CHEMICAL-MINERALOGICAL CHARACTERISATION OF BELITES FROM EXPERIMENTAL SAB CLINKERS

Abstract

Guided by theoretical (sulphoaluminate belitic) SAB clinker chemical compositions, raw materials of known chemical composition (locally sourced from the Carpathian orocline), were mixed and tested by burning in laboratory conditions at different temperatures 1220, 1240 and 1260°C. The resulting materials were analyzed by optical transmission and reflection microscopy (using HF etching) and completed with XRD analyses. In this paper, we describe chemical and mineralogical characterization of clinker belitic components manufactured in laboratory conditions. Belite nests and clusters with complex zoned structure are observed optically in interstitial material of the clinker that is composed of sulfoaluminate, ferrite and anhydrite. These structures display all three morphological types of belite, corresponding to the different polymorphs of belite, depending on the burning and quenching conditions of the clinker. The XRD patterns are interpreted to show that the appearance of β- and α’-belite and different α’àβ transition forms are due to cooling to room temperatures and presence of the belite polymorph stabilizing ions SO₃, Al³⁺, Fe³⁺ and alkali. The conclusions of the study were that belites were formed in various mixtures of raw materials treated at low burning temperatures, relatively slow cooling rates (resulting in different proportions of impurity absorption) and with some experiments run under reducing conditions during firing. Chemically, the lime saturation factor of clinkers is low (LSF= 0.74 - 0.84), such compositions favoring the clinker belite component; the silica ratio is low (SR < 1.5), which can improve the burnability of the clinker; the alumina ratio is medium (AR= 1.5 - 2.5), chemically restricting the solid solution of mineral phases, and the hydraulic modulus (HM) is under 1.7, enhancing resistance to chemical attack.