In recent years, it has frequently been observed at construction sites that, following the installation of refractory castable linings, the surface undergoes "self-pulverization"-manifesting as a growth of "white fuzz" (commonly referred to as efflorescence)-during the curing period. In mild cases, this phenomenon reduces surface strength and causes surface dusting, thereby compromising the quality of the lining material and its installation. In severe cases, it leads to a significant loss of strength in the refractory castable, impairing the lining's normal serviceability. This phenomenon is particularly prone to occurring in high-temperature, high-humidity environments.
The timing and severity of the self-damage phenomenon vary depending on the construction site environment. It typically appears one day after construction is completed and is prone to occurring under conditions of high temperature and humidity. Lightweight refractory castables with high cement content are more susceptible to this issue than heavyweight refractory castables with low cement content. The self-pulverization of refractory castables is primarily caused by factors that can be broadly categorized into chemical and physical processes.
1. Carbonation reaction of calcium aluminate cement
It is well known that the hydration reaction of calcium aluminate cement generally proceeds according to the following pattern:
As indicated by the above equation, there are four hydration products of high-alumina cement: CAH10, C2AH8, C3AH6, and AH. However, at room temperature, only the cubic-crystalline C3AH6 and AH are stable phases; the hexagonal-crystalline CAH10 and C2AH8 are metastable phases that exist only at lower temperatures and transform into C3AH6 as the temperature rises. The hydration of CA2 is similar to that of CA.
It is generally considered that both CAH10 and C2AH8 belong to the hexagonal crystal system; their crystals appear as plates or needles that interlock and overlap to form hard, intergrown crystalline aggregates. AH gel fills the voids within this crystalline framework, creating a dense structure that imparts high strength to the hardened cement paste. In contrast, C3AH6 belongs to the cubic crystal system; it often contains numerous defects-such as dislocations-and typically forms granular crystals with poor inter-crystalline bonding, resulting in lower strength for the hardened cement paste composed of this hydration product.
The densities of the four hydration products-CAH10, C2AH8, C3AH6, and AH-are 1.72, 1.95, 2.52, and 2.42, respectively. These values indicate that the densities of the metastable phases are significantly lower than those of the stable phases. Consequently, the phase transformation leads to increased porosity, the release of free water, and a weakening of the inter-crystalline bonding forces. Furthermore, as aluminum hydroxide transitions from its initial gel state to form gibbsite, its capacity to fill pores diminishes, resulting in a reduction in strength.
Thermodynamic calculations indicate that the following reaction can proceed spontaneously:
Equations (1) and (2) explain why the phenomenon of self-pulverization in refractory castables occurs frequently and is severe under conditions of high temperature and high humidity.
