portland cement is a hydraulic binder forming a paste with water , which sets and hardens due to hydration reactions . the main constituent of portland cements is clinker , which itself is composed of 4080  wt.% tricalcium silicate 3caosio2 ( alite , c3s ) , 1050  wt.% dicalcium silicate 2caosio2 ( belite , c2s ) , 015  wt.% tricalcium aluminate 3caoal2o2 ( aluminate , c3a ) , and 020  wt.% tetracalcium aluminoferrite 4caoal2o3sio2 ( ferrite , c4af ) . alite and belite determine most of the adhesive properties , strength , and durability of portland cement . both alite and belite show the same physical and mechanical properties after complete hydration but the former hydrates much faster . about 9001000  kg co2 per tonne of clinker is released due to calcination of caco3 and fuel combustion [ 2 , 5 ] . the demand for thermal energy equals ~30004000  mj / tonne of clinker for dry process . additionally , ~324540  mj / tonne of cement of electrical energy is required for grinding of raw materials and cement . in contrast , belite has low reactivity than alite , rendering it much less useful . the high reactivity of alite is related to the reactive sites around its more ionic oxygen atoms that are not contained in belite . attempt to improve the reactivity of belite phase in order to reduce energy requirements for cement production is one of the most important challenges . belite has five polymorphs (  ,  ,  , h , and l ) undergoing reversible temperature dependent transformations as illustrated in figure 1 . the h , l , and  polymorphs are derived from the  form by a decrease of the symmetry due to the disorder of sio4 groups and slight changes in the position of ca atoms . -belite is a nonhydraulic polymorph and does not account for the setting and hardening of cement . -c2s crystals are less dense ( more voluminous ) than -c2s crystals , which causes cracking of other -c2s crystals , forming voluminous powder and dust . dusting phenomenon can be prevented if -c2s is stabilized by fast cooling and/or by inclusion of stabilizing ions . fe , al , mg , zn , cr , pb , b , na , and k can replace calcium and/or silicon atoms in the structure and reduce the ca / si ratio of -c2s to less than 2 [ 11 , 12 ] . reactivity and hydraulic properties of stabilized -c2s depend on the type and amount of the stabilizing ions . on the other hand , fast cooling may produce fine -c2s crystals affording --c2s transformation , even without the need for stabilizer . based on density functional theory calculations , stabilizing ions modify charge density localization of the electronic structure of c2s and enhance the reactivity and in turn would broaden the applicability of c2s . synthesis of low - energy belite cement as an alternative to the conventional portland cement was carried out in two steps . the first step is preparation of dicalcium silicate hydrate ( hillebrandite , ca2(sio3)(oh)2 ) by hydrothermal treatment of silica rich waste material / lime mixtures . hillebrandite was synthesized by hydrothermal treatment of quartz and lime with ca / si = 2.0 at 200c for 10 hours or at 250c for 5 hours . the main silicate anion structure of hillebrandite studied by si mas nmr analysis is a mixture of a dimer and a single - chain polymer ( larger than si5o16 ) and that polymerization advanced with an increase of the synthesizing temperature . different polymorphs of calcium silicate hydrates with ca / si = 2.0 , namely , -c2sh [ ca2(sio4h)oh ] , dellaite [ ca6si3o11(oh)2 ] , and hillebrandite , were synthesized and their stability and phase transformation were investigated under hydrothermal conditions up to 400c . -c2sh completely transforms to dellaite at 350c for 1 hour which in turn undergoes a reversible phase transformation to hillebrandite at low temperatures below 300c . hillebrandite was synthesized by mechanochemical treatment of amorphous precipitated silica / lime using a vibration mill at room temperature . moreover , si mas nmr analysis illustrates that the temperature at which -c2s begins to form decreases as the ca / si ratio of hillebrandite becomes higher   or as the temperature of hydrothermal synthesis of