A forum held 8 December 1998 - Sponsored by EHDD Architecture and Pacific Energy Center
Role of Flyash in Sustainable Development
P.K. Mehta
Good evening, it's very nice to see so many of my friends here. You have heard a very eloquent presentation that we all have a responsibility to do something about global warming. All of us, architects, structural engineers, designers and others, we participate in decision making about concrete mix design, the quality of concrete, and it is here that we can do something about global warming. As was mentioned earlier, each ton of portland cement we consume throws out into the environmental loading about one ton of CO2. So if we are producing 1.3 billion tons of portland cement annually, this emits 1.3 billion tons of CO2 into the environment.
And in 25 years we expect that the demand for portland cement in the world will double up. This is because the developing countries with large populations and rapid population growth, mostly in Asia, South America and Africa, where about 5 billion of the globe's 6 billion people live, still have a long way to go in terms of socioeconomic development. And you cannot tell them "please do not do what the developed countries have done to achieve a high standard of living." They require just as we do, reasonably decent housing for their children, reasonably decent roadways for travel, etc. With globalization of technology and economy, their aspirations are very high; every country wants its citizens to participate in an affluent lifestyle.
So there is no way to stop the demand for more concrete. But any additional portland cement clinker capacity that we build is going to load the environment with an additional one ton of CO2 per ton of cement. There is no shortcut, there is no way around it, half of the CO2 comes from the decomposition of limestone, which is a major raw material for making portland cement, and half of it comes from the fuel. Since coal is the cheapest fuel, we are going to continue to use it for making portland cement. The question is can we continue to meet the increasing demand for cement and concrete in a sustainable manner.
Fortunately, within this scenario a new player has emerged that can save us. And that player is flyash. There are some problems because most of the literature on flyash, research on flyash, and the flyash codes are obsolete. They are not updated, and it will take time to do so.
Meanwhile, let's look at the current materials, and the current science and technology. Let's satisfy ourselves intellectually and with the help of some laboratory work, some field tests, and some real buildings; and it's our hope that we can solve the problem.
With this background, my job is to give you a brief introduction as to what flyash is and what flyash can do in concrete today, based on the current state of our knowledge. Although many of you know about flyash, my effort will be to take even those who don't know anything about flyash right from ground level to the latest that we know about it. I will first spend a few minutes talking about the source, because typically people don't know about it.
One of the important issues I will describe are the mechanisms by which flyash influences the properties of concrete. Because if you understand the mechanism, you don't have to worry about codes and standards. Once you have a good understanding of the way things work, you can be courageous enough to take some steps. So I'll dwell on mechanisms, and finally give you a brief introduction to the very latest developments, which 10 years from now will probably become a part of conventional concrete technology.
Characteristics of Flyash
Flyash is a powdery substance obtained from dust collectors, whether they are electrical precipitators or baghouses, in electrical power plants which use ground coal as fuel. The U.S. produces about 60 million tons per year, some say 55 million, some say 50, but I've been hearing 55 million for about the last 10 years so it must be 60 million tons. Because of many of the stigmas attached to flyash, the consumption of flyash in the cement and concrete industry is no more that 6 million tons which is only about 10%. And once I tell you what flyash can do, I think that it's a disgrace that most of it is ending up in the landfills, creating lots of problems with our groundwater, air, and land.
The physical and chemical characteristics of flyash which I'm going to discuss, and their effect on the properties of concrete are mainly from an EPRI Report based on a study funded by EPRI [The Electric Power Research Institute] in the early 1980is at UC Berkeley where I work. I was the principal investigator on the research project. My comments are based on not one flyash, but on about 20 U.S. flyashes from the East Coast, the Midwest, the State of Washington, and from Canada; so it's representative of most flyashes, and this report is available from EPRI as report CS3314, published in January 1984. Most of my comments about the characteristics of flyash are based on this source.
