Cement-Free Refractory Concretes for Stress-Reduced Structures

Dedicated to the 10th Anniversary of refractories WORLDFORUM Refractory concretes have significantly increased their importance for the production of steel, aluminium, cement, lime and the like. Thus, more and more their properties have come into focus, so not only the installation behaviour is important, but drying-out and heating-up procedure and performance regarding thermomechanical and thermochemical behaviour as well. The thermochemical behaviour is mostly characterized by the chemical/mineralogical composition, while the drying-out and heating up behaviour and the performance are also based on microstructural param-eters. Here, cement-free concretes of the sol-gel- and geopolymer-bonded types are the latest state of the art. Their heating-up behaviour can be described as easy to dry, spalling due to steam explosions are largely avoided by reduced contents of crystalline-bonded hydrate water. Faster heating rates save money, time, and energy and are thus ecologically and economically advantageous. Furthermore their perfor-mance during use is characterized by less thermomechanical tensions in the monolithic lining, resulting in an improved performance and longer lifetime. 1 Introduction and definition The ecological and economic production of cement and steel in highly efficient plants is not possible today without the use of high-performance refractory materials. Especially the monolithic materials (refrac-tory concretes, castables, gunnables) have undergone a tremendous development in the recent years. The resultant refractories (vibration, dry-gunning, wet-gunning, self-casting), together with the ever-increasing equipment development, [1–3], not only can match a brick installation, but sometimes even outperforms these linings. This has led to an increase of monolithic installations at the cost of brick linings (Fig. 1, [4]). With bricks and concretes all requirements for steel, aluminium, cement, lime and other industries regarding perfor-mance, lifetime, installation time, geomet-ric requirements, etc. can be fulfilled. Any concessions regarding performance do not have to be made when a monolithic lining is  chosen. Fig. 1: Share of refractory concretes on the world production of refractories The heating-up behaviour of cement-bond-ed concretes requires a slow temperature rise including some optional dwell times between 150–500 °C to evaporate the physical and crystalline-bonded water with-out any structural damage. The stresses in-duced by steam-pressure must not exceed the actual temperature-dependent strength of the material during the heating-up process. The correct heating-up is as important as the correct installation of the concrete itself. On the other hand, an unnecessary long heating-up wastes money, resources, and energy. Fig. 2: Typical microstructures of a low cement, and of a no-cement concrete Fig. 3: Pore size distribution and permeability of a low cement, and of a no-cement concrete Cement-free concretes, or so-called No-Ce-ment Concretes (NCC), contain up to 0,2 % CaO, at least one deflocculant, and > 2 % ultrafine particles < 1 µm, [5]. NCC can be bonded inorganically (chemically/mineral-ogically e. g. by phosphates, water glass, acidic binders, geopolymers, chlorides, sol-gel systems – typically based on silica sol, sulphates, frits, glasses etc.), or organically (e. g. by lignin sulfonate, pitches, resins, or other organic matter). Recently developed NCCs based on sol-gel technology and geopolymer technology are of increased interest in all industries due to altered and more severe requirements on the refractory lining. This increasingly restricts the application of concretes containing calcium aluminate ce-ment (Low Cement Concretes LCCs). 2 Properties of cement-free refractory concretes 2.1 Microstructure and its influence on heating-up behaviour As mentioned above, under economic constraints the heating process has to be as short as possible to allow a production without wasting time and energy [6]. Crucial for cement-free concretes is a high ratio between physically and chemically bonded water (the latter has to be as low as possible) in combination with a microstructure characterized by a high perme-ability for low steam pressure during drying and heating-up [7]. The pore size is be-tween 0,1–10 µm (especially with a higher share of medium-sized pores > 1 µm) and a specific surface area mainly between 4,2 –5,8 m2/g. This results in a high permeabil-ity (measured by the RILEM method) as-shown in Fig. 2–3. Contrarily, the binding phase of LCCs is dense and compact, and the size of mi-cropores < 1 µm is increased. Their gas permeability with a value of approx. 3,5 m2/g is low [8].