hillebrandite becomes higher . on the other hand , xrd analysis illustrates that the crystallographic properties and density of dissociation products of -c2sh depend on the calcination temperature . -c2sh the intermediate phase then transforms to l phase at 920960c and yields -c2s on cooling while hillebrandite yields -c2s at the lowest temperature . hillebrandite synthesized by mechanochemical treatment of amorphous precipitated silica / lime decomposed into -c2s below 1000c . the reactivity of -c2s relates to its large specific surface area taking into consideration the fact that the hydration is chemical - reaction - rate - controlled until its completion . the specific surface area affects the hydration reaction mechanism . in the case of specific areas of 5.5  m / g or less the hydration rate of -c2s enhances with raising curing temperature of hydration . the hydration rate of -c2s enhanced with dynamic more than static hydration condition . at early ages of hydration , -c2s yields c  s  h with a ca / si ratio of 1.81 plus ca(oh)2 . at later ages , c  s  h and ca(oh)2 react to form a c  s  h(ii)-like monophase hydrate having a ca / si ratio of 1.98 with different morphology . recent works presented alternative cements containing higher amounts of -c2s in their composition based upon the use of waste materials such as rice hull and husk ash as well as coal fly ash [ 15 , 26 , 27 ] . silica fume is an abundant material produced in many countries as a waste product from silicon industry around the world containing approximately 9097% os sio2 . silica fume have average particle size < 1  m , specific surface area of 1330  m / g , and specific gravity of 2.22 . reuse of waste materials seems to be a good approach to solve both economic and environmental issues related to cement production [ 2830 ] . freshly prepared lime was prepared by calcination of limestone powder ( purity > 99% ) in an electrical muffle furnace at 950c for 3 hours . lime was cooled to room temperature in desiccator , milled , and stored in tightly closed plastic bottle to avoid carbonation . preparation of hillebrandite by hydrothermal treatment of silica fume and lime with and without 2  wt% bacl2 was carried out as described below . mixtures of silica fume and lime with and without bacl2 keeping the ratio ca / si or ( ca + ba)/si = 2.0 were hydrothermally treated at 110 or 150c for 2 or 5 hours as illustrated in table 1 . mixtures of silica fume , lime , and bacl2 were added to distilled water at water / solid ratio of 5/1 by weight in stainless steel capsule keeping the occupied volume equal to 0.67 of the total volume capacity . the capsule was heated in electric oven for appropriate temperature and time . at the end of the hydrothermal treatment process the product of hydrothermal treatment ( hillebrandite ) was filtered , washed with distilled water , and dried under vacuum . preparation of -c2s was carried out by calcination of hillebrandite in an electric muffle furnace at 600 or 700c for 3 hours . at the end calcination process , the calcined product ( -c2s ) was cooled to room temperature in desiccator , milled , and stored in tightly closed plastic bottles . x - ray fluorescence xrf and x - ray diffraction xrd analyses were carried out by philips x - ray diffractometer pw 1370 co. with ni filtered cuk radiation ( 1.5406   ) . the fourier transform infrared ftir analysis was measured by spectrometer perkin elmer ftir system spectrum x in the range of 4004000  cm with spectral resolution of 1  cm . the thermogravimetric tga and differential thermogravimetric analyses dtg were carried out with the aid of shimadzu corporation thermoanalyzer with dtg-60h detector with 10c / min heating rate from room temperature up to 1000c ; under nitrogen atmosphere at 40  ml / min flow rate , the hold time at the appropriate temperature is zero . the chemical composition results confirm xrd results indicating that limestone is mainly composed of calcite ( caco3 ) while silica fume is mainly composed of amorphous silica as indicated by a broad hump at around 20252.   