Lesson 12 and 13: Basic Building Construction Materials
Special Cements
Some kind of cements are used for special purposes under certain conditions to satisfy specific requirements. Some of them are masonry cement, Trief Cement, Expansive cement and Oil well cement.
For a long time, lime gauged with sand was used for mortar for laying brick work. However, in order to increase the strength and rapidity of gaining strength, it became common to mix Portland cement with the lime. The usual proportion of cement: lime: sand may range from 1:1:6 for heavy loads to 1:3:12 for light loads. Cement sand mortars are too harsh, while lime makes the mortar easier to work. In order to avoid the necessity for mixing cement and lime, masonry cements have recently been introduced. According to Wuerpel, most successful masonry cement are composed of Portland cement clinker, lime stone, gypsum, and air entraining agent. These constituents are ground to an even grater fineness than that of high strength Portland cement. The plasticity and workability of masonry cement are imparted by the lime stone and air entraining agent. The ease of working masonry cement and their water retentive properties helps to increase their adhesion to bricks or other building units and this is further assisted by the fact their shrinkage is fairly low.
Trief cement is practically the same blast furnace cement except that the blast furnace slag is ground wet and separately from the cement. Wet grinding results in a fine products, with a specific surface of atleast 3000 cm2 / gm. Due to this, the slow rate of gain of strength normally associated with blast furnace cement is avoided and strength from early ages equal to those is ordinary Portland cement are obtained. This cement has smaller shrinkage and smaller heat of evolution while setting than ordinary Portland cement.
Expansive cement expands while hardening. Ordinarily, concrete shrinks while hardening, resulting in shrinkage cracks. This can be avoided by mixing Expansive cement with normal cement in the concrete, which will neither shrink nor expand. Another useful application of Expansive cement is in repair work where the opened up joints can be filled with this cement so that after expansion a tight joint is obtained.
Oil well cements:
In the drilling of oil wells, cement is used to fill the space between the steel lining tube and the wall of the well, and to grout up porous strata and to prevent water or gas from gaining access to oil – bearing strata. The cement used may be subject to very high pressure and the temperature may rise to 400 F. Cement used must be capable of being pumped for upto about 3 hours. It must harden quickly after setting. These properties can be achieved by (a) adjusting the composing of the cement and (b) by adding retarders to ordinary Portland cement.
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