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

AGEING OF REFRACTORYCASTABLES; WHICHINTERACTIONS EXIST ANDWHAT CAN BE DONE IN ORDERTO INCREASE SHELF-LIFE?Presented at 54 th International Colloquium on Refractories at Aachen, Germany in October19-20, 2011.

AGEING OF REFRACTORY CASTABLES; WHICH INTERACTIONS EXIST AND WHATCAN BE DONE IN ORDER TO INCREASE SHELF LIFE?Bjørn Myhre and Hong Peng; Elkem Silicon MaterialsABSTRACT:Ageing of castables is a well known problem that has beenattributed to interactions between cement and humidity. Inthe present study, commercial castable mixes have beentested before and after storage. Ageing experiments withoutingredients showed that additives do together with humidityreact with the cement. The ageing manifests as a gradual lossof self-flow combined with a retarded set time. Ifhexametaphosphate is used as deflocculant, there areindications of reactions between the phosphate and thecement or other ingredients during the ageing. The access ofhumidity is still a determining factor though. This additionalageing effect caused by the phosphate can be minimised byuse of other dispersing systems, here the novel dispersantSioxX/SioxX-quick is suggested. Substitution of the additivepackage leads not only to an improved shelf life, but also toa better flow for the tested castable compositions.INTRODUCTION:Ageing of LCC as phenomenon has been mentioned inpublications repeatedly by mainly the cement manufacturers.The ageing is primarily observed as a prolongation of thesetting time, sometimes connected to loss of flow. At somestage, the set gets so delayed that use of the bagged LCCgets difficult. This is of course a big concern for themanufacturers and the different refractory companies havedifferent approaches to get around this. Little of this activityis however available in literature probably due to its obviouscommercial aspects. So much (most?) of the knowledgerelated to ageing may remain hidden as proprietaryinformation. Due to reductions in research staff in therefractory industry, more problems related to refractory rawmaterials have been addressed to the raw-material suppliersand hence the ageing question has to some extent “bouncedback“ to the cement producers. The two large CA-cementmanufacturers Almatis and Kerneos seem to have followedseparate strategies to deal with the cement ageing though.While Kerneos has presented investigations on themechanisms behind the observed ageing 1,2,3 Almatis seemsto have followed a route that focused on packaging anddocumentation of their cements ability to be stored forextended time without degradation 4 , 5 Eventually these twostrategies, almost simultaneously led to introduction of newproducts with improved storage properties. The Almatisapproach has been in the packing, using plastic bags, whilethe Kerneos approach has been based on manufacturingcement that is less prone to ageing itself 6 .The above cited papers generally agree that it is the cementthat ages and the ageing is mainly caused by humidityreacting with the surface of the cement. Some reaction withCO 2 has also been noticed; however the main culprit hereseems to be the humidity. The exact contribution of thehumidity to the ageing has not been entirely mapped, but asthe loss of ignition of the cement seems to relate to theincrease in setting time that the ageing causes, its is temptingto think that the humidity causes the cement particles to reactand produce a hydrate surface-skin that prevents the cementfrom dissolving and thus increases the set time.The dispersant that is used has also been found to haveprofound influence on ageing 7 .EXPERIMENTAL PROCEDURE:The results that will be presented are mainly self-flow valueand set time.The self flow is measured by filling the freshly mixedcastable (4 minutes wet-mixing) into the flow-conedescribed by ASTM C230 (5cm tall, NOT the more recentdescribed cone of 8cm(EN 1402-4:2003) The self flow valueis the %spread of the patty 1.5minutes after removing thebrass-cone.Set time is defined as the time to the first exotherm. This isoften also referred to as the working time. As the measuredtime is dependent on initial temperature of the castable, thevales have been adjusted a start temperature to 22°C byusing adjustment based on empirical data