Alkali Release From Aggregates And Its Effect On Alkali-Aggregate Reaction

Alkali-aggregate reaction (AAR) is one of significant factors affecting durability of concrete structures. Studying and preventing AAR in concrete is an important object in many countries, since concrete constructions suffered from severe deterioration and damage by AAR, which caused heavy loss in these countries. Three essential conditions must be satisfied to initiate and sustain AAR in concrete: the aggregates are potentially reactive, the alkali concentration is high in the concrete pore solutions and the concrete is exposed to high humidity conditions. Using low-alkali cement with less than 0.6% Na2Oe or limiting the alkali content of concrete constitutes the most popular ways to minimize AAR in the presence of potentially reactive aggregates. Aggregate contains large amounts of alkalis, it was considered that only a relatively small proportion of these alkalis are liable to be soluble within concrete and the effect of these alkalis in AAR might be ignored. Since facts have been reported that some field concrete structures, despite built with low-alkali cement, that were undergoing expansive AAR accompanied by the correspondingly high alkali level in the pore solutions, more and more attentions are focused at the problem of alkali release from alkali-bearing aggregates within concrete and the effect on AAR. Aggregate is the main element in concrete, the little amount of alkali released from aggregate would considerably increase the total alkalis content of concrete pore solution. So it is an urgent and significant object to estimate the amount of alkali release from aggregate within concrete and the promotion on expansive AAR.Alkalis in aggregates exist in various alkali-bearing minerals and the amount of alkali release in concrete varies with type of alkali-bearing minerals and its content in aggregates, which make it hard to estimate the amount of alkali release by aggregate in concrete and its effect on AAR. For this reason, there is still no consensus about the problem, though many studies have been carried out. In this paper, aggregates and alkali-bearing minerals which contain nepheline, Na-feldspar and K-feldspar have been investigated, the ability of alkali release from alkali-bearing minerals in alkaline environment, the mechanism of alkali release from alkali-bearing aggregates and the effect of alkali release on AAR have been studied and the results have been obtained as follow.1. Thermodynamic analysis of the capability of alkali release of alkali-bearing mineralsThe theoretical maximum amount of alkali release from alkali-bearing aluminosilicate minerals in alkaline solution has been studied by thermodynamic method. The results show that (1) The capability of alkali release from aggregate is related to the type of alkali-bearing minerals in aggregates. The maximum capability of alkali release from alkali-bearing minerals in order is dawsonite > leucite > nepheline > dehydration analcime > paragonite > muscovite > jadeite > Na-feldspar > K-feldspar in alkaline solution with pH=13~14. (2) The capability of alkali release from alkali-bearing minerals increases with the decrease of pH of solutions (except nepheline), and increases with the increase of pore solution/aggregate ratio. (3) A number of experimental procedures have been used in the past to estimate the alkali...