Concrete is a mix of large aggregate, small aggregate, and cement (a 4:2:1 ratio is a good approximation though designed mixes will be more calculated than that). The size of the large aggregate isn't particularly important, unless you are working in very tight spaces or around reinforcement, in which case you want suitably small aggregate. In this case there won't be a .
Specifiion for Concrete Aggregates should be followed to limit impurities in aggregates. Coarse aggregates should be free from iron oxides and other minerals that can stain the concrete matrix. Aggregates should not contain injurious amounts of organic impurities. Selecting Aggregates The paste (cement, supplementary cementitious materials, ultrafine aggregate.