We all know that cement is a mineral powder which when mixed with water forms a paste type and aggregates.The reaction between cement and water is called hydration.
Hydration is a chemical process in which chemcial compounds in cement reacts with water ions and forms chemical bonds resulting formation of some hydration compounds.
Only C3S & C2S (calcium silicates) compounds in cement constitutes strength.
Here, C3S reacts with water and forms calcium silicate hydrate and calcium hydroxide. Some heat is formed which evolves due to chemical reactions and chemical bonding in cement when it was mixed with water.
The below illustration gives a clear understanding about how cement sets or hardens.
The calcium silicate hydrate ions gets crystallised and becomes more thicker in shape so that even water can't even diffuse into it. The voids gets reduced due to this calcium silicate hydrate gets filled up in the voids.So porosity gets reduced in the area. This makes cement paste stiff and hard so that the cement paste binds aggregates and metal rods together. C3S reaction occurs fastly and contributes early strength (first 7 days strength).
Here the C2S reaction is similar to C3S reaction but the reaction is very slow. So this constitutes later strength to cement (after 7 days).
Calcium silicate hydrate is extremely smallest particle in gel form. Its the major source of concrete strength.
All reactions continues until presence of water and unreacted calcium silicates. The concrete dries after water depletion.
C3A & C4AF reactions are not important as they impart setting of cement and colour.
Cement + water reaction is exothermic. So it liberates more heat. Also C3S, C3A and cement finess are the three basic components which causes more heat liberation while mixing with water.
