Efflorescence is a process that occurs when moisture within building materials, like concrete or masonry, moves to the surface and evaporates, leaving behind salts and minerals. Over time, as this moisture continues to penetrate the material and evaporate, it can lead to a buildup of these soluble salts which crystallize on the surface. This crystallization exerts pressure on the surrounding material, which can result in spalling, where small pieces of the surface material begin to flake or break off.
The presence of excess moisture is the key factor in this process. As moisture seeps into the building materials, it facilitates the transport of soluble salts to the surface where efflorescence occurs. This dynamic can lead to deterioration of the integrity of the materials over time, as the repeated cycle of moisture infiltration and evaporation compromises the structure.
Other options, while potentially relevant in the broader context of building material integrity, do not directly contribute to the specific mechanism that leads to efflorescence and subsequent spalling. Excess moisture is the fundamental cause that initiates the chain reaction resulting in these damaging effects.