Impact of fly ash content and fly ash transportation distance on embodied greenhouse gas emissions and water consumption in concrete

Abstract

Background, aim and scope

Fly ash, a by-product of coal-fired power stations, is substituted for Portland cement to improve the properties of concrete and reduce the embodied greenhouse gas (GHG) emissions. Much of the world’s fly ash is currently disposed of as a waste product. While replacing some Portland cement with fly ash can reduce production costs and the embodied emissions of concrete, the relationship between fly ash content and embodied GHG emissions in concrete has not been quantified. The impact of fly ash content on embodied water is also unknown. Furthermore, it is not known whether a global trade in fly ash for use in concrete is feasible from a carbon balance perspective, or if transport over long distances would eliminate any CO₂ savings. This paper aims to quantify GHG emissions and water embodied in concrete (f′_c = 32 MPa) as a function of fly ash content and to determine the critical fly ash transportation distance, beyond which use of fly ash in concrete increases embodied GHG emissions.

Materials and methods

This paper used previously published and reported data for GHG emissions and water usage in cement production, quarries, transportation and concrete batching to quantify the embodied GHG emissions (CO₂-equivalent) and water in concrete and the critical transportation distance for fly ash.

Results

Fly ash content alone is not a good indicator of embodied emissions in concrete; increasing fly ash content only reduces embodied emissions when there is a corresponding reduction in the mass of Portland cement used. The total embodied GHG emissions in concrete (GHG_concrete, kg CO₂-equivalent m⁻³) can be determined from the mass of Portland cement used (mass_cement, t m⁻³): GHG_concrete = 66 + 790.7 mass_cement. This equation can be used to determine the reduction in Portland cement required to meet specific GHG emissions targets for concrete, if the Portland cement is replaced by fly ash sourced within 100 km of a concrete batching plant. Fly ash content has little effect on embodied water, which was 2.7–4.1 m³ water per cubic metre of concrete.

Discussion

Fly ash can be transported more than 11,000 km by articulated truck, 47,000 km by rail and 54,000 km by sea and still result in a net reduction in GHG emissions if used to replace Portland cement in concrete. At least 70% of GHG emissions embodied in concrete were due to cement production, even for fly ash content as high as 40%. Aggregate production accounted for 17–25% of embodied GHG emissions. While transport of concrete from batching plant to site represented only 3–5% of GHG emissions, this distance is subject to wide variability and hence can be a source of variation in total embodied GHG emissions. Water used in quarrying aggregate is both the largest and the most variable quantity of water used in concrete production, and accounted for at least 89% of water consumption for all mix designs considered in this study.

Conclusions

While this study used values applicable to Brisbane, Australia, results are presented in a generalised form for ready adaptation to other conditions, for example different distances to raw materials sources, transport emissions factors, etc.

Recommendations and perspectives

A global trade in fly ash has the potential to reduce GHG emissions embodied in concrete, if the fly ash is used to reduce the consumption of Portland cement per cubic metre of concrete. Increasing fly ash usage under these conditions will reduce both the volume of fly ash disposal and the GHG emissions from the concrete industry. Efforts to reduce water consumption in the concrete industry should focus on quarrying processes, and on finding replacement materials with lower embodied water, which may include recycled aggregate. While this study has quantified the GHG emissions and water embodied in concrete as a function of fly ash content, a full life cycle study of concrete is required to determine the full impact of substituting fly ash for Portland cement. Structural characteristics, life span and operational requirements of concrete should also be considered in any decision to alter cement and fly ash content.