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The carbon question for SCMs in concrete - burden-free or economically allocated?

Supplementary cementitious materials (SCMs) - fly ash, GGBFS, silica fume, and the like - are the single most pragmatic lever we have to reduce the embodied carbon of concrete today. How we allocate upstream impacts to those SCMs in carbon footprinting matters: it changes reported global warming potentials, procurement choices and, importantly, the incentives for a circular market for secondary materials.

Below we compare how leading EPD frameworks treat SCMs, set out the practical and material issues, and present the Rebuilt position: favouring a zero-burden (burden-free) allocation for bona fide by-products while preserving rigour through governance, transparency and materiality thresholds.

How LCA and EN15804 treat SCMs

  • Ecoinvent: (widely used for LCA modelling) uses a “cut-off” system model (and other system models) in which waste and recycled materials do not carry the burdens of virgin production.
  • EN16908:2017: PCR for Cement and Building Lime. The product category rules that supplement EN15804 suggest that for co-products that contribute to overall revenue of the order of less than 1% are regarded as very low. Where this is the case, impacts from the process may be neglected.

Other approaches

This is an evolving space. Across global EPDs program PCRs you will see various approaches with, economic allocation used in many cases for multi-output processes (eg blast furnace slag treated economically).

  • EPD Australasia / International EPD PCRs. The Australasia and International system’s shared construction PCRs have been updated (PCR 2019:14 v2.0.0) and continue to require allocation only where unavoidable; the hierarchy (avoid allocation where possible, use physical properties, then economic allocation). The updated PCRs for these EPD programs require economic allocation to be used for processes producing co-products for use in cement and concrete.

The issues - rigour, gaming and market signals

  1. Authenticity vs gaming. A blanket zero-burden rule risks overstating carbon benefits if a material is, in practice, produced because of demand from construction (i.e., it’s a deliberately produced co-product, not a waste product from the primary production). Conversely, rigid economic allocation can disincentivise uptake of genuinely residual streams by making SCMs look less favourable on paper than they are in circular reality.
  2. Materiality. For many SCMs the life-cycle impacts of extraction/processing are small relative to avoided clinker production - the choice of allocation method can flip the sign of the benefit only in marginal cases. This makes it vital to focus on materiality thresholds rather than theoretical purity: if the allocation choice changes an SCM’s GWP outcome by an immaterial amount (say <5–10% of the binder system), obsessing over method is counterproductive.
  3. Circular economy incentives. Economic allocation tends to treat SCMs as having some of the upstream burden, which can blunt the procurement signal in favour of recycled/by-product materials. That outcome is the opposite of what a circular market needs: we want demand to absorb residuals, stimulate scale, reduce disposal and minimise depletion of resources from the natural environment
  4. Fair accounting: Economic allocation artificially redistributes emissions across co-products based on their market value. This can create the appearance of a lower carbon intensity for the primary product—even though no physical processes or actual emissions have changed. In effect, it manufactures a theoretical shift on the accounting ledger only.

Because this could allow primary producers (e.g. steelmakers or coal-fired generators) to claim reduced emissions simply by assigning more burden to a by-product, most EN 15804-based PCRs explicitly prohibit economic allocation for integrated processes. This safeguard prevents burden-shifting, maintains comparability, and keeps the carbon account in balance.

A governance-centred middle path (Rebuilt’s approach)

Rebuilt’s uses its governance and review process which is modelled on best practice governance approach, independent technical review and periodic stakeholder consultation to resolve the tension between conflicting guidance, global EPD program PCRs, and its own values and first principles, which are to support a net zero trajectory by 2050 and efficient resource usage.

This governance informs our decision making process as follows:

  • Clear definitions: We codify what counts as a by-product, co-product and recycled material using the information hierarchy (including primary evidence requirements such as origin, production drivers and contractual relationships).
  • Materiality gates: We apply burden-free treatment only where the downstream use is avoiding a primary production (i.e., the material is residual, the primary industry would have produced the output regardless, and the lifecycle burden of processing the SCM is small). If a material fails materiality gates, we require economic allocation.
  • Transparency & verification: We ensure that PCFs disclose allocation choices, data sources and sensitivity checks, and subject high-impact claims to third-party scrutiny.

Periodic review: We revisit definitions and thresholds (annually or when major technical determination is required) to avoid lock-in and respond to market changes.

Summary

Rebuilt favours a zero-burden allocation for bona fide by-products such as SCMs, subject to clear evidence and materiality gates, with physical/economic allocation reserved for true co-products where the primary process is driven by that output. This position:

  • aligns procurement incentives with circular outcomes and rapid clinker-substitution;
  • preserves scientific integrity through mandatory disclosure, verifier checks and sensitivity analysis; and
  • is operationalised via Rebuilt’s governance and review process so the rule is not a free pass but a well-governed, evidence-based preference.

In short: We treat SCMs as partners in decarbonisation, not as accounting liabilities — but only when independent evidence shows they genuinely are residuals and materially reduce system-level emissions. That combination of ambition and rigour is the fastest, fairest route to lower-carbon, circular concrete.

We acknowledge that this is a complex and evolving space and will continue to work in open dialogue with other verifiers and market actors.