金属凝固论文.pdf

journal of hazardous materials b137 2006 267 276 factors affecting hazardous waste solidifi cation stabilization a review rachana malviya rubina chaudhary hazardous waste management laboratory school of energy and environmental studies devi ahilya university indore india received 8 april 2005 received in revised form 27 january 2006 accepted 29 january 2006 available online 13 march 2006 abstract solidifi cation stabilizationisacceptedasawell establisheddisposaltechniqueforhazardouswaste asaresultmanydifferenttypesofhazardous wastes are treated with different binders the s s products have different property from waste and binders individually the effectiveness of s s process is studied by physical chemical and microstructural methods this paper summarizes the effect of different waste stream such as heavy metals bearing sludge fi lter cake fl y ash and slag on the properties of cement and other binders the factors affecting strength development is studied using mix designs including metal bearing waste alters the hydration and setting time of binders pore structure depends on relative quantity of the constituents cement hydration products and their reaction products with admixtures carbonation and additives can lead to strength improvement in waste binder matrix 2006 elsevier b v all rights reserved keywords stabilization solidifi cation hydration unconfi ned compressive strength pore structure carbonation 1 introduction stabilization is a pre landfi ll waste treatment process which has been used for different types of industrial wastes but is par ticularlysuitedtothosecontainingheavymetals 1 2 thesolid ifi cation stabilization s s process utilizes chemically reactive formulationsthat togetherwiththewaterandothercomponents in sludges and other aqueous hazardous wastes form stable solids the material used for solidifi cation stabilization s s not only solidifi es the hazardous waste by chemical means but also insolublizes immobilizes encapsulates destroys sorbs or otherwise interacts with selected waste components the result of these interactions are solids that are non hazardous or less hazardous than the original waste the degree of effectiveness of these s s products is defi ned basically by two parameters strengthandtheleachresistance thecontinuingneedtodevelop abbreviations s s solidifi cation stabilization bdat best demonstrated available technology ucs unconfi ned compressive strength pfa pulver ized fuel ash ggbs granulated blast furnace slag sea sulphur extended asphalt csh calcium silicate hydrate c3a tricalcium aluminate ch cal cium hydrate aft hexacalcium aluminoferrite trisulphate afm tetra calcium aluminate monosulphate srpc sulphate resistance portland cement wopc white ordinary portland cement abfc activated blast furnace slag anc acid neutralizing capacity corresponding author fax 91 731 2467378 e mail address rubina chaudhary r chaudhary economical and improved waste management techniques has increased the potential importance of solidifi cation technology through out the world in a process defi ned as best demonstrated available technology bdat 1 2 methods used for studying effectiveness of s s process are physical chemical and microstructural hills and pollard 3 used setting and strength development as indicators of solidifi cation and leach test to assess the extent of fi xation researches has investigated range of wastes for which the proportions of cement wastes and fi llers which consistently produce satisfactory product solidifi cation may vary consid erably with waste type upon scaling up and in cost s s of wastes has been examined in the laboratory by a number of workers often purechemicalcomponentsofonecontaminants ofconcernincreasinglyorsimplemixturesareaddedtoopcand other stabilizing agents ashes of different origin lime clay sil icates etc to stabilize the contaminant of concern 4 5 than the results are extrapolated to solidifi cation of waste thisreviewisadiscussionofthepublisheddatarelatingtothe ways in which the inclusion of hazardous wastes bearing heavy metals such as sludges fi lter cakes fl y ash and slags affects the propertiesofcementandotherbinders emphasisisgivenonthe mix design effect on unconfi ned compressive strength ucs hydration effect of carbonation pore structure pore volume and environmental factors affecting s s while remaining con sistentwithacriticaloverviewofthesubject emphasishasbeen 0304 3894 see front matter 2006 elsevier b v all rights reserved doi 10 1016 j jhazmat 2006 01 065 268r malviya r chaudhary journal of hazardous materials b137 2006 267 276 table 1 physico chemical quality criteria for solidifi ed stabilized waste physical properties setting time5 72h compressive strength 0 35mpa chemical properties metal concentration in tclp leachates mg l cd0 5 cr5 pb5 zn300 eco toxicity to daphnia magna 750mg l placed in this article on recently published material to evaluate the degree of fulfi llment of the s s objectives specifi c criteria are given in table 1 6 7 cement is most frequently used and widely studied binder solidifi cation with cement is widely understood and simple is easily available and results in a stable product 8 in general otherbinders sulphateresistantportlandcement srpc white cement kiln dust lime fl y ash pulverized fuel ash pfa sili cates etc partially replace opc which can have a positive or negative effect on strength setting time and hydration 2 unconfi ned compressive strength ucs strengthtestdataoftenareusedtoprovideabaselinecompar ison between unstabilized and stabilized wastes unstabilized waste materials generally do not exhibit good shear strength however if the waste is stabilized into a cement like form the strengthcharacteristicscanbeexpectedtoincreasesignifi cantly 9 2 minimum required unconfi ned compressive strength for a stabilized solidifi ed material should be evaluated on the basis of the design loads epa considers a s s material with a strength of 0 35mpa to have a satisfactory unconfi ned compressive strength this minimumguidelinehasbeensuggestedtoprovideastablefoun dation for materials placed upon it in a landfi ll in uk accept able 28 day strength is 0 7mpa but as low as 0 35mpa is also considered depending on test specimen 3 a study by stege mann et al 10 11 reported unconfi ned compressive strength values for 69 stabilized solidifi ed wastes ranging from 0 06 to 19 99mpa ucs test are performed at different time intervals of 1 3 7 14 28 90 etc days to monitor the effect of the changes in the mineralogical composition of waste with increasing time and environmental exposure fig 1 reveals that the ucs increases with an increase in curing time strength of waste binder matrix is less than pure binder matrix 12 13 2 1 strength of the s s product cement contents varying from 5 to 20 are routinely used to solidify waste low cement content does not adequately coat individual waste particles but still tends to result in set and rigidity of the waste form this is assumed to be the result of physicalchangesinducedbynormal hydrationprocess despite fig 1 unconfi ned compressive strength development after different curing days source adapted from malviya and chaudhary 39 the false set mechanism that is precipitation of salts such as gypsum that impart adequate strength 14 15 solidifi ed materials are weak strengths typically 1mpa and contain signifi cantly less cementious materials and more water than for example concrete it is known that compres sive strength is dependent on the quality of pore structure of the cement based materials this quality is moreover dependent on the kind and quantity of the constituents forming the pore structure that is cement hydration products and their reaction products with admixtures 16 precipitation of waste elements as cations or carbonate result in an increased demand for ca2 ions in the waste matrix affecting strength development the unhydrated cement frac tionssatisfi estheincreaseddemandofca2 ionsandaccelerates hydration of c3s in addition residual cement grains are gener ally decalcifi ed and silicate structures are subjected to increased polymerization of the cement 3 17 due to waste addition csh hydration is poisoned in these situation ettringite plays an important role the main product of hydration giving the early strength and stability is ettrin gite which is provided primarily by the self cementing ability of ashes this is supported by the fact that gypsum has been previously reported 18 to be main binding agent in some solidifi ed products and that addition of up to 5 gypsum is known to enhance both setting and strength development of pozzolanic opc mixes 19 this is further supported by the experimental proof by hills and pollard 3 the impor tance of the strength contribution of gypsum formation can be gauged by reference to pfa caoh mixes where the high est waste loading gave the highest strength initial stiffening and shortest setting time in contrast to opc and pfa opc 20 2 2 