The Pantheon in Rome was built by the Emperor Hadrian 1800 years ago, using stone rubble walls cemented together by "Roman concrete" - lime & imported volcanic soils from Pozzuoli, Italy See A brief introduction to Pozzolans , a paper by Bruce King, shown below.

Making Better Concrete — Guidelines to Using Fly Ash
for Higher Quality, Eco-Friendly Structures
[book]
Green Building Press, 2005

American Concrete Institute, Malhotra, V. M, editor
- Proceedings of the Sixth International Conference [and previous conferences]
Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete
[book]
American Concrete Institute, 1999
- Mineral Admixtures / Compilation 22
[book]
American Concrete Institute, 1992

The Iraivan temple on the Hawaiian Island of Kauai is designed to last 1,000 years, in part by using 60% fly ash as replacement for portland cement. Engineering by P.K. Mehta. Hill, Neville, Holmes, Stafford, and Mather, David Lime and Other Alternative Cements [book] Intermediate Technology Publications, London, 1992 Holmes, Stafford, and Wingate, Michael Building with Lime [book] Intermediate Technology Publications, London, 1997 Maharashtra Fly Ash Information Center History of Fly Ash [website] Fly Ash Info from India Malhotra, V. M, and Mehta, P. K. High-Performance, High-Volume Fly Ash Concrete [book] Supplementary Cementing Materials for Sustainable Development, Inc, Ottawa, Canada, 2002 A review of research and experience to date Malhotra, V. M, and Mehta, P. K. Pozzolanic and Cementitious Materials [book] Gordon and Breach Publishers, Amsterdam, 1996 A comprehensive and up to date review of what is available

Malhotra, V. M, and Ramezanianpour, A. A.
Fly Ash in Concrete
[book]
Canadian Centre for Mineral and Energy Technology (CANMET), Ottawa, 1996

Mehta, P. K.
Investigations on the Use of Rice Hull Ash in Concrete
[paper]
University of California, Berkeley, 1994
A technical paper and good introduction to RHA

Moore, David
The Roman Pantheon
[book] Self-published, 1995

Smith, Ray
Rice Husk Ash Cement
[book]
Intermediate Technology Publications, London, 1984

Intermediate Technology Publications also has a series of excellent introductory pamphlets:

Lime	Pozzolanas / an Introduction
Hydraulic Lime	Pozzolanas / Calcined Clays & Shales and Volcanic Ash
Lime-Pozzolan Cement	Pozzolanas / Rice Husk Ash & Pulverised Fuel Ash [fly ash]

ISG Resources-- Fly ash supplier

Additional resources can be found in the bibliographies and websites of the referenced sources - this is by no means a comprehensive list.

A brief introduction to Pozzolans

by Bruce King, P.E.
exerpted in part from
Alternative Construction - Contemporary Natural Building Methods
edited by Lynne Elizabeth and Cassandra Adams
Copyright © John Wiley & Sons, 2000. Reproduced with permission

The modern use of pozzolans¹ as a cement replacing or enhancing admixture in concrete began many decades ago, and is not new or "alternative" to the construction industry, but a trend in the past decade towards greater usage is now redefining acceptable practice. Often restricted by building codes to small fractions of the cementitious material in a concrete mix, pozzolans have held a relatively minor role in the concrete industry. But three trends are now active that would change that minor role to a much bigger one:

1. Economy
Portland cement, the primary "glue" for structural concrete, is expensive and unaffordable to a large portion of the world's population. Some pozzolans, for various reasons, are also expensive, but the most abundant and widely available, fly ash, is not, and typically costs about half as much by weight as cement. Blended cements that replace up to 60% of the portland cement with fly ash have now been successfully used in structural applications. Since portland cement is typically the most expensive constituent of concrete, the implication is greatly improved concrete affordability.

2. Durability
A wide variety of environmental circumstances are deleterious to concrete, such as reactive aggregate, high sulfate soils, freeze-thaw conditions, and exposure to salt water, deicing chemicals, and acids. Typically these problems have been countered, with partial success, by utilizing special cements, increasing strength, and/or minimizing water/cement ratios. But there now exists an overwhelming body of laboratory research and field experience showing that a careful use of pozzolans is enormously useful in countering all of these problems (and others); the pozzolan is not just a "filler", as many engineers think, but a strength and performance-improving additive. (A discussion of the pozzolanic reaction by which all this magic happens is readily available in the cited source. Very generally, it could be described as the siliceous pozzolans reacting with the (non-cementitious) calcium hydroxide in the hydrated cement paste to produce (highly cementitous) calcium silicate hydrates (CH + S + H —> CSH), yielding higher strength and dramatically reduced permeability.)

