Effects of class F ﬂy ash on sulfate resistance of Type V Portland cement concretes under continuous and interrupted sulfate exposures Nader Ghafoori, Meysam Najimi ⇑ , Hamidou Diawara, Mohammad S. Islam Department of Civil and Environmental Engineering and Construction, University of Nevada Las Vegas, Las Vegas, NV, USA highlights  Sulfate resistance of Type V Portland cement.  Inﬂuence of ﬂy ash on sulfate resistance of Type V Portland cement.  Comparative inﬂuence of continuous and cyclic wet–dry exposures on sulfate resistance of concrete. article info Article history: Received 28 July 2014 Received in revised form 3 December 2014 Accepted 2 January 2015 Keywords: Type V Portland cement Class F ﬂy ash Sodium sulfate Expansion Exposure type Ettringite Gypsum Strength abstract The current study evaluates inﬂuence of class F ﬂy ash replacement level on the ettringite-based expan- sion and gypsum formation (strength reduction and mass loss) of Type V Portland cement concretes. To this aim, control concretes were made using Type V Portland cement with different contents of 333, 374 and 416 kg/m 3 . Fly ash contained concretes were designed by replacing 15, 20, 25 and 30% by weight of cement with ﬂy ash. An experimental program was designed to monitor length change, mass loss, and compressive strength reduction of mixtures under interrupted and continuously-immersed sulfate expo- sure conditions. The results of this study revealed improvements in concretes’ resistance to sulfate attack by replacing a portion of cement with ﬂy ash. The improvements, however, were not as much as expected for class F ﬂy ash. The optimum replacement level was different for different cement contents. It was increased with increases in cementitious materials contents. No mass loss and strength reduction were observed within a year of exposure. The performances of mixtures under interrupted and continuous immersion were nearly similar. Both compressive strength and ettringite-based expansion of the studied specimens under interrupted sulfate immersion condition were marginally lower than those of continu- ously immersed specimens. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Concrete can deteriorate for several reasons and sulfate attack is reported to be a major contributor to concrete premature failure. It was reported that ‘‘concrete deterioration due to sulfate attack is the second major durability problem, after reinforcement corrosion’’ [1]. Sulfates can come from a variety of sources such as groundwater, high clay-content soils, seawater, organic materi- als in marshes, mining pits, and sewer pipes, and in different forms including magnesium sulfate, sodium sulfate, calcium sulfate, potassium sulfate, and ammonium sulfate [2]. Magnesium sulfate is reported to be potentially more destructive than sodium sulfate, and both are found to be more damaging than calcium, potassium, and ammonium sulfates [3]. Sulfate attack can be either in the form of internal attack, which is the result of chemical reactions between constituents of cement paste and sulfate ions; or external attack, which mostly manifests itself in the form of surface scaling similar in appearance to that of freezing and thawing damage [3,4]. The chemical sodium sulfate attack, which is the focus of this study, can have adverse effects on concrete in two distinct forms: (1) expansion of cement matrix generally attributed to the formation of ettringite compounds, and (2) progressive loss of strength and mass due to deterioration of hardened cement paste by gypsum formation [5]. There are a number of factors affecting the severity of sulfate attack and resistance of concrete. These factors include parameters related to: (1) concrete mixture ingredients and proportion such as cement type, cement content, water-to-binder ratio, admixtures, and supplementary cementitious materials, (2) curing and hardened concrete properties such as curing condition, pore structure, permeability, diffusivity, and mechanical properties, http://dx.doi.org/10.1016/j.conbuildmat.2015.01.004 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 7023470221. E-mail address: najimim@unlv.nevada.edu (M. Najimi). Construction and Building Materials 78 (2015) 85–91 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat