From ashes to Green concrete

Commentary, Online Exclusive Feature / 2021

From ashes to Green concrete

by Anisa Pinatih; Candice Lim

April 20, 2021

Globally, via the Paris Agreement and the UN Sustainable Development Goals (SDGs), leading cities and companies are committing to a highly efficient and decarbonised building operations by 2050.

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But to achieve a net-zero circular economy, one of the strategies must also unlock the potential of addressing the full life cycle of buildings, including the reduction of virgin material extraction and C&D waste at the construction stage. Thus, the role of the construction and real estate sector in delivering is tremendous.

Concrete is the most widely used material in Asia; and the environmental impact is massive.

Around 900 kilogrammes of carbon are emitted for the fabrication of every ton of cement. It is the source of about 8 per cent of the world’s carbon emissions, contributing more CO2 than aviation fuel (2.5 per cent) and is not far behind the global agriculture business (12 per cent).

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An alternative to this is Green concrete made of agricultural wastes.

Southeast Asian countries produce a myriad of products from oil palm, which grows abundantly owing to their geographical and ecological conditions. With this massive production, Palm Oil Fuel Ash (POFA) is generated as a by-product of the burning of palm kernel shell, fibre and husk. Malaysia, for example, is one of the largest producers of POFA in the world, with around 1,000 tonnes of it being generated annually by the 200 palm oil mills in operation.

This region is also among the top producers of rice. The by-product of rice milling is rice husk ash (RHA), generated from the incineration. Approximately 110 million tonnes of rice husk and 16 to 22 million tonnes of RHA are generated worldwide, with Asia as the major contributor.

Image by carekung/Shutterstock

What potential can we unlock here and how do we do it?

Chemical composition of POFA and RHA

On the surface, POFA and RHA are pollutants. But they are rich in silica, therefore, a good pozzolanic material—which will react with calcium hydroxide in the presence of moisture and form compounds possessing cement properties at ordinary temperatures. As such, they are suitable for replacement of cement and additional material in concrete.

The amount of silica, alumina and iron oxide for POFA is between 52 per cent and 75 per cent. According to ASTM C618—the standard specification for natural pozzolan for use in concrete—POFA is classified between classes C and F pozzolan, which is suitable for concrete mixtures.

Meanwhile, RHA contains substantially higher amount of silica, between 89 and 90 per cent. With composition of silica, alumina and iron oxide in RHA amounting to more than 70 per cent, RHA falls into a class F pozzolan, which is even more suitable for concrete mixtures.

Turning POFA and RHA into Green concrete

With a lower level of calcium than cement, POFA can help to prevent thermal cracking. Calcium in cement generates heat when hydrated. If cement composition is partially replaced by POFA, the heat generated from the exothermic process will decline, so shrinkage and cracks will be minimal.

Adding POFA into admixture can also increase overall concrete quality thorough the optimum compressive strength and modulus of elasticity. The increased strength could be attributed to the filling effect of the fine particles and the pozzolanic reaction that improves the bond between the hydrated cement matrix and the aggregate.

But POFA has a slower pozzolanic reactivity so the curing period is longer than a traditional concrete. It takes at least 28 days for POFA concrete to cure but after that, the product will have similar or higher strength than normal concrete.

Another noteworthy point is that the replacement of cement with grounded POFA should not be too excessive. Thus far, research suggests that the ideal portion is 20 per cent to produce concrete with better compressive, splitting tensile and flexural strength, with the maximum percentage of replacement being at 30 per cent. Higher than that, the concrete will have lower workability because it will be too dry and porous.

Similar to POFA mixes, high content of active silica in RHA will increase the strength of concrete. The curing time to attain maximum compressive strength also takes at least 28 days. Research suggests that the compact microstructure of RHA concrete improves the bonding between cement matrix, and consequently increases the resistance toward tensile loading.

But why is the application still low?

Leveraging waste products such as POFA and RHA is a promising solution to reduce carbon footprint. Research has also shown that this pozzolanic material can yield better compressive and tensile strength of concrete, and increases crack resistance and workability. However, the construction industry requires faster delivery and quick availability of materials. Longer curing time (28 days and 48 hours for regular and Green concrete respectively) is definitely a disadvantage.

Also, sustainable practices like this needs strong commitment and budgeting for the R&D. Finance is a barrier in many Southeast Asian countries, especially in the operational procedures that include collecting, transporting and treating the waste products. R&D needs to be supported too to ensure that structures made with Green concrete have comparable a life cycle with structures built with conventional concrete.

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Achieving sustainability is not just about checking the Green features off the list, such as the passive cooling and smart maintenance. The whole building lifecycle needs to be examined carefully if we are to achieve the ambitious Paris Agreement and the UN SDGs by 2050. Tapping into the potential of agricultural ashes sounds promising, but the question is: Do we want to do it? This requires whole systemic rethinking. Achieving the Paris Agreement and SDGs is a far cry without transforming the way we build and design.

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