Article published in "Civil Engg & Const. Review"

Much research has been carried out on properties of Concrete containing Fly Ash as replacement of cement. It has become a well known fact that Fly Ash does hold much positive advantages in Concrete in terms of resistance to Sulphate attack, Alkali Silica reaction, Carbonation, Chloride attack and in terms of economic benefits to users, or in terms of conservation of resources (since it replaces a part of Ordinary Portland Cement). In addition to these advantages, Fly Ash also reduces Heat of Hydration on account of its comparatively slow reactivity at early ages. These advantages/ facts need no mention and are very well known across the construction community. The main reason Fly Ash is able to perform this way is because of its pozzolanic property by virtue of which it reacts with by product of C3S/ C2S hydration i.e. CaOH2(CH). CH being an unstable material both chemically and physically creates nuisance in the Concrete leading to problems in durability. Chemical instability of CH relates to its tendency to react with:
1) Sulphates to form CaSO4 which further reacts with C3A (after Concrete has hardened) to form expansive Ettringite. This is Sulphate attack.
2) It produces a highly alkaline environment due to which Si-O-Si (Silicate bond present in Aggregates which leads to Alkali Silica Reaction) reacts with water to form expansive Silanol or Silica gel.
3) CH is a crystalline material which possesses some strength, but it has a tendency to react with atmospheric CO2 to form CaCO3, which by nature is an amorphous material possessing no strength.
4) On account of its physical instability, it is highly soluble in water, and leaches out of Concrete forming pores. These pores get interconnected to form a permeable Concrete. Chlorides, Carbon dioxide find their way into the Concrete through these pores, thereby accelerating process of corrosion in the reinforcement (Prakash Mehta, 2008).
It is clear that most of the problems relating to durability involves CH. Solution to this problem has been found through replacement of some percentage of Ordinary Portland Cement with a suitable pozzolanic material.
A pozzolanic material is characterized by its property of reactivity with CH in presence of moisture to form tri Calcium Silicate hydrate Gel, (the binding material in hardened Concrete). Fly Ash produced from thermal power plants, has proven to be a good pozzolanic material, and is widely used to replace certain percentage of OPC in Concrete.
Indian standards, which guide usage of Fly Ash in Concrete, have identified different ways to use Fly Ash in Concrete. IS: 3812 lays down requirement for different uses of Fly Ash in Concrete, they are, for use as admixture, as pozzolana and as fine aggregate in Concrete. It will be interesting to note that Fly Ash can be used in production of OPC in percentages not more than 5% (admixture) to improve the performance of OPC (IS 8112:1989, IS 12269:1987).
Prejudices:
Although most of the advantages relating to Fly Ash are well known among engineers, at least theoretically, it is unfortunate to note that most do not encourage Fly Ash for replacement of OPC in Concrete. Even some Government projects do not have provision for replacement of OPC with Fly Ash.
The main reason for the fear is inadequate understanding of the effect of Fly Ash on Concrete strength. Whenever Fly Ash is used for replacement of OPC, the practice is to equate it with OPC in terms of strength gain. From actual experience it is found that OPC with Fly Ash leads to slow strength gain compared to OPC. Moreover Concrete with Fly Ash is more sensitive towards temperature as compared to OPC. Meaning, decrease in temperature reduces the strength gain rate in Fly Ash Concrete more than Concretes with pure OPC. Probably this has led to so- called failures of Fly Ash Concretes in certain laboratories. The fear is not predominant only in construction industry, but even Cement companies which advocate usage of PPC over OPC; prefer OPC cement for production of Concrete in their RMC plants.



Of course usage of virgin Fly Ash for blending in Concrete at batching plant is much better than using inter-ground Fly Ash and OPC in the form of PPC. The sole reason being, that Fly Ash particles are spherical in shape, due to which they impart better workability to the Concrete in which they are introduced, Whereas when inter-ground with clinker to form PPC, the shapes get distorted, and these particles no more have their shape in spherical form. The result is higher water demand for desired workability. It won’t be wrong to say that water demand is a cumulative effect of particle shape, particle size distribution and fineness, implying that even after grinding of Fly Ash and OPC there may be possibility that PPC cement may have lower water demand up to certain time of grinding, as compared to OPC and un-ground Fly Ash. But the usual observation on site unfolds a different story, with water demand actually being higher for PPC than OPC in combination with virgin Fly Ash. This obviously calls for refining the process for production of PPC, with optimising the time of grinding so that there is minimum water demand. HCC has come across cases when a standard consistency of 26% with a blend of OPC and Fly Ash was achieved, i.e. a reduction by 2% when tested for pure OPC which gave a standard consistency of 28%.