The cement phase present in the LCCs holds the main part of the crystalline-bonded wa-ter in the calcium alumina-hydrate phases monocalciumaluminate (CAH10) and dical-ciumaluminathydrate (C2AH8). When be-ing heated too fast, these phases release high amounts of steamy water in a closer temperature range between 200–500 °C  (Fig. 4) causing explosions of concrete structures. The total vapour pressure and the vapour pressure difference along the lining thickness in cement-free concrete linings is much lower than the ones containing calcium aluminate cement [9]. Fig. 4: Chemically bonded water in a low cement concrete, and in no-cement concrete A stress-reducing structure has thus to be free of calcium aluminate cement to prevent stresses during drying out and heating-up. This can also be seen by the influence of crystalline water on strength: more than 85 % of the water in NCCs is physically bonded and can leave the lining at temper-atures < 150 °C. Only the remaining water (less than 15 %) is chemically bonded and contributes in a low amount to stresses in the drying out and heating-up phase above 150 °C. A setting for 24 h, recommended for cement-containing concretes, is not nec-essary for cement-free concrete linings. The specially adapted microstructure of NCCs (Fig. 2) permits a half-time drying period compared to that of calcium aluminate ce-ment-containing LCCs. Heating rates up to 60 °C/h can be realized (Fig. 5). Fig. 5: Allowable drying procedures for no-cement, and low cement concretes 2.2 Mechanical properties of cement-free concretes The phase of drying and heating-up is just the (inevitable) transition to the main use of a refractory lining, its performance under process conditions at high temperatures. The selection of the concrete itself (compatibility to the kiln feed material, slag resistance, CO-resistance) can be cleared by knowledge of the staff of the heavy industry plant and the refractory manufacturer (aid-ed by laboratory tests, application of ternary systems, etc.). A parameter, which is much more difficult to estimate regarding its influence on the performance of a lining, is the mechanical load. Thermomechanical properties are es-sential for the compensation of mechanical stresses, i.e. for a stress-reduction in the refractory lining. The application of stressreducing structures is the basis for a reduc-tion of mechanical influences. Typical loads are stresses due to excessive temperature, deformation of vessels (ladle, rotary kiln), thermal shocks, etc. There are several works done to calculate these stresses, e. g. by fi-nite element methods, but a transfer to real production conditions is still not easy to do. As a result, in general a refractory material is chosen on the basis of its cold crushing strength or modulus of rupture (e. g. by the data sheet values). A similarly important pa-rameter, which is usually not appropriately recognised, is the elastic behaviour of the refractory, mostly Young modulus E or shear modulus G (connected by Poisson’s ratio µ). Of even more importance is the ratio of strength (cold crushing strength σD) and Young modulus E, as it determines the stresses originating from thermal expan-sion, thermal shocks, and in the case of a rotary kiln, from the kiln shell ovality, [10]. In each case, the ratio σD/E has to be as high as possible (strength itself has to still withstand the requirements of the installa-tion) to keep the stress absorption capacity as high as possible. Only when the ratio σ /E is high, loads by thermal expansion, rapid temperature change and mechanical impact due to vessel deformation can be absorbed elastically without the unwanted appearance of macro-cracks (another possi-bility would be a stress absorption by plastic deformation [11]). Mechanical characteristics of cement-free- and cement-containing refractory concretes up to a temperature of 1600 °C determined at room temperature after firing are shown in Fig. 6. At temperatures < 800 °C, low-cement refractory concretes exhibit very high strengths, induced by the special reactive compounds cement, alumina, and microsilica. The strength of cement-free sys-tems in this temperature range is lower, but still more than sufficient for all refractory concrete applications. Generally, cement-containing and cement-free binding phase concepts show a strength increase at tem-peratures above 800–1000 °C. Fig. 6: Mechanical properties of no-cement, and low cement concretes Above these temperatures, several calcium-aluminate-cement-containing LCC products in particular typically exhibit a high modu-lus of elasticity and thus a high brittle-ness, characterized by a high cold crushing strength and simultaneously a high Young’s modulus. Contrarily, cement-free products show a lower strength and a disproportionally lower Young’s modulus and consequently a high microstructural elasticity at high temperatures. This results in a higher ratio of σD/E and superior performance in areas subjected to thermomechanical stresses. Thermomechanical properties determined at higher temperatures shows this as well (Fig. 7). Fig. 7: Hot modulus of rupture of no-cement concretes, and low cement concretes While it can be stated that all concrete grades are generally suitable for their typi-cal applications, for several applications a careful selection of the concrete grade can improve the performance (lifetime) of the lining. For highest refractoriness, sol-gel concretes offer a very good solution, while geopolymer-bonded concretes offer advan-tages in stress-compensation because of the lower gradient of properties. Due to the formation of CaO-containing minerals (anorthite), the hot strength of LCCs is lower, but at temperatures below 1000 °C their high strength still can be uti-lised if a high abrasion resistance is required. A similar behaviour is observed for the refractoriness-under-load test according to [12] (Fig. 8) i. a. for a bauxite concrete with 75 % Al2O3.
