Designing Concrete with Fly ash
Much research has been carried out on properties of concrete containing fly ash as replacement for cement. It is a well-known fact that fly ash holds many positive advantages in terms of resistance to sulphate attack, alkali silica reaction, carbonation, chloride attack and 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 the heat of hydration on account of its comparatively slow reactivity at early ages. These advantages/ facts 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 a problem in the concrete, leading to problems in durability. The 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 a 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 the process of corrosion in the reinforcement. (Prakash Mehta, 2008.) It is clear that most of the problems relating to durability involve CH. The solution to this problem has been found through replacement of some percentage of Ordinary Portland Cement with a suitable pozzolanic material.
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 as replacement of OPC in concrete. Some government projects, too, do not have the provision for replacement of OPC with fly ash.
The main reason is inadequate understanding of the effect of fly ash on concrete strength. Whenever fly ash is used as a replacement for 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, a decrease in temperature reduces the strength gain rate in fly ash concrete more than in 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 and prefer OPC cement for production of concrete in their RMC plants.
Of course, using virgin fly ash for blending in concrete at the 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 interground with clinker to form PPC, the shapes get distorted, and these particles no more have their shape in a spherical form. The result is higher water demand for desired workability. It won-Æt 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 could be the possibility that PPC cement may have lower water demand up to a certain time of grinding, as compared to OPC and un-ground fly ash. However, 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 per cent with a blend of OPC and fly ash was achieved, i.e, a reduction by two percent when tested for pure OPC which gave a standard consistency of 28 per cent.
What needs to be done?
Figure 1 gives a clear picture of the 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 in OPC concrete, whereas most of cement companies show higher strength of fly ash-based concrete beyond 28 days in comparison to concrete with a 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 the cementing efficiency indices of the fly ash used in a trial, reproduced in Table 1 (Amit Mittal, 2008), it turns out to be something between 0.45 to match strength for 28 days and 0.8 to match strength at 90 days (for 40per cent replacement with Fly ash) and 0.63 to match strength for 90 days (for 50per cent 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, 40per cent mix (total cementitious, percentage fly ash) using W/C ratio of 0.35.
Using Eqn. (i):
W = W
Thus, 0.45= 158
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 the 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 would be much less in case of concrete made with blend of OPC and fly ash. The reason for comparing costs is to point out the inefficient usage of resources by cement companies. If we had to see this problem from the point of sustainability, it would 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 the 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 the case of pre-stressed concrete, pre-stressing is done only after achievement of a 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.
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 per cent 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.
Fly ash reduces water demand say by 12 per cent as compared to OPC (M. L. Gambhir, 2004), so we reduce the water content to 141 litres.
W = W
i.e. 0.5 = 141 . (Since Fly ash is 40 per cent 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 kgs per cubic metre of concrete out of which 150 kgs shall be fly ash and 222 kgs 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).
320 kg of OPC costs much higher than combination of 222 kgs of OPC and 150 kgs of 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 per cent of heat in three days.
Each tonne of cement produced releases 0.95 tonnes of CO2 in atmosphere (including energy consumption, if the heat is coal generated). It has been possible to reduce OPC by one hundred kgs per cubic metre, or by 30 per cent. Thus by replacing 40 per cent cement, we are able to reduce CO2 emissions by 44 million tonnes 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.
It becomes vital 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, the 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 comparisons 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 won't 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 `invisible hand` theory 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 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 costs but the 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 crude step at this moment since not much data is available, it will surely 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.