Ancient Roman Formula Offers Clues to Greening Concrete

Concrete Industry to Slash Carbon Emissions

TL;DR: The concrete industry, responsible for 8% of global CO2 emissions, is pursuing multiple decarbonization strategies—from carbon capture to novel chemistries—while recent discoveries about self-healing Roman concrete offer additional insights. Despite promising innovations from startups and established producers, scaling low-carbon alternatives faces significant hurdles around cost, building codes, and the fundamental economics of a commodity business operating on thin margins.

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The world pours approximately 30 billion tons of concrete annually, making it humanity's most-consumed material after water. This ubiquitous building material also carries an enormous environmental burden: cement production alone generates roughly 2.9 billion tons of CO2 each year—approximately 8% of global emissions, rivaling the output of major industrial nations.

"Concrete is literally the foundation of modern civilization, but it's also one of our biggest climate problems," says Sabbie Miller, associate professor of civil and environmental engineering at the University of California, Davis. "We can't simply stop using it, so we have to reinvent it."

The Intergovernmental Panel on Climate Change estimates that achieving net-zero emissions by 2050 requires cutting cement sector emissions by at least 24% by 2030—a target the industry is nowhere near meeting at its current trajectory. Now a combination of Silicon Valley innovation, industrial-scale carbon capture projects, and even insights from 2,000-year-old Roman engineering are converging to address one of the climate's most intractable challenges.

The Chemistry of the Problem

The environmental challenge stems from concrete's essential ingredient: Portland cement, which binds the mixture of sand, gravel, and water into solid rock. Manufacturing Portland cement requires heating limestone and clay to approximately 1,450°C (2,640°F) in massive kilns—a process that releases CO2 through both fuel combustion and the chemical decomposition of limestone (calcium carbonate) into lime (calcium oxide).

This calcination reaction alone accounts for roughly 60% of cement's carbon emissions, with fossil fuel combustion for process heat contributing the remainder. Even the Romans, whose concrete structures like the Pantheon have endured for two millennia, couldn't avoid this fundamental chemistry. They heated limestone to around 900°C (1,650°F) to produce lime—lower temperatures than modern kilns, but the same calcination reaction still released substantial CO2.

Recent research published in Science Advances by MIT's Admir Masic revealed that Roman concrete's legendary durability came from "hot mixing" techniques that created calcium carbonate clumps with self-healing properties. When cracks formed, water would dissolve these lime deposits and redeposit calcium carbonate, sealing the damage. The Romans also used volcanic pozzolana ash that reacted with seawater to form aluminum-tobermorite crystals—rare minerals that actually strengthened over time rather than degrading.

While Roman concrete produced roughly 0.5-0.6 tons of CO2 per ton compared to 0.9 tons for modern Portland cement—a meaningful reduction—it cured extremely slowly, sometimes taking years to reach full strength. Modern construction timelines and the need for steel-reinforced structures make direct adoption of Roman methods impractical, though the durability insights are influencing contemporary research into self-healing and geopolymer concretes.

Revolutionary Chemistries

A new generation of materials science companies is attacking the carbon problem by fundamentally reimagining cement production. Brimstone Energy, backed by $85 million in venture funding including support from Bill Gates's Breakthrough Energy Ventures, has developed a process that manufactures cement from calcium silicate rock instead of limestone, eliminating calcination emissions entirely while potentially sequestering CO2 in the byproduct magnesium silicate.

"We're making exactly the same product—ordinary Portland cement that meets ASTM C150 specifications—but through an electrochemical process powered by renewable energy," explains Cody Finke, Brimstone's CEO. The company broke ground on its first commercial demonstration plant in Nevada in 2024.

Sublime Systems, another Gates-backed venture with over $80 million in funding, has developed an electrochemical cement production process that operates at ambient temperature, eliminating the need for high-heat kilns entirely. This approach echoes the Roman preference for lower-temperature processing while using modern electrochemistry to achieve it.

CarbonCure Technologies has taken a different tack, injecting captured CO2 into concrete during mixing where it mineralizes permanently—a modern twist on the carbonation chemistry that contributed to Roman concrete's strength. The Canadian company has deployed its technology in over 700 concrete plants across North America.

