By Adam Berg
Introduction
Pozzolans are supplemental cementitious materials that are used to improve the performance of concrete. Pozzolans are used in most concrete mix designs and have several potential benefits including buffering of reactive aggregates and weight reduction, among others. The primary source of pozzolans used throughout the country is the ash produced from coal fired power plants (fly-ash).
Climate change and pollution concerns are resulting in the closure of many coal-fired power plants. With the closure of these power plants across the country, fly ash is becoming a less viable supplementary cementitious material (SCM) option for the concrete industry and an alternative will be required sooner than later. Natural pozzolans are nothing new and date back to the early Romans where natural pozzolans have been used for thousands of years. The term pozzolan is attributed to the town of Pozzuoli in the Bay of Naples just 25 miles east of Mount Vesuvius where the entire region is covered in thick beds of “pozzolana”, a volcanic pumice and ash deposit from previous eruptions of Mount Vesuvius. Those pozzolans were used to build many of the Roman monuments that are still standing to this day, which is a testament to their durability.
Natural pozzolans (Class N) are natural materials that have pozzolanic properties (examples are volcanic tuffs or pumicites, opaline cherts and shales, clays, zeolites, and diatomaceous earths). Some of these pozzolans may require calcination, which refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores). The material is raised to high temperature without melting under restricted supply of ambient oxygen (i.e., gaseous O2 fraction of air). This is generally done for the purpose of removing impurities or volatile substances, and/or grinding the material for further refinement. Calcination produces large amounts of CO2 which is an issue currently being addressed.
Typically, concrete is 60-70 percent aggregates, 15-20 percent water, and the remaining 10-15 percent is cementitious material (depending on the application). In 2016, it was estimated that California produced 9.8 million tons of cement, while consuming approximately 9.5 million tons. However, these are rough estimates since California does import and export some materials.
Since fly ash is the most popular SCM used in concrete, Caltrans is concerned about its availability and continually adjusts their specifications as technology and testing changes. Caltrans specifications require the use of SCMs to reduce alkali reactivity in concrete and greenhouse gas emissions in heavy concrete mixes. Typically, Caltrans is calling for a minimum of 25 percent SCM/fly ash, unless the aggregate source can be shown to be harmless, in which case just 15 percent SCM/fly ash is required. In addition, Caltrans changed its criteria in 2010 to allow for less energy-intensive concrete mixes. This was done by eliminating minimum cement requirements, removing upper limitations on the quantity of fly ash & other SCMs, and allowing the mixing of up to three cement/SCM ingredients.
Fly Ash
Fly ash is classified either as class F or class C. The primary difference between class F and class C fly ashes is the Calcium (Ca) content, where class F fly ash has Ca less than 10 percent, while class C fly ash has Ca content of greater than 10 percent. Class F is typically derived from the burning of anthracite or bituminous coal, and Class C is usually derived from the burning of lignite or subbituminous coal. Class F fly ash is pozzolanic, with little or no cementing value alone but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. Class F fly ash can be used as a Portland cement replacement ranging from 20-30 percent of the mass of cementitious material. Class C fly ash has self-cementing properties as well as pozzolanic properties, but class F fly ashes are more effective SCMs than class C because Class F has higher silica content.
Much of the current fly ash production is located on the east coast of the United States where there is a concentration of coal-fired electric power generation plants with some production located in Arizona, Nevada, Utah, and Wyoming. One concern with the location of fly ash is the need for transporting these materials to California and the rest of the West Coast and a second is the potential for severe bottlenecks as it travels through the transportation network.
In Southern California, there are two primary suppliers of fly-ash: EcoMatrials and Salt River Materials Group. Current prices (including delivery of the materials) are around $115 per ton, which is less expensive that current prices for traditional cement that is between $140-$160 per ton.
Natural Pozzolans
Natural pozzolans are mined from natural deposits located throughout the US (mainly where there has been prior volcanic activity or ancient sea beds), and with the potential for fly ash shortages throughout the Country, natural pozzolans may be an important alternative. These alternatives are already widely used in other countries, where there are no coal-fired power plants, with good results.
Calcined clay, like fly ash, can be used as a substitute for a portion of the supplementary cement mixture; typically, in the range of 15 to 35 percent. Calcined clays can reduce permeability, increase resistance to sulfate attack and reduce alkali-silica reactivity expansion. Metakaolin is made from high purity kaolin clay calcined at high temperatures and is typically used in addition to cement and can significantly reduce permeability and increase strength.
Class N natural pozzolans are located throughout the west coast as they are a byproduct of volcanic activity. There have been numerous studies conducted on the location and viability of these deposits (EnviroMINE Inc. included) with some current mining operations in Arizona and California.
Locating and producing natural pozzolans near the market has two benefits: it would greatly reduce the emissions produced by reducing the distance needed to transport them, while also reducing the cost of transporting these materials to their final destinations. The costs of transportation determine the maximum economic distance the pumice, pumicite, rhyolite, and zeolite can be shipped and still remain competitive with alternative materials.
Conclusion/Looking Ahead
While the prospect of using natural pozzolans has potential rewards for limiting greenhouse gasses and reducing costs associated with transportation, the question of scaling up to meet demand remains to be answered. If production of natural pozzolans cannot meet the demand for these products, are we going to be stuck in the same situation as the fly ash bottlenecks that are currently experienced on the west coast? Perhaps a little of both sources are the answer.
Emerging technologies are primarily focused on the elimination of the calcination process of natural pozzolans, which is the main contributor to the production of CO2 emissions. Many of these technologies and mineral products are being studied and developed and may be the future of green concrete and may require the industry to follow stricter environmental standards and guidelines for use in future construction materials.
As the supply of fly ash decreases, an alternative will be necessary. As regulations and environmental laws change, the need for carbon neutral SCMs will require a much closer look.