C-S-H, calcium silicate hydrate, is the main phase in hydrated plain cement paste.C-A-S-H, alumina-containing calcium silicate hydrates, is the main phase in blends of plain cement containing aluminate phases or with supplementary cementitious materials (SCMs).C-A-S-H has a highly variable composition in blends and generally forms with lower Ca/Si and often higher Al/Si.Its importance in cement chemistry is dee to the fact that C-S-H and C-A-S-H form at least half the volume and is the main contributor to the cohesion and stability of the hardened material.