Terra CO2 Technologies and Solidia Technologies have developed processes to cure concrete blocks with CO2 instead of steam, reducing emissions while accelerating production cycles. Solidia claims its carbonation-curing technology can reduce the carbon footprint of concrete products by up to 70%.

The Geopolymer Alternative

Geopolymers—aluminosilicate materials activated by alkaline solutions—represent another pathway inspired partly by Roman pozzolanic chemistry. These binders can be formulated from industrial waste products like fly ash and slag, avoiding limestone calcination entirely. Earth Engineered Products produces a geopolymer cement it claims achieves carbon-negative status by utilizing waste materials and sequestering CO2 during curing.

Researchers at the University of Tokyo recently demonstrated a geopolymer concrete with compressive strength exceeding 100 MPa while reducing CO2 emissions by 64% compared to conventional concrete. However, long-term durability data remains limited.

"The technical performance is there for many applications," says Dr. John Provis, professor of cement and concrete materials at the University of Sheffield. "The barriers now are primarily regulatory, economic, and getting specifiers comfortable with materials that don't have 150 years of field performance history." The Romans benefited from empirical trial and error over centuries; modern alternatives must prove themselves much faster.

Industrial-Scale Incrementalism

While startups pursue revolutionary approaches, the incumbent industry has focused on incremental improvements. Holcim, the world's largest cement producer, aims to reach net-zero emissions by 2050 through a combination of alternative fuels, carbon capture and storage (CCS), and clinker substitution—replacing portions of Portland cement with supplementary cementitious materials (SCMs) like fly ash, slag, and calcined clay.

Heidelberg Materials has committed €1 billion to carbon capture projects across its European cement plants. The company's Brevik plant in Norway, expected online in 2024, will capture 400,000 tons of CO2 annually—approximately 50% of the facility's emissions. Oxy Low Carbon Ventures and HeidelbergMaterials announced plans for North America's first commercial-scale carbon capture project at a cement plant, targeting 2026 startup at the Mitchell, Indiana facility with capacity to capture 95% of the plant's CO2 emissions—approximately 1.3 million tons annually.

Direct carbon capture from cement kilns represents perhaps the most straightforward decarbonization pathway—but also one of the most expensive, with costs estimated at $100-300 per ton of CO2 captured. The economics depend heavily on carbon prices and government incentives. The Inflation Reduction Act expanded the 45Q tax credit to $85 per ton for captured CO2 that is sequestered geologically and $60 per ton for CO2 used in products, significantly improving CCS economics in the United States.

CEMEX launched its Vertua brand of lower-carbon concrete products in 2020, offering formulations with up to 70% reduced CO2 intensity through optimized mix designs and SCM substitution. However, the availability of traditional SCMs is becoming a bottleneck as coal-fired power plants (the primary source of fly ash) shut down and steel production (the source of blast furnace slag) transitions away from traditional processes.

"We're in a transition where our traditional supplementary materials are disappearing faster than alternatives can scale," notes Dr. Franco Zunino, research scientist at Sika Technology AG. "Calcined clay looks promising as a replacement, but it requires significant investment in new processing capacity."

The Standards Paradox

Building codes and industry standards, designed to ensure structural safety and longevity, have become unexpected barriers to green concrete adoption. ASTM, ACI, and other standards organizations base specifications on decades of field experience with traditional Portland cement formulations.

"We have 100-year-old bridges built with Portland cement that are still standing, so engineers and specifiers are understandably conservative about adopting new materials," explains Dr. Kimberly Kurtis, professor of civil and environmental engineering at Georgia Tech. "Getting new cement chemistries or high-replacement SCM mixes approved requires extensive testing and demonstration projects." The Romans had centuries to refine their concrete through observation; modern innovations face compressed development timelines.

The American Concrete Institute recently updated ACI 318 building code provisions to allow higher volumes of SCMs, but many jurisdictions haven't yet adopted the new language. The federal government's establishment of low-embodied-carbon concrete procurement standards could accelerate acceptance—the Federal Highway Administration issued guidance in 2023 encouraging state DOTs to specify environmental product declarations (EPDs) and set maximum embodied carbon limits for concrete in federally funded projects.

