New population of refractory concretes: Grain size composition and volume factors

Abstract

Previous articles in this series [1][2][3] showed that one of the outstanding characteristics of new refractory concretes -low-cement refractory concretes (NTsOB) or cast vibration thixotropic refractory bodies (NVTOM) -is their high density. Together with other factors the density is largely determined by the choice of the optimum grain-size composition.Selection Principles and Grain Distribution Curves. The choice of grain size in refractory concretes is governed by both production and service requirements. Bearing in mind the need for the minimum water requirement of the concrete mixture and the minimum porosity, the problem is one of obtaining the maximum packing density of the components. On the other hand, concrete mixtures should possess the necessary production properties (mobility and placement facility with the optimum vibration-compaction cycles, the absence of dilatancy preventing mixing and placement). Moreover, the choice of the optimum grain-size composition helps save cement -the most expensive component. In many applications of these concretes it is also true that excess cement impairs the properties of the concrete.The grain-size composition of refractory concretes to a marked degree determines their shrinkage during drying (setting) and in service, the thermal-shock resistance, and the relationship between temperature and strength. Uncompensated shrinkage [4,5] has a special influence on the properties.Questions of the choice and compilation of a grain-size composition for the ceramic concretes and low-cement refractory concretes have been examined in many papers [2,[4][5][6][7][8][9][10][11][12].The production of the new type of concretes usually involves a mixture of continuous composition which is more suitable technologically than discontinuous mixtures. The packing effectiveness can be improved by using equations to determine the best relationship between the contents of the various fractions and the maximum grain size in the mixture dma xIn particular, in several projects with NTsOB the grain composition was chosen taking account of the maximum packing density curve according to the Furnas equation [13][14][15][16][17]. As an example, Fig. i, using data in [13][14][15], presents the optimum curves for mixtures. It follows from curve 6 that for a concrete with dma x = 4.8 mm the content of particles <74 Dm should be 37.6%, and <0.5 ~m 4%. With an increase in dma x there is a proportional increase in the median diameter d m of the mixture, and also the polydispersion factor Kp [5]. Thus, when dma x equals 4.8, 1.2 and 0.3 mm, then Kp = Ks0/K20 is 160, 84, and 50 respectively. In accordance with this, with an increase in dma x for concrete with dense or porefree filler, there is a reduction in the porosity of the concretes. Other "ideal" grain-size distributions curves exist, and are analyzed in [5,18].Analysis of grain distributions of ceramic concretes, e.g., in [19,20], shows that they are close to the curves shown in Fig. i. Thus, ceramic concretes based on high-colloid (VKVS) mullite an...