Table 3 Summary of embodied CO₂ eq values per unit mass assigned to each constituent material for embodied carbon calculations

OPC

0.84

^304,354

Assumed CEM I OPC composition in accordance with the European norm (EN 192-1)⁵⁷: 90.8% clinker, 5.05% gypsum, 5.21% limestone, and 0.2% additives by weight. The mixture proportions are based on sector-aggregated data for the United Kingdom.

Aggregate

0.00747

³⁰⁴

Based on average market composition in the United Kingdom, comprising 64.2% land-won, 27.5% recycled and secondary, and 8.3% marine aggregate, with the consumption statistics reported by British Marine Aggregates Producer Association³⁵⁵.

Quartz sand

0.0102

³⁵⁶

–

Quartz powder

0.0234

³⁵⁶

Quartz powder with particles size ranging from 0–0.22 mm.

Silica fume

0.052

³⁵⁷

Based on the EPD of a commercially available silica fume produced by Ferroglobe LC (UK); material is recovered as a by-product from the smelting of elemental silicon or ferrosilicon alloys in electric arc furnaces.

Superplasticiser

0.5722

³⁵⁸

Based on the average values reported in EPDs of 14 commercially available SP manufactured by Sika^® (Switzerland).

Steel fibre

2.03

³⁵⁹

Based on the EPD of Dramix^® steel fibre for fibre-reinforced concrete, produced by Bekaert (Belgium).

Water

0.000177

³⁶⁰

Estimation based on the UK Department of Energy Security and Net Zero government conversion factors for company greenhouse gas reporting; considers emissions from water supply only (excluding water treatment), assuming water is incorporated into the final product.

GGBFS

0.142

³⁶¹

Upper limit as reported by Black¹¹⁵, with the estimate accounting for the emission required for rapid cooling, grinding, and transportation from the production site to the grinding facility.

FA

0.027

^115,362

–

RHA

0.0314

³⁶³

Estimation based on rice husk, with 94% of emissions attributed to the agricultural phase and 5% to the transportation from paddy fields to mills.

LP

0.0172

³⁵⁶

–

GP

0.142

–

Estimation based on the processes required for sorting, cleaning, crushing, and grinding, including transportation of the raw material from the collection point to the processing facility and subsequent powder milling. The value is assumed to be the same as GGBFS.

SS

0.142

–

Estimation based on emissions from transportation (from the steel production site to the grinding facility) and the energy required to grind gravel-sized material into micrometers particles. Zhang et al.³⁶⁴ reported a value of 0.045 kg CO₂ eq/kg excluding transportation; to ensure a fair comparison, this study conservatively assumes the same as that used for GGBFS.

CC (MK)

0.423

³²⁵

Comparisons by Habert and Ouellet-Plamondon³²⁶ highlighted the significant influence of the heating energy source; therefore, the value adopted herein accounts for clay extraction, the additional amount of unified clay required to achieve the final product’s unit weight (loss of ignition), and assumes a fossil-based energy source for heating.

NBP

0.143

–

Li et al.²⁴³ reported values ranging 0.009 to 0.013 kg CO₂ eq/kg, considering only the emissions from energy consumption during sieving; However, in practice, the materials sourced from the waste excavation rocks (such as those from railway construction) required transportation and grinding. Therefore, to ensure a fair comparison, the value in this study is estimated to be same as that for GGBFS.

NB

0.435

³²⁵

Estimated by assuming the same energy consumption as for MK, with an adjustment for the increased mass loss associated with dehydroxylation during the formation of meta-clay.

NaOH

1.434

³²⁶

Estimated for alkaline solutions based on emissions per unit mass of the dry component, assuming production via the diaphragm cell process for conservatism. (For reference, the more commonly used membrane cell process yields a value of 1.26 kg CO₂ eq/kg.) As the electricity mix strongly influences emissions, a global average value was used in this study.

Ca(OH)₂/Hydrated lime

1.09

^365,366

Based on the average values from EPDs for five commercial products manufactured by Adbri and Boral (Australia).

Na₂SiO₃/Waterglass

1.06

³²⁶

Assumed production via the furnace liqour route, with a global average value reflecting the choice of electricity mix.

nSiO₂

1.69

³⁶⁷

Estimated based on the manufacturing process for sodium silicate solutions, as a proxy for the precipitated material.