3. Environment
Portland cement requires enormous heat in its manufacture, making it expensive not just to the consumer, but to the atmosphere as well. For every ton of cement produced, roughly 1/2 of a ton of CO₂ (greenhouse gas) is released by the burning fuel, and an additional 1/2 ton is released in the chemical reaction that changes raw material to clinker, making the production of cement responsible for more than 8% of all the greenhouse gases released by human activity.  High-volume use of pozzolans is (or can be) not just an effective use of "waste" material, and not just an economic savings, but makes possible a noticeable reduction in greenhouse gas buildup. Seen from another perspective, high volume pozzolan usage in blended cements is a way for the cement industry to supply the ever-growing world market without having to build new production facilities.

Some pozzolans are manufactured to augment concrete mixes in some specific way, others are ground from fired clay soils (such as the surkhi of India, made by grinding fired clay bricks), and others are volcanic tuffs or diatomaceous soils mined directly from the earth (the name "pozzolan" derives from early uses of a cementitious volcanic ash mined near Pozzuoli, Italy). The most commonly used ones, however, are waste products from industry, which can be described in generally descending order of quality as:

1. Cementitious and pozzolanic materials

High-calcium fly ash (class C)
HCFA is the residue collected from the smokestacks of coal-fired power plants generally using lignite and subbituminous coals. Class C fly ashes are in themselves mildly cementitious, and have been combined with lime or even calcium carbonate soils to produce moderately strong concretes.

Ground Granulated Blast Furnace Slag
GGBFS is the ground residue from iron smelters, and is also mildly cementitious in itself, but hugely pozzolanic in combination with water and cement. Its usage dates back 200 years, is widely used in Europe, and is currently restricted by the 1997 UBC to 50% of the cementitious material in a concrete mix.

2. Highly active pozzolans

Condensed Silica Fume
CSF 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.

Rice Hull (or husk) Ash
RHA is the least known of the four pozzolans discussed, but enormously promising on a global scale. The world's primary staple crop is rice, the milling of which generates 100 million tons of hulls, or chaff, annually. Like the straw, hulls have historically been burned in the fields, but the resulting pollution is increasingly causing health problems. Research in India and the United States has found that if the hulls (or straw) are burned at a controlled low temperature, the ash collected can be ground to produce a pozzolan very similar to (and in some ways superior to) silica fume. (Simultaneously, rice farmers have found that their biomass wastes can be burned to produce electrical power; several such powerplants are now operational in the rice-growing regions of the USA, and more are planned in the USA, Brazil, the Phillipines, and Japan. It is now known to be easy to burn the hulls (or straw) in such a way as to produce power and also generate the desired quality of ash for pozzolanic use. The implication, then, is that the hull/straw disposal problem worldwide can be ameliorated by building small ( one to five MW) plants in rice-growing regions that could cleanly dispose of the crop waste, generate electricity for the area, and provide high quality cement.)

3. Normal pozzolans

Low-calcium fly ash (class F, generally less than 10% CaO)
LCFA is the residue collected from coal-fired power plants generally using anthracite and bituminous coals. Though generally less effective than the class C fly ashes, class F fly ash is nevertheless an abundant and useful pozzolan; note that the referenced structure in Nova Scotia, using 60% fly ash, was utilizing class F material. Both classes of fly ash are restricted by the 1997 UBC to 25% of cementitious material "in special exposure conditions" (section 1904), but this restriction is widely interpreted to apply to concrete in all conditions.

Conclusion

Many experts in concrete research are now calling for increased usage and high-volume usage of pozzolans, especially fly ash; some have submitted that all concrete should contain fly ash unless there is a compelling reason not to (John Scanlon, "Mineral Admixtures" (ACI Compilation 22), 1992). In light of economic conditions and the abundant evidence, both from research and in the field, it seems inevitable that regular and high-volume usage of pozzolans will become standard practice in the concrete industry.

¹  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." This definition of pozzolans is taken from "Pozzolanic and Cementitious Materials" by Malhotra and Mehta (Gordon and Breach Publishers, 1996), a primary resource for the preceding article. Other sources are ACI Compilation 22 (1992), ACI 226.3R-87, "Use of Fly Ash in Concrete", ACI 226.1R-87, "Ground Granulated Blast Furnace Slag as a Cementitious Constituent in Concrete", ACI 234R-96, "Guide for the Use of Silica Fume in Concrete", and "Investigations on the Use of Rice Hull Ash in Concrete", Technical Report to RHA Technologies, Inc. by the University of California, 1994.

Return to "pozzolan" footnote, above...