Rate of pozzolanic reaction of two kinds of activated coal gangue

J Shanghai Univ(Engl Ed), 2009, 13(4): 322-326Digital Object Identifier(DOI): 10.1007/ 811741-009-0414-1Rate of pozzolanic reaction of two kinds of activated coal gangueZHOU Shuang-xi(周双喜)121. School of Civil Engineering and Architecture, East China Jiaotong University, Nanchang 330013, P. R. China2. State Key Laboratory of Green Building Materials, Beijing 100024, P.R. ChinaAbstract Two kinds of activated ways are used to prepare activated coal gangue fine powder, one is calcining coal gangueat 800C (gangue A), and the other is calcining coal gangue with a certain calcite at 800"c (gangue B). The experimentshows that strengths of blended cement mortar with coal gangue b are higher than that of blended cement with coal gangueA. Hydration of cements with the two kinds of activated coal gangue is investigated through a differential thermal analysis.The weight loss due to Ca(OH)2 decomposition of hydration products by differential thermal analysis/thermo gravimetricDTA/TG)can be used to quantify the pozzolanic reaction. a new method based on the composition of hydration cement isproposed to determine the degree of pozzolanic reaction. The results obtained suggest that the degree of pozzolanic reactionof gangue B is faster than that of gangue aKeywords thermal activated, hydration, differential thermal analysis(dTa), thermo gravimetric(TG), pozzolanic, degreeof hydrationIntroductionamorphous non-crystal, making the coal gangue activeSince the Cao content in coal gangue is low,gen-Coal gangue is waste residue during the productioncally below 3%, the way of calcining coal gangue withof coal mining and coal washing, and is one of the most a certain calcite was adopted. When the original coalcharge capacity of coal gangue takes 15%-20% of the ture, the cementitious property was improve empera-harmful industrial wastes. Generally the overall dis- gangue with calcite was sintering at a certainoutput of raw coal. According to the State EconomicWhile a number of studies have been conductedand Trade Commission of China, annual emission of on high-performance Metakaolin(MK)mortars,therecoal gangue in this country approaches one-hundred mil- are few studies concerning the hydration progresson tons l2, amounting to an accumulation of more thanMK. or similar MK additives3000 million tons, and covering an area of 120000 haAccording to [6, the degree of pozzolanic reactionBecause of different mineral contents of the coalof MK in these pastes was quantitatively determinedgangue in different places, chemical composition is com-using a selective dissolution method. This method hasplex with about 10 elements[3-4. The major elements been developed to quantify the degrees of pozzolanicare normally silicon and aluminum. Most coal gangue reactions, different from the traditional methods foris a clay rock. Its main mineral composition consists ofsessing the degree of cement hydration by determiningclay mineral, followed by primary mineral quartz, potas- nonevaporable water and calcium hydroxide(CH)con-sium,anorthite, siderite, pyrite and calcite. Previous tentsresearch shows the bonding of fresh coal gangue is veryThis study focuses on a quantitative analysis ofweak, because this kind of coal gangue has steady crystal blended cement with activated coal gangue using ther-structure, and the atom ion and members are arrangedaccording to certain law by order even if its chemicalal analysis. The objective is to determine the degreecomposition is proper and lowly active. After calcinat-of reaction of the pozzolansing at high temperature of 700-850"C, the clay mineral 1 Experimentalis dewatered and disintegrated and the carbon conent is removed while the deteriorative impurity burnedThe 52.5R Portland cement produced by the Huaxinout.The crystal is disintegrated and transformed into Cement Plant in Hubei Province was used in all theReceived Sept 5, 2007; Revised Jun 2, 2008中国煤化工Projectorted by the Opening Project of State Key LaborateKF105), and the Engineering Research Center of Railway EnvironmCNMHG G ant ioG G-igtNo.01306020)CorrespondingauthorZhoUShuang-xi,Phd,E-mail:green55q163.comShanghai Univ(Engl Ed), 2009, 13 (4): 322-326323mortar and paste mixtures. Coal gangue was obtainedThe blended cement paste mixture design is pre-from the Xuzhou Coal Mine. The first activated gangue sented in Table 2. Mixture No. 1 is blended cement withwas simply calcined coal gangue at 800"C, referred to as 30% gangue B. Mixture No. 2 is blended cement withgangue A, and the second activated gangue was calcined 30% gangue A. Mixture No. 3 is blended cement withwith a certain content of calcite at 800 C, referred to as 30% quartz powder, which is an inert mineral additive.gangue B. The chemical composition characteristics ofordinary portland cement(OPC)and the original coal Table 2 Mixture design of different blended cement pastesgangue are listed in Table 1. The XRD patterns for ce-ment