What is Portland Pozzolana Cement (PPC)?
The Portland Pozzolana cement is a mixture of Ordinary Portland cement (OPC) clinkers and pozzolana materials. PPC is a type of blended cement. It consists of 15-35% pozzolanic material, 4% gypsum and the rest is clinker.
The pozzolanic materials are reactive siliceous and aluminous materials which themselves possess little or no cementitious properties. But when the cement is mixed with water, during the hydration reaction, pozzolans react with one of the hydrated products, Ca(OH)2, and produce compounds that possess cementitious properties.
History of Pozzolana cement
The ancient Greeks and Romans became aware of the fact that certain volcanic ash, when mixed with lime yielded mortar that possesses superior strength and durability. Hence Roman builders began to use volcanic ash found near Pozzuoli village near Mount Vesuvius in Italy. These volcanic ashes are mostly siliceous (silica rich) in nature and thus acquired the name Pozzolana.
Later on, the name Pozzolana was applied to any other material either natural or artificial, having nearly the same composition as that of volcanic ash found at Pozzuoli.
Types of Pozzolana
The pozzolanic materials can be grouped into two major categories namely,
- Natural Pozzolans
- Artificial Pozzolans
- Volcanic tuff
- Diatomaceous Earth
- Fly Ash
- Silica Fume
- Ground Granulated Blast Furnace Slag (GGBFS)
- Burnt clay
- Rice husk ash
The natural pozzolans needed further grinding and in some cases require calcining to exhibit pozzolanic activities. The natural pozzolans have lost their popularity because of the availability of more active artificial pozzolans.
It has been mentioned in this article, that the products of hydration are C-S-H gel, Calcium hydroxide Ca(OH)2, Calcium Aluminate hydrate (C3AH6), and Ettringite. Among these, Ca(OH)2 can be seen as one of the most undesirable compounds because it is soluble in water and easily pave the way for sulphate attack and carbonate attacks.
Reactive silica from pozzolans reacts with Ca(OH)2 in the presence of water and produces Calcium Silicate Hydrate (CSH) gel. This reaction enhances the properties and durability of concrete because a non-cementitious product (Ca(OH)2) is converted into a cementitious product (CSH). This is called as Pozzolanic reaction.
Pozzolan + Calcium Hydroxide + Water → C – S – H (Gel)
Advantages of PPC
- Portland Pozzolana Cement (PPC) is more economical than OPC since the costly clinker is replaced by cheaper pozzolanic materials.
- PPC has better durability since soluble Ca(OH)2 is converted into insoluble cementitious products (CSH gel). Hence in places like the marine environment, PPC exhibits better impermeability
- In PPC there is a low risk of being affected by either Sulphate attacks or Carbonate attacks. Because, one of the reactants of the two reactions i.e., Ca(OH)2 amount is significantly reduced in PPC as shown above.
- PPC generates less heat of hydration. Even the rate of hydration is also slower.
- As PPC is finer than OPC, due to pozzolanic reaction, it increases the pore size distribution and reduces the micro-cracks at the transition zone.
- The long-term strength of Portland Pozzolana Cement (PPC) is higher than the OPC because of the conversion of Ca(OH)2, a non-cementitious product into CSH gel, a cementitious product.
Note that the above advantages can only be enjoyed only if the optimum amount of pozzolana is added during the manufacturing process and the curing should be done properly.
Disadvantages of PPC
- The rate of development of strength is initially slightly slower than OPC.
- The reduction of alkalinity (Ca(OH)2) may reduce the resistance to corrosion of steel reinforcement. But it also increases the impermeability nature, hence this reduction may not affect as much.
- The setting and curing time is nominally longer than OPC.
Application of PPC
Portland Pozzolana Cement (PPC) can be suitably used in the following situations,
- For hydraulic structures
- For mass concrete structures such as dams, piers, etc.
- For marine structures
- For sewage disposal works
PPC cannot be used in 2 conditions,
- Where high early strength is a requirement.
- Where the curing period is shorter.
Difference between OPC and PPC
|1. It has higher early stage strength than PPC||1. It has higher later stage strength than OPC|
|2. It has high Heat of hydration comparable to PPC. Hence not preferable in mass concreting works.||2. It has less Heat of hydration than OPC hence preferably used in mass concreting works.|
|3. OPC is not suitable for aggressive environments like marine, coastal areas since it has high amount of Ca(OH)2, it is more prone to sulphate or carbonate attacks. Shortly, OPC is less resistance to chemical attacks.||3. PPC is suitable for aggressive environments like marine, coastal areas since the Ca(OH)2 amount is converted into CSH, it is less prone to sulphate or carbonate attacks. PPC is more resistance to chemical attacks.|
|4. OPC is less impermeable than PPC||4. PPC is more impermeable than OPC because of pore refining process.|
|5. Setting time of OPC is lower than that of PPC||5. PPC has more setting time than OPC provided we can have more time for finishing.|
|6. Curing period is shorter for OPC. Acc. to IS 456, cl 13.5.1, Min curing days for OPC is 7 days.||6. Curing period is longer than OPC. Acc. to IS 456, cl 13.5.1, Min curing days for PPC is 10 days.(if possible increase it to 14 days)|
|7. OPC is costlier than PPC.||7. PPC is cheaper than OPC|
PPC Vs OPC, Which is better?
The one-line answer to this question is it depends. Yes, both cement exhibit certain unique characteristics, hence it is upon us to decide where we can use them to reap the utmost benefits. Consider the following situations as an example,
- If you want to do footing concreting in soil that has aggressive chemicals instead of OPC, we can go for Portland Pozzolana Cement since it provides better impermeability characteristics.
- For mass concreting works, it is advisable to use PPC since it has lower Heat of hydration.
- If you need more time in concreting works like column starter works or finishing works, It is advisable to use PPC cement since it sets slower than OPC and buys us some extra time.
- If you need to do concreting in hot weather conditions, it is advisable to use OPC because PPC requires a moist environment and longer curing time to reap its benefits, as it is not possible in hot climatic conditions we can go for OPC. (If Suitable curing arrangements can be made, then PPC is also a viable choice).
From the above examples, it is seen that it’s all about the choices and the place we decide to use them. Hence, understand the characteristics of cement and decide further by considering all other possibilities.
Hope this article gives complete knowledge about Portland Pozzolana Cement (PPC) and if you have any queries leave it in the comment section.