California's Buy Clean Act, requiring EPDs for structural steel and concrete purchased for state projects, has driven significant supplier engagement with carbon tracking and reduction. Similar legislation is spreading to other states and countries.

Economic Headwinds

Concrete is a commodity business where producers compete primarily on price, typically operating on margins of 5-10%. Low-carbon alternatives commanding price premiums face difficult market adoption, particularly for price-sensitive applications like residential construction and infrastructure.

"A developer building an apartment complex cares about cost per square foot, not carbon intensity," notes one concrete producer who spoke on condition of anonymity. "Until regulations or market forces change that calculus, green concrete will remain a niche product for high-profile projects with sustainability commitments."

Some market dynamics are shifting. Corporate sustainability commitments, particularly among technology companies building data centers, have created demand for low-carbon concrete willing to pay premiums. Amazon committed to using 20 million cubic yards of lower-carbon concrete across its projects by 2025. Microsoft specified CarbonCure concrete for its Silicon Valley campus expansion.

Government procurement policies are becoming increasingly important demand drivers. The U.S. General Services Administration announced in 2023 that new federal construction projects must prioritize low-embodied-carbon materials, including concrete with verified EPDs showing reduced climate impact.

The Global Dimension

China produces approximately 55% of the world's cement—more than the rest of the world combined—making its decarbonization strategy critical to global outcomes. The country has set targets to peak cement production capacity and reduce carbon intensity, but its approach emphasizes energy efficiency and alternative fuels rather than revolutionary cement chemistries.

India, the world's second-largest cement producer, faces similar challenges balancing development needs with climate commitments. The country's rapid urbanization requires enormous volumes of affordable construction materials, potentially limiting adoption of premium-priced green alternatives.

European cement producers, facing the world's highest carbon prices through the EU Emissions Trading System and the impending Carbon Border Adjustment Mechanism, are furthest ahead in decarbonization efforts. However, competitiveness concerns persist around carbon leakage—the risk that production shifts to regions with less stringent climate policies.

Looking Forward

Industry observers suggest concrete decarbonization will likely proceed along multiple parallel paths rather than a single dominant solution. Optimized conventional concrete with SCM substitution and improved energy efficiency may reduce emissions 30-40%. Carbon capture could address another 30-40% of remaining emissions. Novel chemistries and processes could serve specialized applications where performance or sustainability requirements justify higher costs.

Modern research into self-healing concrete draws directly on Roman insights, incorporating bacteria, encapsulated healing agents, or reactive inclusions inspired by the lime clump mechanism. Lower-temperature cements are being developed that reduce both firing temperatures and calcination requirements. Some researchers are exploring seawater concrete applications, though steel reinforcement corrosion remains problematic—a challenge the Romans avoided by rarely using metal reinforcement.

Scientists at MIT demonstrated that concrete made with biochar—charcoal produced from agricultural waste—can achieve mechanical properties comparable to traditional concrete while sequestering carbon. Engineers at the University of Colorado Boulder developed a living concrete using photosynthetic bacteria to mineralize calcium carbonate, though practical applications remain distant. Researchers at ETH Zurich developed concrete formulations using recycled construction demolition waste as aggregates and binders, demonstrating that circular economy approaches could significantly reduce both material consumption and emissions.

"There's no silver bullet, but we have a portfolio of solutions that collectively can get us where we need to go," says Dr. Karen Scrivener, professor of construction materials at EPFL and director of the Nanocem consortium. "The question is whether we can deploy them fast enough."

Time is of the essence. Global concrete consumption continues growing approximately 2-3% annually, driven by urbanization in developing countries. Without rapid decarbonization, the cement and concrete sector's cumulative emissions could consume a substantial portion of the remaining carbon budget for limiting warming to 1.5°C.

The industry that built the modern world—and that two thousand years ago built structures still standing today—now faces perhaps its greatest challenge: reinventing itself for a climate-constrained future while continuing to provide the foundation for human development and prosperity.

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