In OPC, the main alkali is cal. Hydroxides formed from limestone, the cal. Carbonate. The second material in OPC is silica in the form of sio2 and aluminate to some extent. This proportion of silica and alumina wrt ca(oh) 2 mainly decides the properties of cement.
When fly ash or any other substances like slag, rice husk ash, quartz, or feldspar (all contains silica) is added to some extent with OPC - the result is PPC. This PPC takes a long time to harden, the hydration at a considerable rate even continues for months together. But after a prolonged period, it achieves qualities like good waterproofing, weather resistance, and strength equal to or greater than that of OPC, with a lower cost than 100%OPC.
This unique quality of OPC, that it achieves 90 % of its strength in 28 days, makes it vulnerable to replace lime or every cementitious material.
So to achieve the equal or more strength than OPC with less cost - activation of today's pozzolanic material, the coal fly ash, is mixed or activated with sodium hydroxide before it is acted with cal oxide/hydroxide. Experiments with diff. Percentage of activated fly ash has shown very encouraging results with a higher strength but not before three months curing. Research is going on with diff. percentage of silica and aluminate with sod. hydroxide to achieve quick strength.
In future this sod. hydroxide may replace the requirement of limestone completely for the making of cement.