Concrete in its most basic form is paste (cement and water) and aggregates (rock and sand) which is plastic and malleable when newly mixed and strong and durable when hardened. Many people interchange the words “cement” and “concrete”. Technically, this is like interchanging the words “flour” and “cake”.
The oldest SCMs are natural pozzolans, typically volcanic ash and similar non-crystalline minerals. These products are naturally occurring and were successfully used by the Romans to create structures like the Coliseum and Pantheon. Almost two thousand years after it was built, the Pantheon is still the world's largest unreinforced concrete dome. Fly ash works very similar to natural pozzolans and since the mid-20th Century has largely replaced them in the United States because of its availability and very low cost. However, market dynamics are changing the quality and availability of fly ash. As a result, natural pozzolans are becoming more common again.
In designing a concrete mix, many different properties must be considered and balanced. The most basic property is compressive strength. Concrete compressive strength is generally a function of the cement characteristics, water to cement ratio (w/c) and properties of the aggregate. Increasing the amount of cement and/or reducing the amount of water for a given set of materials generally increases the compressive strength. Fly ash pozzolans help reduce w/c ratio because of their spherical bead shape.
There are many more properties of concrete which can become important in certain environments and structural applications, including: alkali-silica reactivity (ASR), efflorescence, freeze/thaw resistance, lightweight/heavy weight, sulfate resistance, low heat of hydration (in mass concrete), and shrinkage. Engineers design the concrete mix to provide the best characteristics for the lowest cost. Pozzolans can improve concrete performance in many of these areas by reacting with unwanted compounds and making the concrete stronger, less permeable, or more dense; resist cracking; or by delaying the chemical reactions to reduce heat.
The cement is a powder that reacts with water to form the glue (calcium silicate hydrates or CSH) that binds everything together. Portland cement, the primary portion of paste in modern concrete, is made by heating limestone and other minerals to very high temperatures, then grinding the mix into a fine powder. Unfortunately, this high heat reaction produces CO2, both as a result of burning the fuel for heating and the chemical reactions in the limestone. Although it varies from cement plant to cement plant, every ton of cement creates about a ton of CO2 emissions. It has been estimated that 5% -8% of the man-made global CO2 emissions are from Portland cement manufacturing. Portland cement is also the most expensive component in concrete.
Modern concrete also contains Supplementary Cementitious Materials (SCM) to improve its properties and reduce cost. The most widely used SCMs function as a pozzolan. When water and Portland cement react, calcium hydroxide (CaOH) is formed as a byproduct that is detrimental to concrete strength and permeability. Pozzolans react with the byproduct to form more CSH glue. The two most common pozzolan SCMs are fly ash and ground granulated Blast Furnace Slag (GGBFS), also called slag cement. Fly ash is produced by the air pollution control systems from coal burning power plants. Fly ash often replaces some of the Portland cement and improves the workability and permeability of the concrete without a significant loss of strength. Slag Cement is a nonmetallic byproduct developed during iron production in a blast furnace. When rapidly chilled by quenching, it changes into a glassy particle. When ground into a fine powder, these particles, like fly ash react with the CaOH to form more CSH or “glue”. Other, more expensive SCMs include silica fume and meta-kaolin. Because of the relatively high costs, these products are typically used only in very special mixes in which high early strength or very low permeability is required.