Supplementary cementitious materials for mitigating degradation of kraft pulp fiber-cement composites B.J. Mohr a, ⁎ , J.J. Biernacki b , K.E. Kurtis c a Department of Civil and Environmental Engineering, Tennessee Technological University, 1020 Stadium Drive, Box 5015, Cookeville, TN 38505-0001, USA b Department of Chemical Engineering, Tennessee Technological University, 1020 Stadium Drive, Box 5013, Cookeville, TN 38505-0001, USA c School of Civil and Environmental Engineering, Georgia Institute of Technology, 790 Atlantic Drive, Atlanta, GA 30332-0355, USA Received 24 May 2006; accepted 2 August 2007 Abstract Kraft pulp fiber reinforced cement-based materials are being increasingly used where performance after exposure to environmental conditions must be ensured. However, significant losses in mechanical performance due to wet/dry cycling have been observed in these composites, when portland cement is the only cementitious material used in the matrix. In this research program, the effects of partial portland cement replacement with various supplementary cementitious materials were investigated. Binary, ternary, and quaternary blends of silica fume, slag, Class C fly ash, Class F fly ash, metakaolin, and diatomaceous earth/volcanic ash blends were examined for their effect on the degradation of kraft pulp fiber- cement composite mechanical properties (i.e., strength and toughness) during wet/dry cycling. After 25 wet/dry cycles, it was shown that binary composites containing 90% slag, 30% metakaolin, or greater than 30% silica fume did not exhibit any signs of degradation, as measured through mechanical testing and microscopy. Ternary blends containing 70% slag/10% metakaolin or 70% slag/10% silica fume were also effective in preventing degradation. A reduction in calcium hydroxide content and the stability of the alkali content due to supplementary cementitious material addition were shown to be primary mechanisms for improved durability. Published by Elsevier Ltd. Keywords: Fiber reinforcement (E); Degradation (C); Supplementary cementitious materials; EDX (B); SEM (B) 1. Introduction Pulp-fiber cement composites have been increasingly used as non-structural exterior building components where performance must be ensured after environmental exposure. However, it is understood that such composites may exhibit significant degradation of mechanical properties (i.e., strength and toughness) when subjected to cycles of wetting and drying [1–7]. Thus, the influence of environmental exposure on the long-term performance of pulp fiber-cement composites warrants further investigation so that appropriate mitigation strategies may be developed. With wet/dry cycling, pulp fibers become embrittled due to the formation of cement hydration products within the fiber lumen, the fiber cell wall, and/or around the fiber [7–10]. According to a model proposed by Mohr et al. [7,10], degrada- tion during wet/dry cycling occurs progressively by: (1) initial fiber-cement debonding, (2) reprecipitation of secondary ettringite within the void space at the former fiber-cement interface (kraft fibers only), and (3) fiber embrittlement due to reprecipitation of calcium hydroxide within the fiber lumen and/ or fiber cell wall. This model, with some modifications, is also applicable to thermomechanical pulp (TMP) fiber-cement composites [6,10]. Two avenues may be explored for mitigation of fiber-cement composite degradation: (1) fiber modifications and (2) matrix modifications. Modification of the fibers, including fiber impregnation [1], fiber treatments [11], and fiber fibrillation [7,12], have been examined as means to minimize fiber-cement debonding and/or moisture migration around and through the fibers during wetting and drying. In addition, modifications affecting fiber chemical composition, dimensional stability, and fiber-cement bond strength have also been evaluated [6,7]. Available online at www.sciencedirect.com Cement and Concrete Research 37 (2007) 1531 – 1543 ⁎ Corresponding author. Tel.: +1 931 372 3546; fax: +1 931 372 6239. E-mail address: bmohr@tntech.edu (B.J. Mohr). 0008-8846/$ - see front matter. Published by Elsevier Ltd. doi:10.1016/j.cemconres.2007.08.001