The Cement Industry Is Retiring Its Own Cheapest Substitutes

The two materials that did most of the work in cutting cement’s clinker factor are both byproducts of industries the energy transition is actively shrinking. Fly ash falls out of coal-fired power stations. Ground granulated blast furnace slag falls out of ironmaking. Neither is manufactured on purpose, neither is priced as a product in its own right, and neither is going to grow.

That matters because the clinker factor is the single largest lever cement has on its own carbon intensity, and the industry has spent thirty years pulling it with materials it does not control the supply of. The arithmetic that governs the whole sector is straightforward: at a clinker factor of 0.71, the 1,800 million tonnes of registered Chinese clinker capacity is worth roughly 2,535 million tonnes of cement-equivalent output. Every decimal point of that factor is worth tens of millions of tonnes of clinker — and therefore of CO₂ — and every one of those decimal points has to come from somewhere other than the kiln.

Where the substitutes actually come from

The traditional answer is the two byproducts. Pulverised fuel ash is a residue of coal combustion, and its availability is a direct function of coal burn in power generation. Granulated blast furnace slag is a residue of primary ironmaking, and its availability is a direct function of blast furnace iron output. Both are therefore exposed to the same transition that is decarbonising the customer.

The exposure is not symmetric and it is not gradual. Coal capacity retirements remove fly ash supply plant by plant, and a station closing in one region cannot be substituted by a station elsewhere because the material is heavy, low-value and uneconomic to move far. Slag is tighter still: the global steel fleet is shifting toward electric arc furnace routes, which produce no blast furnace slag at all, so the supply of the best-performing clinker substitute on the market is structurally tied to the share of steel that is made the carbon-intensive way.

The result is a squeeze that runs in the opposite direction to the carbon price. The cost of emitting rises, so the incentive to substitute clinker rises, while the supply of the two cheapest substitutes falls. An industry that has built its decarbonisation pathway on other industries’ waste streams has built it on a depleting resource.

The United States has already solved this, and nobody noticed

The interesting thing about the American market is that it got there first, for reasons that had nothing to do with climate policy. According to the USGS National Minerals Information Center, 63 per cent of US cement shipments are blended cements — and 95 per cent of those blends are portland-limestone. Multiply the two and 59.85 per cent of all US cement shipments are portland-limestone.

This is the most important number in the substitution debate and it is routinely misread. Portland-limestone cement does not substitute clinker with a pozzolan. It substitutes clinker with limestone — ground, quarried, abundant, and entirely unconstrained by what any other industry happens to be doing. In the market with the weakest carbon policy in the developed world, the industry found the one substitute that scales because it is a mined mineral rather than an industrial residue.

Europe took the harder road, and the harder road is the one with the supply ceiling. The European specification route leant on slag, fly ash and, more recently, calcined clay — technically better performers in many applications, and all of them either byproduct-constrained or dependent on a kaolinitic clay supply chain that has to be built from a standing start. The regulatory framework is now pushing substitution hard: Ireland’s public procurement rules require a 30 per cent clinker replacement minimum on public projects from September 2024, and the EU’s own construction products framework is tightening the declaration requirements around exactly these blends.

What the carbon price does to a supply-constrained substitute

Set a carbon price against that structure and the outcome is not a smooth substitution curve. At 3,800 million tonnes of cement and roughly 0.60 tonnes of CO₂ per tonne of cement, the sector emits about 2,280 million tonnes of CO₂. At EUR 80 a tonne, that is a carbon cost of EUR 48 on every tonne of cement — 42.1 per cent of a German ex-works price of EUR 113.98. The Carbon Border Adjustment Mechanism began its definitive regime on 1 January 2026 and is designed to replace free allocation precisely as free allocation is withdrawn, so that EUR 48 stops being a number on a spreadsheet and starts being a cheque.

A producer who cannot get slag or fly ash pays it. A producer who can, does not. And the producer who can is increasingly the one who never depended on a byproduct at all — which is why the competitive advantage in this industry is migrating from proximity to a steelworks or a power station toward ownership of a quarry and a grinding circuit.

The substitution that is not a waste stream

Calcined clay is the technology the industry is betting on to break the byproduct dependency, because kaolinitic clay is widely distributed and is not a residue of anything. It is also, and this is the part that gets skipped, a mined and thermally treated product with its own kiln, its own fuel bill and its own carbon footprint — so it substitutes clinker at a lower but not negligible emissions cost, and it requires capital that most producers outside the largest twenty have not committed.

The honest reading of the position is that the industry has three substitution routes with three different constraints. Limestone filler is supply-unlimited and strength-limited. Slag and fly ash are performance-superior and supply-declining. Calcined clay is supply-scalable and capital-constrained. No single one of them carries the clinker factor where the carbon price is going, and the producers who will still be earning a margin in 2033 are the ones who have already hedged across all three rather than optimising for the cheapest tonne available today.

Why this is a packaging story as well as a cement story

There is a second-order consequence that the packaging cluster makes visible. A blended cement has a different particle size distribution, a different bulk density and different flow behaviour from a plain CEM I. Those differences land at the packer head, in valve geometry, in fill rates and in the strength of the sack that has to survive a drop test. The industrial paper sacks market is already contracting on the demand side — cement and building materials are 57 per cent of European deliveries and shrinking — so the substitution shift arrives as a specification problem on a shrinking order book, which is the worst possible time to be re-qualifying a bag.

Look at the whole chain and the pattern repeats at each step. Cement is shifting its product mix away from the commodity grade. Ready-mix concrete is shifting away from standard structural mixes. Cement packaging is losing volume to the silo. In each case the value moves toward the formulated product and away from the undifferentiated tonne, and in each case the winner is whoever controls the specification rather than whoever controls the volume.

Where this sits in the cluster

The substitution question is inseparable from the product-mix question, because a clinker substitute is only worth having if somebody is buying the blend it goes into — that is the arithmetic in CEM I Shrinks While CEM II Adds the Growth. It also explains where the capital is going: the American price-engine piece tracks CRH committing USD 2.1 billion to supplementary cementitious materials rather than to batching capacity, which is a company buying the input that dilutes clinker. For the chain-wide version, where the clinker factor sits alongside every other volume-versus-price trade in the group, read Peak Cement, Falling Concrete.

About the method

This analysis draws on 70 per cent secondary and 30 per cent primary sources. The secondary layer is public and primary: USGS mineral commodity statistics and shipment data, the Global Cement and Concrete Association‘s sustainability reporting, European and Irish public procurement instruments, and the CBAM regulation as published. The primary layer is structured fieldwork with producers, plant technologists and procurement leads on substitution limits by strength class and the practical availability of supplementary cementitious materials at plant level; it is weighted into the conclusions as evidence about what is achievable rather than quoted as individual attribution. All arithmetic in this article — the 59.85 per cent blended share, the 2,280 million tonnes of CO₂, the EUR 48 per tonne and its 42.1 per cent share of the German price — was computed deterministically from the published inputs. No third-party market-research estimate is used, and no competing research provider is cited.