5 Challenges with Lower Carbon Concrete

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Today’s article is 980 words… 7 mins reading time.

Australia is producing approximately 70 million tons of pre-mixed concrete each year and demand is expected to increase 40% by 2050.

The materials team at Arup spoke about how they aim to incorporate lower carbon concrete into our civil projects. What strategies our industry can use when transitioning away from Portland cement by using recycled materials, while ensuring it remains cost-effective and without compromising it’s long-term performance.

The Problem

Concrete is the most prolific construction material with the highest carbon footprint;

Portland cement has been the traditional product for concrete since the 19th century, so what are the major challenges in shifting away from this and towards greener/recycled materials?

5 Major Challenges

1. Risk aversion and comfort with the status quo - the reluctance to change and use greener materials as traditional concrete has historically been a cost efficient and structurally reliable product.

2. Lack of awareness of new Supplementary Cementitious Materials (SCM’s) and their applications.

3. Developing and updating specifications and standards to incorporate recycled substitutes.

4. Untested long-term performance of recycled materials in concrete.

5. Cement supply is not increasing, yet demand is increasing at unprecedented levels. The result is a significant demand gap, highlighting the need for replacement materials to keep up with market demand.

5 Steps for Transition

1. Improving awareness and understanding of recycled materials and opportunities for potential use.

2. Collaboration and project working groups (suppliers, contractors, clients…) to take stronger action and enhance industry confidence in greener alternatives.

3. Specifying and mandating a minimum cement replacement on our projects and addressing the regulatory issues with up-to-date standards.

4. Driving demand through sustainable procurement initiatives and additional market information.

5. Seeking opportunities to use higher SCM mixes for non-structural elements - footpaths, parapets, barriers, noise walls etc. Listing out all the elements on a project, which ones require high early strength and which ones don’t? Additional SCM can then be used.

Supplementary Cementitious Materials (SCM’s)

• Recycled Concrete Aggregates (RCA) - recovered from commercial demolition and civil works, can be used as a subbase under full depth asphalt or as a basecourse under low traffic local roads.

• Recycled Crushed Glass - sourced from glass food and beverage containers and is processed and crushed to produce a sand-sized material. It can be used in the construction of embankments, structural and non-structural fill, retaining wall backfill and drainage, with several specifications in place to support its use.

• Fly Ash - A by-product of coal-fired power generation. Fly ash is used as a supplementary cementitious material, a Portland cement replacement, or a filler in asphalt.

• Slag - a by-product of the iron and steel industry, in the blast furnace slag floats to the top of the iron and removed from the final product.

• Recycled Plastics - sourced from various industrial and household waste streams. It has several road applications, including asphalt, pipes, roadside furniture, supplementary aggregate material, noise walls and bike paths.

• Calcined Clay- a manufactured product from clays in soil use to make geopolymer concrete. No cement is required at all (100% reduction) and superior performance to other recycled materials in concrete.

High SCM mixes are currently defined as;

• In-situ: 50-80% SCM

• Precast: 40-50% SCM

Replacing Cement with Calcined Clay

Infrastructure projects will always require the use of concrete so completely replacing concrete altogether is not a practical option.

One alternative to this could be replacing the cement component with Calcined Clay. Typically, fly-ash and slag are the most common alternatives. However, they only replace 15 to 40% of cement used and their supply is not matching the increasing demand.

This is where Dr Fangjie Chen (Materials Engineer ARUP) believes that geopolymer concrete - using calcined clay can be utilised to make the most effective transition to replacing 100% of the cement used.

The key constraints with finding a suitable cement alternative are;

Cost-effective ✅
Local supply ✅
Suitability and performance ✅

Soils with 30-80% clay is cheap, widely available, and easily accessible;

Did you know?

Clinker - is the solid material produced in manufacturing Portland cement which is used as the binding material. It’s produced by heating a mixture of clay, limestone and sand through a kiln - this is what releases C02 and is responsible for over 55% of emissions in concrete.

EcologiQ is a Victorian Government initiative to optimise the use of circular materials in major infrastructure projects.

Sources

• Recycled Content in Use - video examples

• Low Carbon Concrete (ARUP) - presentation 

• Achieving Lower Carbon Concrete - presentation

• Replacement Materials (Infrastructure Aus) - report

• Greening the Concrete Jungle: Calcined clay - article

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