Portland Pozzolana Cement | Manufacturing | Process| Advantages
What Is Portland Pozzolana Cement
The history of pozzolanic material goes back to the Romans’ civilization era. The descriptions and details of pozzolanic material will be dealt with separately under the article ‘Admixtures’. However, a brief description is given below.
What is Pozzolana? [ A Brife Overview]
Portland Pozzolana cement (PPC) is manufactured by the intergrinding of OPC clinker with 10 to 25 percent of pozzolanic material (as per the latest amendment, it is 15 to 35%). A pozzolanic material is essentially a siliceous or aluminous material which while in itself contains no cementitious properties, which will, in finely segregated form and in the presence of water, react with calcium hydroxide, liberated in the hydration process, at ordinary temperature, to form compounds containing cementitious properties.
The pozzolanic materials generally used for the manufacture of PPC are calcined clay (IS 1489 part 2 of 1991) or fly ash (IS1489 part I of 1991). Fly ash is a waste material, generated in the thermal power station when powdered coal is used as fuel. These are collected in the electrostatic precipitator. (It is called pulverized fuel ash in the UK).
It may be recalled that calcium silicates produce a considerable amount of calcium hydroxide, which is a by-product and large waste material from the point of view of strength and durability. If such waste mass could be converted into a useful cementitious product, it can considerably improve the quality of concrete. The usage of fly ash plays a similar role. The
pozzolanic action is shown below:
Calcium hydroxide + Pozzolana + water → C – S – H (gel)
Manufacturing of Portland Pozzolana cement
Portland pozzolana cement generates less heat of hydration and offers a greater amount of resistance to the attack of aggressive waters than ordinary Portland cement. Moreover, it reduces the wastage of calcium hydroxide when used particularly in hydraulic structures. It is particularly useful in marine and hydraulic construction and other mass concrete constructions like dams and ports etc.
Portland pozzolana cement can generally be used, where ordinary Portland cement is usable. However, it is important to appreciate that the addition of pozzolana does not contribute to strength at an early age. Strengths similar to those of ordinary Portland cement can be expected in general only at later ages provided the concrete is cured under moist conditions for a sufficient amount of time. In India, there is apprehension in the minds of the user to use the Portland pozzolana cement for structural works. It can be said that this fear is not justified. If the Portland pozzolana cement is manufactured by using the right type of reactive pozzolanic material,
The Portland pozzolanic cement will not be in any way inferior to ordinary Portland cement except for the rate of development of strength up to 7 days. It is only when inferior pozzolanic materials, which are not of the reactive type and which do not satisfy the specifications limit for pozzolanic materials, are used the cement would be of doubtful quality.
Raw Material Used In Portland Pozzolana Cement
- Pozzolanic Material: like Fly Ash (confirming to the IS: 3812-1981)
- Portland Cement Clinker
- Portland Cement
Process of manufacturing
A typical cement manufacturing flow chart is listed below.
- All the primary raw materials are collected from the quarry using a loader and loaded into a dump truck then transported to the crusher house.
- Then fine particles like limestone (CaCo3) and clay are conveyed to the raw mill to prepare a homogeneous mixture.
- Then the mixture is sent to the preheated chamber under 8000-10000 C temperature where calcination takes place.
- The pre-heated mixture is then headed towards a rotary kiln where the mixture is further heated up to 14500 C after that the mixture is sent to the rotary cooler and molded form comes out which is called clinker.
- The clinker is stored then in clinker storage. Now the clinker is mixed with the additives like gypsum and pozzolanic materials.
- The total mixture is then sent into a finished grinding chamber to obtain a smooth texture and finish of the final material.
- After final finishing, the materials are loaded into the cement silos and finally poured into the bags & despatched from the factory.
