Question

Gypsum is added to cement to:
(A) prolong hydration
(B) increase strength after hydration
(C) decrease the heat of hydration
(D) reduce curing time

Answer 1

Hint: Gypsum is a mineral with the chemical formula: $ { CaSO }_{ 4 }.2{ H }_{ 2 }O$. Cement adheres to other materials and binds them together. Gypsum is added to the cement in order to control its hardening process.

Complete step by step solution:
In order to solve this question we first need to understand the properties of gypsum and what it is chemically.
Gypsum is a mineral with the chemical formula: $ { CaSO }_{ 4 }.2{ H }_{ 2 }O$. It is moderately soluble in water and its solubility decreases with an increase in temperature. When it is heated in air, it first converts to calcium sulphate hemihydrate and then to anhydrous calcium sulphate.
The natural crystals of gypsum are flattened and twinned which are transparent. These crystals are called selenite. Sometimes selenite also happens to present in a silky fibrous form and is called “satin spar”. Selenite could also be granular or compact. For example, alabaster is a very fine-grained white or slightly tinted type of gypsum which is used for ornamental work.
Now let us gain some knowledge about cement. Cement adheres to other materials and binds them together. The cement that is used is generally lime and calcium silicate-based. They can be further divided into non-hydraulic (does not set in the presence of water rather it sets as it dries; reacting with the carbon dioxide in the air) and hydraulic (sets as the dry ingredients and water react chemically).
Non-hydraulic cement is actually slaked lime (calcium hydroxide) which hardens due to carbonation with carbon dioxide present in the air. The reaction is shown below:
$ Ca{ (OH) }_{ 2 }(s)+{ CO }_{ 2 }(g)\rightarrow { CaCO }_{ 3 }(s)+{ H }_{ 2 }O(l)$
Hydraulic cement undergoes hardening when water is added to the clinker minerals. These clinker minerals are Alite ($3CaO.{ SiO }_{ 2 }$), Belite ($2CaO.{ SiO }_{ 2 }$), Tricalcium aluminate ($3CaO.{ { Al }_{ 2 }O }_{ 3 }$) and Brownmillerite ($4CaO.{ { Al }_{ 2 }O }_{ 3 }.{ Fe }_{ 2 }{ O }_{ 3 }$). The manufacture of the cement typically involves three steps:
(i) A mixture of limestone and clay is grounded to make a “raw mix”.
(ii) The “raw mix” is heated to sintering temperature (around $ 1450^{ o }{ C }$) in a cement kiln.
(iii) The resulting clinker is grounded to produce the cement.
A small amount of gypsum is usually added at the final grinding stage in order to control the setting of the cement. Hence gypsum controls the rate of hardening of the cement.

Hence the correct answer is (A) prolong hydration.

Note: If gypsum is not added to the cement, then it can result in immediate condensation of cement clinker on the addition of water. This is caused by the fast dissolution of Tricalcium aluminate in water which produces calcium aluminate hydrate (a clotting agent). In the presence of gypsum, the Tricalcium aluminate reacts with the gypsum on hydration to produce calcium sulfoaluminate hydrate which forms a protective layer on the cement particles that hinders with the hydration of more Tricalcium aluminate leading to a delay in the time of the setting of the cement.