(The story of one manıs trash and the other manıs treasure)

Quote: ³U.S. power plants produce millions of tons of fly ash annually, which is usually dumped in landfills.²

Pozzolans not only strengthen and seal the concrete, they have many other beneficial features you will realize the moment you add them to the mix.


As I did my research for this summary about pozzolans and its uses, I got more and more amazed by the way construction professionals deal with this subject. Though by now pozzolans, particularly fly ash, have become pretty much commonplace, many professionals of the construction industry still seem to look at them as some kind of inert fillers or magic potion. And for some reason they donıt seem to realize that any potion, magic or not, has to be carefully produced and applied to accomplish the desired result.

Producers of fly ash and rice hull ash need to dispose of these substances, and of course they would prefer to do that for a profit. Therefore they are often willing to promise anything that will help them to get rid of what they perceive as "trash". That in turn puts the responsiblity on the construction professionals to thoroughly investigate pozzolans and to demand higher quality standards for them, so they can be used correctly. Given the present situation, my hope is that this publication will help the construction professional to have an influence on how pozzolans are prepared, and to distinguish which ones are suitable for their purposes.

This summary was written with the purpose of helping people who are interested in the subject, mostly the members of the discussion list about ³ferrocement² (FC). No claims are made regarding accuracy or completeness, nor are there any other claims. This publication pursues no commercial interest. Therefore permission is granted to copy the entire text or parts of it, as long as it is not done for commercial purposes, and as long as due credit is given to the author.

Origin of ³Pozzolan²

The term ³pozzolan² is derived from the name of the town Pozzuoli, Italy. It is situated near Mt. Vesuvius and is the place where the Romans more than 2,000 years ago mined the ashes deposited by the occasional eruptions of this volcano. Adding these ashes at a ratio of 2:1 to aged lime putty (aged 2+ years) they were able to construct those sturdy buildings we still admire today.

Given this mineral origin, some purists consider only volcanic ashes, pumice, tuffs, etc. as ³pozzolans². But as the ashes of organic origin, like ³pulverized fuel ashes² (PFA, mostly coal ashes) and rice hull ashes (RHA) also show enhancing properties when mixed with cement or lime, most of the times the origin is irrelevant. What counts are the properties, primarily particle size and purity (absence of carbon), and the results!

Definition and Use of ³Pozzolans²

A "pozzolan" is defined as "a siliceous or siliceous and aluminous material, which in itself possesses little or no cementing property, but will in a finely divided form - and in the presence of moisture - chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties."

Definition taken from "Pozzolanic and Cementitious Materials" by Malhotra and Mehta (Gordon and Breach Publishers, 1996)

Letıs have another look at this definition:

€ A pozzolanic material has to contain reactive silicates or alumino-silicates.
€ The particles must be fine enough to provide a sufficient reactive surface area for the solid-state chemical reactions.
€ The particles react with the alkalis and calcium hydroxide from the cement to produce cementitious compounds (calcium-silicate hydrate gel, calcium-alumino silicates, etc.).

Regarding the ambiguous words ³fine enough², generally 45 µm (micron) are specified as the maximum particle size. Realizing that these particles are too big to allow a timely reaction between the lime that is given off as the cement hardens, and the silicate or alumino-silicate of the pozzolan, fly ash producers often indicate 35 µm as the maximum particle size. Some researchers in the field insist that even this is normally still too big, and they specify a maximum of 10 µm.

Cement gives off lime as it hardens and this lime will inevitably react with silica (silicates or alumino-silicates). The aggregate in the concrete is basically silica, but unfortunately it reacts too slowly, due to its reduced total surface area. In a concrete without pozzolans, the lime produced in the hardening of the cement will slowly react with the aggregate, producing gels. These gels are expansive, and that is welcome as long as they just fill the voids with a slight pressure thus avoiding water penetration and leaching.

The problem starts when too large a volume of gels is produced after the concrete has hardened. That is the case if the lime only has aggregate or large pozzolanic particles to react with. Given the relatively small total surface area of the aggregate or big pozzolanic particles, as well as the slow reaction between lime and silica, most of the gels will be produced after the hardening of the cement, resulting in possible disastrous pressure build-up and a slow destruction of the concrete. This makes it clear that the pozzolan has to be very fine (<10 µm) to ensure that most of the gels are formed before the hardening.

