MgO SiO 2 H 2 O bonded MgO castables

MgO SiO 2 H 2 O bonded MgO castables Part 1: Effect on flow, set and hot properties when substituting microsilica with alumina in pumpable MgO based castables. Cecilie Ødegård, Zhiqiang Chen, Bjørn Myhre, Elkem ASA Materials. Ningsheng Zhou, Sanhua Zhang, Luoyang Institute of Refractories Research. Presented at the Fourth International Symposium on Refractories in Dalian, China, March 24-28, 2003. The fine art of

MgO SiO 2 H 2 O castables Part 1: Effect on flow, set and hot properties when substituting microsilica with alumina in pumpable MgO based castables. Cecilie Ødegård, Zhiqiang Chen, Bjørn Myhre, Elkem ASA Materials. Ningsheng Zhou, Sanhua Zhang, Luoyang Institute of Refractories Research. Abstract MgO-based castables with MgO-SiO 2 -bond have been studied with respect to flowability, flow decay and hot properties. Fine calcined alumina is introduced by increased volume substitution of microsilica. Al 2 O 3 is added to promote spinel formation, which may be beneficial to the slag resistance of such mixes. It was found that the R.U.L tests gave T 0.5 above 1650 C for the MgO-SiO 2 and MgO-Al 2 O 3 systems. From HMOR testing, the MgO-Al 2 O 3 -SiO 2 bonding phase shows evidence of liquid formation at 1500 C. Two mixtures of suitable casting properties and refractoriness were chosen to be installed by wet shotcreting method. Introduction Basic castables based on MgO-SiO 2 -hydrate bonding. The development of basic self-flowing castables continues. Today, however, there is no widespread use of this concept due to some fundamental problems. The tendency of magnesia to hydrate, lack of a good binder and the thermal expansion of magnesia are some of the problems. An appropriate binder has been found for the basic castable. Microsilica and fine magnesia allowed to react with water give strong bodies 1,2,3. No calcium aluminate cement is needed. The set-reaction is considered to be formation of magnesia-silicate-hydrate phases, it has not yet been fully described. Improved flowability is achieved by using more than 4 wt% microsilica in these cement-free castables. Further, at above approximately 1000 C, forsterite (2MgO SiO 2 ) is formed from MgO and microsilica. In one study, it was found that forsterite existed in all samples fired at 1100 C and 1600 C, and in a 9 wt % microsilica-containing castable close to 60 % forsterite was present in the matrix 4. Forsterite has a melting point at about 1890 C, and eutectic temperature in the system 2 MgO SiO 2 MgO is 1860 C. The adding of alumina in this system has at least a couple of implications. Substituting microsilica with alumina usually lowers the flow values, although particle size distribution overlaps between microsilica and superfine alumina. 5 Further, volume expansion 6 may be experienced due to spinel formation. This may counteract the shrinkage as forsterite is being formed. Unfortunately, the combination of SiO 2 with Al 2 O 3 and MgO in the bond phase often gives low melting phases. It is known that at 1500 C even very small amounts of SiO 2 will deteriorate hot strength of corundum-spinel castables 7. Scope of this work This work is a study of the MgO-SiO 2 system as a candidate material for basic self-flowing castables. The effect on placement properties like flow and set time when substituting volume fractions of microsilica with fine multimodal calcined alumina is investigated. Further, the high temperature properties will change, going from a MgO-SiO 2 bond system to a MgO-SiO2- Al 2 O 3 and, finally, to a MgO-Al 2 O 3 bond-system. Hot modulus of Rupture and Refractoriness Under Load test procedures are used to investigate this. Self-flowing castables are easily installed by wet shotcreting. Chosing q-values of 0.25 and 0.28 from the Andreassen Model 8, gives potentially free-flowing and vibratable castables. The mixes found to be best-suited from this work were later installed by shotcreting and tested for hot properties and resistance to basic slags. The results will be found in a separate paper (Part 2). Procedure Castable mixing After the ingredients were dry-mixed in a Hobart mixer for 4 minutes, water was added, and the castable was mixed for an additional 4 minutes. Flow measurements Flow measurements were performed by using a flow cone (height 60 mm, top diameter 70 mm and lower 2

