Alite

Composition and Structure: Alite in Portland cement has a complex composition with CaO and SiO as major components. The formula of alite includes minor amounts of other oxides like MgO. Substitution of calcium ions or orthosilicate ions in alite must maintain electric charge balance. Limited substitution of orthosilicate ions with sulfate ions is possible in alite. Alite composition varies with the bulk composition of the clinker within certain limits. Polymorphs: Tricalcium silicate is unstable below 1250°C but can be preserved at room temperature by fast cooling. Slow cooling of tricalcium silicate leads to the formation of belite and CaO. Polymorphs of tricalcium silicate differ structurally due to minor deformations. Hydration Steps: Alite is crucial in Portland cement for setting and early strength development. Belite contributes to late strength due to lower reactivity. High alite reactivity is achieved by retaining high temperature polymorphs. Initial hydration of C involves partial dissolution of superficial oxide ions. Calcium silicate hydrate (CSH) phase provides strength to the hydrated cement system. Alite as Precursor of Silicate Phases in Medieval Lime Mortar: Alite rich in CaO and poor in SiO may form directly by pyrolizing siliceous limestone. Medieval lime mortars with unexpected hydraulic properties contain silicate phases formed by hightemperature reactions. Tournaisian limestones rich in amorphous silica were used for building lime mortar in the Tournai area. Presence of wollastonite and rankinite in medieval lime mortars indicates hightemperature reactions. Hydraulic properties of lime mortar in Tournai may have been developed unintentionally. Detection: Refer to the article on belite for more information.