Flyash Chemistry
First, unfortunately, the codes, ASTM and so forth, have a very heavy emphasis on the chemistry of flyash. For example "Class F flyash must have more than 70% total of silica, alumina, and iron oxide, and Class C has more than 50% of these oxides" etc. And people get confused because there is really no direct relation between the chemistry of flyash and the properties in concrete. Most of the properties of flyash in concrete are determined by the flyash mineralogy and particle size, and not by chemistry. Don't worry about these various chemical percentages, there is a big range and this range doesn't mean anything. I'm just showing the range to show you that there is a lot of variability, and people get worried about variability. They worry that "what if today I'm getting flyash with 48% silica and tomorrow it goes to 44%". It's not important, nothing will happen.
Flyash Mineralogy
Most important is the flyash mineralogy, and with regard to the mineralogy of flyash, 60-90% is glass. It starts out as impurities in coal, mostly clays, shales, limestone, and dolomite. They cannot be burned so they end up as ash, and at high temperatures they fuse and become glass. Because of the high speed of the flue gases, the molten glass turns into glass beads, or tiny spheres of glass. I'm emphasizing this because it's the kind of material if we didn't have it, we would have to invent it in order to improve the workability and durability of concrete.
For flyash in the U.S., there are two ASTM Classes, Class F and Class C that are based on total amounts of silica, alumina, and iron oxide present. This doesn't have much significance. In Europe and the rest of the world there is a recognition that if you want to make some differentiation based on the chemistry, then divide flyash based on it's calcium content, because the calcium content of flyash has a great influence on the type of glass. And if a material is mostly glass, we should only be worrying about what kind of glass it is. There is too much emphasis on the remaining stuff that is not glass. Remember flyash is 60-90% glass, and modern flyashes are much closer to 70-80% glass.
Low calcium flyash also contains non-reactive crystalline minerals; say you have 80% glass, with the 20% remaining being a non-reactive mineral like quartz, mullite, which is an aluminum silicate, hematite and magnetite, which are iron oxides, and a less reactive alumino silicate glass. There are two glass types. If you have high calcium flyash then the alumino silicate glass has also a lot of calcium in it and that glass is more reactive. So that's why Class C flyash gives you higher early strength compared to Class F, because Class C tends to have much more calcium oxide. High calcium flyashes also contain reactive crystalline minerals such as free lime, tri-calcium aluminate, tetra-calcium alumino-sulfate, and calcium sulfate, depending on the sulfur content of the ash. And all of these are reactive crystalline minerals and the glass is also much more reactive.
Flyash Particle Size
There are two parameters that determine the reactivity of flyash, one is the mineralogy, and the second is the particle characteristics. Now you should pay very careful attention to the particle characteristics. Particles are mostly glassy, solid and spherical. There are some hollow cenospheres and so forth, but let's not spend time on that because most important is that most of the flyash consists of glassy particles that are solid and spherical. There is also some unburned carbon present, depending on the efficiency of burning. Today's furnaces are very efficient; you may have only 1% carbon, and this carbon is in the form of highly micro-porous large particles, they are just like Swiss cheese, they are large but are not round or spherical because they are not a molten material, it's like charcoal or coke.
The particles of flyash range in size from 1 to 100 microns (1,000 microns is 1 mm, so this largest size particle, 100 microns, equals 0.1 mm). The average size is about 20 microns which is similar to portland cement average particle size. Now what is more important for you to remember is that more that 40% of the particles are under 10 microns, and particles under 10 microns, regardless of the type of flyash, are the ones that contribute to the 7 and 28 day strengths. Under 10 microns is the magic number. And particles about 45 microns and larger, which is 325 sieve residue may be considered as inert. They do not participate in pozzolanic reactions, even after one year, so they behave like sand.
So with flyash, don't worry about the Blaine surface area. What is most important is the particle size distribution. Particles below 10 microns are the ones which are really beneficial for early strength. Particles about 45 microns and larger are not so useful. Between 10 and 45 microns are the ones that slowly react between 28 days and one year or so. Most flyashes have less than 15 or 20% particles which are above 45 microns, and more than 40% particles which are under 10 microns.
