Materials in the AI203--ZrSiO, system, as research has shown [1][2][3], are of great interest in view of their high thermal-shock resistance and strength, including performance at elevated temperatures [4].Two methods are known for obtaining suspensions of mixed composition in the AI203--ZrSi04 system: staged suspension and mixing of suspensions. Using both methods, the starting materials consist of powders, dry milled in metal mills, followed by washing to remove tramp iron in hydrochloric acid [5]. In obtaining slips by the first method the ZrSiO~ is added to water with a regulated pH (2.0), and after 5-min mixing the alumina is added, followed by combined mixing for 30 min, and casting.Using the second method individual suspensions are first obtained with the optimum parameters; then they are mixed in the stated ratios [6]. The porosity of casts from the first method came within the range 45-48%, and for the second 35-40%.In this article we examine the results of deyeloping a method for obtaining mixed suspensions in the system AI20s (80%)--ZrSi04 (20%) with increased concentrations, their bonding properties, and the density of the cast articles.Taking previous work [7,8] into account, suspensions were obtained by direct combined wet milling in a ballmill using corundum linings and grinding bodies.The original materials were powdered electrocorundum (about 99% AI=Oa) as specified by OST 2MT71-5-78 and zircon (65.5% Zr02, 32.7% SiO=) as stated by OST 48-82-81 with a specific surface measured with the PSKh-8 apparatus equal to 4000 and 4500 cm2/g. To obtain a high concentration wet milling was done with portion charging of the material [7,8]; and in this case the additions of HCI were used to keep the pH of the medium close to the optimum for deflocculation (pH = 3.5-4.0). The first stages of grinding were done on the electrocorundum (as being the more difficult to grind).Before adding the 20% ZrSi04 the suspension had a density of 2.38 g/cm 3 (C V = 0.46), a viscosity of 1.5 ~ and a predominant concentration of particles measuring not more than i0 ~m. After the zircon had been added combined milling was done in 20 h.The density of the slip thus obtained was 2.60 g/cm 3 (C V = 0.54), water content 17%, and pH = 4.0, and was characterized by a relatively polydispersed grain size composition (Fig. i): the content of particles of diameter up to 5 Dm was 60%.The peculiarities of the resulting slips were the aggregative instability during storage (aging), and the marked coagulation during mechanical mixing.This was due to the fact that the pH of the suspension on standing increased into the nonoptimum region, which was accompanied by a rise in the rheological factors, and as a result in the porosity of the casting (Fig. 2).From an analogy with data in [9] the increase in the pH of the resulting mixed suspension can be explained by the contribution of A1203.It is known that an increase in the pH value of aluminous suspensions in the acid region according to [9] can be explained by the reaction NaA102 + 4HCI = NaCI § A...
