Properties of Ordinary Portland Cement
(i) Chemical properties: Portland cement consists chemical compounds are following:
(a) Tricalcium silicate 3 CaO.SiO2 (C3S) 40%
(b) Dicalcium silicate 2CaO.SiO2 (C2S) 30%
(c) Tricalcium aluminate 3CaO.Al2O3 (C3A) 11%
(d) Tetracalcium aluminate 4CaO.Al2O3.Fe2O3 (C3AF) 11%
There can be small quantities of impurities present such as calcium oxide (CaO) and magnesium oxide (MgO).
When water is added to cement, C3A is the first to react and it cause initial set. It produce great amount of heat. Early C3S hydrates and develops strength in the first 28 days. It also produces heat. C2S is the next to hydrate. It hydrates slowly and it is responsible for increase in decisive strength. C4AF is comparatively inactive compound.
(ii) Physical properties: The following physical properties should be varified before choosing a portland cement for the civil engineering works. IS 269-1967 specifies the method of testing and prescribes the restriction:
1. Fineness
2. Setting time
3. Soundness
4. Crushing strength.
(1) Fineness: It is measured in terms of percentage of weight received after sieving the cement through 90 micron sieve or by surface area of cement in square centimeters per gramme of cement. According to IS code condition weight received on the sieve should not be more than 10 %. In terms of particular surface should not be less than 2250 cm2/gm.
(2) Setting time: For initial setting a time period of 30 minutes as minimum setting time and a maximum period of 600 minutes as maximum setting time is specified by IS code, provided the tests are conducted as per the process prescribed by IS 269-1967.
(3) Soundness: Once the concrete has hardened it is essential to ensure that no volumetric changes occur. The cement is said to be unsound, after hardening if it exhibits volumetric instability. IS code recommends test with Le Chatelier mould for testing this specific property. At the end of the test, the indicator of Le Chatelier mould should not enlarge by more than 10 mm.
(4) Crushing strength: For this mortar cubes are composed with standard sand and tested in compression testing machine as per the specification of IS code. The minimum strength mentions is16 N/mm2 after 3 days and 22 N/mm2 after 7 days of curing.