Concrete ingredients

Concrete is cement, water, fine aggregate (sand) and coarse aggregate (crushed stone or gravel), often with admixtures and mineral additions. Cement and water react to form a paste that binds the aggregates into an artificial rock.

Aggregates make up roughly 60 to 75 percent of the volume. They give bulk, reduce shrinkage and control cost. They must be strong, clean and well graded so that the smaller particles fill gaps between larger ones and less paste is needed. Flaky or elongated stones, silt and clay coatings, and organic impurities weaken the mix, which is why aggregates are tested before use.

Water does two jobs: it reacts with cement and it makes the mix workable. Only part of it is needed for hydration; the rest leaves pores as it dries. More water means more pores and lower strength, which is the basis of the water-cement ratio law.

Mineral additions such as fly ash, ground granulated slag and silica fume replace part of the cement, improve long-term strength and durability, and cut the carbon footprint. Chemical admixtures such as superplasticisers let the mix flow at a lower water content.