Is concrete bad for the environment?

Cement share of global CO₂

about 7%IEA, second-largest industrial emitter

Type IL limestone content

5–15%blended cement under ASTM C595

Type IL CO₂ reduction

up to ~10%versus ordinary portland cement

State DOT approval

all 50 states + DCAmerican Cement Association, 2024

PLC market share

passed portland in June 2023USGS shipment data

Short version

• Cement is the emitter. Aggregate and water contribute very little to concrete’s footprint.
• Type IL portland-limestone cement cuts up to about 10 percent of that, and it is approved everywhere in the US.
• Supplementary cementitious materials replace part of the cement, but fly ash supply is shrinking as coal plants retire.
• Carbon mineralization injects captured CO₂ into fresh concrete, where it turns into a solid mineral.
• Pervious concrete is recognized by EPA as a stormwater practice, which is an environmental benefit, not a cost.

Why is cement the problem and not concrete?

Cement is the problem because it is the only ingredient whose manufacture releases large amounts of carbon dioxide. Concrete is mostly aggregate and water with a relatively small fraction of cement binding it together, and quarrying rock and sand is not carbon intensive. Cement production is, on two counts at once: the kiln burns fuel to reach clinkering temperature, and the chemical conversion of limestone into clinker releases CO₂ from the limestone itself.

Scale is what makes it matter. The IEA identifies cement as the second-largest industrial emitter of carbon dioxide, responsible for about 7 percent of global emissions, and the third-largest industrial energy consumer. Reducing that means either using less clinker per ton of cement, less cement per cubic yard of concrete, or capturing what the process releases.

Every practical decarbonization lever in use today follows one of those three routes. None of them involve making concrete out of something other than concrete.

What is portland-limestone cement?

Portland-limestone cement, designated Type IL, is a blended cement made under ASTM C595 in which finely ground limestone replaces a portion of the clinker. Limestone content typically runs between 5 and 15 percent. Because less clinker is burned per ton of cement, the CO₂ footprint drops by up to about 10 percent while the cement performs as ordinary portland cement does.

Adoption in the United States is essentially complete on the approval side. As of 2024, every state department of transportation and the District of Columbia permits the use of portland-limestone cement, which removes the specification barrier that historically kept it out of public work.

The market followed. Shipments of portland-limestone cement passed those of traditional portland cement in June 2023 according to US Geological Survey data, making Type IL the dominant cement for general concrete construction in the country. For most projects this happens without anyone changing a mix design, which is why it is the single largest carbon reduction already delivered in North American concrete.

What are supplementary cementitious materials and why is fly ash getting scarce?

Supplementary cementitious materials, or SCMs, are reactive powders used to replace a portion of the portland cement in a mix. The common ones are fly ash from coal combustion, ground granulated blast-furnace slag from steelmaking, silica fume, and natural pozzolans. Beyond cutting cement content, they generally improve durability — lower permeability, better sulfate resistance, and mitigation of alkali-silica reaction.

Fly ash has been the workhorse, and its supply is declining. As coal-fired power plants are retired or converted to natural gas, less ash is produced, and the concrete industry competes for a shrinking stream. The precast industry has tracked this closely, and the responses now in use include harvesting and beneficiating ash from existing disposal sites, importing ash, and shifting toward slag cement and natural pozzolans.

Specification practice is adapting alongside supply. State DOTs have modified their SCM requirements, and blended cements produced at the plant increasingly deliver the substitution that used to happen at the ready-mix batch plant.

How does carbon-cured concrete work?

Carbon-cured concrete puts captured carbon dioxide into the material permanently instead of into the atmosphere. In the ready-mix version, CO₂ is injected into the fresh mix during batching, where it reacts with calcium ions from the cement and mineralizes as calcium carbonate embedded in the concrete. CarbonCure, which supplies this technology, describes the injected CO₂ as immediately and permanently mineralized, with producers using the resulting strength benefit to cut cement content.

A different approach changes the cement itself. Solidia’s system uses a low-lime calcium silicate cement manufactured at lower kiln temperatures with less limestone, and cures the concrete by reacting it with carbon dioxide rather than with water. Holcim, which partnered with Solidia to commercialize it, has described the technology as capturing CO₂ in building materials.

Both approaches are additive rather than exclusive. A plant can run portland-limestone cement, replace part of it with slag, and inject mineralized CO₂ in the same mix, and the reductions stack.

Does concrete help the environment in any way?

Concrete does deliver environmental benefits, and stormwater is the clearest one. Pervious concrete is an open-graded mix with interconnected voids that lets rainfall pass through the pavement into a stone reservoir and the soil below, instead of running off a hard surface into a storm drain. EPA lists permeable pavement — specifically including pervious concrete — as a green infrastructure practice for managing stormwater where it falls.

The benefit is water quality and hydrology rather than carbon. Reducing runoff volume reduces streambank erosion, combined sewer overflows, and the pollutants that surface runoff carries into receiving waters. NRMCA’s Concrete in Practice series covers pervious concrete as a material and the placement practices that make it work.

Durability counts too. A pavement or structure that lasts decades longer amortizes its embodied carbon over a much longer service life, and the cement industry’s own decarbonization roadmap treats service life and material efficiency as part of the equation alongside cleaner cement.

Common mistakes

• Blaming concrete instead of cement — aggregate and water carry almost none of the footprint, and the reduction levers all act on the binder.
• Assuming lower-carbon cement is exotic — Type IL is approved in every state and has been the majority cement in the US since 2023.
• Specifying high fly ash content without checking supply — coal plant retirements have made regional availability the binding constraint.
• Treating CO₂ mineralization as an offset — the carbon is chemically locked into the concrete, which is a different claim than a purchased credit.
• Ignoring service life in the comparison — a mix with slightly higher embodied carbon that lasts twice as long can be the lower-impact choice.