Analysis of the hydration heat evolution in cements incorporating construction waste powders by examining phase changes

Concrete is the most widely used building material on Earth, a mixture of sand, water, and a binding powder called cement. When water meets cement, a chemical reaction begins that turns the soft mixture into a hard, stone-like solid. This process, known as hydration, releases heat. In massive structures like dams or high-rise foundations, this heat can build up to dangerous levels, causing the concrete to crack as it cools. To manage this, engineers often look for ways to slow down the reaction or reduce the total heat generated. At the same time, the construction industry produces vast amounts of waste, including crushed bricks, old tiles, and bits of demolished concrete. While the large chunks of this debris are often recycled into new gravel, the fine, dusty powder that results from crushing is frequently discarded. Researchers are now investigating whether this fine waste powder can replace some of the cement in new concrete, turning a disposal problem into a useful resource that also cools down the chemical reaction.

A team of scientists in Algeria set out to test this idea using three specific types of construction waste: powder made from recycled ceramic tiles, powder from recycled bricks, and powder from recycled concrete. They created a series of mortar mixtures, which are similar to concrete but use only fine sand. In their experiments, they replaced 10%, 20%, and 30% of the standard cement in each mix with one of these waste powders. They then placed these mixtures into a special insulated container designed to measure the temperature changes as the materials hardened. By tracking how much heat was released and how fast it came out, the researchers could map the entire life cycle of the chemical reaction from the moment the water was added.

The results showed that adding these waste powders significantly changed the behavior of the cement. In every case where cement was replaced, the total amount of heat released over five days dropped. The more powder they added, the less heat was produced. When they replaced 30% of the cement with recycled concrete powder, the total heat released was only about 70% of what a standard mix would produce. This reduction is particularly valuable for construction in hot climates, where excessive heat can damage the structure before it fully sets. Beyond just reducing the total heat, the waste powders also slowed down the speed of the reaction. The moment when the heat release peaked—the point of most intense chemical activity—was delayed by two to three hours compared to ordinary cement. This delay gives workers more time to pour and shape the concrete before it begins to stiffen.

The researchers also looked closely at the different stages of the chemical reaction to understand why these changes happened. They found that the type of waste powder mattered. The recycled ceramic powder, which is very fine, shortened the initial nucleation and growth phase of the reaction, whereas the brick and concrete powders prolonged this phase. However, regardless of the powder type, the peak heat flux was delayed by 2 to 3 hours for all mixtures. The study revealed that the reaction involves a sequence of events: first, new crystals form and grow; then, these crystals interact with the remaining cement grains; and finally, the reaction slows down as ions must travel through the solidifying material to find new places to react. The waste powders altered the timing of these steps. For instance, the brick and concrete powders extended the initial crystal growth phase, while the ceramic powder made the final, slow diffusion phase much more dominant.

Ultimately, the study confirms that these fine fractions of construction waste are not just inert fillers but active ingredients that change how cement behaves. The waste powders possess properties that allow them to participate in the hardening process, creating a material that is chemically active yet produces less heat. The findings suggest that using up to 30% of these recycled powders is a viable strategy for creating more sustainable building materials. By reducing the heat generated during construction, these mixes could help prevent cracking in large structures, while simultaneously diverting tons of waste from landfills. The research provides a clear picture of how these materials work, offering a practical path forward for making the construction industry more efficient and less environmentally damaging.