Belite

Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 (C2S in cement chemist notation). Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 (C2S in cement chemist notation). The name was given by Törneborn in 1897 to a crystal identified in microscopic investigation of Portland cement. Belite is a name in common use in the cement industry, but is not a recognised mineral name. It occurs naturally as the mineral larnite, the name being derived from Larne, Northern Ireland, the closest town to Scawt Hill where it was discovered. The belite found in Portland cement differs in composition from pure dicalcium silicate. It is a solid solution and contains minor amounts of other oxides besides CaO and SiO2. A typical composition: Based on this, the formula can be expressed as Ca1.94Mg0.02Na0.01K0.03Fe0.02Al0.07Si0.90P0.01O3.93. In practice, the composition varies with the bulk composition of the clinker, subject to certain limits. Substitution of calcium ions or orthosilicate ions requires that electric charges be kept in balance. For instance, a limited number of orthosilicate (SiO44−) ions can be replaced with sulfate (SO42−) ions, provided that for each sulfate ion, two aluminate (AlO45−) ions are also substituted. Dicalcium silicate is stable, and is readily prepared from reactive CaO and SiO2 at 300 °C. The low temperature form is γ-belite, or lime olivine. This form does not hydrate, and is avoided in cement manufacture. As the temperature rises, it passes through several polymorphic states: 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: 2 Ca 2 SiO 4 ⏟ Belite + 4 H 2 O ⏟ Water ⟶ 3 CaO ⋅ 2 SiO 2 ⋅ 3 H 2 O + Ca ( OH ) 2 ⏟ Portlandite {displaystyle {ce {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.