Microsilica containing refractory castables - Elkem

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$ageing of refractory castables; which interactions exist and ... - Elkem$

Set of microsilica containing castablesUp to the first half of the 1980’ies, the commonway of determining set time of refractory castableswas by assessing set with the use of a Vicat-needleset up, or less sophisticated methods likeassessment by knocking, squeezing etc. All thesemethods had a number of difficulties, of which thelabor and lack of precision when used on castablesystems was the most important.In 1984, Fentiman, George and Montgomery 3 ,correlated results from Vicat measurements withtemperature development of castables.Instead of using a Vicat needle (penetration of aneedle into fresh mortar) the temperature of thecastable (or mortar) is recorded. Adiabaticcalorimetric studies of aluminous cement in contactwith water have shown that there are twoexotherms. The first occurs immediately and isassociated with heat of wetting and rapiddissolution. A dormant period then follows duringwhich hydrate nuclei form and develop. Oncecritical nuclei have been formed, bulk precipitationof hydrate occurs giving rise to a second exotherm 4 .This second exothermic reaction may be used todetermine when the setting starts by simplyembedding a thermocouple into a proper amount offresh castable or mortar. If the thermocouple isconnected to a strip-chart recorder, the exothermmanifests as a temperature increase. The time toinitial set(TTS) may thus be measured (Fig. 9). Thetime to temperature increase correlates well withdata obtained by the Vicat needle. Figure 10 showsa typical exotherm and the correlation betweenVicat and the exothermic test. The exothermic testcan, as opposed to the Vicat needle, also givereliable results when applied on castables.TimeTTMTTSSTARTFigure 9. Typical Strip-chart print out showingtemperature in a microsilica containing CA-cementbased refractory castable.Figure 10.Correlation between Vicat initial set andresults from exothermic test for calcium aluminatemortar.For castables with both microsilica and CA-cement,the castable normally shows an additionalexotherm, which has been associated with theformation of a layer of CASH compounds on thesurface of the cement particles that dissolves andenables setting to proceed, giving the finalexotherm 5 . Usually, the appearance of the firstexotherm (TTS) is associated with the end ofworkability of the castable, and the maximumtemperature of the second exotherm (TTM) isregarded as the time to final set. The temperaturemethod has been found easy and reliable, but is notsuited for castables with very low cement contentbecause of the small amount of heat liberated. Theminimum cement content for reliablemeasurements may be around 3-5% depending onsample size and apparatus.At UNITECR 2005 in Orlando, Myhre 6 presented astudy in which some interesting correlationsbetween microsilica and set time were shown. Thesystem investigated was based on the standardcastable composition given in Table 1, with themicrosilica quality as the variable. All microsilicasamples came from the same source, but taken atdifferent occasions during a period of a few monthsfor two successive years. In this context it must bestressed that the microsilica in question is anuncontrolled grade, collected as a by-product fromsilicon production. To further emphasise this point,the original figure from Orlando has beenmanipulated to also include results from mixesusing the controlled microsilica grade 971 sampledand tested during the same months but in 2007.
36:00 12:0030:00 6:0020042005971-200728:48:0024:00:00FS20CalgonDarvan 811DTTS [h:min]24:00 0:0018:0012:00TTS (h:min)19:12:0014:24:009:36:006:004:48:000:0024/1 3/2 13/2 23/2 4/3 14/3 24/3Sampling dateFigure 11. Initial set as a function of productiondate for microsilica from one silicon furnace duringwinter/spring 2004 and 2005. Values obtained withmicrosilica grade 971 is added for reference.The set time as defined by the time to the firstexotherm of the castable, TTS, shows a verypronounced dependency on the addition level, withCalgon being the prominent example. In Figure 12and 13 the set time is shown for the bauxite and thewhite fused alumina system, respectively. In bothcases the set time of the Calgon dispersed castableshas a very long set time at addition levels around0.35mg/m 2 . This coincides in the white fusedalumina example unfortunately also with themaximum flow. In the bauxite example, as themaximum flow was obtained at higher Calgonadditions this is no longer a problem. FS20 shows aflatter response to the addition level, although byvery high addition level there is a tendency towardsprolonged set.TTS (h:min)24:0020:2416:4813:129:366:002:240.00 0.50 1.00 1.50 2.00 2.50 3.00Dosage (mg/m 2 )FS20CalgonFigure 12 The effect of dispersant addition on settime of bauxite based LCC (1 & 2 Table 1)0:00:000.00 0.30 0.60 0.90 1.20 1.50Dosage (mg/m 2 )Figure 13: Effect of additive on set time of thewhite fused alumina based LCC.If the two different systems are compared, at thesame addition level, the two systems reactsimilarly, yielding similar set times and flowvalues. The bauxite however needs more water (5.5as compared to 4.15).Shelf lifeShelf life of castable 1 and 2( Table 2) with 0,05%FS20 and 0,20% Calgon respectively, wasmeasured on dry-mixed samples (25kg, 6minutes inHobart mixer) placed in closed, plastic lined paperbags. Two conditions were chosen, one was anunheated warehouse at the Elkem site inKristiansand Norway, the other was in atemperature and humidity controlled room,>50%RH at 20°C. The differences between the twoconditions were thus that the warehouse fluctuatedwith the weather and seasons while the climateroom had close to constant conditions withrelatively high humidity. This controlled conditionhas earlier been found to accelerate ageingsignificantly and is used in accelerated ageing testsin Elkem. In Figure 14 the self-flow of the castablesare shown as a function of storage time.Although not having a very strong effect on flow,some lowering in self-flow is seen particularly forthe Calgon samples after extended storage in theclimate room.In Figure 15, the set time as expressed by the timeto the first exotherm (TTS) is shown as a functionof storage time.As opposed to the flow, the storage has a strongeffect on set time. Already after 30 days of storage,the set time increases significantly for bothdispersants at both storage conditions. While the
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Microsilica containing refractory castables - Elkem

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