Portland Pozzolana Cement (PPC) For Construction - Pozzolanic Reaction & Strength

Portland Pozzolana Cement (PPC) is also called as fly ash blended cement. The main parameter that makes an ordinary portland cement (OPC) differ from PPC is the fly ash content. Lets discuss each and every feature of PPC in simple explanation.

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Ingredients of Portland Pozzolana Cement (PPC)

The main ingredients of PPC manufacture are:

1. High Quality Portland Cement Clinkers (65 to 80%)
2. Gypsum (3-5%)
3. Pozzolanic Materials (15 to 35%)

All the above mentioned ingredients are blended in required proportions and are grinded thoroughly to produce PPC. 

As per IS: 1489, Part I, the fly ash used must be not less than 15 percent and not greater than 35%, by mass of PPC.

Pozzolana in PPC

The pozzolana used in PPC can either be natural or artificial material. In natural form it is clay & shale, volcanic tuffs, diatomaceous earth. Artificial forms of pozzolana are fly ash and rice husk.

The most commonly used pozzolana are fly ash. Fly ash is a fine material which is not a cementitious material, but it undergoes reaction with lime in the presence of water under ambient conditions to give a similar products and hydrated structure to that of ordinary portland cement concrete or mortar.

Also Read: Hydration Reaction in Ordinary Portland Cement

Hydration Reaction of Portland Pozzolana Cement (PPC)

We know, the important hydration products of OPC mortar or concrete are C-S-H Gel. The reaction results in excess amount of calcium hydroxide Ca (OH)2. These unreacted and problem causing byproduct, calcium hydroxide formed in PPC reacts with fly ash material and produce C-S-H gel. Hence, Ca (OH)2 is utilised in PPC to produce more CSH that glue the concrete aggregate.

The pozzolanic reaction continues till the complete CH produced or the pozzolan used in concrete is consumed.

Hence, quality production of C-S-H gel give a compact structure to PPC concrete. This hence increases the durability of concrete and strength over a period of time.

Benefits of Pozzolanic Reaction in PPC Concrete

1. Pozzolanic reaction in PPC cement undergoes  continued hydration. This provides, higher long term strength, reduced heat of hydration and improved resistance to chemical attack.

2. Pozzolanic reaction consumes less water compared to OPC hydration reaction. It consumes around 7.5 to 4 percent water hence reducing bleeding issues, shrinkage and creep issues and permeability issues.

3. Pozzolanic reaction facilitates cohesion within the mixture. Cohesion thus improved can reduce segregation and issues faced during concrete placement.

How Pozzolana Reaction Improve Concrete Strength?

The pozzolanic reaction performed in PPC is a secondary reaction which is a super charger reaction to the primary reaction of portland cement and water. 

Fine Size of Pozzolana Improve Concrete Strength

One important feature of fly ash or pozzolana are their fine particle size. As they are finer than cement particles, they have larger surface area to perform hydration reaction. Hence, there is reduction of amount of trapped water between the cement particles, as major water get reacted.

Pozzolana Reaction Products in PPC

Particle Packing Effect of Pozzolana Improves Concrete Strength

The fine size of pozzolana have the ability to fill the voids, thus reducing the overall voids in concrete. Fly ash is hence an efficient void filler compared to OPC. This will also reduce the amount of water content to obtain given consistency.

Improve Concrete Resistance Against Chemical Attack

Excess amount of CH in concrete, during age migrated out of the concrete interior through capillary action, which leaves behind a voids and makes concrete porous. These voids, later invite the ingress of water with chlorides, sulphates and other damaging chemical. In cold climates, freeze-thaw damage can also damage the concrete.

The use of pozzolans in concrete reduce the presence of the cement hydration product i.e. the calcium hydroxide (C-H) and boosts concrete to be highly chemically resistant.

Ball Bearing Effect in Pozzolana Particles in PPC

The fly ash particles posses spherical shape, that reduces inter-particle friction. This facilitates mobility of mix and improves rheology of concrete.

Improves Chloride Resistance

Natural pozzolan in concrete provide a densely packed concrete that resist the penetration of liquids or gases that can result in steel reinforcement to corrode.

Mitigate Alkali Aggregate Reaction in Concrete

Alkali-aggregate reaction in concrete is facilitated by the presence of alkalis presence in the concrete either in the aggregates or through the by products of concrete hydration.

As natural pozzolans readily reacts with calcium hydroxide and trap any possible alkalis in concrete, the chances of alkali-silica reaction and efflorescence are reduced.

Also Read: What is Rheology of Concrete?