The push to reduce embodied carbon in construction is stronger than ever. Concrete—one of the most widely used materials on major projects—sits at the centre of this effort. Low-carbon technologies like Ground Granulated Blast-furnace Slag (GGBS), fly ash, and other supplementary cementitious materials (SCMs) are now common in specifications to cut the footprint of concrete mixes.

These measures are essential, but if carbon reduction is considered in isolation, it can create unintended impacts on programme, methodology, and cost.

Early Strength Matters

Ordinary Portland Cement (OPC) concrete is known for its rapid early strength gain. That makes it ideal for techniques that depend on predictable early-age performance, such as slip forming and fast-cycle structural frames.

High-GGBS mixes tell a different story. While they often match or exceed OPC in long-term strength and durability, their early hydration is slower—especially in the first 24 to 72 hours. For programme-critical activities like slip-formed cores, where concrete needs “green strength” within hours, this delay can limit rise rates or even make slip forming unworkable.

Cold Weather Makes It Harder

Lower temperatures slow hydration for all mixes, but GGBS-rich concretes feel it most. A mix that performs well in summer may struggle in winter, forcing teams to slow slip speeds, extend curing, add heat, or change sequences. These fixes add cost, disrupt logistics, and can even increase operational carbon—offsetting the embodied carbon savings you aimed for.

The Carbon–Programme Tension

This is where the tension lies: carbon targets are often set early, but their impact on construction methodology only becomes clear later. When sustainability decisions happen in isolation, the result can be late-stage changes, productivity losses, or reverting to OPC mixes under pressure—undermining both delivery and carbon goals.

Align From the Start

The question isn’t “How low can we go on carbon?” It’s “How do we cut carbon while delivering efficiently and safely?”

Early collaboration between designers, contractors, suppliers, and sustainability teams is key. Together, you can:

• Identify elements where early strength is critical.
• Use faster-curing mixes (OPC or low GGBS) for programme-sensitive elements like slip-formed walls and slabs.
• Maximise low-carbon mixes in areas where early strength isn’t a constraint—such as rafts, foundations, walls, and columns.
• Model seasonal and temperature impacts on strength development.
• Balance embodied carbon savings against programme risk and operational carbon.

Low-Carbon Core Walls? Know the Trade-Off

If your carbon strategy includes low-carbon mixes for core walls, understand the implications: this will almost certainly rule out slipform construction—the fastest method for cores—and will have a significant programme impact. That decision needs to be made early and factored into sequencing and delivery planning.

Smarter Carbon Reduction

Reducing carbon is non-negotiable—but it needs a balanced approach. A blanket specification for low-carbon concrete can compromise programme certainty and efficiency.

True sustainability is about optimisation, not extremes. When carbon targets, methodology, and programme align from day one, projects deliver on both environmental and operational success.

So before you commit to a mix design, ask yourself:

Do your carbon and programme targets truly align?