Most engineers, contractors, builders, masons and others who put up structures of whatever size and shape are most likely to use cement at some point in the process. Even graves or crematoria have cement in them. What is it that has endeared constructors to cement in human activities especially in infrastructure? Dr Alice Nabatanzi gives some clues.
What is cement?
Cement is a fine mineral powder made by finely grinding clinker (produced by burning a mixture of iron, calcium, aluminum and silica-containing materials) together with gypsum and other additives such as slag.
Cement is chiefly of two kinds based on the way it is set and hardened:
- Hydraulic cement which hardens due to the addition of water, and
- Non-hydraulic cement which is hardened by carbonation with the carbon present in the air, so it cannot be used underwater.
Non-hydraulic cement is produced through the following steps (lime cycle):
- Calcination: Lime is produced from limestone at over 825°C for about 10 hours. (CaCO3 → CaO + CO2)
- Slaking: Calcium oxide is mixed with water to make slaked lime. (CaO + H2O → Ca(OH)2)
- Setting: Water is completely evaporated.
- The cement is exposed to dry air and it hardens after time-consuming reactions. (Ca(OH)2 + CO2 → CaCO3 + H2O)
Hydraulic cement is mainly made up of silicates and oxides:
- Belite (2CaO·SiO2);
- Alite (3CaO·SiO2);
- Tricalcium aluminate/ Celite (3CaO·Al2O3)
- Brownmillerite (4CaO·Al2O3·Fe2O3)
The ingredients are processed in the kiln in cement plants.
Portland /Hydraulic cement
The most commonly used cement nowadays is hydraulic cement known as Portland cement. Portland cement is suitable for wet climates and can be used underwater. Different types or blends of Portland cement include Portland blast furnace slag cement, Portland fly-ash cement, Portland pozzolana cement, Portland-silica fume cement, masonry cement, expansive cement, white-blended cement, coloured cement and very finely ground cement.
Composition of Portland cement
The composition of cement is a matter of great interest to engineers. For understanding cement composition, one must know the functionality of cement ingredients. By altering the amount of an ingredient during cement production, one can achieve the desired cement quality. There are eight major ingredients of cement. The general percentage of these ingredients in cement is given below:
|Ingredient||Percentage in cement|
Functions of cement ingredients
The main features of these cement ingredients along with their functions and usefulness or harmfulness are given below:
|a) Lime: Calcium oxide or calcium hydroxide||Helps form silicates and aluminates of calcium||Reduces cement strength and causes cement to set quickly||Makes cement unsound and causes it to expand and disintegrate|
|b) Silica/silicon dioxide||Helps in the formation of di-calcium and tri-calcium silicates which imparts compressive strength, bond strength, and abrasion resistance||Prolonged setting time|
|c) Alumina/ Aluminium oxide.||Imparts quick setting property to the cement||Weakens the cement|
|Magnesia/ Magnesium oxide||It can reduce the burnability of raw meal, increase the content of free lime in clinker and decrease the strength of cement|
|Iron oxide||Imparts colour to cement, acts as a flux, at a very high temperature, it imparts into the chemical reaction with calcium and aluminum to form tricalcium alumino-ferrite and tricalcium alumino-ferrite imparts hardness and strength to cement|
|Calcium sulfate||This is present in cement in the form of gypsum. It slows down or retards the setting action of cement|
|Sulfur trioxide||Causes cement to unsound|
|Alkaline||Increases cement strength||Excess alkaline matter delays time of final setting, reduces comprehensive strength and increases rapid chloride ion permeability and causes efflorescence|
Setting and hardening of cement
Upon addition of water to Portland cement, a brief and intense hydration starts which is called pre-induction period, during which the calcium sulphate and alkalies dissolve completely, then short, hexagonal and needle-like crystals form at the surface of the clinker particles as a response of reaction between calcium and sulphate ions with tricalcium aluminate. Furthermore, calcium silicate hydrates in colloidal shape, caused by the formation of a thin layer of hydrated products on the clinker surface, and then first hydration period ceases.
Once the hydration period stops, the induction period starts, in which time no reaction takes place. The initial hydrated products are very feeble to couple the gap between the clinker particles and do not form a consolidated microstructure. Furthermore, the movement of the cement particles in accordance to one another is slightly affected.
The setting starts after one to three hours, when initial calcium silicate hydrate forms on the surface of the clinker particles, which are finely grounded in the beginning. After completion of hydration of clinker furthermore an intense hydration takes place. The next level of curing starts after four hours and ends within 24 hours. During this period a basic micro-structure forms consisting of C-SH needles and leaves, calcium hydroxide and ettringite ((Ca))6(Al2O3)(SO3)3.32H2O) crystals which grows in longitudinal shape. As the crystal grows, the gap between the cement particles tends to bridge. Furthermore, the hardening process steadily increases, but with a decreasing rate. The density of microstructure increases and pores get filled. The pores filling takes place with respect to time and the hydration process increases the compressive strength of cement.
Why do we blame cracks and shoddy work on cement?
The truth is that all cement in its right form works EXCEPT:
- When the manufacturer has deliberately given out a wrong product for which we have the Uganda National Bureau of Standards to save us
- When sellers adulterate the cement
- When builders adulterate the product on site
- When there is misconception and poor utilization of the product. Poor utilization of cement results in big-time mistakes, the best example being the cracks in Karuma Dam.