(i) Chemical properties
Portland cement consists of the following chemical compounds:
(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 may be small quantities of impurifies present such as calcium oxide (CaO) and magnesium oxide (MgO).
When water is added to cement, C3A is the first to react and cause initial set. It generates great amount of heat. C3S hydrates early and develops strength in the first 28 days. It also generates heat. C2S is the next to hydrate. It hydrates slowly and is responsible for increase in ultimate strength. C4AF is comparatively inactive compound.
(ii) Physical properties
The following physical properties should be checked before selecting
a portland cement for the civil engineering works. IS 2691967 specifies the method of testing and prescribes the limits:
(a) Fineness (b) Setting time
(c) Soundness (d) Crushing strength.
(a) Fineness: It is measured in terms of percentage of weight retained 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 specification weight retained on the sieve should not be more than 10 per cent. In terms of specific surface should not be less than 2250 cm2/gm.
(b) Setting time: A period of 30 minutes as minimum setting time for initial setting and a maximum period of 600 minutes as maximum setting time is specified by IS code, provided the tests are conducted as per the procedure prescribed by IS 269-1967.
(c) Soundness: Once the concrete has hardened it is necessary to ensure that no volumetric changes takes place. The cement is said to be unsound, if it exhibits volumetric instability after hardening. IS code recommends test with Le Chatelier mould for testing this property. At the end of the test, the indicator of Le Chatelier mould should not expand by more than 10 mm.
(a) Crushing strength: For this mortar cubes are made with standard sand and tested in compression testing machine as per the specification of IS code. The minimum strength specified is 16 N/mm2 after 3 days and 22 N/mm2 after 7 days of curing.
