Chemical Properties of Cement
The raw materials for cement production are limestone (calcium), sand or clay (silicon), bauxite (aluminum) and iron ore, and may include shells, chalk, marl, shale, clay, blast furnace slag, slate.
Chemical analysis of cement raw materials provides insight into the chemical properties of cement.
1. Tricalcium aluminate (C3A)
Low content of C3A makes the cement sulfate-resistant. Gypsum reduces the hydration of C3A, which liberates a lot of heat in the early stages of hydration. C3A does not provide any more than a little amount of strength.
Type I cement: contains up to 3.5% SO3 (in cement having more than 8% C3A)
Type II cement: contains up to 3% SO3 (in cement having less than 8% C3A)
2. Tricalcium silicate (C3S)
C3S causes rapid hydration as well as hardening and is responsible for the cement’s early strength gain an initial setting.
3. Dicalcium silicate (C2S)
As opposed to tricalcium silicate, which helps early strength gain, dicalcium silicate in cement helps the strength gain after one week.
4. Ferrite (C4AF)
Ferrite is a fluxing agent. It reduces the melting temperature of the raw materials in the kiln from 3,000°F to 2,600°F. Though it hydrates rapidly, it does not contribute much to the strength of the cement.
5. Magnesia (MgO)
The manufacturing process of Portland cement uses magnesia as a raw material in dry process plants. An excess amount of magnesia may make the cement unsound and expansive, but a little amount of it can add strength to the cement. Production of MgO-based cement also causes less CO2 emission. All cement is limited to a content of 6% MgO.
6. Sulphur trioxide
Sulfur trioxide in excess amount can make cement unsound.
7. Iron oxide/ Ferric oxide
Aside from adding strength and hardness, iron oxide or ferric oxide is mainly responsible for the color of the cement.
The amounts of potassium oxide (K2O) and sodium oxide (Na2O) determine the alkali content of the cement. Cement containing large amounts of alkali can cause some difficulty in regulating the setting time of cement. Low alkali cement, when used with calcium chloride in concrete, can cause discoloration. In slag-lime cement, ground granulated blast furnace slag is not hydraulic on its own but is “activated” by addition of alkalis. There is an optional limit in total alkali content of 0.60%,
calculated by the equation Na2O + 0.658 K2O.
9. Free lime
Free lime, which is sometimes present in cement, may cause expansion.
10. Silica fumes
Silica fume is added to cement concrete in order to improve a variety of properties, especially compressive strength, abrasion resistance and bond strength. Though setting time is prolonged by the addition of silica fume, it can grant exceptionally high strength. Hence, Portland cement containing 5- 20% silica fume is usually produced for Portland cement projects that require high strength.
Cement containing high alumina has the ability to withstand frigid temperatures since alumina is chemical-resistant. It also quickens the setting but weakens the cement.