents. Second, the slag was alkali activated with sodium silicate (moduli 1.7 and 2) at 4%, 6% and 8% %Na 2 O, the strength increased with the amount of slag in the concrete and developed faster as %Na 2 O increased. The microstructures of both type of con- cretes were dense; however, the strengths of activated slag were superior at similar binder loads, indicat- ing that the hydration products of activated slag are of higher intrinsic strength. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Blast furnace slag (BFS) is a by product of pig iron production, a stage process in the production of steel; approximately 300 kg of slag are generated per ton of pig iron [1]. More than 167 million tons of slag were produced worldwide in 1996 [2]. On the other hand, concrete based on Portland cement is the most widely used material in the world; and compared to other materials like steel, aluminium and plastics, is the most viable option for the construc- tion industry considering economical and environmental costs. Nonetheless, it is estimated that 7% of the anthropogenic CO 2 cor- responds to the Portland cement (OPC) production, considering the current world environmental situation, it is obvious that cement and concrete specialists must search for ways to reduce that figure, or at least to avoid its growth. One of the possibilities is the massive usage of industrial wastes like BFS, to turn them to useful environmentally friendly and technologically advantageous cementitious materials; various possibilities exist in this regards, as described below. BFS can be used to partially replace OPC, or as a cementitious material per se after chemical activation. Much research has been developed on both areas; however, comparative studies of both routes on one BFS are not common. Various factors influence the reactivity of BFS, among them are: the surface area, chemical composition, vitreous fraction, etc. The surface area required for BFS as a cementitious material is normally greater than for OPC, this is needed to improve the BFS reactivity and thus the strength attained when it replaces OPC [3]; a range of 550–650 kg/m 2 is ac- cepted, and higher values were reported not to bring further advantages on strength [4]. However, the grinding resistance of BFS is higher than OPC, thus grinding to the mentioned levels in- creases the cost of the BFS [2]. The BFS chemical composition varies widely among plants due to differences in the raw materials em- ployed [2], various indices have been proposed to attempt to pre- dict the quality of an acceptable BFS, one example is the index Ih =(C + 1.4M + 0.56A)/S > 1.65 [5] for BFS from 56 laboratories; other indices are also described in the literature [2,6,7]. According to many authors, a high percentage of vitreous phase is needed for the BFS to react [8], but full amorphization is not required [2,5]. Lang [2] pointed out that it is difficult to predict the BFS contribu- tion to strength based on the chemical composition and glassy fraction, which are in turn governed by specific parameters like the interaction of a specific BFS with a specific clinker and one spe- cific calcium sulphate, or other chemical activators. Considering the above, an investigation to compare the performance of one BFS as a cementitious material in different chemical environments was considered of great interest. This paper presents an investigation of strength development of concretes with BFS as a cementitious component, the aim was to compare the performance of one relatively coarse BFS as a partial replacement of PC and also as a cement per se after alkaline 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.02.002 * Corresponding author. Tel.: +52 844 4389600x9675; fax: +52 844 4389610. E-mail address: Ivan.escalante@cinvestav.edu.mx (J.I. Escalante-Garcia). Construction and Building Materials 23 (2009) 2511–2517 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat