Old is New Again: Pozzolans Lead the Future of Low Carbon Concrete

Obsolete Concrete Practices are Failing the Climate

Concrete is the most widely used building material in the world, and its primary ingredient, cement, is responsible for a significant share of global CO₂ emissions. These emissions arise both from fossil fuel combustion and the chemical release of carbon during limestone calcination. In the pursuit of low carbon concrete, there is ongoing discussion and practice around new and innovative materials to solve this problem.

A core component of my role as an engineer involves keeping pace with innovations in structural materials and systems to deliver solutions that are structurally efficient and climate aligned for low carbon performance. Even so, newer isn’t always better.

Natural Pozzolans, a foundational component of ancient concrete technology, are experiencing a powerful resurgence as one of the most effective supplementary cementitious materials (SCMs) available today. They are increasingly recognized for their proven ability to significantly reduce upfront embodied carbon while meeting, or exceeding, target compressive strengths in modern concrete applications.

Pozzolans Represent Both a Performance Advancement and a Carbon Breakthrough in Modern Concrete Design

At the 2024 ACI Spring Convention in Toronto, I prioritized conversations with top researchers and industry leaders to explore the latest low carbon concrete technologies and how they translate into practice and my structural design work.

Dana Ratkovich, Chair of ACI Committee 332, Residential Structural Foundations, identified pozzolans as the most promising path forward, stating that “pozzolans are the future of sustainable concrete.”

Natural Pozzolans: Old World Tech Solving Today’s Carbon Problem

In her research and ACI conference presentation, Dr. Somayeh Nassiri, professor at the University of California, Davis, and leading scholar on sustainable concrete design, underscored the urgency of reducing clinker content as global cement demand continues to rise. She noted that California alone consumes an estimated 10–12 million tons of cement annually, with concrete production contributing roughly 1.8% of the state’s total greenhouse gas emissions each year.

Dr. Nassiri also highlighted the history of pozzolan use. While ancient Roman engineers richly used volcanic ash in their concrete, California has its own tradition of incorporating natural pozzolans and volcanic ash in major civil infrastructure, from large scale dams to the Los Angeles aqueduct. She noted mid-20th century research documented their effectiveness, particularly in reducing cracking and improving long-term durability.

Furthermore, she cautioned reliance on industrial byproducts like slag and fly ash are not a long-term solution. Supplies are finite and, in many cases, are imported from overseas, which undermines local sustainability goals. Instead, Dr. Nassiri emphasized the opportunity presented by California’s own volcanic ash deposits, a naturally abundant resource that could help meet the state’s climate targets while maintaining concrete performance.

Smart Concrete and Smart SCM Choices

Where the Romans relied on volcanic ash, we now can draw from a broad spectrum of pozzolans including natural minerals, reclaimed materials, and engineered synthetic blends to offer improved performance, greater sustainability, and cost efficiencies. Today, many pozzolans are being rediscovered and re-engineered as pivotal levers for reduced cement and lowered embodied carbon of concrete.

Modern manufactured SCMs include fly ash, slag, and silica fume, while naturally occurring options include pumice, volcanic ash, and calcined clay. Emerging SCMs include carbon active recycled SCM concepts such as biochar-based and recarbonated demolition waste, combining recycled aggregate with CO₂ injection.

One key advantage of natural pozzolans is they are not constrained by the availability of industrial byproducts, making them a consistent and scalable solution. 

Pozzolans Hit SB 596 and SE 2050 Targets 

California Senate Bill 596 establishes ambitious decarbonization targets for the cement industry, beginning with a mandate for the state to develop a comprehensive strategy that ensures all cement used in California reaches net-zero greenhouse gas emissions by 2045. In the near term, the bill sets a critical interim benchmark: a 40% reduction in greenhouse gas intensity below 2019 levels by the year 2035. These targets aim to transform one of the state’s most carbon intensive industries through a phased, accountable approach aligned with California’s broader climate goals.

The Structural Engineers 2050 Commitment Program (SE 2050) is an initiative developed by the Sustainability Committee of the Structural Engineering Institute (SEI) of the American Society of Civil Engineers (ASCE), in response to the SE 2050 Challenge issued in 2019 by the Carbon Leadership Forum (CLF). This challenge calls on all structural engineers to understand, reduce, and ultimately eliminate embodied carbon in their projects by the year 2050.

Endorsed by the SEI Board of Governors in December 2019, the SE 2050 Commitment Program aims to provide an accessible and accountable framework for structural engineers and firms to actively engage in reducing embodied carbon through every phase of design and construction. By fostering a culture of information sharing and carbon conscious design strategies, SE 2050 supports the structural engineering profession in achieving zero net carbon structures by mid-century.

Natural pozzolans offer one of the most promising pathways to meet both Senate Bill 596 and SE 2050 regulations and commitments for embodied carbon reduction in concrete. 

Natural Pozzolans are a Measurable Advantage: Lower Carbon, Higher Performance, Higher ROI

According to Dr. Enrique del Rey Castillo, a professor at the University of Aukland, contributor to ACI Committee 240, Pozzolans, and leading scholar on sustainable concrete design, and Joseph E. Thomas, Executive Director at the Natural Pozzolan Association, raw natural pozzolans currently deliver up to a 95% reduction in greenhouse gas emissions compared to Portland cement, positioning them as a critical ingredient in the shift toward climate aligned structural design.

“Carbon Embodiment/Greenhous Gas Reduction: In today’s world, reduction of the carbon footprint has become an environmental necessity and, increasingly, a license to operate. Governments and other regulatory bodies are now mandating specific decreases in greenhouse gas emissions for a variety of products, including concrete. Certain states and municipalities in the US are mandating up to 30% reductions in the carbon footprint of concrete and requiring Environmental Product Declarations (EPDs) for all concrete produced and placed. Without simultaneous relaxation of strength specifications, which is not happening, such reductions in carbon footprint can only be achieved by the increased use of supplementary cementitious materials. Currently, the only viable options to achieve these carbon reductions while maintaining or increasing performance standards, are slag, fly ash, and Natural Pozzolan (NP). In certain locations around the world fly ash will not reduce the carbon footprint of concrete as the inherent carbon embodiment of the coal derived material is not zeroed out by government fiat – as it is in the USA. To provide a direct comparison of NP to cement, current EPDs in the US indicate raw NPs are in the .05 GHGeq range (Greenhouse Gas Equivalent), versus approximately .95 GHGeq for portland cement. This equates to a 95% reduction in greenhouse gases for the NP compared to the Cement. In other words, for every pound of cement replaced with NP, there is a .95 pound reduction in GHGeq.  Calcined NPs, for obvious reasons, have a higher carbon footprint than raw NPs. However, compared to portland cement, there is still a massive reduction of GHGeq – on the order of 70% reduction, versus a 95% reduction for raw NP. This can be attributed to the fact that calcined clays require calcination temps of only about 700 C° versus up to 1400 C° for cement clinker. Also, carbon is released from calcium carbonate, or limestone, when calcined, thus creating a much higher carbon embodiment in cement clinker. NPs do not contain carbonates. ”¹

¹ Enrique del Rey Castillo and Joseph E. Thomas, Natural Pozzolans (University of Auckland and Natural Pozzolan Association, 2023).

Dr. Castillo and Thomas’ research demonstrate replacing 20–40% of Portland cement with natural pozzolans can deliver equal or greater compressive strength, especially over longer cure periods, with gains of 30–50% observed in some cases.

