Basics in Building Construction

Basics in Building Construction 4(2+2)

Lesson 12 and 13: Basic Building Construction Materials

Cement

The modern Portland cement was invented in the 18th century. In 1824, an English brick layer, by name Joseph Aspadin has invented the modem portland cement. In India the production of cement was started in the early years of the 19th century and at present several big and small industrial units are producing different kinds of cement. The name Portland is imparted to the cement because, after setting it resembles a kind of stone plentily available in Portland of England.

Cement is the chief active binding constituent of concrete and it accounts up to 10% of the volume of the concrete. By adjusting the amounts of cement, concrete of various strengths can be obtained. Cement possesses good adhesive and cohesive properties when mixed with water. This enables cement to bind the inert materials i.e.,aggregates together firmly to form a solid mass.

Hydration of Cement
The process of hardening of cement when mixed with water is called as hydration of cement. Heat developed during the process of hydration is called as heat of hydration. In case of mass concrete works heat of hydration is to be controlled greatly to avoid internal cracks, which otherwise may lead to total failure.

Setting Time
After mixing portland cement with water, it becomes plastic. This paste like form will gradually become less plastic and finally becomes a hard stone like mass. The time taken for a cement paste to become hard after mixing with water is called as 'Setting time'. The setting time is divided into two parts:

1. Initial setting time: The time at which the cement paste loses plasticity after mixing with water is called as 'Initial Selling time'.
2. Final setting time: The time taken for the cement water paste to become hard mass is called as 'Final setting time'.

The setting time can be either increased or reduced by mixing suitable admixtures. As per the Indian Standard Specification for Ordinary Portland cement the initial setting time should not be less than 30 minutes and the final setting time should not be more than 600 minutes.

Classification and Composition of Cement