Belite is the mineral in Portland cement responsible for development of "late"
strength. The other silicate,
alite contributes "early" strength, due to its higher reactivity. Belite reacts with water (roughly) to form
calcium silicate hydrates (C-S-H) and
portlandite (Ca(OH)2) according to the reaction: \underbrace{2 Ca2SiO4}_{Belite} + \underbrace{4 H2O}_{Water} -> 3 CaO . 2 SiO2. 3 H2O + \underbrace{Ca(OH)2}_{Portlandite} This rapid reaction is "chemically analogue" to the slow natural hydration of
forsterite (the magnesium end-member of
olivine) leading to the formation of
serpentine and
brucite in nature, although the kinetic of hydration of poorly crystallized artificial belite is much faster than the slow weathering of well crystallized Mg-
olivine under natural conditions. \underbrace{2 Mg2SiO4}_{Forsterite} +\underbrace{3H2O}_{Water} -> \underbrace{Mg3Si2O5(OH)4}_{serpentine} + \underbrace{Mg(OH)2}_{brucite } The
hydrate phase, [3CaO·2SiO2·3H2O], is referred to as the "
C-S-H" phase. It grows within
interlocking needles providing the strength of the hydrated cement system. Relatively high belite reactivity is desirable in Portland cement manufacture, and the formation of the unreactive γ-form must be rigorously avoided. This is achieved by rapid cooling, forming crystals that are small, distorted and highly defective. Defects provide sites for initial water attack. Failure to cool the clinker rapidly leads to inversion of belite to the γ-form. The γ-form has a substantially different structure and density, so that inversion leads to degradation of the crystal and its surrounding matrix, and can also trigger decomposition of the neighboring
alite. This is observed macroscopically as "dusting": the clinker
nodules fall to a fine
dust. == Detection ==