What needs to be done:
Figure 1 gives a clear picture of effect on strength by replacing cement with Fly Ash. It can be seen that strength developed in Concrete with Fly Ash is always less than OPC Concrete, whereas most of cement companies show higher strength of Fly Ash based Concrete beyond 28 days in comparison to Concrete with equal quantity of OPC.
Fly Ash needs to be characterised by its Cementing Efficiency Index (Peter Hewlett, 2004) for different temperatures at different ages in combination to particular Cement.
W = W . -- - - - - - - - - - - - (i)
Cs (C+FK)
Here W, C & F are the weights of water, Ordinary Portland Cement and Fly Ash respectively for the given mix, and K is the cementing efficiency index of the Fly Ash. W/Cs is the equivalent water cement ratio, i.e. the required water cement ratio for the same strength but without Fly Ash. If we try to find out cementing efficiency indices of the Fly Ash used in a trial, reproduced in Table 1 (Amit Mittal, 2008), it comes out to be something between 0.45 to match strength for 28 days and 0.8 to match strength at 90 days (for 40% replacement with Fly Ash) and 0.63 to match strength for 90 days (for 50% replacement with Fly Ash) (figure 2). The steps to calculate cementing efficiency index is shown below:
From Table 1 we can find that for OPC (without Fly Ash), with 350 Kg Cement and 0.45 W/C ratio the 28 day strength is 37.8 MPa. The closest strength at 28 days is achieved with 450, 40% mix (total Cementitious, Percentage Fly Ash) using W/C ratio of 0.35.
Using Eqn. (i):
W = W .
Cs (C+FK)
Thus, 0.45= 158 .
(270+180*K)
Thus, K= 0.45 (This index is to match strength for 28 days of OPC Concrete).
This data can then be used to design Concretes with the desired percentage of Fly Ash for the required age of Concrete.



Another interesting property of Fly Ash should be incorporated in the mix design procedure, i.e. its ability to produce a better workability with lower water contents. A higher percentage of Fly Ash in cementitious material can yield better workability. M.L. Gambhir proposes multiplication factors both for water content and cementitious content for different percentages of Fly Ash (M. L. Gambhir, 2004).
Concrete made with OPC and Fly Ash when compared to Concrete made with Equal quantity of OPC alone, shows better durability in terms of Rapid Chloride Penetration tests, Sulphate resistance (Peter Hewlett, 2004), ASR, etc. whereas in the limits for Cement content in IS:456- 2000, minimum cement content holds same for all cements. It rather would be more appropriate to specify limits for test results on Concrete/ mortar for various aspects of durability viz. RCPT, Sulphate resistance, Mortar Bar Expansion (ASR) etc. rather than specifying minimum cement content per cubic metre of Concrete.
If PPC cement, available in the market, were to be compared with blend of same brand OPC and same Fly Ash, the cost for production of same grade of Concrete will be much less in case of Concrete made with blend of OPC and Fly Ash. The reason for comparing cost, is to just point out the inefficient usage of resources by cement companies. If we had to see this problem from the view point of sustainability, it might be clear that Energy consumption in producing equivalent grade of PPC Concrete will be much higher than the energy for OPC and PFA blend Concrete. Another reason for stating superiority of OPC and PFA blend is the situational advantage to increase or decrease the Fly Ash content to accelerate the production rate in construction. For example construction projects in Sub- Zero temperatures demand faster strength gain rate of Concrete to avoid damages due to freezing. In case of prestressed Concrete, prestressing is done only after achievement of certain strength, the faster the strength achievement, the more efficiently resources can be handled. In these conditions if one had to use PPC, the cost can work out to be much higher than OPC, since in these cases early age strengths holds more priority than 28 day strength.
