The process by which magnesium oxide expansion agents (MgO) expand in concrete through hydration is the core mechanism by which they function as shrinkage-compensating concrete expansive agents. The primary component of magnesium oxide expansion agents is magnesium oxide (MgO), typically prepared through calcination and grinding of natural minerals such as magnesite and dolomite. When magnesium oxide expansion agents are added to concrete and come into contact with water, the surface of the magnesium oxide particles begins to hydrate with water, forming magnesium hydroxide (Mg(OH)₂). This reaction is the chemical basis for the volume expansion of magnesium oxide expansion agents.
In the initial hydration reaction, extremely fine, hexagonal, and flaky magnesium hydroxide crystals form on the surface of the magnesium oxide particles. These crystals grow closely to the surface of the magnesium oxide particles, with diameters of approximately 100 nanometers and thicknesses of only 10 to 20 nanometers. Due to their tiny size, they form a dense layer of magnesium hydroxide on the surface of the magnesium oxide particles. As the hydration reaction continues, the magnesium hydroxide crystals gradually grow thicker, transforming into hexagonal prisms or tetragonal bipyramidal crystals. During this process, the volume of the magnesium hydroxide crystals increases significantly, beginning to compress the surrounding magnesium oxide particles and cement paste.
The growth of the magnesium hydroxide crystals not only increases their own volume but also produces an additional expansion effect through interactions between the crystals. On the surface of the magnesium oxide particles and within the surrounding pores, hexagonal, flake-shaped magnesium hydroxide crystals overlap, generating crystal growth pressure. This pressure acts on the pore walls, expanding the pore space and, in turn, increasing the macroscopic volume of the cement paste. Furthermore, the magnesium oxide expansion agent particles themselves have a loose, porous structure filled with tiny magnesium oxide crystals. When these crystals hydrate simultaneously, the resulting magnesium hydroxide contacts each other, further increasing the volume of the magnesium oxide particles and thus expanding the surrounding cement paste structure.
The expansion properties of the magnesium oxide expansion agent are closely related to the stability of its hydration products. The decomposition temperature of magnesium hydroxide is as high as 350°C, far above the temperature range of concrete's operating environment. This means that magnesium hydroxide can remain stable in concrete for a long time and will not decompose or lose its effectiveness due to temperature fluctuations. Therefore, after magnesium oxide expansion agents are added to concrete, their expansion effect is long-lasting, effectively compensating for autogenous, drying, and thermal shrinkage during the hardening process, thereby reducing cracking.
The expansion rate and amount of magnesium oxide expansion agents can be controlled by adjusting their preparation process. Calcination temperature and residence time are key factors influencing magnesium oxide activity. Higher calcination temperatures and longer residence times result in fewer lattice defects in magnesium oxide, larger grains, lower hydration activity, and slower hydration rates. By controlling calcination conditions, magnesium oxide expansion agents with varying activity levels can be prepared, allowing the expansion rate and amount to be tailored to specific project requirements. This adjustability allows magnesium oxide expansion agents to meet the shrinkage compensation requirements of different concrete types.
In practical engineering applications, magnesium oxide expansion agents are particularly suitable for large-volume concrete structures. During the hardening process, large-volume concrete experiences large temperature gradients due to the heat released by cement hydration. This results in internal temperature increases and external cooling and contraction, leading to the risk of cracking. The slow expansion of magnesium oxide expansion agent allows it to match the shrinkage rate of concrete, continuously providing expansion compensation during concrete cooling, effectively reducing thermal stress and preventing cracks. Furthermore, magnesium oxide expansion agent can be used to compensate for concrete shrinkage due to drying and water loss, improving concrete's crack resistance and durability.
