Effects of alkali addition on the mechanical properties and durability of concrete

Increasing the concrete alkali content from 0.6% to 1.25% of Na2Oe 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-tocement (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.