Definition of ASR

Talking about potentially destructive reactions, you will often hear the acronym ³ASR². What is ASR? Is it good or is it bad? The answer to both questions is, ³it depends!² One definition for ³ASR² is ³alkali-sulfate reaction². This one is never good and often outright bad! It happens when ettringite is produced, as a result of a chemical reaction between lime and sulfur. It can - but does not always - result in expansion and lead to cracking of the concrete. Gypsum, in addition to ettringite, can also be produced during sulfate attack and is capable of producing expansion. Sulfate attack of concrete may lead to cracking, spalling, increased permeability and a loss in strength.

The other meaning of ³ASR² is ³alkali-silica reaction². This one can be good or bad, and in reality always happens. The construction professional only refers to the unwanted form of ASR when he talks about it, but the desired reaction between pozzolan and the lime from the cement-hardening process is also an ³ASR², though a much faster one.

As mentioned before, cement produces free lime as it hardens, and this lime will eventually react with whatever is available, forming expansive gels. Therefore what we have to do is to ensure that these gels will be formed on time, (before the concrete hardens too much), so they wonıt cause too much pressure build-up in the hardened concrete. Otherwise even a relatively small pressure (load) on the concrete could destroy it.

The only way to achieve this is including very fine (<10 µm) and reactive (i.e. burnt at relatively low temperatures) pozzolans in the mix. The resulting gels will occupy the voids in the still elastic concrete, sealing them from water.

If you choose the right pozzolan, select the right particle size, add the correct amount and mix it well, this concrete will be a coherent and impermeable artificial rock.

Benefits of Pozzolans

Pozzolans not only strengthen and seal the concrete, they have many other beneficial features you will realize the moment you purchase them or add them to the mix. All of the below benefits apply to fly ash and rice hull ash, and most of them to silica fume as well.

Spherical Shape: Fly ash (FA) and rice hull ash (RHA) particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures.

Ball Bearing Effect: The "ball-bearing" effect of FA and RHA particles creates a lubricating action when concrete is in its plastic state.

Economic Savings: Pozzolans replace higher volumes of the more costly cement, with typically less cost per volume.

Higher Strength: Pozzolans continue to combine with free lime, increasing structural strength over time.

Decreased Permeability: Increased density and long-term pozzolanic action, which ties up free lime, results in fewer bleed channels and decreases permeability.

Increased Durability. Dense pozzolan concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Pozzolan concrete is also more resistant to attack by sulfate, mild acid, soft (lime-hungry) water, and seawater.

Reduced Sulfate Attack: Pozzolans tie up free lime that otherwise could combine with sulfate to create destructive expansion.

Reduced Efflorescence: Pozzolans chemically bind free lime and salts that can create efflorescence. Denser concrete, due to pozzolans, holds efflorescence-producing compounds on the inside.

Reduced Shrinkage: The largest contributor to drying shrinkage is water content. The lubricating action of FA and RHA reduces the need for water and therefore also drying shrinkage.

Reduced Volume: As pozzolans can in certain cases substitute for up to four times the mass of cement, besides making the same amount of concrete harder than without pozzolans, less voluminous structures are able to bear the same load.

Reduced Heat of Hydration: The pozzolanic reaction between pozzolan and lime generates less heat, resulting in reduced thermal cracking when pozzolans are used to replace portland cement.

Reduced Alkali Silica Reactivity: Pozzolans combine with alkalis from cement that might otherwise combine with silica from aggregates, which would cause potentially destructive expansion.

Workability: Concrete enhanced with FA and RHA is easier to place, with less effort, responding better to vibration to fill forms more completely.

Ease of Pumping: Pumping of FA and RHA concrete requires less energy, therefore longer pumping distances are possible.

Improved Finishing: Sharp, clear architectural definition is easier to achieve with FA and RHA concrete, with less worry about in-place integrity.

Reduced Bleeding: Fewer bleed channels decreases porosity and chemical attack. Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in enhanced bond strengths.

Reduced Segregation: Improved cohesiveness of pozzolan concrete reduces segregation that otherwise could lead to rock pockets and blemishes.

Reduced Slump Loss: More dependable concrete allows for longer working time - especially important in hot weather.

Very low Chloride Ion Diffusion: Pozzolans make concrete more resistant to salt water (seawater).

Improved Water Tightness: The formation of expansive gels effectively seals the concrete.

Resistance to Freeze-Thaw: As water doesnıt penetrate the hardened concrete, freezing canıt cause destructive expansion.

Resistance to Adverse Chemical Reactions: The example of Dynastone shows how pozzolans can protect against strong acids.