interferences of waste in strength development solidifi ed products are expected to harden over 2 5 day period the effect of waste is studied by adding progressive quantities to opc a severe strength loss could be observed at low waste additions for example introduction of zn pb cd at 1 by weight do not effect strength development 21 a 3 w w total solids addition of zinc containing metal plat ing waste resulted in a 99 reduction in the 28 day strength 3 r malviya r chaudhary journal of hazardous materials b137 2006 267 276269 for the waste stream of electroplating galvanizing metal fi nishing operations and zinc plating waste ucs of 2 1 15 and 1 0mpa resulted from opc combined with ground granulated blastfurnaceslag ggbfs opccombinedwithpfaandopc alone respectively 22 yangandchen 23 stabilizedincinera torashwithopcandggbfs withreplacementofopcbyslag resultinginbettercompressivestrength theyalsotriedpolymer sp a water reducing agent with no effect on ucs lombardi et al 13 reported ucs reduction of about 80 but more than suffi cient for disposal in landfi ll lin et al 24 s s lead bearing waste with sulphur sulphur extended asphalt sea opc type i the 7 day ucs increased with sulphur after the particles and contaminantswerewetted sulphurbinderincreasedthebinding force with particles ucs decreases with increasing sea up to 25 because sea is a viscous material at room temperature theuseofhigherbindercontentthanoptimumincreasedthefi lm thickness of binder which coated the aggregate and resulted in loss of ucs on spiking waste with oil sulphur the strength loss was about 25 30 because oil can reduce the viscosity of asphalt and thus affect the strength and stability of asphalt soil mixture onincorporationofoilfuelashincementmortars 25 thestrengthofoilfuelashmortarswassignifi cantlylessincom parison to control mortar samples the ucs decreased by 56 when the alkali content in opc increased from 0 15 to 2 8wt addition of 2wt sodium sulphatetoopc electroplatingsludgehadlittleimpact although thestrengthofopcreportedlydecreaseswithincreasingsodium sulphateconcentration forsludgecontainingni cr cd andhg metals opc class f fl y ash opc to fl y ash to sludge 0 2 to 0 6to1 andclasscfl yashandlime limetofl yashtosludge 0 3 to 0 5 to 1 impart better strength than opc opc to sludge 0 3 to 1 26 according to the study by macsik and jacobson 27 slag and quick lime cannot contribute to an increase of the undrained shear strength slag must be combined with standard portland cement the combination of 87wt sulphide soil 6 5wt slag and 6 5wt opc gave the required strength adding h3bo3 0 5 and 10wt decreased ucs at 3 days buttheucsapproachedcontrolvaluesafter28days 28 spec imens lose strength after leach testing from an initial strength of 19 85 to 0 46 and 0 05mpa after 7 and 14 extractions the leaching of matrix resulted in dissolution of calcium from csh gel whichresultedindecreaseofstrengthastheleftoutskeleton rich in si with no physical strength 13 many additives have been tried including silica fume and blast furnace slag 29 silica fume is a pozzolanic material the glassy silica fume reacts with lime byproduct made by hydrat ing cement to produce more csh and less calcium hydroxide in the cement paste 30 slag lime combinations were advo cated to result in rapid setting characteristics and production of a dense durable matrix sem investigations refl ect less porosity this porosity reduction makes the paste less vulnerable to the mechanism of deterioration 29 concrete incorporating blast furnace slag results in improved resistance to chloride ingress 31 gypsum induces the structural densifi cation by formation of aft and afm phases therefore the value of compressive strength increases and leaching of metals like pb decreases by using gypsum and fl y ash compressive strength increased via active generation and substitution of aluminate hydrates 7 roy et al 26 reported that the effect of sodium sulphate on cementious binder similar to sulphate attack on concrete a 2wt solution of sodium sulphate can lead to severe dete rioration the failure of opc during sulphate attack is usu ally attributed to the large increase in volume due to ettringite formation 2 3 factors affecting strength development 2 3 1 waste binder and water binder ratio ucs decreases as water increases in most solidifi cation recipes chlor alkali plant sludge was stabilized by a commer cial solidifi cation agent it was found that the smaller the sludge to binder ratio the better the physical