Civil Engineering Materials Mortar and Plaster Prepared by Md. Mahfuzur Rahman Assistant Lecturer, Department of Civil Engineering IUBAT

Mortar Mortar is a paste generally made by mixing cementing or binding material (lime or cement) and an inert material (sand or surki) with water.

Different types of mortar Lime mortar: Lime + Sand + Water Surki mortar: Lime + Surki + Water Lime Surki mortar: Lime + Sand + Surki + Water Cement mortar: Cement + Sand + Water Mud mortar: Mud + Saw dust/Rice husk/Cow dung + Water

Sand should be sharp, angular, porous and free from salts and other impurities. Surki should be perfectly clear, free from foreign matter and sufficiently fine to pass through No. 8 (US) sieve. Lime must be well slaked and cement must be fresh and free from adulteration. Water should be clean, free from salts and other impurities and should be of drinking water standard. Must be free from foreign matter.

Function of Sand in Mortar Sand in mortar is used mainly as an inert material to give volume for economy. It subdivides the paste of the cementing materials into a thin film, which is the basic principle involved in using all cementing materials. It offers the requisite surface areas for the film of cementing materials to adhere and to spread. It prevents shrinkage and cracking of mortar in setting.

A well graded sand adds to the density of the mortar. It allows carbon dioxide from the atmosphere to reach to some depth in case of fat lime mortars and thereby improves their setting power. It is also claimed that some chemical reactions take place between silica (SiO2) of sand grains and the constituents of the cementing materials to form a hardened mass.

Functions of Surki in Mortar In mortar, surki is used as an adulterant(a material which provides volume). Surki imparts strength and hydraulic property to mortar.

Uses of Mortar To bind together the bricks or stones in brick or stone masonry. To give a soft even bed between different layers of bricks or stones for making walls tight. In plastering works to hide the joints and to improve appearance. For moulding and ornamental purposes.

Preparation of Lime Mortar, Surki Mortar and Lime Surki Mortar Prepared by grinding the ingredients in a specific manner. For a steady and continuous supply of mortar as in the case of large construction, power driven mortar mills are employed.

First the cementing material is ground with water to the consistency of a thin and lean mix. Then sand or surki or both is added in the required proportion and grinding is done. During the process of grinding, the mixture is properly turned by a scraper and a requisite quantity of water is again added to impart the proper consistency.

The ratio of lime and sand or sand and surki together should be 1:2 or more. These are measured dry by volume. The mortar which has once set or which has been left for more than 24 hours after it has been mixed, should not be used. Use: temporary and important types of construction works.

Preparation of Cement Mortar Cement and sand in definite proportion are measured by volume. Cement and sand are mixed together in state. The quantity of water is calculated and only half of that is sprinkled on the dry mix. The remaining quantity is then added and the whole mixture is turned over twice or thrice to form cement mortar. Used in all important types of construction.

Curing of works in Lime and Cement Mortar All the mortar works should be kept wet for about 15 days for the purpose of curing. During this period important chemical reactions take place and the whole mass gains strength through the process of hardening. Occasional sprinkling with water is necessary until the work is set and become hardened.

Precautions in using mortars Should be mixed in small quantities so that it can be used conveniently before the mortar starts setting. Bricks or stones to be jointed together should be soaked in water very thoroughly at least for 12 hours. The joints should be well filled. Mortar works should be kept wetted for a week or two after it has been laid to prevent the rapid drying of mortar.

Plaster Is a thin coat of mortar of different composition which is applied on both external and internal faces of walls ceiling, columns, staircases etc. The process is termed as plastering.

Uses of Plaster Used to give a smooth and finished surface to the works. To cover joints and defective workmanship. To preserve surface from the action of weathering. To provide a ground for decoration or white or color washing and for moulding, ornamental and architectural works.

Specification of Good Plaster It should adhere firmly to the surface on which it is applied. It should not shrink or contract in volume on drying and setting, otherwise it cracks and gives an unsightly appearance. In case of lime plaster, it should not certain unslaked particles of lime which absorb moisture from the atmosphere and spoils the plastered surface.

Classification of Plaster Lime plaster – Lime : Sand = 1:2 to 3; 3/8 to ½ inch thick in one coat Surki plaster – Lime : Surki = 1:1.5 to 2; 3/8 to ½ inch thick in one coat Lime-surki plaster – Lime : Surki : Sand = 1:1:1; 3/8 to ½ inch in one coat

Sand Rubbing The external plastered surface of buildings is sometimes finished with sand rubbing which is a plaster composed of cement, lime and clean sand in the proportion 1:2:6. It is laid on plastered surface in thickness of 1/16 to 1/8 inch. The surface is thoroughly rubbed, smoothened and finished. The surface is to be kept moist for at least 2 days.