Fly Ash as a Pozzolan

Fly ash is the most commonly known artificial pozzolan and results from the burning of pulverized coal in electric power plants. The amorphous glassy spherical particles are the active pozzolanic portion of fly ash. It is important that the coal is burnt at relatively low temperatures. At higher temperatures the glassy particles would turn crystalline, rendering them useless as pozzolans. Fly ash is 66-68% glass, on an average. Class F fly ash (see ASTM C 618) readily reacts with lime (produced when portland cement hydrates) and alkalis to form cementitious compounds. In addition to that, Class C fly ash may also exhibit hydraulic (self-cementing) properties.

Concrete made with Type C fly ash (as opposed to Type F) has higher early strengths because it contains its own lime. This allows pozzolanic activity to begin earlier. At later ages, Type C behaves very much like Type F - yielding higher strengths than conventional concrete at 56 and 90 days.

Though fly ash is typically produced in coal-fired power plants, in reality it doesnıt matter at all where the ash comes from, as long as it can produce the benefits listed above. Unfortunately that may not always be true with the kind of ash you would like to use as a pozzolan. For example, coal from the east coast tends to contain sulfur, which is still present in the ash, or the particles of an ash ­ regardless of its origin - might be too big or contain too much carbon. In an attempt to the classify different qualities of ash, categories have been created for coal-derived fly ash.

Fly ashes that comply with ASTM C 618 for mineral admixtures in portland cement concrete come in two classes: Class C is produced from burning sub-bituminous coal and has faster strength gain, while Class F is produced from burning bituminous coal and has higher ultimate strength.

Chemical Composition of
Class C Fly Ash

Substance or Property

(ASTM C 618) %

SiO2 plus Al2O3 plus Fe2O3, min


SO3, max.


Moisture content, max.


Loss on ignition1, max.


1³Loss on ignition³ basically refers to the carbon content in the ash. The more carbon that is still present, the more weight you will lose upon burning the ash. Ideally, there should be no weight loss at all. As little as 3% of coal in the cement mix (without aggregates) will prevent the hardening of the concrete. On the other hand, 1% doesnıt seem to be a problem at all, so the gap is pretty narrow.

In combination with portland cement, Class C fly ash can be used as a cement replacement, ranging from 20-35% of the mass of cementitious material. Class C fly ash must replace at least 25% of the portland cement to mitigate the effects of alkali silica reaction.

Note: If the fly ash has high calcium content, it should not be used in sulfate exposure or hydraulic applications.

Chemical Composition of
Class F Fly Ash

Substance or Property

(ASTM C 618) %

SiO2 plus Al2O3 plus Fe2O3, min


SO3, max.


Moisture content, max.


Loss on ignition1, max.


In combination with portland cement, Class F fly ash can be used as a cement replacement ranging from 20-30% of the mass of cementitious material.

Note: If the fly ash has high calcium content, it should not be used in sulfate exposure or hydraulic applications.

Improving the Quality of Fly Ash

Realizing that the quality of the average fly ash often leaves much to desire, at least one company has been working on methods to improve it. The next two paragraphs are copies of this companyıs press release. Note that both ultrasonic conditioning and wet scrubbing are used. If somebody goes to such extend to boost the quality of fly ash, obviously there is also a market and good use for this quality-enhanced product, and that should be food for thought.

MATERIALS ISG process lands patent for fly ash carbon removal. Salt Lake City-based ISG Resources, Inc., a developer of technology for the use of recycled coal combustion by-products (CCB), announced that it has been issued U.S. Patent No. 5,840,179 for a process to improve carbon removal from fly ash.

"The subject patent 'Ultrasonic Conditioning and Wet Scrubbing of Fly Ash' is an important improvement to the conventional froth flotation process used to remove unburned carbon from fly ash," says ISG Vice President of Engineering Dr. Rafic Minkara, P.E., process co-inventor. "Our team of scientists discovered that some unburned carbon particles are porous and trap micro-spherical pozzolan less than one
µm in size. The release of these fine pozzolan particles from the carbon improves the recovered-carbon quality and adds a desirable component to the processed fly ash."

Note that both the quality of the removed carbon (now a valuable commodity) and the fly ash are improved! Below is an example of what can be done with this improved pozzolan (copy of press release).

