What are the classification of cement?

Classification of cement:

Cements used in construction can be characterized as being either hydraulic or non-hydraulic, (see also hydraulic and non-hydraulic lime plaster).
Non-hydraulic cement is prevented from setting in wet conditions or underwater, and is attacked by some aggressive chemicals after setting.
Hydraulic cement is made by replacing some of the cement in a concrete mix with activated aluminium silcates, pozzolanas, such as fly ash, to activate cement setting in wet condition or underwater and further protects hardened concrete from chemical attack. (e.g., Portland cement) hardening because of hydration.
The chemical process for hydraulic cement found by ancient Romans used volcanic ash (activated aluminium silicates), to activate cement hardening between the anhydrous cement powder and water or plaster and water instead of relying on water drying out and simultaeously reacting with airborn carbon dioxode.
Presently cheaper, pollution free fly ash from power stations or other waste or by products are used as pozzolanas with plain cement to produce hydraulic cement. Pozzolanas can replace up to 40% of Portland cement.
Thus, cement can harden underwater or when constantly exposed to wet weather. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water and from chemical attack. Non-hydraulic cements and plasters do not harden in wet conditions.
The most important uses of cement are as an ingredient in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material.