Effects of alkali addition on the mechanical properties and durability of concrete N. Smaoui a , M.A. Be ´rube ´ b, * , B. Fournier c , B. Bissonnette d , B. Durand e a Service d’Expertise en Mate ´riaux Inc., 1400, Boulevard du Parc Technologique, Que ´bec City, PQ, Canada G1P 4R7 b De ´partement de ge ´ologie et de ge ´nie ge ´ologique, Universite ´ Laval, Sainte Foy, PQ, Canada G1K 7P4 c ICON/CANMET, Natural Resources Canada, 405 Rochester, Ottawa, ON, Canada K1A 0G1 d De ´partement de ge ´nie civil, Universite ´ Laval, Sainte Foy, PQ, Canada G1K 7P4 e IREQ, Hydro-Que ´bec, 1740 Boulevard Lionel-Boulet, Varennes, PQ, Canada J3X 1S1 Received 23 January 2003; accepted 4 May 2004 Abstract Increasing the concrete alkali content from 0.6% to 1.25% of Na 2 O e of the cement mass by adding NaOH to the mixture water has harmful effects on most mechanical properties (compressive, splitting, direct tensile, and flexure strengths) of concrete made with a water-to- cement (w/c) ratio of 0.41 and limestone aggregates not susceptible to alkali – silica reaction (ASR), however not on the elasticity modulus measured under compression or direct tension. Shrinkage tests at 50% RH and 23 jC started after 7 days at 100% RH and 23 jC show that the low-alkali concrete shrinks more than the high-alkali one, despite similar water losses. Freeze – thaw tests performed on air-entrained concretes show that the two concretes resist well to freezing and thawing while showing similar air – void systems. When examined under the scanning electron microscope (SEM), the hydrates in the two concretes present similar microstructure; however, the high-alkali concrete shows a more reticular and porous microtexture, which could explain the reduction in strength. D 2004 Elsevier Ltd. All rights reserved. Keywords: Alkalies; Concrete; Shrinkage; Freezing and thawing; Mechanical strengths 1. Introduction The effects of alkalies on various properties of cement paste, mortar, and concrete, including early hydration and setting, bleeding, strength development and ultimate strength, drying shrinkage, susceptibility to cracking, micro- texture of cement hydrates, and durability, have been a subject of concern since many decades. Reviews on the subject can be found in Burrows [1] and Jawed and Skalny [2]. From the literature then available, the latter [2] con- cluded that significant proportions of alkalies in clinker can cause a quick setting, reduce the ultimate strength of concrete, and increase expansion under water and shrinkage under drying conditions. As discussed hereafter, based on limited or conflicting information in many respects, the present paper addresses the effects of alkalies on strength development, drying shrinkage, freeze–thaw durability, and microstructure of concrete. 1.1. Effects of alkalies on cracking and drying shrinkage Burrows [1] participated in a long-term test program that consisted in the fabrication of 104 concrete panels made with 27 different cements and that were placed in 1943 at Green Mountain dam in Colorado. Fifty-three years later, he found that the amount of cracking observed on these panels was directly proportional to the alkali content in the cements used, thus suggesting that high-alkali cements are more susceptible to drying shrinkage. Blaine et al. [3] performed shrinkage tests on cement pastes made with 199 different cements. They found that cement pastes containing more alkalies were undergoing more shrinkage; however, they did not observe any correlation between the alkali content and the amount of drying shrinkage for their companion con- crete specimens. Based on the work by Carlson [4], Burrows [1] attributes this behaviour to the development of micro- 0008-8846/$ – see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2004.05.007 * Corresponding author. Tel.: +1-418-656-3930; fax: +1-418-656-7339. E-mail address: berube@ggl.ulaval.ca (M.A. Be ´rube ´). Cement and Concrete Research 35 (2005) 203 – 212