“Compressive Strength and Flexural Strength: Most engineers in North America specify concrete strength at 28 days. A straight cement concrete has nearly reached its ultimate strength in that amount of time, so it was an acceptable timing for portland cement concrete. However, in concrete mixes with NP replacing 20~40% of the cement, strength parity with a straight cement mix might not be achieved until after 28d, although many NPs mixes do in fact achieve parity at 28d. The important point is not when parity is reached but rather that in 56d, 180d, and 365d, NP based mixes can easily achieve 30-50%greater strengths than a straight cement concrete. The implications of this well-known phenomenon are myriad. For one, if specifiers will simply move from 28d compliance to 56d or later, concrete mix designs can be optimized by reducing the cementitious content, in some cases significantly, to achieve ultimate strength requirements. It is the ultimate strength that is most important to project and building safety, as opposed to a 28d specification. Secondly, this will allow for much greater reductions in the carbon footprint by taking advantage of the higher ultimate strengths gained by using NPs. Additionally, testing at 3M labs in Minnesota has found that the flexural strengths of NP based concrete mixes are higher, even at 28d, than similar mixes using straight cement. NP increases ductility in concrete”.2

2 Enrique del Rey Castillo and Joseph E. Thomas, Natural Pozzolans (University of Auckland and Natural Pozzolan Association, 2023).

The Data’s Clear. The Materials Exist. So Why the Lag? 

The biggest barrier to implementing low carbon concrete isn’t a lack of innovation, it’s a lack of clarity. Most engineers don’t know which SCMs are available, what mix designs to ask for, or who to call. And when every phase of a project is running on tight timelines and risk-averse defaults, Portland cement wins by inertia.

Specifying concrete without first assessing local SCM options is a systemic missed opportunity. Part of the challenge is the absence of streamlined pathways to connect specifiers with viable, low carbon alternatives. Prescriptive concrete design isn’t neutral, it’s a carbon heavy standard reinforced by convention. Even so, change doesn’t require waiting for the system to evolve, it begins with a single decision: your next mix specification.  

National Database Efforts Have Been Defunded

As Nathan Forrest, Technical Director at California Nevada Cement Association explained to me, “A national effort to build an interactive database of supplementary cementitious material (SCM) availability was already in motion, led by the National Ready Mix Concrete Association (NRMCA), the California Nevada Cement Association, CalCIMA, and USGBC California. The plan was to develop a GIS-based tool that mapped ready-mix plants across the country, identifying whether each plant had Environmental Product Declarations (EPDs), and listing which pozzolans or SCMs: such as fly ash, slag, natural pozzolans, calcined clays, or PLC, were locally available, along with contact information for technical leads. Unfortunately, this initiative was halted when the $10 million in FHWA federal grant funding was rescinded. As such, securing funding support may need to be revisited with USGBC or other aligned organizations.”

This obstacle presents an opportunity. Rather than attempting to revive a stalled national initiative, I propose a regionally focused SCM database, beginning on the West Coast and expanding across the U.S. This tool would support engineers, designers, and concrete producers in identifying local SCM options to lower embodied carbon through practical, targeted decisions. By anchoring the initial scope locally, the project remains cost-effective and inherently scalable. Where the national vision faltered, this pilot model charts a pragmatic path forward advancing low-carbon concrete innovation region by region. Would USGBC and NRMCA step forward to fund a pilot model?

How to Make Pozzolans and SCMs Easy to Source, Specify, and Scale

Make a Phone Call and Shift the Industry

An impactful yet underutilized action an engineer can take is to pick up the phone and call regional concrete suppliers. Ask directly which pozzolans or supplementary cementitious materials (SCMs) are available for your project location.

This single question can determine whether your mix is a carbon heavy default or a design which meaningfully reduces embodied carbon and improves performance. With SCM availability varying by region and supplier, proactive outreach becomes a baseline strategy for sustainable design, not an optional extra.

Once you’ve identified regional SCM options, such as natural pozzolans, fly ash, slag, or advanced technologies, communicate your performance requirements to the concrete supplier, who can customize the mix design accordingly. You’ll gain the ability to reduce cement content, maintain or exceed structural performance, and lower greenhouse gas emissions; a move that supports SB 596, SE 2050, and broader climate commitments.

Make It Official: Spec the Mix on Your Plans

The next critical step in carbon smart design is simple: document your SCM intent directly on the structural drawings. Don’t leave concrete mix decisions to chance or worse, a default prescriptive design with Portland cement.

