Pozzolans General Description

A pozzolan is broadly defined as an amorphous or glassy silicate or aluminosilicate material that reacts with calcium hydroxide formed during the hydration of Portland cement in concrete to create additional cementitious material in the form of calcium silicate and calcium silicoaluminate hydrates. The first pozzolans were used by the Romans to make cement from burned limestone and Santorum earth from volcanic eruptions. These ancient concrete mixes were extremely durable and many architectural elements survive today. They underline the fact that one of the compelling reasons for incorporating pozzolans in concrete today is to improve quality and to extend service life by enhancing the durability of this ubiquitous construction material.

To function properly, pozzolans must be amorphous or glassy and generally finer than 325 mesh (45 microns) in particle size. Finer particle sizes generally have greater reactivity, meaning they more quickly convert to supplementary cementitious material, helping in early strength development as measured by standardized tests such as ASTM C618/C109.

Pozzolans can continue to react in concrete for many years, further strengthening the concrete and making it harder and more durable during its service life. Pozzolans also serve to densify and reduce the permeability of concrete, which helps to make it more resistant to deterioration and swelling associated with various exposure conditions.

Types of Pozzolan

Pozzolans commonly used in modern concrete construction include coal fly ash (aka pulverized fuel ash or PFA), ground granulated blast furnace slag, silica fume, and metakaolin (calcined clay). The common feature of all these pozzolans is that they are silicates or aluminosilicates that have been converted to amorphous or glass phases in a high temperature furnace or combustion chamber, followed by rapid cooling or quenching under various conditions. The amorphous or glassy form allows the silicates to react readily as the concrete cures. For use in modern cement and concrete applications, pozzolans must be low in alkalis (Na2O and K2O) which cause long-term durability problems in concrete by expansion due to the alkali-silica reaction (ASR). Chemically, pozzolans are comprised principally of oxides of silicon, aluminum and calcium. The composition of the common pozzolans, including VCAS™ pozzolans, relative to Portland cement are compared in the ternary diagram (CaO-SiO2-Al2O3) below.

The Pozzolanic Reaction

There are several steps involved in the so-called pozzolanic reaction in concrete. As Portland cement reacts with the gauging water, the tricalcium silicate (C3S) and dicalcium silicates (C2S) react to form calcium silicate hydrates (C-S-H), largely responsible for strength development, together with a by-product of portlandite, or calcium hydroxide. At the same time, the alkalinity of the water (now referred to as pore fluid) increases to pH 13 or higher. This combination of events provides ideal conditions under which the pozzolan can react. The high pH first causes the silicate network structure of the pozzolan to break down to smaller units which then react with the calcium hydroxide to form more calcium silicate hydrate binder. The net effect is that the calcium hydroxide in the concrete — which itself has no strength-forming properties and is also a potential site of weakness for certain forms of chemical degradation — is converted by the pozzolan to additional C-S-H binder which is deposited in pore spaces. This leads to a general densification of the cement matrix which contributes to increased strength, reduced permeability, and increased long-term durability.

Portland cement requires only about 25% water to completely react to form concrete. However, most concrete mixes employ considerably larger percentages of water content to allow the mix to flow properly into the formwork or to achieve proper workability. This excess water inevitably introduces void spaces in the concrete that make the concrete porous and provide conduits for the passage of water and aggressive solutions. It is therefore easy to understand that the water-to-cement (w/c) ratio is a major controlling factor in the strength potential of a given concrete mix design: low w/c ratios are associated with higher strengths; whereas high w/c ratios are associated with lower strengths. The conversion of the calcium hydroxide to C-S-H by the pozzolanic reaction and the filling of the void spaces both contribute to improved concrete quality, compressive and flexural strength, and long-term durability. 

Mix Design with Pozzolans

Over the last 50 or more years, engineers and scientists have developed a number of strategies for effectively designing concrete with pozzolans. Depending on the type of pozzolan used and the design goals for the concrete, adjustments will be made to at least the cement replacement factor and the w/c ratio. Pozzolans such as blast furnace slag and fly ash have relatively low water demands and can be used to replace up to 40% of cement in some concrete mixes. In contrast, silica fume has much finer particles with very high water demand, necessitating the use of high range water reducers or superplasticizers to even achieve a 10% cement replacement. At the same time, the fine particle size renders this pozzolan more reactive, allowing concretes to achieve higher strengths more quickly. Metakaolin requires somewhat less water than silica fume and can be used at replacement levels of 10-15%.

Depending on the reactivity of the pozzolan, the cement replacement factor, and the w/c ratio, the initial strength (up to 3 days) of concrete with pozzolans may be reduced by up to 20% compared to the strength of a control concrete without pozzolan. Between 14 and 28 days, however, the pozzolanic concretes typically have similar or higher strengths than the control concrete, with great benefit to the long-term durability, hardness, and strength of the concrete structure or component. High performance pozzolans, such as silica fume and metakaolin, typically have the least impact on the early strength. VCAS™ pozzolans fall into this high performance category and can reach or exceed the control strength by 3 days. Specific strength development data on the performance of VCAS™ pozzolans can be found in the VCAS™ technology sections, Data Sheets, and Technical Bulletins.

VCAS™ pozzolans are comparable in reactivity with silica fume and metakaolin, and are engineered to achieve their performance both by uniform materials chemistry and quenching and by fine particle size. As such, VCAS™ pozzolan have low surface area and smooth surfaces that have 10% less water demand than silica fume or metakaolin. This allows VCAS™ pozzolans to be used at cement replacement rates up to 30% or higher.

Low water demand, high reactivity, and white color after cure are the hallmarks of an excellent pozzolan for white Portland cement concrete.

Benefits of Using Pozzolans

The following are benefits that are generally obtained from using pozzolans in concrete. Specific information for VCAS™ pozzolans are given in the Technology Section and accompanying data sheets and technical bulletins.