diameter 100 mm). The testing was performed on a vibration table set at 50 Hz at a double amplitude of 0.75mm. Immediately after mixing, the castable was filled into a dozen cones, cured at ambient temperature at around 20 C, and relative humidity at around 70%. When time had elapsed, one flow cone was removed and the castable was allowed to spread by the action of gravity alone. When the spreading stopped, the percentage increase in the diameter of the castable was taken as the free-flow value. The spread castable was then subjected to 15 seconds of vibration, and the resultant spreading taken as the vibra-flow value. When time had elapsed, another flow cone was taken off to measure flow-value with time. Flow decay When vibration value became below 70%, both for self-flowing and vibration castables, this was regarded as the end of working time. Time was recorded. Cold modulus of rupture Cold M.O.R. was measured on samples of 40x40x160 mm (ASTM C348), cured for 24 hours before drying at 110ºC. The samples were then fired at 600ºC, 1000ºC, 1200ºC, 1400ºC and 1600ºC for 5 hours. Subsequently, strength was tested on the dried samples (110ºC) and on the fired samples. Coldstrength-testing was performed according to ASTM C348 (flexural strength). Hot modulus of rupture Hot M.O.R. testing was performed on samples of 25x25x150 mm in accordance with PRE/R18. The samples were cured for 24 hours before drying at 110ºC. Samples were then préfired for 24 hours at test temperature and allowed to cool before being loaded into the testing machine. The heating rate was 300ºC/h for both préfiring and testing, and the samples were allowed to equilibrate at test temperature for 30 minutes prior to testing. Refractoriness under load, R.U.L R.U.L was measured on cylinders 50 mm high with central bore 12.5 mm, as described in ISO R1893 (1970). The constant load on the test samples was 0.2 MPa. The samples had been préfired for 24 hours at 1500ºC, unless otherwise stated. Heating rate was 300ºC/h for both testing and préfiring. The equipment was designed in accordance with descriptions of ISO R1893 (1970); maximum accessible temperature was 1800ºC for testing under rising temperature. Raw materials Chinese fused magnesia with MgO-content higher than 98% was the main raw material. The other materials were reactive alumina CTC 50 from Alcoa and Elkem Microsilica Grade 971 U. For dispersion Vanisperse CB from Borregaard (Norway) was used. Table 1 shows the recipes with q = 0.25 for self-flowing castables, while Table 2 shows the recipes with q = 0.28 for vibration castables. Table 1. Composition of the self-flowing castables (wt.%), q = 0.25 MS/RA ratio 10/0 8/2 6/4 4/6 3-5 mm 10 10 10 10 Fused MgO 1-3 mm 27 27 27 27 0-1 mm 25 25 25 25-44 micron 28 26.4 24.8 23.2 Microsilica (MS) 971U 10 8 6 4 Reactive Al 2 O 3 (RA) CTC50 0 3.6 7.2 10.8 Dispersant Vanisperse CB 0.25 0.25 0.25 0.25 Water (15.2 vol.%) 5.32 5.25 5.16 5.09 Table 2. Composition of the vibration castables (wt.%), q = 0.28 MS/RA ratio 6/0 4/3,6 2/7,2 0/10,8 3-5 mm 10 10 10 10 Fused MgO 1-3 mm 27 27 27 27 0-1 mm 27 27 27 27-44 micron 30 28.4 26.8 25.2 Microsilica (MS) 971U 6 4 2 0 Reactive Al 2 O 3 (RA) CTC50 0 3.6 7.2 10.8 Dispersant Vanisperse CB 0.25 0.25 0.25 0.25 Water (15.2 vol.%) 5.19 5.12 5.04 4.96 3

Results and discussion Flow value Fig 1. Relationship between MS/RA ratio and flow values At q = 0.25, both self-flow and vibration-flow were highest at MS/RA = 6/4. Fig. 1 (left). There is not a very close overlap of PSD between CTC50 and microsilica. Instead of filling the same particle sizes as when substituting microsilica with CT 3000 SG 5, the CTC fills the particle-size-void between microsilica and 325 mesh MgO. The multimodal particle size distribution of CTC50 is beneficial to the particle-packing. The result being the increasing contents of CTC50 increase free-flow in mix 8/2 and 6/4 (Figure 1, left). The mix 4/6 contains the most CTC50, but this mix has only 4 wt% microsilica, so the free-flow-value is lower than for the 60/40 mixture. For q = 0.28, self-flow and vibration flow decreased with increasing reactive alumina content. These mixtures are potentially vibraflow. Only the 6 wt% microsilica-containing castable shows free-flow behaviour. Flow decay For q-value 0.25 the recipes with MS/RA = 6/7.2 and MS/RA 4/10.8 gave the longest working times. With 8 wt % microsilica, the initial free-flow of almost 90 % is lost after 45 minutes only, Fig 2 left. For the vibra-flow mixes with q = 0.28 there is no initial free-flow for recipes with 2 wt % and 0 wt % microsilica. There is not sufficient working time with low microsilica contents. With 6 wt % microsilica, there was more than 120 minutes working time, Fig. 2 right. Fig 2. Decay of free-flow value, left q = 0.25, right q = 0.28. 4