Dynastone gained national recognition for inventing the technology to produce an acid-resistant cement which, when used in concrete, can withstand sulfuric acid exposure with a pH of 1.0. Since the fly-ash-bearing product is as cost-effective as ordinary cement, Dynastone officials note, it is well suited for many applications, including:

Sanitary sewer pipe. Present day concrete pipes typically have a life span of 50 years and cannot be used in some sewer systems because of the high acid content. Dynastone pipe has an estimated life of 100+ years and can be used anywhere in sewer construction because of its high acid-resistance.

Mortar. Dynastone mortar has a bond-strength of 100 psi. When subjected to hurricane force wind and rain (75 mph), it resisted water penetration for four hours. Commercial mortar (20 psi strength) resists water for five seconds under the same conditions, the company reports, while special mortars can resist such exposure for up to one hour.

It looks like at present only fly ash is enhanced this way, but any other ash ­ like rice hull ash ­ could be treated in the same manner. If your goal is an exceptionally high-quality concrete - like FC/ECC - have a look at enhanced pozzolans

For more info about this process go to

For more info about fly ash go to: http://www.iflyash.com/whatisflyash.htm

Rice Hull (or Husk) Ash as a Pozzolan

Rice hull ash (RHA) is frequently referred to as a pozzolan superior to fly ash. Some people even claim that it is superior to silica fume (see below). Unfortunately, there is hardly any in-depth information available in the public domain, though there should be a lot of proprietary information. Therefore most likely the average user is betting on good luck when purchasing this product. RHA does not come by nature as a ³finely divided powder², one of the requirements to be a good pozzolan. So if anybody wants to experiment with it, he should make sure that it is either already finely ground or that he can use a suitable mill and screen.

As rice hulls are an organic product, they contain carbon. The technology for burning rice hulls has improved a lot, but that doesnıt mean that each and every plant that burns these hulls is using the latest technology. Even if they do, the result will not necessarily be a suitable pozzolan. The modern furnaces for rice hulls are probably mostly designed to produce as little NOx emission as possible. For that the hulls would have to be burnt with the minimum possible amount of air (oxygen). That in turn would unfortunately mean that the carbon content measured in ³LOI² (loss on ignition) might be high.

The mere fact that Iım just speculating here - due to the lack of precise info ­ should alert the future user. It is clear that some people had very good results with RHA, but it isnıt clear why, and why others had poor to disastrous results. Given the potential of pozzolans in general - and RHA in particular - it would be good if user groups like ³ferrocement² could conduct research into this matter. Among other points of research they might consider ultrasonic conditioning and wet scrubbing as a previous stage before including this pozzolan in their mixes.

Silica Fume (SF)

Silica fume is a waste product of the silicon metal industry, and is a super-fine powder of almost pure amorphous silica. Though difficult (and expensive) to handle, transport and mix, it has become the chosen favorite for very high-strength concretes (such as for high rise buildings), often in combination with both cement and fly ash.

Silica fume is a by-product resulting from the production of silicon or ferrosilicon alloys or other silicon alloys. Silica fume is light or dark gray in color, containing typically more than 90% of amorphous silicon dioxide. Silica fume powder collected from waste gases and without any further treatment is generally called ³undensified silica fume², to distinguish it from other forms of silica fume.

Undensified silicon fume consists of very fine vitreous spherical particles with an average diameter about 0.1µm, which is 100 times smaller than the average cement particle. The undensified silica fume is almost as fine as cigarette ash and the bulk density is only about 200 - 300 kg/m3. The relative density of typical silica fume particles is 2.2 to 2.5. Because the extreme fineness(!) and high silicon content, silica fume is generally a very effective pozzolan.

High-strength silica fume concretes of up to 300Mpa have been achieved in some countries. Applying silica fume in concrete fertilizer storage silos effectively reduced calcium nitrate attack. Condensed silica fume (CSF) has been used in repairing a dam stilling basin to improve abrasion erosion resistance; it has also been employed as an essential additive to prevent alkali-silica reaction.

Though condensed silica fume is much easier to handle and transport, uncondensed silica fume (normally in the form of a slurry) is more effective. The smaller, already wetted particles mix much easier and distribute better, hence reactivity is better, too!

Chemical composition of silica fume

Chemical composition of SF varies depending on the nature of the the manufacture process from which the SF is collected. The main constituent material in SF is silica (SiO2), the content of which is normally over 90%. The following table lists a chemical analysis of a commercially available silica fume.