In the EPRI study we also worried about whether the furnace design would have any affect on the reactivity of flyash, but we found that the furnace design did not affect flyash reactivity. We found that modern furnaces generally produce a flyash that is low in carbon. ASTM has a 6% limit on carbon in flyash used in concrete, but flyash produced today typically contains 1.0% to 1.5% carbon. And today's flyashes are high in glass, they are 80-90% glass, and have good reactivity. A flyash of this composition will have a good reactivity when its composed of a large proportion of fine particles. So don't go by the residue of the 325 mesh; it only tells you the particles which are inert. To judge flyash reactivity, you will have to find out what percentage of the particles are below 10 microns.
What is the significance of any unburned carbon particles? Unburned carbon particles influence mostly the water demand and the air entraining agent required. In the East Coast and the Midwest where concrete is exposed to freezing & thawing cycles, there is always air entrainment in concrete. In this case, the carbon content is something to worry about because it would influence the dosage of the air entraining admixture.
Again, to continue the conclusions from the EPRI study, we found that except for calcium, flyash chemistry has little influence on reactivity. So except for calcium, don't worry about silica, alumina, iron oxide, etc., they have nothing to do with the properties. The superior reactivity of high calcium flyashes is related to the composition of glass and the presence of reactive crystalline phases.
How Flyash Works
Now that you have learned about what flyash is, let's look at how it works. The first equation in the illustration (see figure 2.2) shows you the chemistry of hydration of portland cement. About 50% of portland cement is composed of the primary mineral tri-calcium silicate, which on hydration forms calcium silicate hydrate and calcium hydroxide. If you have a portland-pozzolan cement, and flyash is the pozzolan, it can be represented by silica because non-crystalline silica glass is the principal constituent of flyash. The silica combines with the calcium hydroxide released on the hydration of portland cement. Calcium hydroxide in hydrated portland cement does not do anything for strength, so therefore you use it up with reactive silica. Slowly and gradually it forms additional calcium silicate hydrate which is a binder, and which fills up the space, and gives you impermeability and more and more strength. This is how the chemistry works.
Figure 2.1 Photograph of flyash enlarged many times
Figure 2.2 Flyash combines with excess and unwanted large crystals of calcium hydroxide (CH) to form additional useful binder (C-S-H)
Now in order to understand the benefits of flyash use we have to look at the physical manifestations of the chemical reaction. There is something called a transition zone in concrete. So far we have looked only at the cement paste, but in concrete you have sand and gravel, and many properties of concrete are controlled by the strength of the interfacial bond between the aggregate and the cement paste, and that interfacial bond is called the transition zone. The transition zone in portland cement concrete is very weak because of the presence of large crystals of calcium hydroxide which find space here due to the wall effect next to the coarse aggregate particles. You can see very clearly in the slide, these large calcium hydroxide crystals, they do not really bind with the aggregate, and they can be easily detached and cracked. And it is these cracks which are the ones that are responsible for the lack of impermeability, or lack of water tightness in concrete. So if you are able to build a stronger transition zone, then you can eliminate micro-cracking, and improve the impermeability, as well as improve the chemical durability, and thus end up with a highly durable concrete.
Also, the particles of coarse aggregate, due to the wall effect, tend to trap water next to the aggregate, and therefore what you see on the surface of the concrete as the visible bleed water is only part of the mixing water. A large amount of mixing water ends up as a locally high water-cement ratio type of cement paste next to the aggregate particles. When you vibrate concrete this is what happens: you have part of the extra mixing water on the surface as the visible bleed water, and a very large amount of bleed water due to internally trapped water next to the coarse aggregate.
The next slide shows you what happens (see figure 2.3). As a result of local high water cement ratio paste next to the coarse aggregate, you form the large crystals of calcium hydroxide, very large crystals, and you have large pores left over making this an area of weakness. If there is any stress, if there is any drying shrinkage, any thermal shrinkage, any loading and unloading effect, then these stress effects could very easily rupture the concrete. It would rupture next to the coarse aggregate particles because of the high porosity, and because this area is filled up with something which is not very strong, these are large plates of calcium hydroxide which can be cracked very easily.
The next slide shows you that this is what actually happens in the field. (see figure 2.4). From a thin section of concrete that deteriorated in a few years, you can trace the micro-cracks with a fluorescent dye. Sea water or de-icing chemicals can permeate very easily through these micro cracks, many of which are interconnected with the cracks which exist next to the aggregate particles. Most of the causes for lack of durability of reinforced concrete, whether it's alkali aggregate reaction, or the corrosion of steel, or sulfate attack, they can be very easily linked to the permeability of concrete, to its lack of water tightness. And this lack of water tightness is not there in freshly cured concrete; it comes later due to environmental effects: heating and cooling, wetting and drying, and because you have built in areas of weakness which micro-crack very easily. When these micro-cracks interconnect, you have channels of flow from outside, and that's how the aggressive chemicals get into the concrete.
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Figure 2.3 Large calcium hydroxide crystals create a plane of weakness next to the coarse aggregate
Figure 2.4 Cross section of concrete showing interconnected micro-cracks adjacent to coarse aggregate.
CANMET Research
Next slide. There are two recent developments regarding high volume flyash concrete, one is the CANMET study which I mentioned earlier, and I want to show you some of the results using these mixes, and how they work. What happens to the voids in the transition zone if you increase the portland cement content of the concrete mixtures? The fine particles of cement in the internal bleed water would dissolve and you will still have voids although smaller in size. Now imagine what would happen if, instead of using the more reactive material (that is portland cement), you add fine particles of a less reactive material such as flyash and also reduce the water content of the concrete mix. You will end up with a less porous transition zone because these tiny glass beads of flyash will obstruct the channels of flow and will make the water-cement ratio more homogeneous in concrete by preventing the formation of local bleed water. And this is exactly what CANMET has found, by limiting the water content, and by introducing a large amount of flyash in the concrete mix. In this mix (see figure 2.5) there is 60% or 215 kg per cubic meter of flyash (360#/CY), 120 kg of mixing water (200#/CY), and 150 kg of cement (260#/CY). The water to cementitious ratio is limited to 0.32 due to the use of a super plasticizer, as well as due to air entrainment which is always required in Canada. So this is a typical CANMET mix, and the next slide shows the properties.
The bleeding with this mix ranges from very low to negligible due to the very low water content of this type of concrete, and also due to the obstruction of channels of flow, you don't expect bleeding on the surface. One of the negative side effects of this is, that you have to take proper care to prevent plastic shrinkage cracking. There are a lot of benefits to this material, but you will also have to learn about the negatives, what is the flip side of the coin. The flip side of the coin is you don't have the luxury of too much bleed water. The construction workers cannot take a coffee break and go away until the sheen is gone, and then come back and finish it. Because there is no sheen to go away, there is no bleed water at all, so you have to be aware of that.
Next slide. With the CANMET mix, the setting time is somewhat longer because remember, the cement content is less that 300 pounds here, and it is the cement that hydrates very, very quickly. It is the hydration of cement that forms calcium silicate hydrate, and as these fibers of calcium silicate hydrate grow, they tend to weave into each other and give you the time of set and strength. So if the cement content is low, naturally the time of set will be slow. But in general, the experience both from the laboratory and the field mixture is that high volume flyash concrete does not show unacceptable retardation in setting time, and demonstrates enough strength development to produce adequate strength at one day. They have obtained 10 MPa (1,500 psi) strength at one day in many of these mixes.
Next slide. Another very important advantage of flyash in concrete is the reduction of thermal cracking. Well known structural engineers who have been in the business for a long time, Professor T.Y. Lin, Professor Ben Gerwick, from their field experience can tell you that many of the problems in concrete are due to thermal cracking. Heat of hydration impacts are usually only considered in mass concrete, such as massive dams. But even in structures which are much less massive, only two or three feet thick, it is massive enough to accumulate enough heat of hydration to cause thermal cracking. So CANMET did a study on 10 foot x 10 foot x 10 foot cubes with a high volume flyash mix. Using this mix the temperature rise was only 35 degrees C compared to 65 degrees C in the control mix using only portland cement. This is a very significant advantage for the durability of concrete because thermal cracking would reduce the watertightness.
The typical compressive strengths that we get with this mix (see figure 2.6) is 8-10 MPa at one day, 35 MPa at 28 days, about 43 MPa at 91 days, and about 55 MPa at one year.
Next slide. Very important from the standpoint of sustainable development is the durability of structures. A lot of structural damage, especially in reinforced concrete, occurs due to the corrosion of steel. There is an ASTM test, rapid chloride penetration test ASTM C1202, where at 28 days 500 to 2,000 coulomb rating was found for the CANMET mixes (see figure 2.7). There is a table showing that anything less than 2,000 coulombs is a very low permeability concrete, so the permeability to CO2, and to chlorides which are responsible for the corrosion of steel is very low in the high volume flyash concrete. And this coulomb rating continues to improve, because many flyash particles react very slowly, pushing the coulomb value lower and lower.
A field test was undertaken by the University of Toronto. Preliminary data is shown in the next slide (courtesy of Professor Michael Thomas). In ten years you can see that the chloride penetration is negligible at about 1 inch depth of cover if you have 50% flyash in the concrete mix. It is an ongoing study in a tidal zone exposed to sea water, with 25 MPA concrete.
The next slide shows another advantage, this is based on a study by Professor Schiessl at the University of Aachen. In Europe they are very much concerned about dumping of industrial by-products because of the potential for ground water contamination. Flyash contains very small amounts of toxic elements, so they wanted to find out about the potential for the leaching of elements such as zinc, chromium, etc. Schiessl and his coworkers found out in the study that flyash concrete is very effective at immobilizing even externally added heavy metals in mortars. Test B is not a very good test, test C is the correct test for mortars and concrete, it is called the tank test and is a leach test on uncrushed specimens. They introduced into the mix, 185 mg of zinc per kg of mix. Then in the leach test they could only leach out 0.09 mg in 56 days. So it's not only that flyash concrete would prevent outside ions from getting in, but it also keeps whatever is in the concrete all tied up, it will not permit it to get out. If you have a toxic metal that has been immobilized by using flyash in concrete, rest assured that it will stay there. In the case of chromium from 53 mg of added chromium, only 0.15 mg could be leached out from the flyash mortar specimens.
Next slide. So this is the mix for the future. In the future, because of so many of these advantages, and much concern about sustainable development, we'll have not only portland cement in the concrete mix, we'll have silica fume and other pozzolans. And in this witches brew a super plasticizer, an air entraining admixture, or other chemical admixtures may be incorporated. This is the future.
Let me go back to a few more transparencies and then I'll finish. I mentioned the ASTM C1202 which is based on AASHTOis T277 test. A 1,000 to 2,000 coulombs current flow in a 6 hour test is a rating of low chloride permeability (see figure 2.8). This is usually a portland cement concrete with less than a 0.4 water cement ratio. If you want very low permeability, less than 1,000 coulomb rating, this is typical of an internally sealed pore system such as with a latex modified concrete. Such low permeability ratings can be obtained with ternary cement blends which I will discuss next.
Chloride Permeability Ratings per AASHTO T-277
Charge Passed
coulombs
Chloride Permeability
Typical of:
> 4000
High
Portland Cement Concrete
W/C > 0.6
2000 - 4000
Moderate
Portland Cement Concrete
0.4 < W/C < 0.6
1000 - 2000
Low
Portland Cement Concrete
W/C < 0.4
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