It’s possible to make great concrete without the huge emissions from cement. Why isn’t this common?

Concrete is the most used human-made material on Earth. Billions of tonnes are produced every year.

Unfortunately, concrete is responsible for 5-8% of global carbon dioxide emissions. Concrete is typically made of sand, gravel and cement, the binding agent. The manufacturing of cement is the main reason why concrete is so high-emitting.

For two centuries, cement makers have relied on the Portland cement method. This requires heating limestone to about 1400°C in kilns. At these temperatures, limestone breaks down, releasing carbon dioxide. Kilns often run on coal or gas, which release even more CO₂ when burned.

Low-emissions concrete is entirely possible and it depends on substituting limestone cement with another binder. One of the best alternatives is geopolymer concrete, which often uses industrial byproducts. Emissions from geopolymers are up to 80% less than for Portland concrete. Australia has one of the largest geopolymer cement project in the world – Queensland’s Wellcamp Airport near Toowoomba, which used about 40,000 cubic metres of geopolymer cement.

But over a decade after the airport was built, geopolymer cement hasn’t taken off. Here’s why – and what we can do about it.

How do you make geopolymer concrete?

A polymer is a long molecule made up of repeating smaller chemical units. Geopolymers is the term for polymers made from minerals. They’re typically produced from mining byproducts such as clay or industrial byproducts such as fly ash and slag from blast furnaces. Alkaline chemicals such as sodium hydroxide are used as activators to convert these byproducts into a ceramic-like material able to bind together.

Australia produces about 10-15 million tonnes of fly ash a year – comparable to its annual cement consumption of about 11-12 million tonnes. Fly ash is challenging to dispose of and can become a major waste problem. Geopolymers offer a clever way to convert it into something valuable.

Australian companies such as Wagners (Queensland) and Zeobond (Victoria) started commercialising geopolymer cement in the 2000s. The 2014 construction of Wellcamp Airport by Wagners showed concrete without cement could be deployed at commercial scale.

So why hasn’t geopolymer scaled up?

Geopolymers have major advantages over traditional concrete. But there are major barriers to overcome.

One is a lack of long-term performance data. Cement concrete has 150 years of real-world evidence, while geopolymers have only been around since the 1980s. The construction industry can’t yet confidently guarantee or insure geopolymer structures.

Public infrastructure projects can help by trialling geopolymers and monitoring long-term performance. This would build the evidence base needed if the industry is to accept it.

In the meantime, there’s already enough evidence to show geopolymers are well suited for masonry, mortar, renders, plasters, paving and precast products.

Workability and handling is another challenge. Construction workers know how to handle cement-based concrete. But geopolymers have to be handled differently, as they can be corrosive until they set. This means they need extra safety measures and operator training. The way this concrete sets and hardens is also different to cement.

While Australia’s existing standards and operating procedures primarily focus on conventional concrete, geopolymer guidance has recently been released. More will need to be done to boost familiarity.

Variation in product quality is another major issue. Because geopolymers are often produced from industrial by-products, feedstock quality can vary significantly and give inconsistent results.

Fly ash is a good example. After coal is burned, it leaves this fine, powdery ash behind. But the ash itself differs depending on type of coal – brown coal (lignite) makes fly ash with more calcium while black coal (anthracite) produces ash with little calcium and more silica and alumina. These differences can lead to concrete with very different characteristics, from strength to setting time to durability.

Geopolymers can also be made from feedstocks such as mineral processing waste, which can give it useful properties such as resistance to acids.

For the industry to mature, it makes clear sense to classify geopolymers based on their precursor type. Kevlar and cling film are both made from plastic polymers, but it wouldn’t make sense to simply call them “polymers”.

Finding more feedstock options is sensible.

One final challenge is the local availability of feedstocks and chemical activators such as caustic soda and water glass (sodium silicate). Both of these can be produced in Australia, but supplies of fly ash depend on coal plants. As these retire, fly ash may become harder to source.

A chicken and egg problem

These technical challenges can be overcome. But large-scale adoption will mean overcoming a classic chicken-and-egg problem.

Concrete producers are reluctant to invest in large-scale geopolymer production if the demand isn’t there, while builders, developers and infrastructure firms are hesitant to rely on less familiar materials without established supply chains and a long-term history of performance.

Government procurement policies and demonstration projects could help solve the problem by creating early demand and building industry confidence.

Geopolymers are unlikely to replace cement any time soon. But as the construction industry seeks to reduce emissions, geopolymers have real promise.

With continued research, appropriate standards, targeted government support and stronger industry engagement, geopolymer concrete could play a significant role in Australia’s efforts to cut emissions, tackle industrial waste and make infrastructure more sustainable.