Alkali–silica reaction, pessimum effects and pozzolanic effect

The pessimum proportion and pessimum size effects for alkali–silica reaction-induced deterioration of concrete (ASR) and the pozzolanic effect of fine siliceous admixtures in concrete have been explained based on the proposed ASR model [T. Ichikawa, M. Miura, Modified model of alkali–silica reaction, Cem. Concr. Res. 37 (2007) 1291–1297.]. The attack of alkali hydroxide to aggregate particles composed of ASR-reactive minerals generates the layer of hydrated mature alkali silicate and the layer of less hydrated immature alkali silicate under the mature layer. The mature alkali silicate preferentially reacts with Ca2+ ions to convert to fragmental calcium alkali silicate, because the reaction accompanies a significant volume contraction. The immature alkali silicate gradually reacts with Ca2+ ions to cover the surface of the reactive minerals with tight layers of calcium alkali silicate called reaction rims. The reaction rims allow the penetration of alkaline solution but prevents the leakage of viscous alkali silicate generated afterward, so that the alkali silicate is accumulated inside the rims to give an expansive pressure enough for cracking the aggregate and the surrounding concrete. Due to the absorption of Ca2+ ions by mature alkali silicate, too much increase of the proportion of reactive aggregate causes the deficiency of Ca2+ ions for the formation of reaction rims, so that the ASR expansion decreases after passing the pessimum proportion. Very fine reactive aggregate and admixtures with the grain size less than ~ 50 µm preferentially react with alkali hydroxide to convert to mature alkali silicate without leaving any reactive minerals. Homogeneous mixing of the sufficient amount of very fine siliceous admixtures in concrete therefore inhibits the ASR by absorbing Ca2+ ions for the rim formation. The resultant fragmental calcium silicate fills the pores in concrete to increase the strength and the durability of the concrete. The admixtures thus act as pozzolanic materials.