Limestone filler implementation in geotechnical concrete mixes-Paper

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SB3, a geotechnical subcontractor on a major UK high-speed railway project, introduced limestone filler into three concrete mixes in June 2023 following laboratory and concrete-plant trials. The mixes are used for diaphragm walls, piles and rigid inclusions, within a project requiring very large volumes of geotechnical concrete.<br /><br />Limestone filler’s fine, largely inert particles improve the packing of concrete’s granular components. This can increase paste stability, reduce water demand and help limit bleeding and segregation—important risks in deep foundation pours. It can also replace some cementitious material while maintaining the minimum fines content required for stable geotechnical concrete.<br /><br />The trials showed that filler mixes achieved long-term strengths comparable to the original mixes, with no meaningful deterioration in bleed performance or other fresh and hardened properties. Since implementation, no out-of-specification bleeding or filtration results were reported, and mild bleeding events declined.<br /><br />The filler reduced the calculated embodied carbon of the three mixes by approximately 5.2–7.4% per cubic metre. From June 2023 through January 2025, the project placed about 74,400 m³ of filler concrete, saving an estimated 761 tonnes of CO₂. Further savings may be possible as the project continues.<br /><br />The authors conclude that limestone filler can improve the stability of geotechnical concrete while lowering binder use and emissions, without compromising performance. They suggest that future revisions to relevant standards could distinguish minimum cement requirements for durability from minimum fines requirements for stability, allowing lower-carbon fillers to replace some cementitious material.

Keywords

geotechnical concrete

limestone filler

embodied carbon

high-speed railway

diaphragm walls

deep foundations

concrete mix design

bleeding and segregation

cement replacement

CO2 savings