Sustainability in Concrete: Fly Ash (PFA) as a Cement Replacement

Introduction

Sustainability has become a focal point across many industries, including construction, where the demand for eco-friendly practices is pressing.

The construction industry relies heavily on concrete, yet one of its primary components—cement—comes with a significant environmental cost. Cement production alone accounts for roughly 8% of global carbon emissions, making it crucial to explore ways to minimise its impact.

One great option for reducing environmental impact in concrete production is using fly ash—also called Pulverised Fuel Ash (PFA). PFAs are sustainable cement alternatives, which can replace up to 30% of cement content in concrete mixes.

Fly ash is the by-product of burning coal at power plants. What was once considered waste is now being reused in various ways across the construction industry. PFAs are used in everything – from ready-mix concrete to paving products and even soil stabilisation.

With cement production’s high environmental cost, finding ways to lower its impact is essential. PFA offers several benefits to concrete’s strength and durability. This makes PFAs sustainable cement alternatives for traditional cement.

Why Use Fly Ash (PFA)?

PFAs offer several benefits when used as a partial cement replacement:

• Stronger Concrete: Fly ash boosts the strength of concrete over time.
• Better Workability: Concrete with PFA flows and pumps more easily, which is ideal for construction.
• Lower Water Needs: This can mean fewer cracks over time, making for more durable concrete.
• Reduced Shrinkage and Permeability: Less cracking and reduced water absorption extend the concrete’s lifespan.
• Lower Carbon Footprint: Replacing some of the cement with fly ash helps reduce the concrete’s environmental impact.

Other Sustainable Cement Options

In addition to fly ash, other industrial by-products are used to make concrete greener:

1. Portland Limestone Cement (PLC): By adding limestone into the mix, the overall carbon footprint is reduced by 5-10%.
2. Granulated Blast-Furnace Slag (GGBS): A by-product of steel production, slag can replace up to half of the cement in a concrete mix.
3. Silica Fume: Helps concrete resist water and improve durability.
4. Agricultural Waste: Ash from rice husks or palm oil production can also work well in concrete.

Challenges with Fly Ash

Fly ash is a fantastic material, but it does come with a few considerations:

• Slower Setting Time: PFA slows down the drying time of concrete, which may require adjustments to construction timelines.
• Early Strength: PFA may not provide as much early strength as traditional cement, although it strengthens well over time.
• Scaling and Appearance: Using too much fly ash can lead to surface issues, like scaling or streaking.
• Limited Impact: PFAs can’t fully replace cement in concrete, therefore, limiting its positive impact.

Fly Ash in the UK: Then and Now

In the UK, fly ash from coal plants, known locally as PFA, has been a popular cement substitute in concrete for decades. Around a quarter of all ready-mix concrete in the UK uses fly ash to replace a portion of its cement. However, with the phase-out of coal plants, there is no fresh UK-produced PFA. This means that we are now looking to “urban mining” to meet demand.

Urban mining refers to recovering buried ash from historic sites. There are currently over 100 million tonnes of quality ash stored in the UK that could be processed and reused in concrete. This not only supports sustainability but also brings back a valuable resource that would otherwise go unused.

Case Study: Sizewell B Nuclear Power Station – Dry Fuel Store

At Eastern Concrete, we’ve seen first-hand how well PFA can work in challenging projects. For the Dry Fuel Store project at Sizewell B Nuclear Power Station, we needed a concrete mix that could handle high heat and seismic conditions. Fly ash played a key role in delivering concrete with the required strength and durability, showcasing its effectiveness in even the toughest conditions.

Conclusion

With a limited supply of fresh PFA in the UK, reusing historical ash through urban mining could be the way forward. It’s sustainable, helps us meet performance standards, and reduces concrete’s carbon footprint.

As demand grows for low-carbon building materials, PFA offers a familiar, proven solution to create more sustainable concrete without sacrificing quality.

Using fly ash is a sustainable and practical way to make concrete that’s strong, durable, and better for the environment. Now is the perfect time to consider PFA as we continue moving toward greener construction practices.