Concrete is a construction material composed of three main materials: cement, water and aggregates (sand, natural gravel or crushed stone). Sometimes an admixture is added in order to change or modify certain properties of concrete. The chemically-active ingredient in concrete is cement.
The reactivity of cement is only guaranteed and achieved when mixing it with water. Aggregates do not have a role to play within the chemical reactions in concrete, but they’re very useful because they act as economic, filler materials with good resistance to volume changes, which occurs in concrete after mixing.
Another important aspect of aggregates is that they improve the durability of concrete. Concrete aggregates can sometimes contain impurities like clay particles, which can have an impact on the concrete properties.
Clay coatings consist of clay particles that are held tightly to aggregate the surface. Because the material usually adheres to the aggregate even after the concrete is mixed, it is believed to interfere with the aggregate-cement paste bond. Unlike clay coating, dust coating is easily removed during mixing and affects concrete performance by increasing the amount of fine particles dispersed in the mix.
The presence of clays in the fine aggregate used to make concrete is known to be detrimental to the structural properties of the concrete. The Texas Highway Department presently makes use of the results of the ‘Sand Equivalent’ and ‘Loss by Decantation’ tests as a means of detecting such clay and controlling the quality of fine aggregate to be used in Portland cement concrete. It has been found that the amount of minus no. 200 mesh fraction of concrete aggregates affects the properties of concrete. Clay contaminants in concrete aggregate affect concrete properties primarily through their effect on water demand. Concrete strength and shrinkage correlate to a high degree with sand equivalent value and to a slightly lesser degree with water-cement ratio.