This Simple Cement Swap Could Help Save 2 Billion Tons of CO2 a Year

By Nidhi DhullReviewed by Bethan DaviesMar 25 2025

A recent study published in ACS Sustainable Chemistry & Engineering examined the use of thermoactivated recycled cement waste (RC), dehydrated at 500 °C, as a high-substitution (>75 wt.%) replacement for ordinary Portland cement (OPC) when combined with limestone filler (LF). Using a reverse filling approach, researchers assessed the blended RC’s reactivity, water demand, strength, porosity, and CO₂ emissions.

Background

Globally, about one gigaton of cement waste is generated each year—roughly 25 % of total cement production. This volume surpasses the availability of most supplementary cementitious materials (SCMs), positioning recycled cement waste as a promising resource for new concrete applications.

Heating cement waste to 500 °C (thermoactivation) removes water and hydroxyl groups, restoring partial reactivity. But the process also increases surface area, which in turn raises water demand and limits strength development—two key challenges for practical use.

To overcome these limitations, researchers have blended thermoactivated RC with LF to improve particle packing and reduce water demand. Micronized cement (MC)—ultra-finely ground OPC—was also introduced to fill capillary pores and boost strength. This study aimed to test how these engineered blends (eRCs) perform under real-world construction conditions.

How the Study Was Designed

Researchers developed four cement systems:

• 100 wt.% RC
• RC₈₀-MC₂₀: 80 wt.% RC + 20 wt.% MC
• RC₇₅-MC₁₀-ML₁₅: 75 wt.% RC + 10 wt.% MC + 15 wt.% micronized limestone
• RC₃₅-MC₁₅-L₅₀: 35 wt.% RC + 15 wt.% MC + 50 wt.% regular limestone

The last three were classified as engineered RC systems (eRCs).

Physical properties like density and surface area were measured using helium pycnometry and BET analysis. Particle size was assessed with laser granulometry. Thermal behaviors were tracked using thermogravimetric (TG) and differential TG (DTG) analysis from 23–1000 °C.

For performance testing, standard-consistency pastes were cast into molds to measure compressive strength over various curing times (6 hours to 28 days). Post-fracture samples were further analyzed using TG/DTG and quantitative X-ray diffraction to track phase evolution. Mercury intrusion porosimetry was used to study porosity and pore size distribution at 1, 7, and 28 days.

CO₂ emissions were also calculated, excluding those from crushing and grinding processes other than the production of MC.

Key Findings

The study found that RC and MC components interacted synergistically in the eRC blends. RC contributed to fast early-age hydration, while MC helped reduce large capillary voids over time—especially after the first day of curing. This combination resulted in both early and long-term strength gains, making eRCs viable for general construction use.

Environmentally, RC alone produced the lowest CO₂ emissions but didn’t meet strength requirements. In contrast, the eRCs—blended with just 10–20 wt.% MC—achieved solid performance with substantially lower CO₂ emissions (198–320 kgCO₂/ton) than OPC (846 kgCO₂/ton) and conventional blended cements (427–657 kgCO₂/ton).

The study also highlighted a critical environmental advantage: the dehydration–rehydration mechanism responsible for early strength gain operates at 500 °C, far lower than the 900 °C required to decompose carbonates in traditional clinker production. This allows for meaningful performance gains without high-carbon calcination.

Looking ahead, the research suggests that widespread use of eRCs—along with full utilization of SCMs and carbonated cement waste—could cut cement industry emissions by up to 61 % by 2050 (2.31 Gt CO₂/year). This projection significantly outpaces the Global Cement and Concrete Association’s current roadmap for net-zero concrete.

Takeaways and Next Steps

This study demonstrates that engineered recycled cement (eRC) blends can reduce CO₂ emissions while maintaining structural performance—especially when paired with MC and LF to balance water demand and refine porosity.

While the results are promising, future research will need to address long-term durability, validate the use of real-world cement waste, and explore field-scale applications. A broader integration of recycling strategies like carbonation and thermoactivation will also be key to scaling this solution.

Journal Reference

Zanovello, M., John, V. M., White, C. E., & Angulo, S. C. (2024). Engineered Blended Thermoactivated Recycled Cement: A Study on Reactivity, Water Demand, Strength-Porosity, and CO₂ Emissions. ACS Sustainable Chemistry & Engineering, 13(2), 800–814. DOI: 10.1021/acssuschemeng.4c06567, https://pubs.acs.org/doi/10.1021/acssuschemeng.4c06567

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