Table 3. Heating explosion resistance q = 0.25 q = 0.28 MS/RA wt. 10/0 8/3.6 6/7.2 4/10.8 6/0 4/3.6 2/7.2 0/10.8 350 C - - - - - - - - - - - - O O O X 550 C O O O O O O O O O O O O O O - - 650 C O O O O O O O O O O O O O X - - 750 C O O O O O O O O O O O O - - - - 950 C O O O O O O O O O O O O - - - - 1000 C O X O X O O O O O O O O - - - - O: no explosion; X: explosion or cracked Heating explosion resistance After having been cured for 24 hours, samples were put into a hot furnace, in which the temperature had been raised to the desired level in advance. After having been soaked at the given temperature for 20 minutes, the samples were checked. If the sample had exploded or cracked, X was marked in Table 3. If the sample was intact, O was recorded. Very good heating-explosion-resistance was found for the free-flowing mixtures (q = 0.25). Only one had sample cracked when heated to 1100 C for the mixtures with highest microsilica content (10 wt% and 8 wt%). Using 2 wt% or 0 wt% in the vibra-flow mixtures (q = 0.28) gave exploded samples at lower temperatures (650 C and 350 C). Cold modulus of rupture From 110ºC to 1000ºC, CMOR for all castables decreased with temperature. Common for the MgO-SiO2 bonded castables is that they are rather strong after demoulding. For example 11.3 MPa dried at 110ºC with 8 wt % microsilica. Above this temperature they gradually lose strength up to 1000ºC. The reason for this drop in strength is not fully understood, but may be connected to crystallisation of an amorphous bond-phase. At higher temperatures, above 1000ºC, the samples regained strength. The sample without microsilica MS/RA = 0/10.8 has low strength at all temperatures (Fig. 3, down). At 1600ºC CMOR is higher (25 MPa) for the 6 wt% (q=0.28) than the 10 wt% (q=0.25) alumina-free castable. Fig 3. CMOR of MgO castables at q = 0.25 (upper) and q = 0.28 (lower). 6

Permanent linear change PLC reflects the phase formation and sintering taking place in the castables bond phase. The microsilica-free sample with 10.8 wt% alumina shows the characteristic expansion when spinel is being formed. A permanent expansion of 2.3% is seen for the MS/RA= 0/10.8 sample at 1200 C (Fig. 5). Also the alumina-free mixture MS/RA =10/0 expands at 1200 C, but only 0.6 %. At 1600 C the castable has a negative PLC of 0.6%. The samples with alumina have a higher shrinkage, up to 1.0% for the castables MS/RA =8/3.6 and MS/RA= 6/7.2 (Fig. 4). Fig 4. PLC of MgO castables with q-value 0.25 as a function of temperature. Fig 5. PLC of MgO castables with q-value 0.285 as a function of temperature. Hot modulus of rupture In Fig. 6, it is seen that the sample MS/RA = 8/3.6 has the highest HMOR from 1200ºC to 1400ºC, but at 1500 C, this sample has the lowest HMOR. At 1500ºC; only the MgO-SiO 2 sample MS/RA=10/0 gives 10MPa. There is a trend of decreasing HMOR with increasing alumina-content from 3.6 wt % to 10.8 % in the temperature range 1200-1400ºC. For the q = 0.28 group, there is a similar pattern. MS/RA = 4/3.6 has the highest HMOR at 1200 C and 1300 C and decreases as more alumina is being introduced. At 1500ºC, the alumina-free sample has a HMOR of 5.5 MPa. At 1400 C and 1500ºC, the microsilica-free MgO-Al 2 O 3 sample has a higher HMOR than the MS/RA = 2/7.2 sample. At 1500 C, the hot-strength of the alumina-free MgO-SiO 2 sample with q-value 0.25 is higher than when the q-value is 0.28. When 10 wt % microsilica is being used, more forsterite (2MgO SiO 2 ) is likely to be formed, giving higher strength. 6

