Published October 2026 · NuGreen
Why look beyond Portland cement?
Portland cement is the binder in most of the world’s concrete. Its key ingredient, clinker, is made by heating limestone to about 1,450 °C. That process releases CO₂ twice: from the fuel that heats the kiln, and from the limestone itself as it breaks down (calcination). Cement production is widely estimated to account for around 7–8% of global CO₂ emissions.
Every alternative below reduces carbon in the same basic way: by using less clinker, or none at all. They differ in how much clinker they remove, how they behave on site, and how easy they are to buy today.
1. Blended cements with fly ash or slag
The most widely used option. Part of the clinker is replaced with supplementary cementitious materials (SCMs) — usually fly ash from coal power plants or ground granulated blast-furnace slag (GGBFS) from steelmaking. The SCMs can be blended at the cement plant or added at the ready-mix plant. Portland-limestone cement (Type IL in the U.S.), which replaces some clinker with ground limestone, is a related and now common option.
- Carbon: lower than Portland cement; the saving grows with the share of clinker replaced.
- Early strength: high replacement levels can slow early strength gain, particularly in cold weather.
- Where it is used: almost everywhere — ready-mix, paving, structures and mass concrete.
- Availability: wide, though regional fly ash and slag supply can be tight.
2. Limestone calcined clay cement (LC3)
LC3 replaces roughly half of the clinker with a combination of calcined clay and limestone. Clays suitable for calcination are widespread, and they are calcined at a much lower temperature than clinker.
- Carbon: substantially lower than Portland cement, while still containing clinker.
- Early strength: generally comparable to Portland cement at typical ages, depending on the mix.
- Where it is used: general construction; it behaves much like conventional cement on site.
- Availability: emerging; commercial production is still limited to a number of regions.
3. Clinker-free geopolymer cement
Geopolymer cement (part of the wider alkali-activated family) uses no clinker at all. Aluminosilicate-rich materials such as fly ash and slag are activated with an alkaline chemistry at ambient temperature, forming a hard binder without a kiln.
- Carbon: the largest reduction potential of the three, because clinker is removed entirely; the result depends on the activator and feedstock used.
- Early strength: formulation-dependent; some systems are designed for rapid strength gain.
- Where it is used: infrastructure, precast and repair, typically qualified project by project.
- Availability: fewer suppliers; usually specified through performance-based standards such as ASTM C1157 rather than prescriptive ones.
At a glance
| Option | Clinker | Carbon vs Portland | Early strength | Availability |
|---|---|---|---|---|
| Blended (fly ash / slag) | Reduced | Lower, scales with replacement | Can be slower at high replacement | Wide |
| LC3 | About half | Substantially lower | Generally comparable | Emerging |
| Geopolymer | None | Largest potential reduction | Formulation-dependent | Fewer suppliers |
These are general characteristics. Actual performance and carbon depend on the specific product, local materials, mix design and curing, so every option should be compared on product-specific data and confirmed for the project.
Where NuGreen fits
NuGreen makes a clinker-free geopolymer binder from fly ash and slag, activated at ambient temperature and used with standard ready-mix equipment. You can read how the technology works, review the embodied carbon data and methodology, or see the technical data sheet. For a specific project, the specifier support page explains how to request a technical evaluation or pilot pour.