Advantages of PPC in Fresh Concrete
Portland pozzolana cement (PPC) has following advantages when concrete is in its fresh state.
Portland pozzolana cement has spherical cement particles and they have higher fineness value. Due to the spherical shape concrete move more freely and more fineness of particles allows better filling of the pores. This type of cement also gives better cohesiveness to concrete. PPC cement also reduces the rate of slump loss of concrete as compared to concrete made with ordinary cement, particularly in hot weather condition.
Bleeding is a type of segregation in which some of the water in the concrete mix tens to rise to the surface of fresh concrete. As a result of bleeding, the top surface becomes too wet and concrete will become porous, weak and non durable. PPC cement reduces bleeding by providing greater fines volume and lower water content for a given workability. This also helps to block bleed water channels.
PPC cement helps to produce more cohesive concrete and is less prone to segregation & bleeding. The spherical shape of particles serves to increase workability and pumpability by decreasing friction between aggregate particles and between concrete & pump line.
4. Setting Time & Finishability
PPC cement slightly prolongs the setting time of concrete which helps the mason for good finishing of concrete or cement mortar. The cohesiveness of concrete mix helps for better finishing of concrete.
Advantages of PPC in Hardened Concrete
Portland pozzolana cement (PPC) has following advantages when concrete is in its hardened state.
1. Compressive Strength & Rate of Strength Gain
The strength & rate of strength gain of concrete made with PPC will be equivalent to ordinary concrete at 28 days. The silicate formation of PPC continues even after the rate of hydration of ordinary cement slows down. This results in increased strength gain at later ages. This higher rate of strength gain will continue with time and result in higher later age strength.
2. Modulus of Elasticity
The modulus of elasticity of PPC concrete is somewhat lower at early ages and little higher at later ages than ordinary concrete.
3. Bond of Concrete to Steel
The bond or adhesion of concrete to steel is dependent on the contact area of steel with concrete, the depth of reinforcement & density of concrete. PPC being finer in nature usually increases paste volume & reduce bleeding thus the contact will be increased, resulting into improved bond with steel.
4. Heat of Hydration
The hydration of PPC is a slower process than hydration of ordinary cement, resulting into slower heat generation and lower internal stresses in concrete. Thus PPC becomes ideal cement for mass concreting like dams, retaining walls, large foundation etc.
5. Reduced Shrinkage
PPC in concrete helps to reduce drying shrinkage & plastic shrinkage. Drying shrinkage is reduced because of lower internal concrete stresses & slower heat generation. Plastic shrinkage is also reduced considerably because concrete bleeds less at a given slump or workability by using PPC.
If concrete has interconnecting void spaces, then the concrete becomes permeable. In PPC concrete, the lime [Ca (OH)2] liberated during initial hydration is consumed by reactive silica & forms an insoluble cementitious compound instead of leaching on the concrete surface. This helps in reducing void spaces & also blocks capillary channels & subsequently reduces permeability of concrete.