Sol-gel concretes offer the highest refrac-toriness or the lowest thermomechanical deformation. Cement-free concretes based on the activated geopolymer system and LCCs are characterised by a higher defor-mation due to the presence of alkali/earth alkali-containing compounds; still the NCC is able to compensate mechanical stresses much better than the low cement concrete, thanks to its thermoelastic behaviour. LCCs are state of the art, and cement-free concretes already have reached this standard. The installation of all materials is similarly unproblematic if the installation in-structions of the manufacturer are obeyed. Fig. 8: Refractoriness-under-load of typical no-cement, and low cement concretes based on bauxite 2.3 Chemical resistance of cement-free concretes The chemical resistance of refractory mate-rials depends mainly on the chemical and mineralogical composition of the base ma-terial. Additional influences are the struc-ture (mostly the porosity) and additional compounds like SiC, forming a protective layer on the surface of the refractory lin-ing and preventing damage due to alkali-alumomsilicatic reactions (accompanied by volume-expanding reactions like formation of feldspars and feldspathoids). Investiga-tions on the alkali resistance are manifold, the behaviour is mostly satisfactorily inves-tigated [13, 14]. Still, one reaction which may need more in-vestigations in the future is the resistance to acids, especially acids which contain sulphur (sulphuric acid, sulphurous acid). These can be formed in the kiln by the increasing use of sulphur-containing alternative fuels. The acid corrosion can occur if sulphuric acid is formed from SOx and H2O during cooling of process gases. The dew point of sulphuric acid depends on the concentration of the acid component and the gas humidity; it is always above the dew point of water. To avoid acid corrosion, the inner surface temperature of the steel shell material must be maintained above the dew point, char-acteristically above 140 °C. If this is not the case, then sulphuric acid can form. In refractory concretes, a deterioration of the lining by acid attack is directly related to the content of CaO. Calcium oxide and its mineralogical com-pounds in concretes can be attacked by sulphur-containing acids. Formation of langbeinite minerals and calcium sulphate is possible, accompanied by volume change and destruction of the binding phase. If the actual strength is exceeded, spalling and lining destruction takes place. Fig. 9: Advantageous acid resistance to H2SO4 of no-cement concretes compared to a low cement concrete A comparison of cement-free NCCs (sol-gel- and geopolymer-bonded) and an LCC shows the advantage of a CaO-free system after exposure to sulphuric acid at 20 °C to different acidic concentrations and ex-posure times (Fig. 9). The LCC is destroyed due to the attack on the binding phase, the CaO-free NCCs are nearly unaffected by the acid attack. As a result, stress reduc-tion and an increased chemical resistance is achieved by bonding phase modification. 3 Applications in the heavy industry Typical industrial applications of these stress-reducing monolithic linings can be found in plants of the heavy industry. 3.1 Application in the steel industry While the lining in converters or ladles shift-ed generally to a basic one due to increased thermochemical loads, some areas are preferably lined with non-basic concretes.(Fig. 10). The lip ring of a ladle was lined with a cement-free sol-gel-bonded refrac-tory concrete (Fig. 10, l.). After 75 cycles the lining had to be refurbished, exceeding the service life by roughly 10 % due to a reduc-tion of the mechanical stresses (Fig. 10, r.). No premature spalling was observed. This clearly shows the effect of stress reduction (in any form, thermal, mechanical, chemical) on the performance of refractory linings. Fig. 10: Installation of a sol-gel-bonded concrete after 1st heating-up (l.), and after 63 cycles 3.2 Application in the aluminium industry For the repair and relining of aluminium melting furnaces no-cement sol-gel-bond-ed concretes provide several advantages. Fig. 11 shows a service repair of an alumini-um melting furnace after 4 years of produc-tion due to a premature wear in the bottom and the ramp. A sol-gel-bonded concrete was installed on the existing lining to allow only a short stop and a quick restart of the production. Fig. 11: Service repair of an aluminium melting furnace After cooling down and breaking out, the furnace was relined. Apart from the use of the liquid silica sol the procedure of mixing, casting and vibrating is comparable to that of any other typical refractory concrete. Due to the perfect adhesion and bonding to the existing refractory lining no special pre-treat-ment was required. A repair of the safety lining was not necessary. After installation and a short drying time at ambient temperature the heating-up procedure was started. After only 22 h of heating-up and 12 h of sintering with liquid metal at 850 °C the furnace was ready to return to production. In total the complete relining of the bottom and the ramp took only 1 week [15]. 