Chemistry of alkali–silica reaction and testing of aggregates

Engineers and real state owners are demanding that concrete aggregates should be tested for their alkali-silica reactivity, before their use, by some accelerated test method. An analysis of the consumer's requirements shows that actually two, contradictory, demands are made on the test methods. These are: (a) to ascertain alkali-silica reactivity of an aggregate in a reasonably short time and (b) to evaluate and set an acceptance limit of long-term expansion of a reactive aggregate-cement combination. This contradiction is often ignored in national standards and only one test method is specified. However, it is known that concrete expansion, due to alkali-silica reaction, could be suppressed by using sufficient quantity of a pozzolan even when the concrete is exposed to a strong alkaline solution. Most of the available test methods for the evaluation of alkali-silica reactivity of aggregates are empirical. In this paper, I propose to examine different test methods in the light of recent fundamental understanding of the mechanism of alkali-silica reaction and expansion. The emphasis is on the alkali-silica reactivity of aggregates themselves and not on the acceptance limit. The mechanism of alkali-silica reaction and expansive pressure generation suggest that the most appropriate condition of testing is to expose the test specimens to a solution of moderate hydroxyl ion concentration, preferably Ca(OH)2 solution, and high ionic strength. The high ionic strength should be achieved by adding sufficient quantities of an "indifferent" electrolyte like a neutral alkali salt to the alkaline solution. The fundamental understanding of the mechanism of alkali-silica reaction suggests a simple chemical test method for the evaluation of aggregates themselves. This method has been tested with Danish aggregates and an acceptance criterion has been suggested. Preliminary experiments show that the reactivity of samples of Japanese andesite, British Greywacke, Swedish porphyritic rhyolite, Norwegian mixed aggregate of rhyolite and quartzite and a silicified limestone aggregate from Belgium could be detected within 24 h. This is one of the quickest methods for the evaluation of alkali-silica reactivity of aggregates. (A) "Reprinted with permission from Elsevier".