Editor’s Note: This is the final article in a four-part series detailing the types of supplementary cementitious materials (SCMs) available and the role they play in concrete’s strength. To read the third article in the series on silica fume, click here.

Natural pozzolans have been used as a cementitious material for thousands of years. The earliest use shows lime and diatomaceous earth being used together as a cementitious binder by ancient civilizations near the Persian Gulf. The invention of “modern” portland cement in the early 1800s – nearly 7,000 years after the first recorded natural pozzolan use – did not end their use as a binding agent in concrete. Rather, the U.S. Bureau of Reclamation began extensive research to better understand natural pozzolans’ impact on heat of hydration and concrete durability in mass concrete applications. Natural pozzolan use in North America expanded during the early-to-mid 1900s with the construction of a series of public works projects including numerous dams and the Los Angeles Aqueduct. Unlike earlier civilizations that relied on raw natural pozzolans, most modern applications call for calcined or heat-treated natural pozzolans.

Natural pozzolan basics

A pozzolan is a siliceous material that possesses little cementitious value by itself. However, if it is finely divided in the presence of moisture, it will react with calcium hydroxide to form calcium silicate hydrate and other cementitious compounds. It can be used as a substitute for cement in concrete mixtures. The most common natural pozzolans used in concrete applications today include calcined clay, calcined shale and metakaolin. Other types of natural pozzolans that are used less in modern applications include volcanic ash, volcanic glass (pumicite and obsidian) and rice husk ash, among others.

Natural pozzolans are sourced from natural mineral and volcanic deposits. Some minerals like clay or shale require heat treatment to transform them into pozzolans, while others like volcanic ash exhibit pozzolanic behavior with minimal processing.

Calcined versus raw

The process of turning clay or shale into calcined clay, metakaolin or calcined shale involves applying significant heat to the material. The process causes reactions that incorporate oxygen into the material’s structure, effectively altering its makeup and behavior, and transforming the material into a pozzolan. After the calcination process, the calcined material may be further processed before finally being ground into a fine powder suitable for use as a supplementary cementitious material. Metakaolin is considered a unique type of calcined clay. Metakaolin is produced by using a higher-temperature calcination process specifically on kaolin clay before being ground finer than traditional calcined clay. The high material fineness along with the different calcination process results in metakaolin exhibiting behaviors different from ordinary calcined clays.

Some materials, like volcanic ash, can exhibit pozzolanic behavior in their raw form without calcination or extensive processing. These are sometimes referred to as true natural pozzolans.

In the plant

As an SCM, natural pozzolans are typically used as a partial replacement for ordinary portland cement or as a substitution for fly ash. The replacement or dosage rates vary depending on the pozzolan and the desired concrete characteristics and behavior.

“For true natural pozzolans (non-calcined), replacement rates tend to mirror the replacement levels of fly ash,” said Ken McPhalen, manager of technical services at Advanced Cement Technologies. “On average, you’re looking at anywhere from 20% to 30% cement replacement by weight. For metakaolin, it’s much lower – around 8% to 10% for most high-performance applications.

“Dosage levels could be bumped up in those situations when you’re addressing a specific challenge, like mitigating alkali-silica reactivity.”

During batching, natural pozzolans may be incorporated into the mixer as an individual component separate from cement, or as a blended cement. ASTM C618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete,” dictates chemical and physical requirements for natural pozzolans, which fall under Class N. ASTM C595, “Standard Specification for Blended Hydraulic Cements,” outlines requirements for hydraulic cement blended with other cementitious materials like slag, limestone or pozzolans. ASTM C595 Type IP blended cement defines portland-pozzolan blends, while Type IT indicates a ternary blend, which could include OPC, a pozzolan and a third cementitious component.

Pozzolanic reactions

Like other pozzolans, natural pozzolans react with water and calcium hydroxide to produce calcium silicate hydrate. CH and CSH are both products of portland cement hydration reactions. However, CH is porous and contributes little strength to cement paste while CSH is denser and serves as the main strength-building agent as concrete cures.

The reactivity and behavior of any SCM will vary depending on the parent material and the final makeup of the processed SCM, as well as the dosage rate, constituent materials, curing and other parameters.

Generally, pozzolanic reactivity is influenced by particle size, material composition and temperature. As particle size decreases, the total surface area of the particles increases and allows the reactions to occur faster. The composition, and specifically the glass and calcium contents, will also impact reactivity. Pozzolans tend to react faster when used in conjunction with a high-alkali portland cement, or with increasing temperature.

Physical properties

Calcined clay and calcined shale are often used in general purpose concrete applications. These natural pozzolans range in size. However, roughly 2/3 of the particles must be smaller than 45 micrometers, in accordance with ASTM C618.

Metakaolin, on the other hand, is often employed in special applications where very low permeability or high strength is needed. On average, metakaolin particles exhibit diameters ranging from 1 to 3 micrometers.1 For comparison, silica fume particles are less than 1 micrometer, with average particles having diameters of 0.1 micrometer. Although ordinary portland cement grains have a broad particle size distribution that depends on the ASTM C150 cement type classification, only a small percentage of particles may be less than 5 micrometers while the majority of particles are less than 45 micrometers.

