Q: What is Cement?
A: Cement is a fine mineral powder with hydraulic properties. Mixed with water it becomes apaste able to set and harden slowly, even without air, and under water. There are four main oxides in cement, these are:
- lime (CaO)
- silica (SiO2)
- alumina (Al2O3)
- iron oxide (Fe2O3)
These elements are obtained from natural materials and sometimes from industrial by-products.
The most common type of cement in use is known as Portland Pozzolana Cement.
Q: What is blended hydraulic cement?
A: Hydraulic cement consists of two or more inorganic constituents (at least one of which is not Portland cement or Portland cement clinker) which separately or in combination contribute to the strength-gaining properties of the cement. Ref. ASTM C219
Q: Why is Portland cement so named?
A: Joseph Aspdin gave Portland cement its name in 1824 after realizing that its colour after hardening, resembled that of the natural limestone quarried from the isle of Portland in the English Channel.
Q: What are the wet and dry processes for the manufacture of clinker for cement?
A: Clinker for cement can be manufactured by either of two generic processes: the wet or the dry process.
The wet process (used by TCL) involves the introduction of water into crushed limestone to form aslurry. This slurry is then dried in the kilns at a temperature of approximately 200 degrees Celsius before being “cooked” in the same kiln at a temperature of about 1300 degrees Celsius. The hot raw meal is now converted into a form known as clinker. This is then milled together with gypsum and pozzolan at certain proportions (depending on the type of cement manufactured) to a specific particle size to form cement.
The dry process on the contrary, introduces limestone in a dry form into the kiln instead of as the slurry form. The clinker emerging from the kilns then follow the same processing as in the wet process.
Q: What is the purpose of Gypsum in cement?
A: The main function of gypsum in cement is to regulate the setting time of the cement paste, however, it is also known to play a part in controlling the rate at which the cement paste develops strength at early ages and to control shrinkage of the cement products during drying. Ref Gypsum & Anhydrite in Portland Cement.
In the absence of gypsum, freshly mixed concrete will not have a “working life” and will set immediately after mixing. The presence of gypsum in cement must be tightly controlled. Too much gypsum causes the concrete mix to get pliable, like putty. In such a condition, the mix will not set, a phenomenon known as “false setting”. Too little gypsum causes what is known as flash setting, where the concrete mix sets too quickly before proper working or placement.
Q: What are the basic ingredients of cement?
A: Regular Ordinary Portland Cement consists primarily of clinker (approximately 95%) The other 5% is accounted for by gypsum.
Depending on the quality of limestone (predominantly calcium carbonate), it can contain varying quantities of iron oxide, Silica, Aluminum Oxide, Calcium Oxide and Magnesium Oxide. The heating of the limestone in the kilns initiates a number of chemical reactions, which transforms these oxides into various other chemical compounds some of which include Tri Calcium Silicate, Di Calcium Silicate, Tri Calcium Aluminate, and Tetra Calcium Aluminoferrite. All of these compounds being contained in clinker.
Blended cement is a combination of clinker, gypsum and pozzolan.