Highly concentrated ceramic bonding suspensions. Mechanism and features of structure formation during drying

Abstract

The processes of the formation of a solid material in VKVS (highly concentrated ceramic bonding suspensions) technology are characterized in the light of a number of changes, accompanied by alterations in the nature of the bond between the particles [1 ]. The primary structure formation of VKVS is accomplished during their partial dewatering, for example, on account of the porous mold, the filler, etc. The green article obtained from VKVS of acid or acidamphoteric composition, as a rule, is characterized by a rather high mechanical strength and elastobrittle destruction. Despite the fact that, both for the original casting in which all the pores are filled with liquid (volume proportion of kinetically bonded liquid Cws), and also for the casting after partial drying (right upto a moisture content of 0.5-I%), there is a characteristic lack of water resistance [2]. The water resistance, taken as the criterion for assessing the bonding properties of VKVS [3] is attained after drying the material to a moisture content of less than 0.1-0.2% [2]. The latter is explained mainly by the formation in the system of siloxane bonds. The probability of their formation between the microparticles of colloidal particles and the active colloidal covering of the grains of mineral constituent of VKVS in the undried material is low.Previously [2] using high-precision indicators for linear displacement it was shown for the first time that during the drying out of castings of siliceous composition, after a certain shrinkage, there is an expansion. It was postulated that it is due to polymerization of the interstitial silicic acid.Starting from the assumption about the interaction of the expansion of the casting and the water resistance and strength after drying, in the present article we deal with the problem of investigating the mechanism and features of the structure formation during drying out, bearing in mind the influence of such parameters as the type, dispersion and deduced concentration of the colloidal component cPrkk, and the critical concentration C v cr of the original VKVS.The linear changes in the castings in relation to their moisture content were studied on equipment shown in Fig. 1. The use of the induction sensor was useful for registering the change in the linear dimensions of the casting with an error of less than _+ 1 #m.The original VKVS (see Table 1) were studied in the form of suspensions of fused quartz, ordinary chamotte (27.2% A12Os), and mixed suspensions in the system ordinary ehamotte--quartz sand in the ratio 85:15 [4]. The suspensions were obtained by wet milling using the known technology, and stabilization [3]. The specimens measuring 10 • 10 • 8 mm were shaped in gypsum molds. Directly after the completion of the casting up of the specimen the casts were extracted, and then one of them was established on the equipment for measuring the linear dimensions, and using two parallel measurements we established continuous weighing in order to construct the relationship ACre 1 = f(W).Taking into account the ...

New-generation refractory concretes. Interrelation between structure formation and strengthening

“…First of all, these binders are characterized by the fact that the shrinkage of castings (binder) occurs only at the fast drying stage (reduction of the moisture content by 1 -1.5% with the initial moisture content being 6-7%). The casting grows by 10-20% of the value of the shrinkage [12,18]. In the course of shrinkage at the first stage of water removal the stren~h of the binder growth intensely (by a factor of 2-2.5), which is natural in accordance with expression (9).…”

“…In the course of shrinkage at the first stage of water removal the stren~h of the binder growth intensely (by a factor of 2-2.5), which is natural in accordance with expression (9). The mean value of the thickness of the surface films on particles, which determines the shrinkage, lies within 2 -3 rim, i.e., is on the order often monomolecular liquid layers [18]. Further removal of water is not accompanied by intense growth of streng~, which seems attributable to the stresses caused by the forces bringing about the growth of the casting.…”

Refractory concretes of new generation. General characteristics of the binder systems

“…As was shown earlier [19], during the final stages of drying, the castings based on HCBS of acidic and acidic-amphoteric systems are characterized by some linear growth (up to 0.08%).…”

“…It is assumed that the mechanism is determined to a large extent by the capillary forces developed when drying the finely dispersed structure and by the development of a stress state in the system. This is indicated by the fact that the growth of the castings increases with increasing content of the colloidal component Ccc in the original HCBS [19].…”

“…To the best of our knowledge, this aspect has not been analyzed thus far with regard to argillaceous materials. Figure 6 compares the data concerning the granuiometric composition of two types of HCBS that are characterized by fairly high valeus of Kp (12 and 17 for curves 1 and 2, respectively) and kaolin and a ciay whose degree of dispersion was reported earlier [19] in a tabulated form.…”