Portland Pozzolana Cement (PPC)

Portland Pozzolana Cement (PPC)

Pozzolana essentially means a silicious material having no cementing properties in itself. But in finely divided form it reacts with Ca(OH)2 in presence of water at ordinary temperature and forms compounds possessing cementing properties. In this process Ca(OH)2

is also utilized which otherwise is destined to leaching out of concrete leaving a porous and weaker concrete.

 The chemical reaction which takes place is as given below:

 2C3S + 6H = C3S2H3 + 3 CH

Ca(OH)2 + SiO2 + Al2O3 = C3S2H3+ other compounds

 Generally fly-ash, shale, volcanic ash, opaline, diatomaceous earth etc. are used as Pozzolanas. It is easier to grind together OPC clinkers with pozzolana than mixing the pozzolana afterwards at the time of mixing. Mixing afterwards is difficult because of Pozzolana being finer than cement. Fineness of Pozzolana is 300 m2/kg as compared to 225 m2/kg for OPC.

 Fly-ash is a waste product of thermal plants which is collected by electro-statically precipitating the exhaust fumes of coal-fired power stations. The use of fly-ash is an eco-friendly process as precipitation of fly-ash reduces the pollution. The quality of fly ash is tested. The code specifies that percentage of silica plus alumina should be at least 70% and maximum loss on ignition should be maximum 12%. Generally much better quality is available from power plants as compared to the codal specifications. The obvious advantage of using PPC is that impermeable and denser concrete is produced by blending OPC with fly-ash.

The early strength of PPC is contributed by OPC fraction and Pozzolana also starts contributing after sometime. This early strength should normally be not less than 22 MPa in 7 days and not less than 31 MPa in 28 days. Other properties of PPC like soundness, setting time etc. are same as OPC.

 Advantages of Pozzolana cement

 1.   Less heat of hydration

2.   Reduced leaching of Ca(OH)2 as it is utilized in reaction with Pozzolana.

3.   It can be used at almost all the places where OPC is used.