A structural note, like below, empowers your team to deliver on carbon and performance goals:
PROVIDE CONCRETE MIXES WITH MINIMUM 30% FLY ASH (CLASS F OR APPROVED EQUIVALENT) CEMENT REPLACEMENT AT FOOTINGS AND MINIMUM 15% FLY ASH (CLASS F OR APPROVED EQUIVALENT) CEMENT REPLACEMENT AT SLAB ON GRADE FINISH WORK.

SCM Tracker: Your Shortcut to Smarter Mix Specs

It’s long overdue. The concrete design community needs a centralized, searchable database of SCM availability. This single tool could empower engineers to match local material options with both performance criteria and carbon reduction goals clearly and efficiently.

A simple database = faster decisions, lower emissions, better builds.

California’s unique convergence of high seismic demands, progressive building codes, and ambitious environmental policy creates an ideal landscape where innovation, regulation, and opportunity intersect to lead the future of low carbon concrete.

Access Is the Future of Low-Carbon Concrete

The next shift in sustainable design isn’t just innovation, its access, because engineers can’t specify what they can’t see. 

It’s essential to develop a comprehensive SCM availability database, or a centralized tool to connect engineers with real-time, regionally available SCMs. This resource could align concrete mix with performance and carbon goals, accelerating progress toward SB 596 and SE 2050.

What’s needed now is alignment, from research lab, to batch plant, to project site. From innovation to execution, from awareness to action. This is how we scale sustainable concrete. This is the move.

How EA Structural Engineering Is Advancing SCMs to Lower CO2

At EA Structural Engineering, each design decision is an opportunity to reduce embodied carbon without compromising performance. We actively work to incorporate SCMs, including natural pozzolans and emerging advanced technologies, into structural designs to reduce cement and align with sustainable design goals.

By staying at the forefront of SCM research and application, and maintaining strong relationships with suppliers, researchers, and builders, we are driving measurable carbon reductions across each phase of design and construction.

Each design choice counts, and as I like to say: 

Keep it light. Keep it right. Keep it tight.

Keep it light

Prioritize approaches and materials that reduce carbon output. Whether it’s through pozzolanic and SCM replacements, performance based design, or lifecycle thinking.

Keep it right 

Stay informed. Reach out. Collaborate with local suppliers to identify what’s possible based on your project and site specific conditions. Act.

Keep it tight

The elegance of engineering lies in its precision; where intention, efficiency, and strength converge in a unified solution. Precision means more than structural integrity; it reflects intelligent material use and reduced embodied carbon. Awareness matters. Every decision contributes to the carbon story of a building. Even small refinements in approach can yield measurable results. Designing and detailing with intention means optimizing, simplifying, and building for constructability, all in service of streamlining effort, minimizing waste, and advancing climate aligned outcomes. Pozzolans aren’t optional, they’re essential.

Cement Can’t Compete. Pozzolans and SCMs Outperform and Outlast with More Strength, More Ductility, Less Carbon, Smarter Returns. 

Structural engineers influence nearly 15% of the world’s CO2 emissions through the materials they specify. Outdated codes lead to overdesign, overbuilding, wasted materials, and avoidable carbon emissions.

Improved alignment between SCM availability and concrete specifications are vital. Incorporating pozzolans or other SCMs lower cement content, reduce CO₂ emissions, and in many cases, enhance durability and long-term performance. This is not just a sustainability measure; it is a technical and strategic design decision. Advancing sustainable practices in structural engineering begins with actions that may seem small, yet yield substantial results. A single conversation with a supplier can become the catalyst for positive change across successive projects.

The science is clear. The materials are available for immediate use. The mandate is clear. Implementation isn’t optional, it’s the next step in responsible design. As engineers, we can choose to reclaim proven, effective materials, like natural pozzolans and redefine sustainable design. 

The era of default design is over, and resilient and optimized structures are powerful. Let’s build like the future depends on it, because it does. By blending research, outreach, and documentation, we empower our projects, our clients, and our planet.

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