Effects on fresh concrete

Calcined clay and calcined shale tend to have similar effects on many fresh concrete properties. Both materials generally have no impact on water demand, but both have a tendency to increase workability. Unlike metakaolin, calcined clay and shale usually show no significant impact on air content. Additionally, calcined clay and shale have little effect on bleeding and segregation because the particles are, on average, slightly larger than portland cement grains. Significantly finer particles, as with silica fume or metakaolin, lower the bleed rate and the bleed capacity. Calcined clay and shale have little to no effect on setting time.

“One of the benefits you will achieve with true natural pozzolans is a slower hydration, which for larger sections will reduce the build up of heat,” McPhalen said. “The true natural pozzolans tend to retain water, which, if you can balance your mix design correctly, will release that water much more slowly into the system which tends to lead to less cracking over time as well. True natural pozzolans have an affinity for water and you really need to take that into consideration in your mix design.

“They are much slower reacting too, so you may not be able to turn forms as fast.”

Although metakaolin is a type of calcined clay, it behaves differently than traditional calcined clays. Metakaolin’s extremely small particle sizes cause it to increase water demand and decrease workability. Additionally, concrete made with metakaolin often displays lower air content. An added benefit of metakaolin’s fineness is its tendency to decrease bleeding and segregation. Also, like calcined clay and shale, metakaolin displays no measurable impact on setting time.

The general assumption regarding natural pozzolans’ impact on heat of hydration is that it will be measurably lowered. However, the heat of hydration when working with metakaolin is assumed to be about equal to, or slightly greater than, that of ordinary portland cement, typically ranging from 100% to 125% that of OPC.2

“With metakaolin, you can turn the forms faster simply because the product is going to react quicker and it won’t penalize you for slower hydration,” McPhalen said.

Effects on hardened concrete

Calcined clay, calcined shale and metakaolin can all improve concrete’s durability. Specifically, natural pozzolans are known to increase corrosion resistance and sulfate resistance, decrease permeability and absorption, and reduce alkali-silica reactivity. These benefits are largely attributed to the materials’ pozzolanic behavior, which decreases concrete’s permeability and increases density and strength by consuming CH and producing CSH. Like fly ash, slag cement and silica fume, natural pozzolans also contribute to increased long-term strength gain.

Other characteristics related to concrete durability like impact resistance and abrasion resistance are unaffected by the use of natural pozzolans or other SCMs. These attributes are related to concrete’s compressive strength as well as aggregate characteristics. Additionally, natural pozzolans show no significant impact on freeze-thaw durability, which is heavily dependent on concrete’s water-cementitious material ratio and air void system as well as concrete’s compressive strength and aggregate characteristics.

In terms of architectural products, McPhalen had a few words of caution.

“Some natural pozzolans that are calcined become exotic colors, while those that don’t get calcined tend to have a fairly consistent color,” he said. “With architectural precast, you may want to think more about metakaolin, which is generally produced under color specifications to prevent variations. It’s used more for high-performance applications where you’re looking for higher strengths or ultra-low permeability, as well as in architectural applications, where the white color is a benefit to the aesthetics of the project.”

Availability and opportunities in your plant

Although natural pozzolan availability is not dependent on manufacturing industries as is the case with fly ash, slag cement or silica fume, the existence of appropriate clays or shales does vary by region.

Natural pozzolans, whether raw or heat-processed, are valuable in many unique applications including mass concrete pours or where enhanced durability, improved ASR resistance or increased resistance to chemical attack are desired.

“They lend themselves incredibly well to mass concrete applications, be it dams or foundations, anything where you need to maintain a low heat of hydration and keep water in the system for a long period of time,” McPhalen said. “Those are the applications where natural pozzolans will really shine.”

If you are new to using natural pozzolans or SCMs altogether, McPhalen said to ask yourself these questions first, “What is your true purpose for wanting to use these materials? Is it the technical benefits? Is it to reduce your CO2 footprint in your mix design?” Once you know that, it’s a little easier to find the right ingredient that will meet those demands.

It’s important to weigh the benefits and drawbacks associated with each SCM and carefully proportion and adjust any mix design in accordance with the manufacturer’s recommendations, governing standards and specifications.

“As with any new product, they all have their quirks,” McPhalen said. “Full testing is imperative to determine your ultimate water demands and recalibrate your chemical admixture demands within those mix designs. You really have to go start to finish and do full and thorough testing to truly understand the product you’re now introducing into your mix.

“The next step beyond that is to make sure the quality control and production staff understand what it is that they’re now dealing with, because as best as you can make your mix design, it can be destroyed in a hurry if it’s not handled properly. It’s to everybody’s benefit to understand what the new concrete is going to look like and how it will behave.”

Kayla Hanson, P.E. is a technical services engineer with NPCA.


1, 2 Portland Cement Association Design and Control of Concrete Mixtures