Which cement is good?
As such there is no cement called good or bad cement. Blended cements (PPC & PSC) are superior to OPC as they have better technical benefits for improved durability.
Amongst blended cements, PSC is superior to PPC. Chemical composition of Slag and Fly ash differs. Chemical composition of Slag is almost similar to that of cement where as in case of Flyash does not contain lime. Hence Slag cement (PSC) helps in getting better and uniform properties in concrete in comparison to Flyash (PPC) cement.
Is Setting Time of PPC/PSC is slow in comparison to OPC?
It is a myth that blended cements sets slower. BIS specification for setting time is same for all these cements. In fact we compare the setting time of concrete / mortar made with these cements.
The rate of hardening of concrete made with PPC/PSC is slightly lower in comparison to concrete made with OPC. This is advantageous as higher rate of hardening will result in early loss of water from concrete which is highly undesirable. In fact concrete made with PSC will have a higher ultimate strength.
What is the significance of colour on the quality of cement?
Colour is not a property of the cement and does not have bearing on any of the properties of cement. Depending on oxide composition the shade of cement varies. In fact lighter coloured cements are preferred due to some benefits such as lesser consumption paint to plastered surface; lighter coloured surface absorbs less heat …etc.
Why the colour of PSC is lighter compared to other cements?
GGBS (Ground Granulated Blast furnace Slag) is off white in colour which is used in producing PSC. Hence the colour of PSC is lighter. Always lighter colour has advantages as stated above.
Why the colour of PPC is darker?
Flyash – a byproduct from thermal power plant is used in producing PPC. This fly ash consists of traces of unburnt carbon and other impurities hence PPC is darker in colour. However the colour of the cement has no bearing on the quality of cement.