Example:
An OPC Concrete gives 30 MPa strength at 28 days for W/Cs ratio of 0.5. The water content is 160 litres and cement content 320 Kg per cubic metre of Concrete. Now we desire to use 40% Fly Ash for replacing OPC, which has a Cementing Efficiency Index of 0.4 for 28 days, with the available OPC, so that the strength achieved is equivalent to OPC Concrete at 28 days.
Solution:
Fly Ash reduces water demand say by 12% as compared to OPC (M. L. Gambhir, 2004), so we reduce the water content to 141 litres.
W = W .
Cs (C+FK)
i.e. 0.5 = 141 . (Since Fly Ash is 40% of total cementitious)
(0.6Cm + 0.4Cm*0.4)
So, Cm= 372 Kg per cubic metre (total cementitious content).
Now the cementitious content is 372 Kg per cubic metre of Concrete out of which 150 Kg shall be Fly Ash and 222 Kg shall be OPC. The water cement ratio required now will be 0.38.
If the strength required was at 90 days instead of 28 days, and the cementing efficiency index found was 0.8, the total cementitious content then would have been 307 Kg per cubic metre of Concrete and water cement ratio required would be 0.46 (based on similar calculations shown above).
Economics:
320 kg of OPC costs much higher than combination of 222 kg of OPC and 150 Kg Fly Ash. The difference could be somewhere near Rs. 250 per cubic metre of Concrete (OPC cost- Rs. 5/Kg and Fly Ash Cost- Rs. 1.6/Kg). The heat of hydration from 320 kg of OPC at 3 days has been found out to be somewhere near 17.7 Mcal(Mega Calories), Whereas with the alternative combination, the heat of hydration comes down to 14.9 Mcal per cubic metre of Concrete(Based on actual test results as shown in table 2 and interpolation from SP 23: 1982 considering linear relationship between Heat of hydration and Fly Ash content), i.e. a decrease by 15% of heat in 3 days.
Each ton of cement produced releases 0.95 tons of CO2 in atmosphere (including energy consumption, if the heat is coal generated). It has been possible to reduce OPC by 100 Kg per cubic metre, or by 30%. Thus by replacing 40% cement, we are able to reduce CO2 emissions by 44 million tons per annum (Considering 155 million tonnes cement production per annum in India). Moreover the Fly Ash which otherwise creates an environmental nuisance will be used up in something productive.



Conclusion:
It becomes very necessary for the standards to look into this matter, and make necessary changes, in the mix design procedures for Concrete. It also is very necessary to include Cementing efficiency index and capacity to improve workability when used for replacement of OPC. Keeping in view that durability of Concrete increases when Fly Ash is used to replace OPC, same limits of cementitious content for durability does not seem justified for different types of cement. Rather limits on test results of durability for various tests of Concrete should be specified. Production of PPC is done by inter- grinding clinker of OPC and Fly Ash, which consumes up energy/ resources. If comparison of cost of Concrete made with PPC and Concrete made with blend of OPC and Fly Ash were to be done, the latter would mostly outperform the Concrete made with PPC. Cement companies should treat blend of OPC and virgin Fly Ash as benchmark, in terms of workability, cost, strength etc. when setting the performance targets for production of PPC. Although usage of PPC or blend of OPC and Fly Ash has become need of today to maintain sustainability in construction, it wont be beneficial to completely stop production of OPC, as it proves economical in comparison to Fly Ash based Concrete when high early age strengths are required from Concrete.
Adam Smith in his theory of ‘Invisible hand’ proposes that allocation of finite resources is done by an invisible hand. This invisible hand is referred to as price in terms of economics, if it were to be defined in a single word. The scarcer the resources are, the higher is the cost of the product made from these resources. So, if we have to choose an indicator for sustainable construction, the best indicator would be the cost. Thus two different Concretes made with different cost but same strength can easily indicate which is better in terms of sustainability. Standards can look into the problem of sustainability by also including cost of production of cement (since cost reflects the efficiency of usage of resources) per MPa strength of Cement. Although this might be a very crude step at this moment since not much data is available, but it surely will lead to better usage of resources in future. To start with, there could be data generated on effects of grinding of cementitious material on Workability, Strength etc. Then a suitable method can be devised to find optimum solution from the available data.