Development of glass-ceramics by sintering and crystallization of fine powders of calcium-magnesium-aluminosilicate glass D.U. Tulyaganov a , M.J. Ribeiro b , J.A. Labrincha a, * a Ceramics and Glass Engineering Department UIMC, University of Aveiro, 3810-193 Aveiro, Portugal b ESTG, Polytechnic Institute of Viana do Castelo, 4900 Viana do Castelo, Portugal Received 8 October 2001; received in revised form 9 December 2001; accepted 19 December 2001 Abstract Natural raw materials normally used in the ceramic and glass industry were studied for the production of calcium–magnesium– aluminosilicateglassorglass-ceramicmaterials.Sinteringandcrystallizationprocessesoffinepowdersofparentglasswithchemical composition (wt.%) 46.00 SiO 2 , 15.90 Al 2 O 3 ,1.20Fe 2 O 3 , 0.42 TiO 2 , 23.50 CaO, 9.37 MgO, 0.04 Na 2 O,0.98K 2 O,1.95P 2 O 5 and 0.35CaF 2 werestudied.Crystallizationkineticsofglass-ceramicswascarefullyexaminedbyDTA,XRD,andSEMtechniquesand by dilatometric studies. The desired sequence of events, i.e. nucleation, sintering and devitrification occurred by heat-treating the glassy powder. However, crystallization of the parent glass did not follow phase diagram predictions, since anomalous appearance of akermanite phase was detected along with expected anorthite and diopside precipitation. A reasonable explanation for this unexpected observation is given. # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: A. Sintering; D. Glass ceramics; D. Silicates; Ca–Mg–Al 1. Introduction Glass-ceramics are normally obtained by a controlled crystallization process of suitable glasses. Internally or externally nucleation is promoted to develop micro-het- erogeneitiesfromwhichcrystallizationcansubsequently begin. As a result, the amorphous reservoir of the par- ent glass transforms into a uniform microcrystalline ceramic. The composition of the crystalline phases and the crystallite sizes define the properties of the final material. Therefore, the major components and the composition of parent glass are selected to ensure pre- cipitation of crystals that provide desired properties on a glass-ceramic [1–4]. In the case of internally nucleated glass-ceramics, the use of high specific surface glass-powders will act as uniformly scattered nuclei and no addition of a special nucleating agent is required. Subsequently densification of the glass-powder compact must take place prior to devitrification. This sequence of events starting near the end of the sintering stage comes before crystallization starts which allows dense materials to be obtained [5,6]. Other relevant aspects of the development of glass- ceramic materials include environmental and econom- ical issues. The use of cheap and abundant raw materials as glass batch constituents is logically pre- ferred [7–9]. In our previous work, a cross-section join intheglass-formingregionofthefluorapatite–anorthite– diopside ternary system that corresponds to a constant fluorapatite (4.8 wt.%) content, was selected for the determination of the base glass compositions [10]. The rangeofbaseglasscompositions,inwhichdiopsideand anorthite phases are able to precipitate during heat treatment, was then established. Such type of glasses tends to show easy surface crystallization. Sintering and crystallization behaviour of glass-powder with the nominal composition (wt.%) 47.31 SiO 2 , 15.71 Al 2 O 3 , 24.24 CaO, 9.66 MgO, 2.01 P 2 O 5 and 0.37 CaF 2 has beencarefullystudied.Theaimtoreproduce,ascloseas possible, the alkali-free glass composition by using nat- ural sedimentary rocks as starting materials has been attempted. As major changes, about 0.5% of alkali oxides and slightly more than 0.6% of Fe 2 O 3 were incorporated with natural raw materials. A frit was obtained by melting at 1350–1380  C. Different shaped samples were then obtained by low-pressure injection mouldingfollowedbysuitablede-bonding,sinteringand crystallization steps. The formation of two crystalline 0272-8842/02/$22.00 # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(02)00004-4 Ceramics International 28 (2002) 515–520 www.elsevier.com/locate/ceramint * Corresponding author. Tel.: +351-2343-70250; fax: +351-2344- 25300. E-mail address: jal@cv.ua.pt (J.A. Labrincha).