This is the first installment in our four-part blog series examining where the industry stands today in capturing the value of supplementary cementitious materials (SCMs). Since the benefits of SCMs scale directly to how much portland cement is replaced, having a reliable supply is critical for success. Heidelberg Materials has taken a comprehensive approach to increasing SCM supply, both by opening and upgrading facilities and by developing new technologies that boost output. The result is enhanced access to SCMs, supporting stronger performance in your concrete and greater sustainability on your projects!
Installments of this series were first published as the article "What’s Up with SCMs?” in the NRMCA’s Concrete InFocus magazine, Winter, 2025.
Supplementary cementitious materials (SCMs), when used in conjunction with or to replace a percentage of portland cement in concrete mixtures, can improve the properties of concrete both during construction and during the service life of the built structure. SCMs are defined by ACI CT-23: Concrete Terminology as “inorganic material such as fly ash, silica fume, metakaolin, or slag cement that reacts pozzolanically or hydraulically.” They are also the subject of recently published ASTM C1912/C1912M – 25, Standard Specification for Supplementary Cementitious Material for Use in Concrete.
During construction, SCMs can delay set time by lowering the rate of hydration. This is especially useful in hot weather, as it allows for more time to place and finish the concrete. Lowering peak hydration temperatures can reduce the risk of future thermal cracking in large volume concrete elements. SCMs can also improve the workability of a mix, making it easier to place or pump.
When properly applied, SCMs result in stronger hardened concrete and improve durability by reducing permeability. Including SCMs in a mix can help mitigate alkali-silica reaction (ASR), because many SCMs bind alkaline materials through pozzolanic reaction and reduce porosity. The improvement to strength and durability brought by SCMs is especially valuable for infrastructure projects where extended service life is of paramount importance.
In addition to increasing concrete’s durability and longevity, the use of SCMs delivers environmental benefits. The cement and concrete industries have been exploring multiple routes to net-zero and using SCMs is one of the biggest levers for CO2 emission reduction in concrete. According to the National Ready Mixed Concrete Association (NRMCA) document, The Top 10 Ways to Reduce Concrete’s Carbon Footprint, “To give an idea of how effective the use of SCMs are in reducing carbon footprint, going from a 100% portland cement mix to a 50% fly ash/slag cement mix can reduce carbon footprint by roughly 40%.”
Benefits of using SCMs include:
- Increased ultimate strength
- Greater durability and longer-lasting concrete
- Improved workability
- Mitigation of Alkali-Silica Reactivity (ASR)
- Reduced segregation and drying shrinkage
- Improved sulfate resistance
- Reduced bleeding
- Improved pumpability
- Improved water-tightness
- Reduced heat of hydration
- Reduced efflorescence
- Improved resistance in marine environments
The key to maximizing the benefits of SCMs are high replacement levels and reliable availability. In the past few years, high levels of cement replacement have taken off as a way to produce low-carbon mixes. More reliance on SCM’s makes their secure supply a critical success factor between now and 2030.
An important consideration in using SCMs is that they “play well” with other concrete mix ingredients, including ones that have durability, longevity and sustainability benefits of their own. The primary SCMs used in North America, slag cement and fly ash, have good track records of effective, beneficial use in practically all concrete applications.
Portland limestone cement (PLC) is the most widely used blended cement in North America. It is accepted by state departments of transportation and building codes for concrete. PLC reduces the carbon footprint of a mix by about 10% when replacing ordinary portland cement and is therefore considered an important strategy for achieving sustainability. An additional benefit of PLC is its compatibility with SCMs. Both fly ash and slag cements can be used at similar replacement levels with PLCs and portland cements. Many concrete producers report a synergistic effect in mixes combining PLCs and slag cement, which yields slightly higher concrete strengths than would be achieved with portland cement. Proprietary low-carbon concrete mixes, such as Heidelberg Materials’ EvoBuild®, typically combine PLC and SCMs to replace ordinary portland cement. Depending on carbon reduction targets and regional availability, individual EvoBuild mixes may use slag cement, fly ash, silica fume and/or other pozzolans in the mix.
Learn more here and look for Part 2 of our series, “Expanding Availability of Slag Cement.”