What is Portland Cement?
Portland cement (PC) is the principal binder that, upon hydration and setting, holds aggregates (coarse and fine) together in concrete. It has the ability to stand in water and increase its strength over time. This ability elicits the term hydraulic cement for this type of cement.
What is the Composition of PC?
PC is a manufactured product formed within narrow confines of specific elements and subjected to heat treatment to form a semi-finished product called clinker. Raw materials used include: Limestone, Shale, Iron Ore and Sand.
The principal elemental ingredients in the raw materials are:
o Calcium Oxide (CaO)
o Silicon (SiO2)
o Aluminum Oxide (Al2O3)
o Iron Oxide (Fe2O3) in addition to other elemental oxides.
It is possible sometimes to have all these elemental oxides in some limestones. Where the principal source, limestone, is deficient in any of the elemental oxides then the other ingredients are added in the right proportion in preparation for the manufacturing process.
The latter process comprise of three phases:
1. Grinding the materials into a powder form
2. Heating the powdered material in a kiln up to about 2700°F
3. Grinding the clinker formed with gypsum to form PC
When PC is mixed with water either to form a cement paste or concrete, in a process called hydration, a chemical reaction takes place and for simplicity sake two principal products are formed: 1) calcium silicate hydrate, C-S-H and 2) calcium hydroxide
Calcium Silicate Hydrate (C-S-H) is insoluble in water. It gives the title hydraulic to set PC's ability to stand in water without any detriment. Calcium hydroxide is soluble in water and leaches out of set concrete and in a two step approach first, the solution is drawn to the exterior wall of the structure and secondly, react with carbon dioxide of the atmosphere to form chalk, the white, efflorescence salt found on concrete walls.
MINERAL ADMIXTURE - Mineral admixtures used in concrete formulations contain oxides of silica and other oxides that are found in Portland cement. Examples of mineral admixtures include fly ash, silica fume and volcanic ash.
Mineral admixtures used in concrete may be referred to as Pozzolans, or materials that won't harden in water but in the presence of lime, react and harden to form a stone-like mass.
One important use of mineral admixtures in concrete is the ability of the silica component to react with the soluble calcium hydroxide formed in the hydrating cement to form an insoluble calcium silicate hydrate thus helping to form a dense, compact concrete/cement paste.
Concrete formulated with mineral admixtures are not likely to form efflorescence salts. The use of mineral admixtures in concrete helps resist sulfate attack and alkali-silica reactivity. In the fresh concrete the presence of mineral admixtures often improve the workability of the mix.
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