It is an open secret that the main reason why human beings live together in a community is because they are not self-sufficient. Living in a community or city makes it easier to acquire goods that one lacks the capacity or competitive advantage to produce. What strengthens and holds together the fabric of this community is virtue like honesty, industry, fortitude and bravery. The opposite scenario-a society where people look down upon virtue, if taken to its logical extreme is a nightmarish gangland anarchy. That is why virtue is critical and should never be taken lightly because it makes a big difference in the quality of life people lead.
In building construction, cement works more or less like virtue holding building parts together by binding foundation and wall courses together permanently, one brick at a time. Besides that, cement also gives additional strength and durability to structures as well as an eye catching finish.
Cement is a grey powder with a fascinating characteristic that makes it essential in building construction. A chemical reaction is set off, when it’s mixed with water, which makes the cement material to hold fast together and harden with passage of time. This is why it’s used to make mortar for boding foundation and wall courses as well as making concrete.
Depending on the construction specifications, the Mason may use Lime or Portland cement. The Lime may be used on its own as a bonding material in mortar- to give the mortar higher water retention capacity and increase its cohesion so that it can be spread more easily. However Lime is also used in the manufacture of Portland cement .In other words, cement is lime that has undergone further refining processing during which time other ingredients are added to it.
Cement is used in variety of ways like in making mortar (mix of sand, cement and water), making Ballast (mix of cement, gravel and water), or mixing concrete (mix ballast, sand and water)-to name a few applications.
Lime like Cement is manufactured from Limestone rocks. These rocks are found in handsome supply in Makueni and Coast counties in Kenya, explaining why all cement factories are located in these regions.
Lime is manufactured by changing calcium carbonate (crushed limestone rocks) to calcium oxide.
CaCO3 ——————–950°C————————–⇒CaO + CO2
Calcium carbonate Lime + Carbon dioxide
The crushed limestone rocks is burnt at temperature of 950 degrees Celsius to produce Lime and carbon dioxide as a byproduct. When water is added to lime the chemical reaction produces hydrated lime powder which is calcium hydroxide.
CaO + H2O——————————————⇒Ca (OH) 2
Lime + water Calcium hydroxide
Lime is used in mortar and in manufacture of certain aerated concrete blocks. Mortar made up of lime is flexible and the presence of un-carbonated lime may make minor cracks in finished work to heal by the action of rain water. This because lime hardens by absorbing carbon dioxide from the air which progressively reverse the process used to manufacture it – converting calcium oxide to calcium carbonate.
Hydrated lime react with moisture and carbon dioxide from the air and should be kept in a sheltered cool dry room and used while fresh from opened packaging.
The common Cement you are familiar with, is often referred to as Portland cement (although there are other cement types that do not qualify to be called as such). The name Portland is derived from Portland stones (which is a type of limestone rocks) mined in the isle of Portland, Dorset-England. The Portland stones has been used extensively in major public buildings in Great Britain. This begs the question :how did this name come to be applied to common cement? .Well the name Portland cement was , initially used to market and promote cement by likening to Portland stones-a historically prestigious building material that was selling like hot cakes. With passage of time, cement became a household name and a product of necessity in building construction. As a result the original meaning and purpose lost their significance but the name stuck. Therefore it fair to say that, there nothing “Portland” in any Portland cement. It is just name-end of that discussion!
Cement is a child of hydrated lime-that is to say it was developed from the Lime. The raw material used to make cement are limestone or chalk, shale and clay which are low cost and widespread -making cement a low cost material. The manufacturing process involves burning crushed limestone or chalk in kiln to eliminate carbon dioxide and then adding to the powder clay and iron oxide plus other minor mineral like sodium and potassium.
Portland cement is hydraulic or uses water to work. When it is mixed with water it forms a paste which sets and hardens as a result of chemical reactions between its constituent compounds and water. This setting and hardening does not depend on drying but on the chemical reaction. In fact Portland cement will harden under water. Additional water is needed to make it easy to work with cement when aggregated are added to it.
To prevent corrosion of reinforced steel in concrete, manufacturer will limit the amount of sodium and potassium oxides in cement they produce and sell.
Water proof cement.
This is manufactured by adding hydrophobic materials such as stearates and oleates which works by coating the surface of pores and by surface tension effects which discourage penetration of water molecules. Also the compounds reduces the water –cement ratio which in turn reduces the pore sizes in concrete. This cement is used in making underground water cisterns among other uses.
Price and uses
As already mentioned above, cement is used to make mortar for binding brick, stone and concrete block works. Again cement is also used in concrete block manufacture as well as a plastering finish on floor and walls.
The price of cement in Kenya range between Kshs 500 to Kshs 800 per bag. Common Portland cement brands in Kenya are Bamburi Cement, Portland cement, Rhino cement and Savannah cement.
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Sources:Material for Architects and Builders by Arthur Lyons -PhD.