The setting and hardening of cement after addition of water is due to:

The process of setting and hardening of cement after the addition of water is a complex chemical reaction that involves various compounds present in cement. The main factors responsible for this process are:

1. Hydration of constituent compounds:
When water is added to cement, a series of chemical reactions known as hydration takes place. The main compounds present in cement, such as tricalcium silicate (C3S) and dicalcium silicate (C2S), react with water to form calcium silicate hydrate (C-S-H) and calcium hydroxide (CH). These reactions contribute to the setting and hardening of cement.

2. Binding action of water:
Water plays a crucial role in the setting and hardening of cement. It acts as a medium for the chemical reactions to occur and also provides the necessary fluidity for the initial mixing and placement of the cement paste. The binding action of water helps in the formation of a solid and cohesive structure.

3. Evaporation of water:
While water is necessary for the hydration reactions, its evaporation is also an important factor in the setting and hardening process. As the cement paste sets, some of the water present in it evaporates, leading to a decrease in the water-cement ratio. This reduction in water content contributes to the strength development and hardening of the cement.

4. Presence of gypsum:
The presence of gypsum in cement can also influence the setting and hardening process. Gypsum, in the form of calcium sulfate dihydrate (CaSO4 · 2H2O), is added to cement to control the setting time. It acts as a retarder, slowing down the hydration reactions and extending the time available for workability. However, gypsum does not directly contribute to the strength development of cement.

In summary, the setting and hardening of cement after the addition of water are primarily due to the hydration of the constituent compounds of cement, the binding action of water, and the evaporation of water. The presence of gypsum can also affect the setting time of cement. Understanding these processes is crucial for the proper use and application of cement in construction.