figure 3 illustrates the ftir spectrum of silica fume . the most significant absorption bands of silica appear at 1101 , 796 , and 467  cm corresponding to asymmetric stretching vibration of si  o  si , symmetric stretching vibration of si  o  si , and bending vibration of o figures 5 and 6 illustrate the ftir spectra of lime / silica mixture hydrothermally treated at 110150c for 25 hours . hydrothermal treatment initiates the reaction between lime and reactive silica leading to the formation of intermediate calcium silicates hydrate ( hillebrandite ) which gives rise to strong absorption band at 950  cm ( vibrations of sio4 groups ) and strong absorption bands at 2530 , 1800 , and 330  cm . unreacted silica gives rise to its absorption band at 1101 ( si  o  si asymmetric stretching vibration )   and unreacted lime gives rise to its absorption band at 3643 and 14501500 cm ( vibrations of oh group and ca calcite arising from partial carbonation of unreacted lime gives its absorption bands at 2530 , 1800 , 1420 , 840 , and 700  cm . the existence of unreacted lime and silica indicates that the hydrothermal treatment of the lime / silica mixture ( ca / si = 2/1 ) at 110c for 2 hours does not drive the reaction to completion . ftir indicates that the chance of formation hillebrandite was improved by increase of both time ( figure 5 ) and temperature ( figure 6 ) of hydrothermal treatment . this is confirmed by disappearance of lime and the decreasing amount of unreacted silica . from economic point of view , the best conditions ( heating at 110c for 5 hours ) were selected for the hydrothermal treatment of silica / lime mixtures . figure 7 illustrates tga / dta thermogram of lime / silica mixture hydrothermally treated at 110c for 5 hours . finally , -c2s transformed to -c2s at 955c . when allowed to cool to room temperature , -c2s may transform -c2s instead of -c2s as illustrated in figure 1 . figure 8 illustrates the ftir spectra of lime / silica mixture hydrothermally treated at 110c for 5 hours and calcined at 600700c for 3 hours . as determined by tga / dta results , c2s gives rise to the following absorption bands : three maxima at 1000 , 910 , and 840  cm ( si  o asymmetric stretching modes ) , medium intensity band at 430  cm ( si  o bending mode ) , and a strong band at 530  cm ( si  o  si out of plane bending mode ) [ 3537 ] . c2s does not clearly appear in sample that was calcined at 600c ( figure 8(b ) ) but clearly appears in sample that was calcined at 700c ( figure 8(c ) ) . in general , formation of c2s was accompanied by liberation of lime ( absorption bands at 3643 and 14501500  cm ) . this confirms the tga / dta results which show that hillebrandite completely dehydrated forming -c2s at 734c . figure 9 illustrates xrd patterns of lime / silica mixture hydrothermally treated at 110c for 5 hours calcined at 600700c for 3 hours . hillebrandite does not appear in the xrd analysis ( figure 9(a ) ) because of its gel structure . c2s does not appear in the sample that was calcined at 600c ( figure 9(b ) ) . the mixture of -c2s ( major ) and -c2s ( minor ) appears in the sample that was calcined at 700c ( figure 9(c ) ) . this proves that -c2s was not stabilized by fast cooling and transforms to -c2s [ 11 , 12 ] . figure 10 illustrates xrd patterns of ( lime + bacl2)/silica mixture hydrothermally treated at 110c for 5 hours calcined at 600700c for 3 hours . in this case the mixture of -c2s ( major ) and -c2s ( minor ) appears in the sample that was calcined at 700c ( figure 10(b ) ) . this indicates that -c2s was stabilized by ba ions with minor transformation to -c2s . in other words , ba ions replace calcium and/or silicon atoms and hence it stabilizes the structure of belite and prevents the transformation of -c2s to -c2s . figures 11 and 12 illustrate the sem micrographs of mixture of silica fume and lime hydrothermally treated at 110150c for 25 hours . the mixtures of silica fume and lime that were hydrothermally treated at 110c for 2 hours contain little amount of hillebrandite of low density with web - like morphology ( figures 11(a ) and 11(b ) ) . on the other hand , the mixtures that were hydrothermally treated at 110c for 5 hours contain higher amount of hillebrandite