The cement which is made by the intergrinding of Ordinary Portland Cement (OPC) clinker with some percent (usually 10 to 25% by mass of PPC) of pozzolanic material is known as Portland Pozzolana Cement.
Pozzolanic Material – A pozzolanic material is essentially a siliceous or aluminous compounds, which do not have its own cementing property, which will, in finally divided form and in presence of water, reacts with Ca(OH)2 (Calcium hydroxide), liberated in the hydration process, at ordinary temperature, to form compounds possessing cementitious properties.
The pozzolanic materials generally used for manufacture of PPC are calcined clay (IS 1489:1991 Part-II) or fly ash (IS 1489:1991 Part-I).
Raw Materials of Portland Pozzolana Cement
a. Pozzolana Materials
- Fly ash is a waste material, generated in the thermal power station, when powdered coal is used as a fuel. These are collected in the electrostatic precipitator. Fly ash used in the manufacture of Portland pozzolana cement shall confirm to IS 3812:1981.
- Fineness and average compressive strength in lime reactivity of fly ash that is to be blended with finished Portland cement to produce Portland pozzolana cement, when tested in accordance with the procedure specified in IS 1727:1967, shall not be less than 320 m3/kg and 4.0 MPa respectively.
b. Portland Cement Clinker
The Portland cement clinker used in the manufacture of Portland pozzolana cement shall comply in all respects with the chemical requirements of IS 269:1989.
c. Portland Cements
Portland cement for blending with fly ash shall confirm to IS 269:1989.
d. Other Admixtures
When Portland pozzolana cement is obtained by grinding fly ash with Portland clinker, no material other than gypsum (natural or chemical) or water or both, shall be added. Such air –entraining agents or surfactants which have been proved harmless may be added in quantities not exceeding 1%.
Manufacturing of Portland Pozzolana Cement
- Portland pozzolana cement shall be manufacture either by intimately intergrinding Portland cement clinker and fly ash or by intimately and uniformly blending Portland cement and fly ash.
- For blending of Portland cement and fly ash, the method and equipment used shall be the one well accepted for achieving a complete uniform and intimate blending.
- Gypsum (natural or chemical) may be added if the Portland pozzolana cement is made by intergrinding Portland cement clinker with fly ash.
- The fly ash constituent shall be between 10 to 25% by mass of Portland Pozzolana cement.
- The homogeneity of the mixture shall be guaranteed within ± 3% in the same consignment.
Chemical Reactions of Portland Pozzolana Cement
Simply the chemical reactions takes place on addition of water is as follows
OPC + H2O (water) à C-S-H gel + Ca(OH)2 + Heat
Pozzolanic material + Ca(OH)2 + H2O à C-S-H gel
C-S-H gel is Calcium Silicate Hydrates. C-S-H are the most important products, it is the essence that determine the good properties of concrete. It makes 50-60% of the volume of solids in a completely hydrated cement paste.
Here Calcium Hydroxide is not so much desirable compound and useless compound from the point of view of strength or durability. It reacts with sulphates present I soils and water to form calcium sulphate which further react with tri-calcium aluminate and cause deterioration of concrete. This is known as sulphate attack. If such useless mass could be converted into a useful cementitious product, it considerably improves the quality of concrete. The use of fly ash performs such a role by reacting with Calcium hydroxide and converted it into C-S-H gel.
Properties of Portland Pozzolana Cement
a. Fineness
When tested by the air permeability method as per IS 4031:1988 (part – II), the specific surface of Portland pozzolana cement shall be not less than 300 m3/kg
b. Soundness
When tested by “Le Chatelier” method and autoclave test as per IS 4031:1988 (Part – III), unaerated Portland Pozzolana cement shall not have an expansion of more than 10 mm and 0.8% respectively.
c. Setting Time
The setting time of Portland Pozzolana cement, when tested as per IS 4031:1988 (Part 5), shall be as follows:
Initial setting time = 30 min (minimum)
Final setting time = 600 min (maximum)
d. Compressive strength
The average compressive strength of not less than 3 mortar cubes (area of face 50 cm2) having ratio 1:3 (cement : sand) by mass and (P/4 + 3)% water (of combined mass of cement and sand)(P is the percentage of water required to produce a paste of standard consistency) is as follows:
| a. | At 72 ± 1 h (3 days) | 16 MPa. (Min) |
| b. | At 168 ± 1 h (7 days) | 22 MPa. (Min) |
| c. | At 672 ± 1 h (28 days) | 33 MPa. (Min) |
e. Drying Shrinkage
The average drying shrinkage of mortar bars prepared and tested as per IS 4031:1988 (Part-10) shall not more than 0.15%.
Advantages of Portland Pozzolana Cement
Technically PPC has considerable advantages over OPC when made by using optimum percentage of right quality of fly ash.
- 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, all-round durability, particular in hydraulic structures and marine construction
- It generates less heat of hydration and that too at a low rate.
- PCC have finer particles than OPC and also due to pozzolanic action, it improves the pore size distribution and also reduces the micro-cracks at the transition zone.
- Due to the fineness of PPC 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 higher than OPC if enough moisture is available for continued pozzolanic action.
Disadvantages of Portland Pozzolana Cement
- The rate of development of strength is initially slightly slower than OPC.
- Reduction in alkalinity reduces the resistance of corrosion of steel reinforcement.
Grades of Portland Pozzolana Cement
In India, there is no grade for Portland pozzolana cement and 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.
In many countries, PPC is graded like OPC depending upon their compressive strength at 28 days. In India many manufacturers have requested BIS for grading of PPC just like grading of OPC. They have also requested for to increase the upper limit of fly ash content from 25% to 35%.
Applications of Portland Pozzolana Cement
PPC can be used in every 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 suitable for the following:
- For the hydraulic structures
- For marine structures
- For mass concrete structures like bridge piers, dams and heavy foundations
- For sewers and sewage disposal works etc.