and chemical proper ties of the solidifi ed monoliths this conclusion resulted from durability unconfi ned compressive and tclp testing 32 the reasons for reduction in ucs as reported by todorvic et al 33 is due to variable pore structure and visible holes if mixtures are too wet when opc is blended with fl y ash 13 with the increases in the fl y ash to opc ratio the water demand of the mixincreaseandadecreaseinthestrengthwasobserved similar resultsarereportedforthestudyconductedonsteelfi nishingunit sludge and opc 26 lower strength resulted from the lower portland cement and much higher water contents generally a waste binder ratio of 0 4 0 5 is used with a water solid ratio of 0 4 0 6 super plasticizers 20ml kg of cementious material are added to improve fl uidity and to reduce water binder ratio 34 cement content and curing were reported as the most impor tant factor with the water to solids ratio being less important for the ranges investigated 35 the infl uence of water cement ratio was reported to be secondary compared to the constituents of matrix 30 similar comment is given by zivica 16 com pressive strength depends on the quality of pore structure of the cement based materials this quality is moreover dependent on the kind and quantity of the constituent forming pore structure cement hydration products and their reaction with admixtures wang and vipulanandan 36 reported that the strength of metal laden cement mortar is affected by the water binder ratio at 1 3 and 14 curing days water content of cement fl y ash soil mixture decreases as curing time and cement content increases the infl uence of cement content was more pronounced than that of curing time 37 effectofhumiditywasstudiedonthecompressivestrength of atmospheric fl uidized bed combustion ash and pulverized fl y ash relative humidites of 65 and 90 were maintained in climate cells and the high relative humidity in the stabilized material led to higher ultimate compressive strength however this may induce the formation of more ettringite principally increasing the chances of carbonation 12 theeffectofvariableparticlesizeonstrengthofopc fl yash mortars was studied by erdogdu and turker 38 concluded that different size fractions fi ll the pores in the original ash mortar mixes results in higher strength values than expected 270r malviya r chaudhary journal of hazardous materials b137 2006 267 276 2 3 2 bulk density the increases in the binder content opc opc sulphur opc sea increases the bulk densities because the binders fi lled the void space of solidifi ed specimen 14 39 knoll and andres 40 compared unit weight of the fresh samples to the bulk density of the cured samples and assessed volume changes duringthes sprocess a28daycuredunitweightislargerthan that of the freshly mixed sample this increase in unit weight with curing time results from hydration of the pozzolan fi lling void spaces with csh 3 initial setting time and hydration the calcined product of the cement consists mainly of four mineral phases alite c3s belite c2s aluminate c3a and ferrite c4af other phases are free lime cao and gypsum caso4 2h2o aliteandbelitehydratetocshandportlandite when lime and gypsum are present aluminate hydrates into calcium aluminate hydrate c4ah13 41 ucs decreases with increasing crystalline phases 26 42 hydrationofopcisexothermicandisstudiedbyisothermal conduction calorimetery as hydration proceeds a character istic rate of heat is produced that is a function of composition hydrationeffi ciencyofthesystem temperatureandothereffects 43 44 apureopchydrationcurvecloselyresemblesthatpro duced by c3a and can be corrected to the amount of calcium hydroxide evolved 45 waste opc mixes tend to produce heat of hydration curves that are dissimilar to a pure hydraulic or blendedbindersystem hydrationofopcinstabilized solidifi ed productdependonthenatureandrelativequantityofwaste the maximumrateofheatevolvedisdefi nedasthegradientofhydra tion curve hills et al 46 established that cement hydration is poisonedbyindustrialhydroxidesludge whenthewaste cement ratio is above a certain limit this threshold depends on the natureofthewaste themixingofmetalhydroxidesludgeofcr 0 06 7 zn 0 5 10 and fe 1 2 25 resulted in hin fig 2 poisoningofhydrationduetowasteaddition source adaptedfromhills and pollard 3 drance of the hydration process characterized by an initial peak followed by lack of an exothermic reaction the initial peak is due to precipitation of a solid phase which hinders hy