3.3 Applications in the cement industry The superior performance of cement-free concretes in areas subjected to mechanical and thermomechanical stresses are proven by installations in the nose ring and the kiln outlet of a cement rotary kiln (Fig. 12). The pictures were taken after 7 months and after 9 months. The lifetimes were more than 2 years and more than 1,5 years, re-spectively, exceeding the typical lifetime of the previous installations by more than 3 months. Key to the longer lifetime is the higher mechanical resistance of cement-free concretes and their advantageous stress absorption capacity. Fig. 12: Outstanding performance of cement-free geopolymer-bonded refractory concretes in the nose ring/kiln outlet of a rotary cement kiln 4 Conclusion Cement-containing refractory concretes (ultra low cement, low cement, medium cement, regular cement castables and gun-ning concretes) are still state of the art for lining kilns and vessels of the heavy indus-try. Still, during service some of their disad-vantages become obvious, e.g. sensitivity to stress, acid, and rapid heating-up. Latest developments in the refractory con-crete technology showed that these disad-vantages can be significantly reduced when concretes with a cement-free bonding are installed. The development of cement-free concretes turned mainly into two philoso-phies. The first is a two-component sol-gel system: a sol (mostly a colloidal silica sol) is added to the dry refractory concrete sup-plying workability and generating the necessary strength. The second one is a one-component system based on an activated geopolymer bonding. Both systems are characterized by the ability of a high stress absorption potential thanks to the mechani-cally advantageous ratio of strength to Young’s modulus. Mechanical stresses are absorbed elastically much better compared to LCCs despite the higher strength of the latter. Installations in rotary kilns of the ce-ment industry and in ladles of the steel in-dustries prove this impressively. As the amount of hydrate water in cement-free concretes is significantly reduced, spallings during the heating-up proce-dure are mainly a thing of the past. Rapid heating up of metallurgical vessels saves energy, time, manpower, and money. The installation of NCCs thus even helps to reduce stresses on the environment, as energy-wasting heating-up procedures can be shortened and the lifetime of the lining itself prolonged. Although this already is a tremendous success story for refractory development, research on refractory concretes will generate new and improved properties for the benefit of the heavy industry produc-tion in the future. References [1] Allen, William G.: State-of-the-Art Refractory Shotcrete Techniques and Practices. Shotcrete Spring 2008, 20–24 [2] Beimdiek, K.: Wet-gunning technology: Practical experience as a refractory system solution in ce-ment plants. ZKG International 28 (2005) [11] 48–58 [3] WO2018/108679A1: Mixing nozzle for a gunned concrete application device, gunned concrete application device having such a mixing nozzle, and gunned-concrete application method. June 21, 2018 [4] PRE Annual Statistics 2015.The European Re-fractories Producers Association, Brussels 2015 [5] Routschka, G. (ed.): Handbook of Refrac-tory Materials, 4th edition, Vulkan Verlag, Essen 2012, 120 [6] Groger, P.; Klischat H.-J.; Puntke, S.: Influence of the refractory lining on the cost efficiency of clinker production. Cement Int. 15 (2017) [3] 66–73 [7] da Luz, A.; Braulio, M.; Pandolfelli, V.: Drying behavior and design of refractory concretes F. I.R.E. Compendium series, refractory concrete engineering, Göller Verlag GmbH, Baden Baden, (2015) 317–418 [8] Beimdiek, K.; Kesselheim, B.; Klischat, H.-J.; Schemmel, T.: Cement-free refractory concretes as technology leap for several industries and applications. Proceedings Unitecr 2017, 6th Bi-ennial Worldwide Congress, Santiago de Chile, 26–29 September [9] Schemmel, T.; Thieser, G.; Pfitzmer, N; Kremer, U.: A novel method of online measurement to develop specific heating-up procedures for industrial furnaces. 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– Differentialverfahren mit steigender Tem-peratur (ISO 1893:2007) (Refractory products
– Determination of refractoriness under load
– Differential method with rising temperature (ISO 1893:2007); Deutsche Fassung EN ISO 1893:2008, Beuth Verlag, Berlin 2008 [13]  Leupold, H.; Santowski, K.; Wieland, K.: Verbesserung der Alkalibeständigkeit feuerfester Werkstoffe aus dem System SiO2–Al2O3 für Zementdrehrohröfen im Temperaturbereich bis 1300 °C. Proc. 26th Int. Coll. Refractories, Aachen, Oct. 1983, p. 236-256 [14]  Liever, H.; Klischat, H.-J.; Wirsing, H.: Alka-libeständige Zustellung der Sicherheitszone von chemisch belasteten Zement- und Kalkdrehöfen. Alkali-resistant linings for the security and preheating zones in rotary cement and lime kilns subject to chemical attack. ZKG Interna-tional 55 (2002) [6] 66-75 [15]  Schemmel, T.; Jansen, H.; Kesselheim, B.: Na-nobond – The new cement free castable for quick lining and fast repairing. International Scientific Conference, Refractories, Furnaces and Thermal Insulations, High Tatras, Slovakia, 2012 Kai Beimdiek, Hans-Jürgen Klischat
Refratechnik Cement GmbH Bertram Kesselheim, Thomas Schemmel
Refratechnik Steel GmbH
37079 Göttingen
Germany Corresponding author: H.-J. Klischat
E-mail: hklischat@refra.com
www.refra.com