when the ash ran out

Problem statement

For four decades the cheapest way to cut the clinker in concrete — and to make it more durable — has been to replace 20–50% of the Portland cement with supplementary cementitious materials (SCMs), overwhelmingly coal fly ash and blast-furnace slag. But fly ash is a by-product of burning coal, and "since 2010, the availability of coal ash has been in continuous decline, prompted primarily by the conversion of power-generating plants from coal to natural gas." A 2025 survey of U.S. state departments of transportation (43 of 52 responded) found that "28 of the 33 DOTs that responded indicated that they have been, are currently, or expect to be experiencing shortages of fly ash," and — in the Utah case example — that "precast concrete companies tended to be the most affected." Alternatives exist — natural pozzolans, harvested and beneficiated ash reclaimed from landfills and ponds, ground-glass pozzolan and other alternative SCMs (ASCMs) — yet only "six allow the use of ASCMs, while seven plan to allow their use in the future," and most agencies that permit natural pozzolans have no specification unique to them, relying on the coal-ash standard ASTM C618/AASHTO M 295. The unsolved problem is that the concrete supply chain built its low-carbon strategy on a waste stream that decarbonisation of the power sector is eliminating, and the acceptance machinery for substitutes — reactivity tests, variability limits, field-performance evidence, agency specifications — is years behind the shortfall.

Why this matters

Cement is roughly 7–8% of global CO₂ emissions and SCM substitution is the largest near-term lever in every cement decarbonisation roadmap; a shrinking SCM supply means either more clinker per cubic metre or a scramble for imports (seven DOTs "have begun to import fly ash"), each with its own carbon and cost. Fly ash also protects durability — it is specified against alkali-silica reaction, sulfate attack and chloride ingress — so shortages threaten service life as well as carbon. The same coal-retirement dynamic applies wherever coal power is being phased out (a 2026 EU scenario study of SCM supply is listed in the Source Notes), and blast-furnace slag is tied to a steelmaking route that decarbonisation is also expected to shrink — so the shortfall is not a U.S. peculiarity. Whoever solves regional qualification of substitute pozzolans first will set the specification template other agencies copy.

What’s been tried and why it hasn’t worked

Agencies have responded in five ways: "13 DOTs resort to industry solutions; 11 modify specifications to allow the [use] of other available SCMs, NPs, and ASCMs; seven import fly ash; three adjust concrete mixture designs; and three modify specifications ... to limit the use [of] fly ash for only durable concrete." Harvested ash was written into ASTM C618-23 (which now calls both fly ash and harvested ash "coal ash") and 19 DOTs allow it, but "the reluctance of some state DOTs to use harvested and beneficiated coal and bottom ash may stem from concerns about variability in the material." Natural pozzolans are geographically uneven — Utah's suppliers use volcanic ash, pumice and expanded shale because they are nearby, and 16 DOTs allow metakaolin and 15 calcined clay — but "there seems to be notably less information on laboratory evaluation and field performance of concrete produced using individual NPs, since their properties may vary based on the material type and origin." For ASCMs (ground-glass pozzolan, recycled-concrete fines, non-ferrous slags, biomass and municipal-waste ashes, mining and dredging wastes), "the information currently available on approval and potential performance of ASCMs is not sufficient to prompt some state DOTs to allow their use," and only two DOTs are running field trials. The deeper constraint is that acceptance is organised around a material category (coal ash) rather than a performance property: agencies lack "approaches ... to evaluate and predict the reactivity (and any negative impacts on hydration, performance, and durability) of SCMs, NPs, harvested ash, and ASCMs given their composition and characteristics." A few agencies show the way out — Caltrans derives allowable SCM combinations from measured characteristics via equations in its specifications, and Colorado's performance-based provisions let contractors choose among ten listed materials up to 50% replacement — but these are exceptions.

What would unlock progress

The reframing is from "find more fly ash" to "qualify any reactive material by what it does": a validated, rapid reactivity-and-variability protocol (building on ASTM C1897 isothermal calorimetry/bound-water methods) tied to performance-based acceptance, plus regional maps of pozzolan and by-product resources matched to demand, so that a precast plant or DOT can turn a local clay, ash pond or glass stream into an approved SCM in months rather than years. Adjacent precedents: performance-based specifications for asphalt binders (Superpave) that replaced prescriptive grades, and food/pharma raw-material qualification that accepts variable natural inputs through characterisation plus process control.

Entry points for student teams

A materials team could characterise two or three locally available candidate pozzolans (a clay, a harvested ash, a waste glass) with reactivity tests and small mortar/concrete trials, and produce the data package a DOT says it lacks. A data/GIS team could build a regional SCM supply–demand map — remaining coal-plant ash output and retirement dates, ash ponds, clay and volcanic deposits, glass and slag streams — against concrete demand, identifying where shortfalls will bite first. A specification team could draft a performance-based SCM acceptance clause for one agency, modelled on the Caltrans and Colorado approaches, and test it against the synthesis's list of ASCMs. Relevant skills: cement chemistry and materials testing, GIS and supply-chain analysis, standards writing.