F. 7. Read the texts and then discuss them

a) Mineral Binding agents are powdery materials capable, when mixed with water, of forming a plastic paste which hardens gradually and becomes stone-like under the effect of different physical and chemical processes. This property of binding materials is utilized extensively in the preparation of building mortars and concrete, and also in the manufacture of artificial stone and products without burning.

Depending on their ability to harden and preserve strength in air or water surroundings, mineral binding agents are differentiated into two groups: air and hydraulic.

Air binding agents harden, preserve or increase strength only in air (building gypsum, air hardening lime, etc.). Hydraulic binding agents possess such properties not only in the air, but in water as well. This group of binders includes portland cement, pozzolana cement, clinker cement, etc.

b) The plaster mortar must be dense, possess a sufficient resistance to water or be waterproof to increase its strength. Mortars employed for interior plastering of walls and ceilings of dwellings must adhere well to the base and no shrinkage cracks are permissible.

The common basic properties of plasters of all kinds are: workability (placeability), good adhesion to the base, and absence of shrinkage cracking during hardening. Usually plaster is applied upon the porous base capable of drawing water from the applied fresh layer of plaster, besides, the outer side of the plaster layer being exposed to atmospheric air, and especially to sun rays and wind, also loses moisture rapidly. This two-sided loss of moisture, characteristic of lime and lime-gypsum mortars, accelerates the hardening process. But in the case of hydraulic mortars such loss of moisture is undesirable, for it results in dehydration of the mortar and disturbs the process of hardening. Therefore, hydraulic mortar plasters must be protected from the loss of moisture by periodic moistening, covering with wetted bast mats, plastic films, etc.

c) Air-hardening lime is widely used in building. Since lime possesses a high water-retaining ability, it is employed in the preparation of masonry. When mixed with portland cement, lime is employed for the preparation of mixed mortars, where it replaces the costly cement and serves as a plasticiser. Great quantities of air-hardening lime are used in the preparation of lime pozzolana cement. Lime is widely used in the production of artificial stone: lime-sand brick, silicate and foam-silicate products, slag-concrete blocks, etc. Lime is also used in the manufacture of paints.

Hydraulic lime is the name of the binding agent produced by moderate burning (not to caking) of marly limestones containing from 8 to 20 per cent of clay admixtures. Same as air-hardening lime, hydraulic lime is produced by burning the raw material mostly in kilns at a temperature ranging from 900 to 1000°C. Hydraulic lime can be used instead of other costly binding agents and can be procured from locally available raw materials. Hydraulic lime can also be employed in laying foundations underneath the level of ground water, provided for the first 7-14 days the structures are protected from a direct action of water.