Chemical properties of portland pozzolana cement
Table as per IS: 1489 Part -I
|Sl No||Characteristics||Requirements||Ref: IS code|
|1||Loss on ignition, percent by mass, Max||5.0||IS 4032:1985|
|2||Magnesia (MgO) percent by mass, Max||6.0||IS 4032:1985|
|3||Sulphuric anhydride (SO3) percent by mass, Max||3.0||IS 4032:1985|
|4||The insoluble material percent by mass, Max||x+4.0(100-x)/100||IS 4032:1985|
* Where ‘x’ is declared percentage of fly ash in the given portland pozzolana cement
Physical properties of portland pozzolana cement
|Sl No||Characteristics||Requirements (value)||Ref: IS code|
|1||Fineness||Shall be not less than 300 m2/kg.||IS 4031 (Part 2): 1988|
|2||Soundness||The average drying shrinkage of mortar bars shall not have an expansion of more than 10 mm and 0.8 percent respectively||IS 4031 (Part 3): 1988|
|3||Setting Time||IS 4031 (Part 5): 1988|
|4||Drying shrinkage||Shall not be more than 0.15 percent||IS 4031 (Part 10): 1988|
|5||Compressive strength||IS 4031 (Part 6): 1988|
Types Of Pozzolanic materials
Pozzolanic materials can be categorized into two types :
- Artificial Pozzolanic Material– Rice husk, silika fume, blust furnace slag, etc.
- Natural Pozzolanic Material– Burnt clay, pumicite, diatomaceous earth, etc.
Portland Pozzolana Cement Grades
In many countries, PPC is graded like OPC depending upon their compressive strength at 28 days. In India, so far PPC is considered equivalent to 33 grade OPC, strength-wise, although some brand of PPC is as good as even 53 grade OPC. Many cement manufacturers have requested BIS for grading of PPC just like grading of OPC. They have also requested for
upper limits of fly ash content from 25% to 35%. Recently, BIS has increased the fly ash content in PPC from 10–25% to 15–35%.
Portland Pozzolana Cement Uses
Portland pozzolana cement can be used in all situations where OPC is used except where high early strength is of special requirement. As PPC needs enough moisture for sustained pozzolanic activity, a little longer curing is desirable. Use of PPC would be particularly suitable for the following situations:
- For hydraulic structures
- For mass concrete structures like a dam, bridge piers, and thick foundations like Silo, Chimney foundation, etc.
- For marine structures like ports and canals
- For sewers and sewage disposal works etc.
Advantages of Portland Pozzolana Cement
Technically PPC has considerable advantages over OPC when made by using an optimum percentage of the right quality of fly ash. However, The advantages of PPC can be summarised as follows;
- In PPC, costly clinker is replaced by cheaper pozzolanic material – Hence economical.
- Soluble calcium hydroxide is converted into insoluble cementitious products resulting in improvement of permeability. Hence it offers, durability characteristics, particularly in hydraulic structures and marine construction.
- PPC consumes calcium hydroxide and does not produce calcium hydroxide as much as that of OPC.
- It generates reduced heat of hydration and that too at a low rate.
- PPC being finer than OPC and also due to pozzolanic action, improves the pore size distribution and also reduces the microcracks at the transition zone.
- Reduction in permeability of PPC offers many other advantages.
- As the fly ash is finer and of lower density, the bulk volume of 50 kg bag is slightly more than OPC. Therefore, PPC
gives more volume of mortar than OPC.
- The long term strength of PPC beyond a couple of months is higher than OPC if enough moisture is available for
continued pozzolanic action.
All the above advantages of PPC are mainly due to the slow conversion of calcium hydroxide in the hydrated cement paste into cementitious products. In one investigation, 20 percent calcium hydroxide in one-year-old OPC paste was found to be only 8.4 percent calcium hydroxide in a similarly hydrated paste containing 30 percent pozzolana. It may be noted that due to the dilution and leaching also certain reduction in calcium hydroxide may have taken place. Giving consideration to that effect, the calcium hydroxide should have been 14%. But the fact is that only 8.4% has remained goes to prove that 5.6% of
calcium hydroxide was converted by the pozzolanic activity.
Disadvantages of Portland Pozzolana Cement
A few of the disadvantages are;
- The rate of development of strength is initially slightly slower than OPC.
- The reduction in alkalinity reduces the resistance to corrosion of steel reinforcement.
- The setting time is nominally longer.
- It contains more fine material, handing of concrete is difficult.
To Conclude, PPC is the most commonly used cement, though it is meant to be used for water retaining structures. Due to its weather-resisting properties, it has become widely popular.
Which is better OPC or PPC?
The initial setting time is less in terms of OPC but higher in PPC which means OPC develops faster strength but, PPC is better against atmospheric effect. The choice of cement is depends upon the purpose of use.
Why is pozzolana added to cement?
Pozzolana reduces the excessive heat hydration and bleeding form concrete because of it’s extra fineness.
Is Ultratech OPC or PPC?
Ultratech manufactures both the type.