Cement-Free Refractory Concretes. Part 5. Cement-Free Refractory Concretes Based on Hydraulic Alumina Binders
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Vol. 61, No. 4, November, 2020
SCIENTIFIC RESEARCH AND DEVELOPMENT CEMENT-FREE REFRACTORY CONCRETES. PART 5. CEMENT-FREE REFRACTORY CONCRETES BASED ON HYDRAULIC ALUMINA BINDERS1 Yu. E. Pivinskii2,3 and P. V. Dyakin2 Translated from Novye Ogneupory, No. 7, pp. 25 – 35, July, 2020.
Original article submitted March 26, 2020. CFRC based on alumina hydraulic binders are similar with respect to hardening mechanism to low-cement refractory concretes (LCRC) and ultralow cement concretes (ULCRC). They are characterized by significant strengthening in the heat treatment temperature range 200 – 300°C and severe weakening in the range 600 – 1000°C. By introducing silica sols or microsilica into their composition it is possible not only to reduce or eliminate the weakening effect, but also to increase their strength after firing due to mullite formation. CFRC based on alumina binders have improved thermomechanical properties compared with LCRC. Comparative evaluation of CFRC based on hydraulic binders with other types of refractory concretes is provided. Keywords: low-cement (LCRC), ultra-low-cement (ULCRC), cement free concretes (CFRC), matrix system, alumina binder (AB), silica sol, microsilica (MS), thermomechanical properties.
300 and Alphabond 500 [10 – 13]. According to [11] the Al2O3 content is 91 and 84% respectively. Weight loss within them due to physically bonded water in the range 25 – 250°C is 2.9 and 6.0%, and due to chemically bonded water removed in the range 250 – 1000°C it is 6.0 and 9.2% respectively. Therefore, the overall bonded water content within these forms of alumina may be within the limits of 9 – 15%. Taking account of the agglomerated state of the original powder in spite of the nano-size of individual particles (d ~ 20 – 30 nm) their median diameter d50 according to different data is 2.3 – 3.6 and 5.2 – 6.3 mm for grades Alphabond 300 and Alphabond 500 respectively. According to [12] Alphabond 300 alumina has a particle specific surface Ssp of 195 m2/g, and true density 2.72 g/cm3. In view of the hydration mechanism of concrete hardening based on Alphabond alumina there is typically clear weakening during heat treatment. However, the deformation temperature for these concretes is considerably higher than for LCRC and ULCRC of comparable composition. Refractory concretes based on a p-alumina binder are intermediate between LCRC and ceramic concretes [4, 14 – 17]. For concretes based on Alphabond alumina there is typically a repeated reduction in ultimate strength in bending sben with an increase in temperature from 400 to 1000°C
A disadvantage of the forms of both LCRC and ULCRC as a rule containing microsilica (MS) is reaction with CaO contained within HAC. This is accompanied by formation of readily melting compounds, i.e., anorthite and helenite with melting temperatures of 1380 – 1450°C [1 – 4]. Therefore an important step in development of LCRC technology should be consideration of developing CFRC using hydration hardening binders based on material made from p-alumina [1 – 9]. A modificatio
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