Natural Pozzolans: A Historical Answer to Modern Demands

By Brian P. Lemay

Natural pozzolans are an ideal supplementary cementitious material in nearly any application, from structural concrete for buildings to major infrastructure.

Intriguingly, the use of natural pozzolans in concrete is both an innovative method of addressing several exigent issues facing the concrete industry today while also playing an integral role in some of the earliest historical applications of concrete in ancient Rome. Indeed, pozzolans are named for Pozzuoli, Italy from which the Romans sourced volcanic pumices and tuffs for their concrete mixtures. Natural pozzolans are those which are found in naturally occurring reserves, whether usable in their raw state as in the case of the aforementioned volcanic rocks, or requiring calcination to maximize pozzolanic qualities as in the case of many clays and shales. Concrete producers may choose between these natural pozzolans and those formed as industrial waste products such as fly ash, slag, and silica fume.

Kaolinite mined from a quarry such as this can be calcined, yielding the natural pozzolan metakaolin.

As interest in environmental sustainability continues to grow among both the general public and construction industry professionals, both waste pozzolans and natural pozzolans will continue to play an important role in reducing the embodied carbon of concrete. Additionally, pozzolans enhance the performance of concrete in several key ways. Unfortunately, traditional sources of pozzolans such as fly ash are in short supply even as demand for concrete mixtures containing pozzolans continues to grow each year. Natural pozzolans, being widely used and well understood, are thus a convenient and proven substitute for fly ash or other waste pozzolans in environmentally sustainable projects, as well as those requiring concrete with improved integrity and longevity.

Pozzolans combine with Calcium Hydroxide (CH) within concrete, a byproduct of cement hydration, to form additional Calcium Silicate Hydrate (CSH), the crystalline compound that binds concrete together. This process can mitigate several harmful forms of weathering which can drastically reduce the strength and lifespan of the material. Namely, pozzolans prevent freezing and thawing damage as, in concrete made with ordinary portland cement, the excess CH would migrate out of the concrete, leaving small channels that allow the concrete to be penetrated by water which eventually damages it as it expands and contracts due to fluctuations in temperature. Calcined clay pozzolans in particular improve the early strength of concrete as well as its durability over time. This early increase in strength makes calcined clays an ideal additive for projects using precast concrete.

Raw natural pozzolans perform similarly to waste pozzolans, meaning that, in contrast, they are less ideal for precast concrete applications and require more water during production. However, the benefits of these materials still result in more durable and sustainable concrete compared to concrete made with ordinary portland cement. Further, by increasing concrete’s resistance to chloride penetration, pozzolans can also help protect steel reinforcements within concrete from rusting and damaging surrounding concrete. Pozzolans in concrete also alleviate the problems of alkali-silica reactions, efflorescence, and thermal cracking while setting.

The natural pozzolans used in the construction of the Glen Canyon Dam make concrete less permeable, and thus better able to resist weathering from Lake Powell, the dam’s reservoir.

The ubiquity of concrete in construction and the resulting emissions have led to a particular concern for the industry’s sustainability. The majority of the carbon emitted in the concrete production process comes from the production of cement clinker. The addition of supplementary cementitious materials such as natural pozzolans to concrete mixtures reduces the amount of clinker required in the production of concrete, significantly reducing embodied carbon of concrete produced in this manner. Currently, waste pozzolans are used in concrete far more frequently and in greater quantities than their natural counterparts. According to the American Coal Ash Association (ACAA), 11.9 million tons of fly ash were used in concrete mixtures and over 28 million tons total were produced in the United States in 2021.

Comparatively, the Natural Pozzolan Association (NPA) estimates that the production capacity of natural pozzolans in 2021 was 1 million tons. However, the NPA predicts that this capacity will continue to grow for the foreseeable future. Conversely, while consumption of fly ash remains relatively constant, the quantity produced each year is falling. One reason for this is that the political initiative of sustainability is of course, not limited to the concrete industry, perhaps most heavily affecting the energy sector. Though the primary causes of the decline in coal power are likely not tied to any specific climate legislation, the stated long term goals of recent administrations, much of the public, and even many major energy companies have been to gradually replace fossil fuels with cleaner sources of energy, meaning that the supply of fly ash from coal cannot be expected to rebound in the long term.

Though natural pozzolans do not have the dual benefit of recycling waste material as well as reducing the amount of clinker needed in concrete, as does fly ash, their comparative availability during a shortage of waste pozzolans may allow for a general increase in the use of pozzolans and thus a decrease in embodied carbon in concrete overall. Because of this, it is prudent for concrete producers and contractors to consider the ways sustainability is changing the industry and the existing practical solution natural pozzolans represent in addressing these obstacles.

As the supply of fly ash has become more scarce, maintaining product consistency and economy has become more difficult. It is no longer the cost-saving supplement to cement as it was in the past, and although fly ash will continue to be used in the foreseeable future, it is prudent for concrete producers and contractors to explore more consistently available and affordable alternatives.

Natural pozzolan reserves are abundant in the United States. Thus, it is already possible to easily source pozzolans locally in many parts of the country, and where this is not the case, there often exist future opportunities to exploit untapped reserves. According to a report made by the American Concrete Institute, various raw pozzolans as well as opaline shale are found most commonly in Oklahoma, Nevada, Arizona, and California. American reserves of kaolin clay, which yields pozzolanic metakaolin when calcined, are mainly found in Georgia and many southeastern states. The variety of pozzolanic materials found in nature, whether raw or calcined, is an indication of the many sources of pozzolans within the country. Moreover, given that the transportation of materials is a significant expense in the production of concrete, the availability of natural pozzolans to be sourced near to concrete production plants and construction sites is very important to their future in the concrete industry.

Fortunately, there are numerous firms around the country which provide natural pozzolans for concrete projects, many of which can be found on this list from the Natural Pozzolan Association. Natural pozzolans already see widespread application that is only becoming more common each year, and the technology to implement their production and use more widely does not require further research to be economically viable. In this way, natural pozzolans fill the void left by the supply issues associated with fly ash, while also improving concrete’s sustainability and durability.