Background

It is apparent that ASR occurs between certain forms of silica present in the aggregates and the hydroxide ions (OH) in the pore water of a concrete (Hobbs 1988, Diamond et al. 1992). Hydroxide ions in a pore solution result from the hydration of portland cement (Diamond 1983). The pore solution composition depends on the amount of alkalies present in the pore water which, in turn, is related to the amount of soluble alkalies in the concrete. This dissolution leads to a pH in excess of 12.5 (Diamond 1983). The hydroxide ions in solution will attack sites on a silica surface of the aggregates. If the silica is well-crystallized, there are few highly reactive sites, whereas in a poorly-crystallized or amorphous silica there are many more such reactive sites. In this case, hydroxide attack may lead to complete conversion of the silica to a calcium and alkali silicate gel (Figg 1983; Helmuth et al. 1992).

The formation of the gel per se is not deleterious. The deterioration of the concrete structure is usually due to water absorption by the gel and its subsequent expansion. The gel can creep into existing pores or cracks, where it can do relatively little damage. Of course, this scenario is based on the fact that the viscosity of the gel is such that it can flow into the pores and cracks. However, if the void volume is not sufficient to accommodate all the gel, or is not all accessible by the gel, and if the tensile strength of the matrix is locally exceeded, cracks will form and propagate radially from the reactive aggregate. The sites of crack initiation are often randomly distributed in the specimen, with no preferential direction of crack propagation. The points of crack initiation are determined by the location of the reactive silica in the aggregates and the local availability of hydroxide ions. If the relative humidity inside the specimen is higher than 80 %, (Stark 1994) the gel can expand. Therefore, the permeability and the self-dessication of a concrete specimen play an important role.

Some mineral admixtures have been reported to suppress the deleterious expansion due to ASR (Duchesne et al. 1994). While the mechanisms are not clearly determined, several possibilities have been proposed:

• reduced permeability that prevents the ingress of water
• increased strength, probably due to decreased porosity around the aggregates,
• reduced alkali concentrations in the pore solution because of the reduction of cement and/or the mineral admixture contains less alkali than does the cement it is replacing, and
• increased pozzolanic reaction, leading to the mineral admixture reacting rapidly with the ions, producing CSH that traps alkali ions and therefore reduces their concentration in the pore solution.

The results of this paper will be used to discuss the validity and relative importance of each of these mechanisms.