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Portland-pozzolana cement can be produced by grinding together Portland cement clinker and artificial pozzolana (Fly ash) with addition of gypsum or calcium sulphate.
Portland-pozzolana cement produces less heat of hydration and offers greater resistance to the attack of aggressive waters than normal Portland cement.
It reduces the leaching of calcium hydroxide liberated during the setting and hydration of cement.
It is particularly useful in hydraulic construction and other mass concrete structures.
The ball bearing shape of fly ash significantly aids the workability of concrete. This allows for lower sand content than conventional mixes while handling remains similar. As the proportion of sand is reduced, all performance aspects of the concrete are enhanced.
Because of fly ash’s spherical particle shape, less water is required to achieve the same level of slump as in the control concrete. Concrete mixtures made from PPC typically reduces the water needed by 5% to 10% over the concrete made from OPC
The specific gravity of PPC is much lower than that of OPC; therefore, 100# of fly ash has a much greater solid volume than the same weight of OPC. In PPC concrete, the cementitious materials (cement and fly ash) volume is higher. This higher quantity of cementitious materials greatly assists in the finishing process.
The inter-grinding of fly ash with clinker and gypsum maximizes the pozzolanic potential of fly ash with more consistent product with good control on variability, leading to a better performance of PPC concrete.
The results of investigations indicate that the performance of concrete using Fly Ash is better than the OPC concrete, especially with respect to durability indicator due to pozzolanic action of fly leading to pore refinement and denser concrete matrix.
The beneficial effects of fly ash are seen more pronounced in case of factory-ground Portland Pozzolona Cement (PPC) than site mixing of fly ash with OPC under normal Nepali construction site conditions.
Ordinary Portland cement has a high calcium base affecting the microclimate of concrete and mortar. The interface bond between the cement paste and aggregates can be improved with better pore structure and minimized micro cracks using mineral admixtures like fly ash and granulated blast furnace slag.
Inter-grinding of fly ash with clinker and gypsum produces a highly homogenous mix with a high level of inter-particle contact between cement and fly ash grains
The inter-grinding process also maximizes the pozzolanic potential of the fly ash with more consistent product with good control on variability, which results in to better performance characteristics of concrete.
The inter-grinding makes fly ash particles finer, which imparts additional reactive surfaces for hydration and improves the overall particle size distribution of the resultant cement.
The values of water permeability are lower for PPC concrete due to pore refinement and this is more pronounced in factory produced PPC concrete, due to greater secondary reaction products, because of more number of reactive fly ash particles and their desirable particle size distribution.
In addition, the pozzolanas can reduce the content of calcium aluminates in cementations material, leading to increase of sulphate resistance of concrete from the results
A siliceous or siliceous and aluminous material (Pozzolan) that in itself possesses little or no cementitious value but that will in finely divided form and in the presence of moisture, chemically reacts with calcium hydroxide (lime) at ordinary temperature to form compound having cementitious properties.
To form a compound that has cementitious value, a pozzolan (Fly ash) must react with lime which comes from Portland Clinker. For this, two assumptions must be defined;
How much lime (Ca(OH2)) is available in Clinker for reaction, and
What is the chemical reaction of Pozzolan (Fly ash) with Lime (Ca(OH2))
1. Lime (Calcium Hydroxide) available
Amount of lime is the calcium hydroxide produced during the hydration of OPC. It will depend on the amount of C3S and C2S in cement. Each of this compounds react with water to form Calcium Silicate Hydrate (C-S-H) and Ca(OH)2 Lime. Not all the lime produced is available or free for reaction with Pozzolans (Fly ash). Generally 20-25 % Lime is assumed to be available for Pozzolanic reaction. That is 75% of the reaction product of cement by mass produce strength and remaining is free lime.
The amount of lime is also related with amount of hydration takes place. At early ages, the amount of cement hydration is slow thus less quantity of lime will be available initially.
In addition, when pozzolan (Fly ash) is used it also decreases the amount of free lime percentage.
2. What is the chemical reaction of Pozzolan (Fly ash) with Lime (Ca(OH2))?
It is assumed that all pozzolanic reactions are lime reaction with siliceous pozzolanic material. Reactions of lime with alumineous pozzolanic materials are considered less significant. The dominant hydraulic product of Portland cement is C –S – H (Calcium Silicate Hydrate) which is responsible for strength and durability. It is assumed that the reaction of Calcium hydroxide (lime) and Fly ash (Pozzolan) produce the same hydraulic product (C - S – H) as Portland Cement. The reaction is as follows;
Reaction occurred in OPC in presence of water;
For C3S and C2S as a major constituents of OPC Clinker
2[3CaO.SiO2] + 6[H2O] = [3(CaO)2(SiO2)3(H2O)] + 3[Ca(OH)2]
(OPC) (Water) (C – S – H) (Free Lime)
2[2CaO.SiO2] + 4[H2O] = [3(CaO)2(SiO2)3(H2O)] + Ca(OH)2
(OPC) (Water) (C – S – H) (Free Lime)
When Pozzolans (Fly ash) is added in cement clinker and made PPC, the additional reaction takes place between free lime produced from OPC and reactive silica of fly ash to form an additional calcium silicate hydrates (Cement gel). The reaction is as follows;
3[Ca(OH)2] + 2[ SiO2] = [3(CaO)2(SiO2)3(H2O)]
(Free lime from OPC Clinker) (Silica from Fly ash) (C – S – H) additional Cement gel
OPC Clinker and Water react together and Produce C – S – H that gives strength on compaction and drying while making concrete and also liberate lime (Calcium hydroxide).
However, in case of PPC, this free lime is also utilized by making reaction with reactive silica available in fly ash to produce additional C – S – H giving more cement strength value and leading to pore refinement and denser concrete matrix and also leaving no free lime. This action also reduces the liberation of CO2 by decreasing Heat of Hydration process which will ultimately make the environment more green and eco-friendly as compared to OPC. Hence we can term Jagdamba PPC as a ‘Green Cement’
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