clinker and the Xuzhou original coal gangue are Mix No. OPC/%Additives/%shown in Fig. 1.Gangue b Quartz powderTable 1 Chemical compositions of the cement clinker and Control 1000.32the original coal gangue03200.32ChemicalXuzhou coal20compositionclinker/%gangue/%21,065795The mortar specimens were prepared accord.ing to GB/ T17671-1999, which specifies a water-to-Fe?O33.63cementitious material ratio of 0.5 and a sand-to-3.16cementitious material ratio of 3 by weight. Compres-267082sive strengths of the mortar specimens were determined3rd and 28th days, res3.15strengths of mortar are presented in Table 3To optimize the use of blended cements, pozzolanic▲-C3Sreaction degree of mineral additives must be quantifiedThe differential thermal analysis(DTA)combined withmetric analysis(TGA)is more suitable forfCtudying hydration at later stages. DTA locates theranges corresponding to thermal decompositions of dif-ferent phases in the paste, while TG simultaneouslymeasures the weight loss due to the decompositionsThe ages of testing were 0.25, 0.50, 1, 2, 3, 7, 14, 21, 28,and 60 d and completely hydration was accomplished.a)XRD patterns for cement clinker2 Results and discussion2.1 Compressive strength of OPC andblended cement mortarsFrom Table 3, we can observe that the compressivestrength of blended cement mortar decreases as com-pared to the OPC mortar. The blended cement mortarcontaining coal gangue B exhibits higher compressivestrength than that of blended cement containing coalgangue A.56602.2 Hydration of blended cement pastes8121620242832containing activated coal gangueq-quartz, k-kaolin, c-calcite m-micaActivated coal gangue powder is a mineral additiveIt can improve microstructure and mechanical properXuzhou original coal gangueties of blended cement when used to partially replaceFig 1 XRd patterns for cement clinker and the Xuzhou Portland cement. The reaction mechanism can be diTH中国煤化工 pectslt-9, The physticles fill the voids inA water-to-binder ratio(w/b)of 0.32 was chosen. cemerCN MH Structure of cementThe value is quite common in practice. The blended mortar denser. The chemical effect is the reaction ofcement is mixed with 30% activated coal gangueMK with cement hydratesJ Shanghai Univ(Engl Ed), 2009, 13(4): 322-326Table 3 The 3rd and 28th day strength of different blended cement mortarFlexural strength/MPaCompressive strength/MPaMix No. Cement/% Gangue A/% Gangue B/%3rd day28th day3rd day28th day06.517.446.65.69719.33Figure 2 shows the xrd patterns of blended ceto aFt also indicates that more aFt formes in blendThe main Ca(OH)2(CH)peaks in cement OPC are the strength development of cement mortar. The metdrlyment pastes with activated coal gangue and OPC pastes. cement with gangue B which is helpful to the earlyhighest among the four cement pastes, where the ch nism of reaction is as followspeaks of blended cement with coal gangue B of are thelowest,and the CH peaks in simple thermal activated C3S+H2O-C-S-H+CH,are medium. This was caused by reaction of CH with SIO2+CH+H20-C-S-HMK. CH cannot directly produce strength of the ce-ment paste. Only after it was translated to C-S-h gelAl2O3+CH+CaSO4+H2Oby pozzolanic reaction with active minerals could thisCaO·Al2O3·3CaSO4·31H2O(AFt)was formed, and the higher strength of cement reached. 2.3 DTA/TG of blended cement pastesOn the other hand, the size of the Xrd peak attributedcontaining activated coal gangueTwo important quantities were obtained fromDTA/TG, First the chemically bound water, definedthe mass due to decomposition between the boiling tem-perature and 1100C(wb), which can also be under△CSV CAAFstood as the amount of water needed for cement to react▲oAFtThe value of w was calculated within 105-1100C(seeig. 3). The initial mass loss within 600-780C was as-sociated to decomposition of calcium carbonate. Thismass loss, about 1% of the initial weight, is illustratedin Fig 4 as COloss. Since the paste sample consists of unhydrated cement, a correction to the calculation of wbshould be made. This was done by subtracting the massloss within a 600-780C from wb. The second impor-tant quantity is the mass corresponding to the decom-81216202428323640448525660position of Ca(OH)2 which occurs between 440"C and(a)3d520C. Variations of wb and CHloss with time are re-▲cHported after being normalized by the ignited weight ofw Quartzi..ixe8121620242832364442660中国煤化工(b)28dCNMH1000Fig 2 XRD patterns for 1, 2 and OPC28d cementFig 3 wb content of different cement pastJ Shanghai Univ(Engl Ed), 2009, 13(4): 322-326reduction of Ch due to pozzolanic reactions. To calcu-late such a difference, the data points need to be curve-fitted. The result of data fitting is illustrated in Fig. 5The degree of pozzolanic reaction is calculated fromCHOPC+inertCHOPC+activate gangChIOPCtinert (wb, o)-CHIosS tactivate-gangue (wb, oo)004006008001000where ap stands for the degree of pozzolanic reactionIt can be seen from Fig. 5 that the main CH contentFig 4 DSC/TG measured hydration