Low-Carbon Concrete 101: Reducing Carbon on the Jobsite

Concrete is the backbone of modern construction. Yet cement production alone accounts for about 8% of global CO₂ emissions. In response to the carbon emissions of concrete production, low-carbon concrete has emerged as an alternative to concrete made with traditional cements like Type 1/2, also known as ASTM C-150. The intention? To cut emissions while still delivering the strength and durability these projects demand, whether they are driveways of new residential developments, or DOT and infrastructure projects. Whether these new cement types (categorically known as ASTM C-595 Blended Hydraulic Cements, one of which is Type 1L cement, also called Portland limestone cement or Portland 1L cement) have delivered on the strength and durability claims, is up for contention. 

What Is Low-Carbon Concrete?

At its core, low-carbon concrete is a mix engineered to minimize the emissions associated with cement, the binding component that drives most of concrete’s embodied carbon. This reduction comes from adding raw, or uncooked, limestone to the mix, blending in supplementary cementitious materials (SCMs), utilizing recycled inputs, and optimizing mix proportions. Emerging low-carbon cement technology also contributes to this outcome.

Professionals evaluating what is low-carbon concrete should think in terms of both material inputs and performance outputs: a concrete that requires less clinker, incorporates substitutes like fly ash or slag, and achieves the same compressive strength benchmarks under ASTM testing.

Why Cement Drives Carbon Emissions

Cement production releases CO₂ from two sources: the fuel burned to heat kilns and the chemical reaction (calcination) that occurs when limestone is converted into clinker. This dual release is why conventional mixes have such a large footprint.

Reducing carbon here is not meant to sacrifice structural performance. The intent is to use the right mix design and substitutions to keep the paste strong enough to bond aggregates, resist cracking, and handle long-term loading. That balance is what defines low-embodied-carbon concrete.

Low-Carbon Concrete Mix Options

There are several strategies available to reduce the cement load in concrete mixes. Each comes with its own field considerations.

Supplementary Cementitious Materials

Fly ash, slag cement, and calcined clays replace portions of Portland cement while maintaining binding properties. These SCMs react with calcium hydroxide in the mix to form additional cementitious compounds, strengthening the cured product. When specified properly, they cut emissions significantly and can enhance long-term durability. Regional availability matters — not all markets have consistent supplies, and sourcing must be confirmed early in design.

Recycled Aggregates and Fines

Recycled crushed concrete and industrial byproducts can replace a percentage of virgin aggregate. This does not lower binder-related emissions directly but reduces extraction, transport, and landfill impacts. Mixes with recycled aggregate must be tested carefully, as variability in particle strength and absorption can affect consistency and curing. When verified, these substitutions contribute to both material efficiency and reduced project waste.

Mix Design Optimization

This approach prevents overdesign while meeting structural code requirements. Lowering water-to-cement ratios, incorporating admixtures, and calibrating paste content all are strategies designed to reduce unnecessary cement use. With proper testing, it has the potential to produce reduced-carbon concrete that performs as expected in the field.

Benefits of Low-Carbon Concrete

The advantages of adopting low-carbon concrete extend beyond emissions accounting.

First, the environmental impact is measurable. Lower clinker content directly reduces CO₂ release, bringing embodied carbon of projects down to meet stricter benchmarks. Lifecycle assessments consistently show that mixes designed this way carry less total carbon compared to conventional alternatives.

Second, concrete that resists sulfate attack, alkali-silica reaction, and thermal cracking lasts longer, reducing replacement cycles and lowering cumulative carbon over its service life. This durability translates into fewer repairs, fewer material deliveries, and lower long-term environmental costs.

Concrete’s carbon footprint comes mainly from cement content. Even modest substitutions can yield measurable reductions:

• Replacing ~30% of cement with fly ash typically reduces embodied CO₂ by 15–25%, depending on the baseline mix and LCA boundaries.

• Using ~50% slag cement in place of Portland cement often cuts emissions by 35–45% while improving sulfate resistance.
  

Optimizing mix design to eliminate 50 kg of unnecessary cement per cubic meter saves roughly 40–45 kg of CO₂, based on typical cement emission factors (~0.8–0.9 kg CO₂ per kg cement).