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ELNASYA AZZAHRA
Writer & Corporate Communications Intern @ PT Wijaya Karya Beton Tbk (WIKA Beton)
Concrete depends heavily on cement as its binding component, and cement production carries a substantial carbon footprint. As the construction industry looks for ways to use resources more efficiently, industrial byproducts such as fly ash and ground granulated blast-furnace slag (GGBFS) are gaining attention as supplementary cementitious materials in concrete. Their use raises a broader question: can materials once treated as waste become part of a more circular approach to construction?
The word “waste” in the title is deliberate. By the end of this piece, the word itself may be worth questioning.
When we think about sustainable construction, we often picture energy-efficient buildings, renewable energy or green building certifications. Yet one of the most important sustainability questions may begin much earlier: what are the buildings actually made of?
Concrete is one of the most widely used construction materials in the world, which makes the environmental footprint of its cement content particularly significant. Greenhouse gas emissions are associated with cement manufacturing, making the materials used to build our cities an important part of the sustainability conversation.
For years, discussions about sustainable buildings have focused heavily on how buildings perform once they are completed. But their environmental impact begins much earlier, through the extraction, processing and manufacturing of the materials that go into them. This is also where the environmentalmes tangible: in the resources companies use, the emissions they generate and the choices they make across their value chains.
One possible pathway is to reduce the amount of conventional cement required in concrete while maintaining the technical performance needed for construction. This is where supplementary cementitious materials, or SCMs, become increasingly relevant.
What if part of the solution to construction’s material challenge is already being produced somewhere else?
SCMs can partially replace Portland cement in concrete. Fly ash and ground granulated blast-furnace slag, commonly known as GGBFS, are among the materials that have been widely studied for this purpose. Research has examined their potential to reduce cement demand while supporting the strength, durability and other performance requirements of concrete when appropriately formulated.
This creates a connection between industries.
Fly ash originates from coal combustion, including coal-fired power generation. GGBFS, meanwhile, is produced as a byproduct of ironmaking in blast furnaces. After appropriate processing, both materials can be incorporated into concrete as supplementary cementitious materials.
The significance is not simply that one industry produces a byproduct and another can use it. It is that the relationship between industries can be reconsidered through a circular economy lens.
Instead of asking only how an industrial byproduct should be managed, the question becomes whether it can retain value as an input for another process.
Fly ash (left) and ground granulated blast furnace slag/GGBFS (right), two industrial by-products used as supplementary cementitious materials in concrete. Source: ResearchGate.
Fly ash has pozzolanic properties, meaning it can react with components in cement-based systems and contribute to the development of concrete properties. Depending on the material characteristics and mixture design, its use can support long-term strength and durability.
GGBFS has a different origin but a similar opportunity. Produced from ironmaking slag and processed into a finely ground material, it has latent hydraulic properties that allow it to contribute to the cementitious binder in concrete. Research has examined its effects on workability, strength, permeability and durability.
These characteristics give both materials value beyond their original industrial processes. Yet describing them simply as “waste materials” can also be misleading.
Their suitability depends on their characteristics, processing, quality and intended application. A more useful distinction may be between a material with no further productive use and a byproduct that can become an input for another process.
That distinction matters when considering sustainability.
There is an important caveat to the idea of turning industrial waste into construction resources: reuse does not automatically equal sustainability.
A material does not become sustainable simply because it is classified as a byproduct or because it replaces another material. Its environmental value depends on the system around it.
How far does the material have to travel? How much processing does it require? What proportion of cement can realistically be replaced? Does the resulting concrete meet the required strength and durability standards? And what happens to the material over its life cycle?
These questions matter because the performance of SCMs varies according to factors such as material characteristics, replacement levels and concrete mix design. Studies of fly ash, for example, have identified both environmental benefits and performance limitations, including slower early-age strength development at certain replacement levels.
The objective, therefore, should not be to maximize the amount of an industrial byproduct used at any cost.
The goal is to find the right material, in the right proportion, for the right application while considering both technical performance and environmental impact.
This is where sustainability moves beyond a simple substitution exercise and becomes a question of life-cycle thinking.
For construction companies, this shift could influence more than concrete formulation. It can affect procurement decisions, material innovation, product development and how environmental considerations are incorporated into engineering and business strategy. In ESG terms, material choices can become part of how companies manage environmental impacts across their operations and value chains.
The opportunity is particularly interesting because construction and other heavy industries are connected through their material flows. Steelmaking, power generation and construction do not operate in isolation. The byproducts of one industrial system may become inputs for another when the technical, environmental and economic conditions allow it.
In Indonesia, PT Wijaya Karya Beton Tbk (WIKA Beton) offers one example. According to the company, fly ash and GGBFS are used as substitute materials in the precast concrete mixtures of WIKA Beton Home (WHOME), its modular housing concept, which the company describes as a green product.
The example is useful not because one company has solved the sustainability challenge, but because it illustrates how material innovation can connect industrial byproducts with new construction applications.
The broader lesson extends beyond one company or one product. For construction businesses, the question is increasingly shifting from “What material can we use?” to “What is the environmental and technical value of this material throughout its life cycle?”
The future of sustainable construction may not depend entirely on inventing new materials. It may also depend on looking differently at the materials we already have.
Fly ash and GGBFS illustrate this shift. Materials originating from power generation and steel production can, under appropriate conditions, become inputs for concrete rather than being viewed only through the lens of disposal.
But the bigger opportunity is not simply to turn waste into concrete. It is to rethink how resources move between industries, how materials are valued and how sustainability is measured.
For a sector as resource-intensive as construction, that change in perspective could be as important as the material itself.
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