Fig 6. HMOR of MgO castables with q = 0.25 (left) and q=0.28 (right) for 1200-1500 C. Refractoriness under load, R.U.L The sample with q = 0.25 and MS/RA = 10/0 in Fig 7 showed the best RUL curve, thermal expansion of 2.0 % reached at the highest temperature of 1540 C. The T 0.5 of this sample was measured to 1650 C. The other mixes with alumina present showed the same patterns and had T 0.5 of approx 1610 C. For q = 0.28 in Fig 8, the SiO 2 -MgO sample will start subsiding at 1480 C after expanding 1.9 %. The 2/7.2 and 4/3.6 samples have a similar pattern, with T 0.5 of about 1630 C. The microsilica-free sample of MS/RA = 0/10.8 has slowest creep-rate above 1600 C. This is due to the in-situ formed magnesium aluminate spinel. Fig 7. R.U.L. test of MgO castables with q=0.25 7

Fig 8. R.U.L. test of MgO castables with q=0.28 Conclusion For the vibratable mixes (q=0.28), low CMOR was observed in mixtures MS/RA=2/7.2. and MS/RA = 0/10.8. To develop the low temperature (110 C and 600 C) strength (10-18 MPa), the addition of more than 4 wt% microsilica gave better results. The Al 2 O 3 - MgO system needs an alternative binder for example hydratable alumina, since it does benefit from the MgO-SiO 2 -hydrate bonding at low temperature. High flowability is achieved by a lower q-value, improved particle-packing and sufficient amounts of microsilica. The highest free-flow value was found for the mix with a q-value of 0.25 and a content of 6 wt % microsilica and 7.2 wt% CTC 50 alumina. At 1500 C, HMOR of 10 MPa was found for the forsterite-bonded MgO castable with 10 wt% microsilica. The RUL curves were most promising for the aluminafree castable with 10 wt% microsilica and the microsilica-free sample with 10.8 wt% alumina. This led to the conclusion that the two-component systems gave better high-temperature properties. When choosing two mixtures to be installed by shotcreting, the flowability needs to be good enough for pumping and working time must be long enough for operation. Recipes with 6 wt % microsilica were chosen for both q-value 0.25 and q-value 0.28. The castables were pumped and sprayed by using an Allentown Pump. The installation, hot properties and slag resistance of pumped and sprayed MgO castables are the subject of the following paper, part 2. 8

References 1. B. Myhre: «Cement-free castables in the system MgO-SiO 2 ; The effect of bond-phase modifiers on strength». Presented at The American Ceramic Society, 93rd Annual Meeting, Cincinnatti, 1991. 2. B. Sandberg, B. Myhre and J. L Holm: «Castables in the system MgO-Al 2 O 3 -SiO 2» in Proc. Of UNITECR 95, The Techn. Ass. of Refr., Japan. Kyoto, Japan Nov. 19-22 1995, vol. II, p. 173-80 3. B. Myhre, B. Sandberg and A. M. Hundere: «Castables with MgO-SiO 2 -Al 2 O 3 as bond phase» In Proc. Of XXVI ALAFAR Congress, San Juan, Puerto Rico, Oct. 29 - Nov. 1, 1997, p I/1-I/10 4. Nan Li, Yaowu Wei, Hongpeng Wu, B. Myhre and C. Ødegård: «Properties of MgO Castables and Effect of Reaction in Microsilica-MgO bond System» In Proc. Of UNITECR 99, The German Refractory Assosiation, Sept. 6-9, 1999, Berlin, Germany, p 97-102. 5. B. Myhre, A.M. Hundere: «Substitution of Reactive Alumina with Microsilica in Low Cement and Ultra Low Cement Castables. Part I: Properties related to Installation and Demoulding» in Proc. of UNITECR 97 in New Orleans, USA, Nov. 19-22 1997, vol. I, p. 43-52. 6. F. N. Cunha, R. C. Bradt: «Reactions of constituents for in-situ bonds of MgAl2O4, Mg2SiO4 & 3 Al2O3. «SiO2 in refractories» in Proc. of the 57th Electric Furnace Conference, Pittsburgh PA, Nov. 14-16, 1999, p.143-152. 7. G.W.Kriechbaum et al.: «The Matrix Advantage System, a new approach to low moisture LC selfleveling alumina and alumina spinel castables.» In Proc. XXXIX Int. Feuerfest-Koll, Aachen, Sept 24-25 1996, p. 211-218. 8. A. H. M. Andreassen and J. Andersen, Kolloid Z.50 (1930) p. 217-228. 9

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