Simulation of Cement Grinding Process for Optimal Control of SO3 Content

Cement is produced by co-grinding clinker, gypsum and other components defined in the Cement Standards like limestone, pozzolans, fly ash and slag. The grinding is usually performed in horizontal ball mills. Gypsum is typically the basic source of sulphates (SO3) in the cement. The effect of gypsum on the cement quality is critical and twofold. It affects two of the main properties of this product, setting time and strength.1 For this reason gypsum is added during cement grinding in the mill feeding, requiring a weight feeder of high accuracy. Clinker is mainly composed of four mineral phases: Tricalcium silicate (3CaO ∙ SiO2 or C3S), dicalcium silicate (2CaO ∙ SiO2 or C2S), tricalcium aluminate (3CaO ∙ Al2O3 or C3A) and tetracalcium aluminoferrite (4CaO ∙ Al2O3 ∙ Fe2O3 or C4AF). Bogue 2 in an historical study, established the mathematical formulae for the calculation of the clinker compounds – C3S, C2S, C3A, C4AF – and presented data for the heat of hydration of these compounds. During cement hydration, one of its main mineral phases, tricalcium aluminate, reacts with water at a high rate, which could result in a false set. Gypsum addition slows down this fast and exothermic reaction and false set is prevented during the concrete production, transfer and placing. Conversely, the addition of an excessive amount of gypsum leads to detrimental expansion of concrete and mortar. The optimum gypsum content in cement has been investigated by several researchers due to its importance. Lerch,3 in his excellent study, found that the optimum sulphates content in cement, with respect to mortar strength, is closely related to various parameters like hydration heat, length changes of mortar specimens cured in water, alkalis, C3A content, and cement fineness. The influence of SO3 on the hydration of the cement mineral phases has been investigated by several researchers.3–6 Soroka et al.4 concluded that gypsum accelerates the rate of hydration, when its addition is below the optimum SO3 content, but produces significant retardation when the addition exceeds the optimum. Jansen et al.5 analysed the changes detected in the phase composition during the hydration process. They concluded that the cement phases involved in the aluminate reaction (bassanite, gypsum, anhydrite and C3A) reacted successively. Several researchers have also investigated the impact of limestone addition to the hydration process and sulphates optimum.7–8 Tsamatsoulis et al.9 determined the effect of cement composition and mortar age on the optimum sulphates content using as criterion the maximization of compressive strength. Extensive experimentation has been performed to achieve this purpose by utilizing four cement types and measuring the strength at ages ranging from 2 days up to Simulation of Cement Grinding Process for Optimal Control of SO3 Content