Methods for Extracting Alkalis from Spanish Aggregates

The release of alkalis from aggregates can be a source of alkalis in concrete that has been exposed to a moist environment for a long time. Several types of aggregates can lixiviate different amounts of alkalis; this depends not only on an aggregate´s mineralogy, but also its shape, that is, if it is a crushed or a naturally abraded aggregate. In concrete production, it is required to estimate the potential amount of alkalis releasable in order to determine the concrete mix proportions. Several solutions and test conditions are used to evaluate aggregate alkali release. The potential alkali release of different Spanish aggregates was assessed in this study utilizing different solutions and test conditions; the effect of the ions’ type, concentration, and temperature in the solution was studied. The main findings of this work were that (1) the ultrapure water extraction method provides an easy and fast way to estimate the potential leachable alkali content from aggregates, and (2) in the method of extraction on concentrated alkaline solutions, reactors at 150 ºC are used; (3) the test times are reduced, allowing a greater extraction of alkalis in less time. To estimate the potential contribution of alkalis from the aggregates in a concrete structure, the more accurate extraction methods are those that partially simulate the concentration of the porosity phase of concrete.