When
cement or
concrete are exposed to and ions simultaneously present in
seawater, the
precipitation of the poorly
soluble brucite contributes to enhance the formation of
gypsum in the sulfate attack : : The precipitation of insoluble helps to considerably drive the
chemical equilibrium of the reaction to the right. It exacerbates the
sulfate attack resulting in the formation of
gypsum and
ettringite (an
expansive phase) responsible for the
mechanical stress in the hardened cement paste. However, brucite, a phase with a small
molar volume (), may contribute to clogging the porous network in the hardened cement paste, hindering the diffusion of these harmful reactive species in the cement matrix. This can delay the decalcification of the
C-S-H phase (the "glue" phase in the hardened cement paste responsible for the cohesion in concrete) and its transformation into an M-S-H phase. The exact mechanism of brucite degradation of hardened cement paste remains a matter of debate. If brucite had a high molar volume, it could be
de facto considered a swelling phase (like
ettringite, or highly hydrated minerals), but this does not appear to be the case. It is unclear if it causes expansion or not, and how. If it replaces another phase locally (topo chemical replacement), in cases where its molar volume is smaller than that of the phase it replaces, no expansion is expected; rather, a decrease in
porosity is anticipated. However, if it crystallizes in a large number of tiny crystals growing between existing ones, even with a small molar volume, it could exert a considerable crystallization pressure in the cement matrix, resulting in tensile stress, expansion and cracking. Anyway, prolonged contact between
seawater, or naturally rich Mg-
brines, and concrete may induce durability issues for regularly immersed concrete structures, and their components, especially if they also contain
steel reinforcements (
pitting corrosion caused by
chloride ions). The use of
dolomite as
aggregate in concrete made with a cement with a too high alkali content can also cause brucite precipitation, driving the dedolomitization reaction, as observed in the
alkali-aggregate reaction. : Consequently, the use of dolomite is prohibited as aggregate for concrete. == Gallery ==