Last week, a newly published international analysis underscored the urgency of change in the construction materials sector. The industry is accustomed to loud alarm bells from the research community, but the latest proclamation was especially weighty. They said that without significant change, the sector’s carbon footprint could double by 2050, consuming much of the global climate budget. Cement production alone accounts for roughly 7–8% of global CO2 emissions.
Luckily, many of the stakeholders who have the ability to change that course coalesced in Baltimore for the American Concrete Institute’s Concrete Convention for some candid conversations. Much of the programming centered around one huge question: What does practical decarbonization look like in real specifications, mixes and job sites?
What do we do with ACI 323?
Last year marked the debut of ACI 323-24, Low-Carbon Concrete – Code Requirements and Commentary. ACI says it is the world’s first model code dedicated to embodied carbon in concrete. The organization also made it very clear that the code was created under an expedited timeline due to the industry’s urgent need for a resource to address the climate impact of concrete construction at some level. As past ACI president Antonio Nanni explained in 2023, low-carbon concrete cannot be monolithic because material mixes need to be adapted to a wide range of climates across the world. He said ACI 323 will serve as a “living document” over time that eventually offers real guidance on durability.
Members of ACI Committee 323 reiterated similar caveats at this year’s convention. They noted that rather than trying to dictate chemistry, ACI 323 simply defines a performance framework for documenting and reducing the Global Warming Potential of mixes. The code applies to new construction, pavements, bridges and repairs with compressive strengths between 2,500 and 8,000 psi. The code currently does not apply to precast, shotcrete or auger-cast concrete. Projects with different sizes and scopes come with different reporting requirements. Project teams can determine GWP using Type III Environmental Product Declarations or Life Cycle Assessments and calculate a weighted project-average GWP using ACI’s public spreadsheet.
Shana Kelley of KPFF Consulting Engineers walked through how ACI 323 requirements can be written directly into Division 03 specifications, showing model language for 033000 Cast-in-Place Concrete:
- Products Section: Require mixtures meeting ACI 323’s “Tier BL1” targets and reference Appendix A regional benchmarks.
- Submittals Section: Call for Type III EPDs, mix-level GWP data, and a project-wide reduction calculation.
She also outlined what happens once a jurisdiction adopts the code:
- Projects > 50,000 ft²: must show a 15 % reduction.
- Projects 5,000–50,000 ft²: must document GWP but need not reduce it.
- Projects < 5,000 ft²: report mix information only.
Kelley also noted that benchmark values must be localized. The default ACI Appendix A table provides regional benchmarks in kg CO₂e/m³ by strength class (e.g., 17–28 MPa = 345 kg CO2e/m³ for normalweight concrete in the Eastern region). However, ACI encourages jurisdictions to establish local benchmarks using actual EPD data from regional ready-mix producers. This flexibility allows the same performance framework to function in markets with differing materials, SCM availability and climate targets, Kelley explained.
Kelley’s presentation also connected ACI 323 with recent revisions in ACI 318-25, which modernize material allowances for sustainability.
Key changes include:
- Expanded acceptance of ASTM C595 Type IL (Portland-limestone cement) and ASTM C1697 blended SCMs.
- Recognition of alternative cements if approved by the design professional and building official, provided performance and durability are verified.
- Removal of legacy limits on SCM content from Exposure Class F3, which formerly capped fly ash at 25%, slag at 50%, and total SCMs at 50%.
These limits are being phased out. However, until the 2027 International Building Code cycle, they still apply to exterior concrete exposed to deicing chemicals.
Nathan Forrest, technical director at the California Nevada Cement Association, followed Kelley with examples of how ACI 323 could be used in action. He presented three progressively larger hypothetical projects to show how GWP compliance scales:
- Small Project – Starbucks Drive-Thru
- ~52 yd³ of 4500 psi pavement concrete
- Required documentation only: mix EPDs, sources, and reduction strategies
- The mix incorporated 15% fly ash, 10% recycled aggregate, and locally sourced materials — meeting the documentation criteria
- Medium Project – Low-Water Crossing Bridge
- ~23,400 ft² deck area.
- Required submission of GWP averages, benchmark comparisons, and reduction strategies.
- Forrest used the ACI spreadsheet to calculate a weighted average GWP from EPD data, demonstrating compliance with Tier BL1 even when some mixes exceeded benchmark values individually.
- Large Project – 60,000 ft² Office Building
- Required a 15% GWP reduction below regional benchmark (α = 0.85).
- The project’s final weighted average was 84.6% of the benchmark, achieving compliance through strategic mix selection and SCM optimization.
Forrest’s key message is that ACI 323 compliance is as simple as basic math and documentation, and should not be a matter of guesswork. Engineers can easily check unit conversions (m³ vs. yd³), collect current EPDs, and document reduction strategies—even if they’re measures already in use (e.g., SCM substitution, local sourcing).
Getting real about PLC
Although ACI 323 sets the framework for low-carbon specification, portland limestone cement (PLC, ASTM C595 Type IL) likely represents the most immediate pathway to compliance. PLC replaces 5–15% of traditional clinker with ground limestone, cutting production emissions roughly 8–10% per ton of cement.
Field results, however, suggest there’s a substantial learning curve. Larry Taber (Black & Veatch Special Projects Corp.), Ben Wiese (Green Umbrella Systems) and contractors from McHugh Concrete and Concrete Strategies described mixed experiences:
- Reduced bleed water and faster surface crusting, especially in warm or windy conditions.
- Greater sensitivity to curing and evaporation, making early fogging or evaporation retarders essential.
- Variable setting times and slightly higher water demand due to finer particle size.
- Inconsistent finishing when admixtures are not adjusted.
The consensus across speakers was that PLC is fully viable, but success depends on placement discipline. Recommended practices include:
- Keeping evaporation rates below 0.15 lb/ft²/hr
- Using viscosity-modifying admixtures (1–4 fluid oz per 100 lbs of cement) to control bleed rate
- Applying finishing aids and fogging to maintain surface moisture
- Using ACI 305 hot-weather guidance year-round
Laboratory and field testing by the Kansas City Metro Materials Board confirmed that optimized PLC mixes can achieve equal or higher compressive strength than ordinary portland cement, with no observed shrinkage cracking when properly cured.
Carbon capture at scale
The day concluded with CarbonCure Technologies, a company that has developed a CO2 mineralization system that has become one of the most commercially deployed decarbonization technologies in concrete. The process injects captured CO2 into fresh concrete, forming nano-calcium carbonate that enhances hydration and strength while enabling 3–5% cement reduction.
As of Oct. 2025, the company has installed in 570+ ready-mix plants worldwide (North America, APAC, EMEA, LATAM). Its system has been used in more than 9.8 million truckloads of concrete, leading to 681,000 metric tons of CO2 permanently mineralized into concrete, said CarbonCure Chief Technology Officer Dean Forgeron. He added that the company’s global validation program has screened more than 250 cements since 2023 to establish compatibility across diverse chemistries and SCM combinations, with data feeding predictive modeling tools for mix optimization.
Forgeron also touted the company’s partnerships with MIT’s Masic Lab, which he says is advancing the microstructural science of CO2 mineralization, tracking how carbonation influences hydration kinetics and strength development at nano- to macro-scales.
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This is part 1 of SmartBrief’s recap of the American Concrete Institute’s annual convention. For more insight into the latest news and trends affecting the concrete industry, subscribe today to ACI SmartBrief.