for similarsystems.The castables were stored in a climate controlled room withstable temperature of 20°C and a relative humidity of above50% relative humidity or alternatively, in an unheatedwarehouse, dry but with fluctuating temperature.RESULTS AND DISCUSSION:The experiments that will be quoted in this paper are allcommercial mixes and the description of them willconsequently not be too detailed. However they will bedescribed in sufficient detail to make a fair assessmentpossible. The treatise will be as a ”case study“ presentingseveral cases each adding to the overall knowledge ofageing:Case 1: Bauxite based LCCThe problem: Castables subjected to storage, deteriorates,particularly with respect to set-time which gets prolonged.Table 1 shows composition of the castable in question,bauxite based castable with calgon as additive and 4%cement.The castable was stored in the climate room or in anunheated warehouse. To check if interaction between thecomponents had an effect, raw materials were also stored inthe warehouse but individually packed. All the samples werestored in plastic lined paper bags. On the day of testing theindividually packed ingredients were combined and testedfor comparison.Table 1: Castable used in case-study 1:Microsilica (971U) 8Cement (70%CAC) 4Chinese Bauxite 68calcined alumina 20calgon 0.2water 5.5

Self-flow value [%]4540353025201510504141 413637351224Fresh 1 month 3months 6 months 12 months280Climate roomWarehouseIndividually packedFigure 1: The effect of storage conditions and duration onself-flow.In Figure 1, the self flow of the castable is shown for thedifferent conditions, and it is seen that flow decreasessignificantly with storage. Most severely reduced is the flowof the castables that have been stored under the humidclimate-room conditions, least severe are those that had notbeen assembled prior to storage. In other words, humidityseems to be a dominant factor here; which is in line with theprevious papers findings; but one cannot rule out that theremight be some interactions between the components thatincrease the ageing effect even further.Figure 2 shows the set time of the samples shown in Figure1.Set time [hours]302520151050Climate roomWarehouseIndividually packed6.456.45 6.4522159.68.3 8 7.975.74828>2525 >25 >2525 2523.65.95Fresh 1 month 3months 6 months 12 monthsFigure 2: The effect of storage and storage conditions on settime.Figure 2 shows beyond doubt that ageing is something thatshould be carefully considered for LCC’s. After even a shorttime the set time starts to increase and reaches levels thatmakes the castable practically useless. The packing andstorage conditions are obviously important in this context.The assumption of humidity playing a major role seemsstrengthened as well. The important observation is that if theingredients are individually packed, the set time does notdeteriorate significantly. This must imply that there is somesort of interaction between two or more of the componentsduring storage that accelerates the ageing.Case 2: Self-flow SiC LCCThe next study is on a self-flowing castable based on SiC asaggregate and with 6% cement. The aim was to investigatepossible interactions between the individual ingredientsduring ageing. The test set-up was to mix the ingredientsexcept the one that was tested and then store the mix and themissing ingredient in the climate room for 9 months. Testingwas done fresh and after 9 months in the climate room. Theadditives (here Calgon + Alphabond) were stored in a sealedplastic box, i. e. not exposed to humidity.self flow value [%]1009080706050403020100Table 2: Castable used in study 2.Microsilica 10Cement 6SiC 72calcined alumina 12Calgon 0.2Alphabond 1water 5.5088 884970Full mix without MS without cement withoutadditives9288956fresh9 monthsFigure 3: The effect of ageing on self-flow for the mixesaged without certain ingredients.Set time [hours]80706050403020100>72 >7272 729.2 10.1 9.5 9.62.4fresh9 monthsFull mix without MS without cement withoutadditivesFigure 4: The effect of ageing on set-time for the mixes agedwithout certain ingredients.So, what do these results tell us? Figure 1 and 2 shows thatageing affects both self-flow and set time. The self flow getslower, and the set time increases. The figures indicate thathumidity plays an essential part in the ageing, as has beenfound in the earlier investigations cited above. It also25

