Concrete is a mixture of cement, aggregate and water; which when fresh, forms a paste that later on sets and hardens by means of hydration reactions to form a solid (stone-like) material. CEMENT According to BS 12, cement is hydraulic binder (finely ground inorganic material), which when mixed with water forms a paste that sets and hardens by means of hydration reactions and processes; and which after hardening retains its strength and stability even under water. a) Type of cement: However there are many types of cement for different construction purposes, the most frequently used types are: Ordinary Portland Cement (OPC): used in safe environment (free from chemical attack). Sulphate Resisting Portland Cement (SRPC): used in sulphate exposure (e.g. in sea waters, chemical industries, etc).