Commercially Available
Silica Fume


Percentage %















Properties of (problems with) fresh concrete with silica fume:

Workability, water demand
: Use of silica fume in concrete usually increases water demand. The increased water demand causes an increase in water to cement ratio and could negate the benefits of adding silica fume. For this reason, silica fume concrete (SFC) normally incorporates a water reducing agent or superplasticiser.

Stability: SFC is more cohesive than conventional concrete. This is true for SFCs both with and without superplasticiser. Increased cohesiveness reduces the likelihood of bleeding and segregation. This increased cohesiveness could however increase the required compaction energy.

Plastic shrinkage: Increased cohesiveness of SFC encourages the potentiality of plastic shrinkage and cracking that appears when the bleeding water cannot compensate for the water loss on the surface, due to evaporation. Under conditions of fast evaporation, curing measures should be taken immediately after placing the concrete.

It should be noted that to overcome the above shortcomings, sometimes FA and/ or RHA are also added to the concrete, together with SF.

Properties of hardened concrete with SF

Combining SF with the appropriate aggregates and water-reducing agent can produce high-strength concrete with a cube compressive strength of around 100Mpa, in extreme cases up to 300Mpa.

The impermeability of SFC is higher than that of similar concrete without SF.

Tests have proven that one part of silica fume can replace up to 3-4 parts of cement without any loss of strength. Replacing 10% by weight of cement with SF is a good starting point for experiments.

Unfortunately, some types of SF cannot be used in concrete. The combination of Si and FeSi-75% condensed silica fume has proven to work effectively, while mixtures of FeSi-75% with FeSi-50% and FeSi-75% with CaSi have proven to be ineffective.

Physical Characteristics

The silica fume particle consists mainly of ³vitreous² silica particles. It has a specific gravity of about 2.20, which happens to be the accepted value for the specific gravity of any vitreous silica. Nevertheless, it has been proven that the higher the amount of impurities in silica fume, the higher the specific value. Certain impurities such as iron, magnesium, and calcium (note: but not CaSi) have shown to increase this value.

"Condensed Silica Fume in Concrete", Malhotra, Ramachandra, Feldman, Aitcin, p. 9-10

As stated before, silica fume was first looked at as a replacement for cement, but today only a portion of the cement is replaced with a much smaller amount of condensed silica fume in the concrete mix. Besides that, silica fume - just like any other pozzolan - is not an inert filler, but plays an active role in the performance of the concrete.

Other Pozzolans and Applications

The first uses of pozzolans were for lime as a construction material. To enhance the properties of lime, the most adventurous additives have been used. Among them are ground burnt clay bricks, bone ash, animal blood, and clay. Nowadays metakaolin has also become a pozzolan. As this is basically calcined clay, it should produce results similar to ground burnt clay bricks.

As clays are silicates or alumino-silicates, they should be useful in both lime and cement. For lab tests with raw clays I would recommend to first dissolve the clay in almost the total calculated volume of water needed for the concrete mix. This can be accomplished by placing the clay in the water and left to soak for a day or two. After that it should be finely screened to remove any clumps and foreign matter.

When using pozzolans in mortars, the softer pozzolanic materials (like brick dust from clay bricks fired at less than 950°C) will normally make them more permeable and flexible, while the hard-burned materials, for example PFA (pulverized fuel ash, like fly ash), will tend to produce a harder mortar, closer in its characteristics to cement. In any case, carbon content and particle size have to be known and taken into account.


If after reading this summary you go out and buy ³any old² pozzolan to improve your concrete, most likely you wonıt obtain the expected great results. In a way I compare pozzolans to grease. Both might be dirty-looking, unpleasant substances, but they can do great things for you if you select the right product and apply it correctly. Anything else is an invitation for trouble.

Regardless of the origin of the pozzolan, make sure that the maximum particle size is 10 µm or less, or that at least more than 90% of the particles fulfill this requirement. Also make sure that you are dealing with a product with a low carbon contents (<1%). The lower the better! You can only expect to produce a superior concrete if your pozzolan fulfills these two requirements.

Last of all, an apparently insignificant detail that came to my mind while writing this summary. There are quite a few substances that can negatively affect your concrete, though the most important ones are probably sulfur and salt (NaCl). These ­ as well as many other unwanted substances ­ can be present in our water for the mix. And who would first analyze the tap or well water he is going to use? Therefore I feel like for extremely delicate applications only demineralized or distilled water should be used. Otherwise you might encounter strange problems and never know the cause.

Useful Links

Here are some useful links to start an in-depth investigation into pozzolans:

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