Producing a tonne of Portland clinker means heating limestone and clay to 1,450°C in a rotary kiln, a process that emits CO₂ from two distinct and equally significant sources: burning the fuel needed to reach that temperature, and the chemical decarbonation of the limestone itself (CaCO₃ → CaO + CO₂), which alone accounts for roughly 60% of the process's total emissions and is intrinsic to the reaction — not something you can eliminate just by switching fuels. That's why decarbonising cement is structurally harder than decarbonising steel or glass: the problem isn't just energy, it's chemistry.
LC3: the most industrially mature route
Limestone Calcined Clay Cement (LC3), developed by a Swiss-Indian-Cuban research consortium (EPFL, IIT Delhi, University of Havana), replaces up to 50% of clinker with a blend of low-temperature calcined clay (750°C versus clinker's 1,450°C) and ground, uncalcined limestone. The clay-limestone combination triggers a synergistic pozzolanic reaction that almost fully offsets the mechanical strength lost by reducing clinker content. The result: emissions cut by roughly 40% compared with ordinary Portland, with comparable mechanical performance over longer timeframes (28-90 days) and raw materials — common clays, not pure kaolin — available almost everywhere in the world, including many areas where good-quality limestone for clinker is scarce. LC3 is already in commercial production in India, Colombia and several African countries, often exactly where access to quality clinker is more expensive.
Belite cements (BCSA): less energy, different crystals
Belite Calcium Sulfoaluminate cements (BCSA) modify the mineralogical composition of the clinker itself, favouring the formation of belite (C₂S) instead of alite (C₃S) — the dominant phase in ordinary Portland. Belite requires a firing temperature roughly 100-150°C lower and a lower limestone/clay ratio, cutting process emissions by 10-15%. The practical limitation is that belite hydrates more slowly than alite: lower early strength in the first few days, a problem for industrial construction schedules that requires blended formulations (belite plus a small share of alite or sulfoaluminate to speed up setting).
Geopolymers: the most radical leap
Geopolymer binders — also known as alkali-activated cements (AAC, not to be confused with autoclaved aerated concrete, which shares the acronym) — abandon clinker almost entirely. The raw material is an industrial-waste aluminosilicate (fly ash from coal power plants, granulated blast-furnace slag) activated by a strong alkaline solution (sodium hydroxide or sodium silicate). The polymerisation reaction, at room temperature or with mild heating, produces a chemically very stable three-dimensional amorphous structure of Si-O-Al bonds — it is, in a way, a distant engineered relative of the Roman pozzolanic reaction (see the in-depth piece on Roman cement versus modern cement), but activated chemically rather than over time. Emissions drop by up to 90% compared with Portland, because the high-temperature firing and the limestone decarbonation are almost entirely eliminated.
Brisbane West Wellcamp Airport in Australia (2014) was among the first large infrastructure projects to use geopolymer (the E-Crete brand) for pavements and aprons, demonstrating real-scale feasibility. The main limitation isn't technical but regulatory: as of 2025 there's still no harmonised EN standard for structural geopolymers in Europe, which means every structural use requires a case-by-case technical assessment (ETA, European Technical Assessment) instead of a standard certification — a bureaucratic hurdle that slows adoption more than the chemistry itself does. There's also the issue that fly-ash availability is declining as coal power plants close, which poses a scaling problem for the future: geopolymer risks staying a niche unless activators based on more abundant raw materials are developed (calcined clays, steel-mill slags).
CO₂ curing: capturing instead of emitting
A complementary, more recent approach doesn't change the binder but the curing process. Technologies such as CarbonCure inject industrially captured CO₂ directly into the fresh mix during mixing: the CO₂ reacts with the available calcium to form calcium carbonate, which is permanently mineralised into the cementitious matrix (not released into the atmosphere) and also contributes a modest mechanical-strength gain. The practical advantage is that this technology integrates into existing ready-mix plants without changing the mix design, offering a modest emissions cut (roughly 5-8%) but at a very low adoption cost — it's the fastest route to scale up broadly, even if less decisive than the first three.
None of these routes, on its own, zeroes out concrete's footprint. The most likely direction over the next ten years is a combination: LC3 or composite cements for most current structural uses, geopolymers for non-structural applications or wherever local regulations already allow it, and CO₂ curing applied broadly as an incremental reduction across all production, regardless of binder type.