Waste-Derived Thermal Insulation Bricks from Kaolin–Sawdust–Periwinkle Shell Ash Composites: Multi-Response Taguchi–Grey Relational Optimization for Sustainable Construction

Bricks are the silent workhorses of the built world, holding up roofs and walls for centuries. Yet, the traditional way of making them creates a paradox: the very process that makes them strong also makes them poor at keeping heat inside. When clay is fired in a kiln, it becomes a dense, solid block that conducts heat easily, meaning buildings lose warmth in winter and gain it in summer, forcing heating and cooling systems to work harder. For decades, engineers have looked for ways to make these blocks lighter and more insulating without sacrificing their ability to hold up a structure. The solution often lies in adding tiny pockets of air, or pores, into the clay. These pores act as barriers to heat flow, but creating them usually weakens the brick. The challenge is finding a way to introduce enough air to stop heat transfer while keeping the material strong enough to be used in construction.

In a recent study, researchers at Ahmadu Bello University in Nigeria tackled this problem by turning to two common waste products: sawdust from wood processing and the shells of periwinkles, a type of sea snail abundant in coastal regions. Instead of treating these as trash, the team mixed them with clay to create a new kind of brick. They treated the sawdust as a temporary placeholder; when the brick is fired in a kiln, the organic wood fibers burn away completely, leaving behind a network of tiny holes. The crushed shells, which are rich in minerals, were added to the mix to help the brick stay strong despite these holes. To find the perfect recipe, the scientists did not guess. They used a systematic method called the Taguchi design, which allowed them to test different amounts of sawdust, different amounts of shell ash, and different firing temperatures in a structured way. They then used a statistical tool called Grey Relational Analysis to balance two competing goals: making the brick as good at stopping heat as possible while keeping it strong enough to support a building.

The researchers began by carefully examining the raw materials to understand what they were working with. They used a technique called X-ray diffraction, which acts like a fingerprint scanner for minerals, to see exactly what the clay, the sawdust, and the shell ash were made of. They found that the clay contained a mineral called kaolinite, which transforms into stronger, heat-resistant crystals when heated to high temperatures. The sawdust was confirmed to be made of cellulose, the tough fiber found in plants, which burns cleanly. The shell ash was mostly calcium carbonate, a mineral that breaks down when heated to release gas and leave behind reactive minerals that can help bind the brick together. This detailed understanding of the ingredients gave the team confidence that their mix would work as intended.

Next, they created a series of small test bricks, each with a slightly different recipe. Some contained more sawdust, some more shell ash, and some were fired at lower temperatures while others were fired hotter. They measured how well each brick stopped heat from passing through it and how much weight it could hold before cracking. The results were clear: the temperature of the kiln was the single most important factor for how well the brick insulated. The hotter the brick was fired, the better it stopped heat, because the high heat ensured the sawdust burned away completely and the clay minerals transformed into their strongest forms. However, the amount of sawdust was the most critical factor for strength. Adding too much sawdust created too many holes, making the brick too weak to hold up a wall. The shell ash played a supporting role, helping to strengthen the brick at moderate levels but offering less benefit if added in excess.

By combining all these measurements, the team identified a single "golden" recipe that offered the best balance. This ideal mix contained ten percent sawdust, ten percent shell ash, and was fired at a temperature of 1100 degrees Celsius. When they made a full-sized brick using this formula, the results were striking. The new brick conducted heat at a rate of 0.407 watts per meter per kelvin, which is a massive improvement over the 1.8 watts per meter per kelvin of a standard brick. In simpler terms, this new brick is nearly four times better at keeping heat inside than a traditional one. At the same time, it remained strong enough for construction, with a compressive strength of 7.83 megapascals, which is significantly higher than the 5 megapascals required by building standards.

The study confirmed that this approach works not just in theory, but in practice. The statistical models they built to predict the brick's performance were highly accurate, matching the actual test results almost perfectly. The researchers also noted that the process creates a double benefit for the environment. It diverts two types of waste—wood dust and sea shells—from landfills and turns them into a valuable building material. This is particularly relevant in regions like West Africa, where these waste materials are plentiful. The final product is a brick that is lighter and cooler than the ones we are used to, yet strong enough to build with. The study concludes that by carefully balancing the ingredients and the firing process, it is possible to create sustainable building materials that solve the age-old trade-off between insulation and strength, offering a practical path toward more energy-efficient construction.