of low density with web - like morphology ( figures 11(c ) and 11(d ) ) . the morphology of the hillebrandite greatly changes with raising the temperature of the hydrothermal treatment . the mixtures of silica fume and lime that were hydrothermally treated at 150c for 25 hours with / without bacl2 addition contain high amount of short fibrous hillebrandite of high density ( figures 12(a)12(d ) ) . figure 13 illustrates the sem micrographs of mixtures of silica fume and lime hydrothermally treated at 110c for 25 hours after being calcined at 600700c . as determined by tga / dta results , mixtures that were calcined at 600c contain partially dehydrated calcium silicate with glass - like morphology ( figures 13(a)13(d ) ) . on the other hand , mixtures of silica fume and lime that were calcined at 700c contain fine -c2s crystals ( figures 13(a ) and 13(c ) ) . the amount of fine -c2s crystals markedly increases in case of mixture of silica fume and lime with bacl2 addition ( figures 13(b ) and 13(d ) ) . figure 14 illustrates the sem micrographs of mixtures of silica fume and lime hydrothermally treated at 150c for 25 hours after being calcined at 600700c . mixtures that were calcined at 600c still contain partially dehydrated calcium silicate with glass - like morphology ( figures 14(a)14(d ) ) . the amount of fine -c2s crystals formed in mixtures that were calcined at 700c is not affected by raising the temperature of the hydrothermal treatment even in presence of bacl2 addition ( figures 14(a ) and 14(d ) ) . the following is concluded.(1)ftir results show that hillebrandite was successfully prepared by hydrothermal treatment of lime / silica fume mixtures with ( ca + ba)/si = 2 at 110c for 5 hours.(2)tga / dta results show that hillebrandite partially dehydrates in two steps at 422 and 508c and transforms to -c2s at 734c that in turn transforms to -c2s at 955c.(3)xrd results show that -c2s transforms to -c2s upon cooling . in absence of ba ions , -c2s undergoes major transformation to -c2s . whereas , in presence of ba ions , -c2s could be stabilized with formation of little amount of -c2s.(4)mixture of silica fume , lime , and bacl2 with the ratio of ( ca + ba)/si = 2 was successfully utilized for synthesis of -c2s by hydrothermal treatment at 110c for 5 hours followed by calcination of the product at 700c for 3 hours . ftir results show that hillebrandite was successfully prepared by hydrothermal treatment of lime / silica fume mixtures with ( ca + ba)/si = 2 at 110c for 5 hours . tga / dta results show that hillebrandite partially dehydrates in two steps at 422 and 508c and transforms to -c2s at 734c that in turn transforms to -c2s at 955c . xrd results show that -c2s transforms to -c2s upon cooling . in absence of ba ions , -c2s undergoes major transformation to -c2s . whereas , in presence of ba ions , -c2s could be stabilized with formation of little amount of -c2s . mixture of silica fume , lime , and bacl2 with the ratio of ( ca + ba)/si = 2 was successfully utilized for synthesis of -c2s by hydrothermal treatment at 110c for 5 hours followed by calcination of the product at 700c for 3 hours .
this paper describes the low temperature synthesis of belite ( -c2s ) from silica fume . 
 mixtures of lime , bacl2 , and silica fume with the ratio of ( ca + ba)/si = 2 were hydrothermally treated in stainless steel capsule at 110150c for 25 hours , calcined at 600700c for 3 hours , and analyzed by ftir , xrd , tga / dta , and sem techniques . 
 dicalcium silicate hydrate ( hillebrandite ) was prepared by hydrothermal treatment of lime / silica fume mixtures with ( ca + ba)/si = 2 at 110c for 5 hours . 
 hillebrandite partially dehydrates in two steps at 422 and 508c and transforms to -c2s at 734c which in turn transforms to -c2s at 955c which in turn transforms to -c2s when cooled . in presence of ba2 + ions 
 , -c2s could be stabilized with minor transformation to -c2s . 
 mixture of silica fume , lime , and bacl2 with the ratio of ( ca + ba)/si = 2 was successfully utilized for synthesis of -c2s by hydrothermal treatment at 110c for 5 hours followed by calcination of the product at 700c for 3 hours .