Effect of magnesium and sulfate ions on durability of silica fume blended mixes exposed to the seawater tidal zone Eshmaiel Ganjian a, * , Homayoon Sadeghi Pouya b a School of Science and Environment, University of Coventry, Sir John Laing Building, Priory St., Coventry CV1 5FB, UK b Department of Civil and Structural Engineering, University of Sheffield, Sir Fredrick Mappin Building, Mappin St., Sheffield S1 3JD, UK Received 25 May 2004; accepted 17 September 2004 Abstract The effect of silica fume on deterioration resistance to sulfate attack in seawater within tidal zone and simulated wetting–drying condition has been studied in Portland cement concretes and pastes containing silica fume (SF) with/without ground granulated blast furnace slag (GGBS). Changes in the compressive strength and capillary water absorption of specimens as a function of SF content have been investigated combined with phases determination by means of scanning electron microscopy and X-ray energy dispersion analysis. The strength change factors (SCFs) of specimens with SF (the more SF content, the higher strength loss) were greater than that of the mixes without SF or cured under tap water. Mg 2+ ion originated attack found to be the dominating deterioration mechanism as confirmed by X-ray and chemical analyses. Further, the incorporation of GGBS with SF mixes in different exposure conditions led to the worst performance in all of the test environments. Lower cement content and hydration rate accompanied with particular chemical composition of GGBS made concrete and paste specimens to be more susceptible to deleterious seawater environment. D 2004 Elsevier Ltd. All rights reserved. Keywords: Concrete; Durability; Silica fume; Granulated blast furnace slag; X-ray diffraction 1. Introduction Silica fume has been used as high pozzolanic reactive cementitious material to make high-performance concrete in the severe conditions [1,2]. This mineral admixture has highly been used in severe environmental conditions despite its several mixing and curing problems, because of its acceptable early age strength development [3,4,5]. The hydration mechanism and properties of secondary C–S–H made by pozzolanic reaction have been studied by many investigators [4]. However, pozzolanic C–S–H formed by silica fume–calcium hydroxide reaction might be different in respect of the amount of molecular water, C/S ratio and density [6]. Moreover, because of its rather different characteristics, pozzolanic gel has high potential to contrib- ute in reactions with other internal or external ions such as Al, Cl and alkalines [7,8]. Deterioration and durability of concrete structures which are exposed to harmful ions or chemicals are subject of the major discussions for service life of high cost or key concrete structures. Sulfate attack has been reported to be a cause of damage to concrete for over a century. Many of the reinforced and unreinforced structures exposed to seawater will suffer from deleterious chemical reactions between hardened cement compositions and different sulfate ions in cementitious matrix of the structures.At least six types of reactions could be described by sulfate attack. The most common reactions often used to explain the defined sulfate attack are ettringite and gypsum formation [9]. It is generally accepted that sodium and magnesium sulfate attacks of hydrated cement matrix take place due to the reaction of 0008-8846/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2004.09.028 * Corresponding author. Tel.: +44 24 7688 7625; fax: +44 24 7688 8296. E-mail address: E.Ganjian@Coventry.ac.uk (E. Ganjian). Cement and Concrete Research 35 (2005) 1332 – 1343