quantitiesin cement OPC is the highest among the four cementpastes. The second is OPC containing quartz powder2.4 Degree of hydration of activated coal The content of CH of cement with activated coal ganguegangue from DTA/TB is the lowest. CH contents of blended cement pastesPozzolans can react with hydration products (cal- containing coal gangue B are lower than that of blendedcium silicate hydrated and calcium hydroxide )of OPC cement containing coal gangue Aand water. CH is known to react with activated coalThe degree of pozzolanic reaction determined by thegangue. This becomes more evident as the amount of above calculation is shown in Fig. 6. It can be seen thatCH is plotted against curing time. For OPC pastes, Ch the degree of hydration of activated coal gangue is aincreases as the age increases indicating a proportionaelation with the degree of hydration. In pastes contain-ing activated coal gangue, CH initially increases thendrops as it is consumed increasingly by pozzolans. How-ever, from the experimental data it is evident that Chproduced by blended cement with activated coal ganguehydration is less than CH produced by blended cementcontaining quartz powder, which is an inert mineral ad-OPCditive. Based on this difference variation of the amountof CH with time can be used as a measure of pozzolanicreaction. In this work, only the weight loss associatedwith the decomposition of CH(CHloss)can be accu-rately measured. However, as was shown by Midgley lothis weight loss is very close to the amount of water inCH and therefore, is proportional to the amount of CHFig5 Ca(oH)2 content of different cement pastesThe two most important quantities needed to quan-tify the degree of hydration are the amount of chemi-=00008x09525cally bound water(wp)and the weight loss correspond-R2=09096ing to the decomposition of calcium hydroxide(chloeIn correlating CH,oss to wb, it was assumed that CHwas produced only by OPC, not by OPC and pozzolan0.1Therefore CHloss and wb were normalized by the weightof oPC in each mixture, which was estimated from the100010000proportion between OPC and pozzolan, e.g., 30% OPCcontaining 70 activated coal gangue. For OPC pastes(a)Coal gangue ACHloe changes almost linearly with wb. This comeswith no surprise since the hydration products, which0.5Fy0006.%6are proportional to wb, include CH. The degree of hy-90482-095dration of oPC pastes are obtained by normalizing Wbto its maximum valueIn order to estimate the degree of hydration of poznolans, a reference CHloss-wt plot is needed. Clearly,中国煤化工100010000this would be the plot for OPC containing quartz powder. The difference in CHloes-w plot obtained for eachCN MHGBblended cement system relative to CHloss-wrs plot forOPC containing quartz powder is assumed to define theFig 6 Degree of activated coal gangue pozzolanic reaction326J Shanghai Univ(Engl Ed), 2009, 13(4): 322-326function of time. From the above regression equationsy=000080525andy=0.0006086, we can see[3] LI Dong-xu, SoNG Xu-yan, GoNG Chen-chen. Researchthat the degree of hydration of coal gangue B is fasteron cementitious behavior and mechanism of compositethan that of coalcement with coal gangue J. Journal of the ChineseCeramic Society, 2004, 32(3):358- 363 (in Chinese)inclusions4 Hunan University. Building materials [M. BeijingBased on this study, the following conclusions can beChina Building Industry Press, 1997(in Chinese)drawn5 POoN C S, LAM L S C. Rate of pozzolanic reaction ofThe blended cement mortar containing coalMetakaolin in high-performance cement pastes J. Ce-gangue B exhibits higher compressive strength than thatment and Concrete Research, 2001, 31(9): 1301-1306of blended cement containing coal gangue A(6 PANE I, HANSEN W. Investigation of blended cement(ii)The main CH content of blended cementhydration by isothermal calorimetry and thermal analwith activated coal gangue is lower than thatsis J. Cement and Concrete Research, 2005, 35(6)paste containing the same contents of quartz11551164Variation of the amount of CH with time can be used [7 WANG A Q, ZHANG C Z, SUN W. Fly ash effects:ITheas a measure of pozzolanic reaction. The degree ofmorphological effect of fly ash [J. Cement and Concretehydration of coal gangue B is faster than that of coal):20232029gangue A(8 WANG A Q, ZHANG C Z, SUN W. Fly ash effects: II.ReferencesThe active effect of Ay ash J]. Cement and ConcreteResearch,2004,34(11):2057-2060[1] National State Economic and Trade Commission. Pol9 WANG Ai-qin, ZHANG Cheng-zhi, SUN Wei. Fly ash ef-icy point of coal gangue utilization put out by nationalfects: Ill. The microaggregate effect of fly ash J. Ce-state economic trade commission J]. Coal Cleanment and Concrete Research, 2004, 34(11): 2061-2066Technology, 2000(1):5610] MIDGLEY H G. The determination of calcium hydrox-2 WU Zhong-wei, LIAN Hui-zhen. High performance con-ide in set porland cements J Cement and Concretecrete IM. Beijing: China Railway Press, 1999(in ChiResearch,1979,9(1):7782中国煤化工CNMHG