Think of all the concrete structures you encountered today, like buildings, roads, tunnels and bridges. Concrete is important to modern society, but something as harmless as everyday table sugar can set these mighty structures tumbling.
Sugar interferes with the cement binding process in the concrete. Note that concrete and cement are not the same. Concrete is made by combining cement, water and a filler material called an “aggregate,” like gravel or sand. Most cement is made from limestone and clay. It reacts with water to form a hardening paste that binds the aggregate together to produce hard concrete. This hardening process is called hydration.
One theory suggests that when the concrete mixture contains sugar, the sugar molecules attach themselves to the hydrating cement and inhibit the chemical reactions involved in stiffening the material.
A different theory, called the “precipitation theory,” suggests that the addition of sugar increases the concentrations of calcium, aluminum and iron in concrete. The sugar molecules combine with these metals to form insoluble chemical complexes that coat the cement grains. Several key chemical processes that harden the concrete are then impeded. Hydration slows down the process and the concrete takes longer to set. For this reason, sugar is known as a retarder. Retarders increase the setting time of concrete.
All sugars do not retard cement hydration equally. Refined white sugar is one of the best retarders. Lactose, the sugar found in milk, is a moderate retarder. Trehalose, a sugar produced by various fungi, is non-retarding and does not affect setting time.
Different theories and chemical processes are used to explain retardation for each type of sugar, and one or more mechanisms may be operating in the concrete. No one theory can explain the behaviour of all sugars in concrete under all conditions.
