Scale: The Size of the Solution Must Address the Problem

The Declining Legacy: Slag and Fly Ash

For decades, the cement industry relied on two industrial byproducts — ground granulated blast furnace slag (GGBFS) from steelmaking and fly ash from coal power plants — as supplementary cementitious materials. These are proven, effective, and widely used. But their supply ceilings are structurally constrained.

GGBFS is capped at ~430 Mt/yr — the total output of blast furnace steelmaking globally — and will shrink as the industry transitions to electric arc furnaces. Fly ash has a larger theoretical base (~800 Mt/yr including stockpiled material) but fresh supply is declining fast as coal plants close across the US and Europe. These are legacy feedstocks with contracting ceilings.

The Emerging Supply: Natural and Engineered Alternatives

This supply decline makes the search for alternative SCMs existentially important. Several promising categories are emerging:

• • LC3 (Limestone Calcined Clay Cement): Kaolinitic clay exists on every continent in enormous quantities — a theoretical supply ceiling of ~2,000 Mt/yr. Already commercial in Colombia, Ghana, India, and France. The most credible replacement at scale. [4]
• • Natural Pozzolans: Billions of tons of volcanic ash, pumice, and tuff deposits exist globally — a ceiling rivaling calcined clay. Unevenly distributed but abundant in Latin America, East Africa, Southeast Asia, and western North America. [5]
• • Recycled Glass Pozzolan: If all global glass waste were collected and ground as pozzolan, theoretical supply reaches ~200 Mt/yr. Currently constrained by collection infrastructure, not by geology. [6]
• • CO₂ Mineralization: If CO₂ injection technology (CarbonCure, Solidia) were deployed across global concrete production, theoretical capacity could reach ~50 Mt/yr. Currently sub-1 Mt. [10]
• • Algae / Biocement: Theoretical ceiling of ~500 Mt/yr if bio-manufacturing were industrialized. Lab-stage only today — zero commercial production — but the biological supply chain is inherently renewable.

CCS and CCU: Super-Scalable if the Technology Works

Carbon capture technologies deserve special attention because their theoretical scale is enormous. CCS (Carbon Capture and Storage) could sequester over 5 Gt of CO₂ per year by 2050 according to IEA projections [7]. CCU (Carbon Capture and Utilization) — where captured CO₂ is mineralized into concrete, converted to fuels, or used as chemical feedstock — could utilize an additional 6.2 Gt CO₂/yr globally [9]. At this scale, both pathways would dwarf every SCM category combined.

But each pathway faces a different binding constraint. CCS is limited by how much CO₂ we can actually store. The geological storage ceiling — saline aquifers, depleted oil and gas reservoirs, deep coal seams — is estimated at 1,460 Gt under prudent engineering assumptions [8]. That is large but not infinite, and it presupposes a global network of CO₂ transport pipelines that does not yet exist. As of 2025, operational CCS capacity across all industries is approximately 50 Mt/yr [7] — less than 1% of what is needed.

CCU is limited by how much CO₂ we can capture and economically utilize. The current bottleneck is not the concrete industry's willingness to use CO₂ — it is the cost and availability of captured CO₂ at sufficient purity and pressure. GCCA aims to have CCUS fully operational at just 10 cement plants globally by 2030. The Norcem Brevik plant in Norway — the world's first full-scale cement CCS facility — captures 400,000 tonnes of CO₂ per year, a landmark achievement that still represents 0.01% of the industry's annual emissions.

Retrofit vs. Greenfield

There are roughly 4,000 cement plants operating globally. Building new facilities is slow: permitting, environmental review, community engagement, financing, construction — a 5–8 year cycle for a single plant. Greenfield projects also carry enormous CAPEX risk.

By contrast, retrofitting existing plants — adding SCM blending capability, modifying kilns for alternative fuels, bolting on carbon capture — can happen in 1–4 years. Solutions that leverage existing infrastructure scale faster, cost less, and face fewer political obstacles.

Regional Deployment: No Single Solution Fits Everywhere

Cement is not traded globally. Concrete is rarely transported more than 100 miles. Markets are inherently regional ecosystems with different feedstock availability, regulatory regimes, energy costs, workforce capacity, and infrastructure maturity.

A solution that only works in Europe — where carbon pricing, abundant gas infrastructure, and skilled workforces enable CCS — does not solve cement in sub-Saharan Africa, where clay is abundant but capital is scarce. A pathway dependent on wealthy consumer demand is irrelevant to the Global South, where most new building will occur.

Therefore, global decarbonization requires a portfolio of regionally appropriate solutions. This table shows the structural fit between pathways and geographic contexts: