Контрольное задание 6 вариант 1

I. Прочтите и переведите текст Mortar

Mortar is the matrix used in the beds and side joints on brick-work and for plastering walls and floors. Mortar consists of an inert aggregate bound by a cementing material. The usual cementing materials used for constructional work are hydraulic limes or Portland cement. 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, 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.

Hydraulic lime can also be employed in laying foundations, provided for the first 7-14 days the structures are protected from a direct action of water.

II. Ответьте на следующий вопрос:

What are the basic properties of plasters?

Вариант 2

1. Прочтите и переведите текст Building Gypsum

Building gypsum is the name of an air binding agent produced by heat treatment of natural gypsum and subsequent grinding of the burned product. The basic process in the production of building gypsum is dehydration of natural gypsum. When heated to a temperature ranging from 140 to 170°C, natural gypsum rapidly decomposes into calcined gypsum and water. In manufacturing building gypsum careful attention should be paid to the burning temperature.

High-strength gypsum is produced by burning gypsum rock in special hermetic autoclaves at a steam t° of 125° C. As the moisture evaporates from the gypsum rock, the pressure in the autoclaves rises to 1.3 atm (gauge). The gypsum paste forming when gypsum is mixed with water gradually thickens and turns into a hard stone-like state. One must bear in mind that gypsum products must be dried at a temperature not exceeding 65-70° C, so as to prevent decomposition of the formed gypsum, for this may decrease its strength.

Strength and fineness of grinding are the most important properties of gypsum. When mixed with water, the gypsum paste rapidly sets, losing plasticity and binding ability in the process of setting, and these properties disappear at the end of setting altogether. The beginning of setting starts in 4-5 minutes and ends in 7-30 minutes after water is added.

Building gypsum is used in the manufacture of gypsum-concrete products and for casting moulds, patterns, architectural details and decorative products (cornices, rosettes, corners).

II. Ответьте на следующий вопрос:

How is high-strength gypsum produced?