Figure 5: Self-flow of SiC based self flowing LCC as afunction of additive package and storage. Samples stored athigh humidity in climate room.Figure 5 shows that flow degrades somewhat during the 3months storage time independent of the additive package.Interesting is to note the effect a change from Calgon(sodiumhexametaphosphate) to SioxX as deflocculantcauses, an increase in self flow from 92 to 120% for freshsamples. This is approximately 30% increase in self flow, asignificant increase as compared to that obtained by thephosphate. It is also very interesting that the addition ofSioxX-quick to the castable does not influence flownegatively. This is a trait of many accelerators that has to betaken into consideration.Set time [hours]807060504030201006.3553.6>7232.327.33.35 2.7fresh1 Month3 Months2317.3Calgon SioxX SioxX/SioxX-quickFigure 6: Set time of SiC based self flowing LCC as afunction of additive package and storage. Samples stored athigh humidity in climate room.In Figure 6, the set time as measured by temperature isshown for the set of compositions and conditions. It isnoticed that the addition of SioxX-quick not only reduces theset time for fresh compositions, but also seems to decreaseageing somewhat. Form Figure 4, it may be estimated thatthe inherent setting time after storage without additiveshould be around 25 hours. This ageing was due to hydrationof the cement etc. In Figure 6, the results with SioxX/SioxXquickget close to that number. Due to the differences inadditive package one should be careful not to draw too firmconclusions, but the results using SioxX is a majorimprovement as compared to results with the phosphate(Calgon).Based on the results presented in Figure 6, it seems fair tosay that additive/cement interaction are minimised if thephosphate dispersant system is replaced by the SioxXadditive package, and improved by use of SioxX-quick inconjunction with SioxX.cement hydrates, but additionally the additive reacts with thecement or some other constituent of the castable mix. Theresult is a very long setting. The obvious way to protect acastable mix from ageing is to keep humidity away. This canbe achieved by use of air-tight packing. In practise this maybecome both difficult and expensive though. The rightchoice of deflocculating system is an alternative way to easethe ageing even with moisture present. This was shown inthe examples utilising the SioxX/SioxX-quick additivesystem.1 Mathieu A., Bayoux J.P., Vialle M. ”Ageing of AluminousCement in LCC” proc. UNITECR 1995, Kyoto Japan p. 451-4552 Parr C., et al. „Ageing of Low Cement Castables“ proc.UNITECR 1997, New Olrleans USA, p. 81-893 Fryda H. et al. ”A new insight into the characterisation ofageing and mechanisms that determine the shelf life of lowcement castables” proc. UNITECR 009, Salvador Brasil4 Gierish D., et al. ”Aging behaviour of Alphabond andCalcium Aluminate Cement bonded castables” proc. 49thInt. Coll. on Refr. Aachen 2006, p. 137-1425 Scmidtmeier D. et al. ” Cement Ageing: Something of thePast!” proc. 53rd Int. Coll. on Refr. Aachen 2010, p. 22-266 Parr C. et al. ”New Solutions to Increase the Shelf-life ofLow Cement Castables” proc. 53rd Int. Coll. on Refr.Aachen 2010, p. 27-307 B.Myhre and H. Peng ”Effect of dispersants and rawmaterials on placing properties and shelf life of bauxitebased low and ultralow cement castables” proc. 53rd Int.Coll. on Refr. Aachen 2010, p. 62-668 B. Myhre and K. Sunde: “Alumina based castables withvery low contents of hydraulic compound. Part I: The effectof binder and particle-size distribution on flow and set”,Proc. UNITECR´95, Kyoto, Japan, Nov. 19-22, 1995, p.II/309-316CONCLUSIONAgeing of low-cement castables is a problem that most (all?)LCC manufacturers sooner or later will be confronted with.In the present paper, studies of ageing of two differentLCC’s have been presented, and the results confirm thathumidity is an important factor. This has previously beendescribed in literature. What is also found is that with thepresent phosphate dispersant (hexametaphosphate), reactionsbetween the dispersant and other components are ofimportance. Under the action of humidity, not only the