Alkali-aggregate reactions in concrete

Concrete durability is adversely affected by expansive reactions between cement alkalies and certain rocks and minerals sometimes used as aggregates. Present trends towards production of portland cements containing higher alkali contents than formerly indicate that the problem may become more acute. Recent work shows that the rocks and minerals involved fall into three groups which leads logically to a three-fold subdivision of the reactions termed here: (I) the alkali-silica reaction, (II) the alkalicarbonate rock reaction, and (III) alkali-silicate reactions. A clearer understanding of the mechanisms of these reactions has practical implications particularly because of the light thrown on the criteria for recognition of potentially expansive rocks and minerals in aggregate source materials. Appropriate preventive measures prior to concrete placing make it possible to avoid the worst effects of these reactions. These include use of low-alkali cement, incorporation of a suitable pozzolan, design to reduce wetting and beneficiation of aggregate. The effectiveness of these measures varies however, depending on the mechanism and severity of the reaction.