Reactivity of calcium dialuminate (CaAl4O7) refractory cement with lithium carbonate (Li2CO3) admixture

ABSTRACT The reactivity of calcium dialuminate CaAl4O7 refractory cement containing little amount of gehlenite Ca2Al2SiO7, belite Ca2SiO4, calcium aluminum iron oxide Ca3(Al,Fe)2O6 and calcium iron aluminum oxide Ca3.18Fe15.48Al1.34O28 was studied with and without Li2CO3. The granularity of the cement is d97 = 112.58 µm, d50 = 9.72 µm and d10 = 1.41 µm with water cement ratio W/C = 0.375. Little amount of Li2CO3 from 0.1wt.% to 0.6wt.% was added to the cement paste, and the result from calorimetry showed hydration mean peak time reduced from 17 h to 32 min and showed that the hydration heat of CA2 cement paste with Li2CO3 was 123 J/g lower than the one without Li2CO3 of 168 J/g. XRD of stabilized hydrates show that CaAl4O7 and Ca3(Al,Fe)2O6 participate in the hydration reaction to form C3AH6, AH3 and Ca3Al1.54Fe0.46(OH)12. The results from SEM images show nucleation site with plate-like crystal of CAH10 and small particles of AH3 forming a membrane on the cement surface.


Introduction
The development of new processes and increased requirements for special mortars has led to constant research on the amelioration of traditional binders. The use of calcium dialuminate refractory cement as binders is not well known [1]. For the long setting time required since the production of building motars results mainly from a high speed of strength development.
Calcium dialuminate (CA 2 ) is an important constituent of high alumina refractory cements [2], compared to calcium aluminate (CA), the other principal component. CA 2 hydrates very slowly at room temperature [3], and the reactivity of CA 2 may increase in the presence of CA [3]. Calcium dialuminate exhibits very low thermal expansion and excellent thermal properties and thus considered a desirable phase for refractory applications [4].
Calcium aluminate cement with 70-80wt.% Al 2 O 3 can be used to produce castables with a temperature resistance ranging from 1800 to 1900°C in order to improve technical performance of Low Cement Castable (LCC) of 5-8wt.% for high performance refractory material [5]. For a long time, the hydration of CA 2 was an open issue [2]; and CA 2 was even assumed to be inert. CA 2 tends to react very slowly within the first 48 h. For this reason, in many earlier studies, it has been the hydration of CA alone, which has been intensively investigated.
Chudak et al. [6] reported on the hydraulic reactivity of CA 2 ; they investigated on pure CA 2 with 70wt.% Al 2 O 3 . Using the DCA technique and describes a slow CA 2 cement hydration occurring over several days. Negro et al. [7] concluded that the solubility of CA 2 is much lower than that of CA. This could explain the very low hydration reaction activity of CA 2 in secar71 (calcium aluminate cement with 71% of alumina), which begins after 23-48 h and finish after several months. And AH 3 was observed during hydration of CA 2 at ambient temperature [7,8].
Klaus et al. [3] observed that CA 2 hydration proceeds together than 22 h, and it is known in the investigations that CA 2 was completely dissolved after few days, and they attributed it to the very slow precipitation of C 2 AH 7.5 , C 2 AH 8 . These components were occurring not at the particle surfaces on CA 2 but from pure solution and therefore hydration can proceed until CA 2 is completely dissolved [3].
To accelerate the setting time of alumina cement, Kurdouski et al. [9]., Bensted et al. [10], Robson et al. [11], used mixtures of alumina cement and portland cement as binders in type F tile adhesives [PN-EN12004]. The setting time is shorter when the content of one cement in the mixture is between 20 and 80wt.% [2].
Bensted et al. [10] pointed that the literature concerning possibilities to accelerate the setting and hardening of alumina cement is very extensive. Lithium salts, sodium and potassium hydroxides, portland cement, calcium oxide, and calcium sulfate hemihydrate are the components that mainly influence the hydration of alumina cement. The presence of alkali metals salts in the alumina cement paste and mortar was subject of the investigations by Matusinovic and vrbos [12,13], Venuat et al. [14], Malgorzata Niziurska [1,15].
It is known that hydration products from pure CA 2 forms after 24 h [1][2][3]. With conventional CA 2 , the initial CAH 10 hydration products appear after 3 days followed by the formation of C 2 AH 8 and its conversion to C 3 AH 6 between 3-14 days [7].
The objective of this work was to study the mechanism and conditions that appear when used Li 2 CO 3 as accelerator of setting time of CA 2 cement. In this case, Li 2 CO 3 were used from 0.1 to 0.6wt.% in calcium dialuminate refractory cement made. Calorimetry heat flow, XRD, and scanning electron microscope were used to observe the hydration of CA 2 with lithium carbonate and the microstructure of cement paste without and with lithium carbonate after 14 days.

Powder cement preparation
The calcium dialuminate cement was obtained by solid state reaction of a mixture of high quality bauxite (Al 2 O 3 ≥ 80wt.%) with lime (CaO ≥ 95wt.%) ( Table 1). The mixture was homogenized in a milling jar for 10 min then dried at 105°C. The agglomerate is treated at 1550°C for 2 h. The clinker obtained was milled in a SiC jar with SiC mill. The medium diameter of the particles was d 50 = 9.72 µm, d 10 = 1.41 µm and d 97 = 112 µm.

Cement paste preparation
To the prepared powder CA 2 cement, varying amount of Li 2 CO 3 0wt.%, 0.1wt.%, 0.2wt.%, 0.3wt.%, 0.4wt.%, 0.5wt.% and 0.6wt.% is added, respectively. The Li 2 CO 3 is weighed and added to the cement powder and mixed before add the water in the calorimeter for injection method. For external mixing or ready-mixed mixture, Li 2 CO 3 is weighed and added to cement and mixed with water in the rotary mixer before put in the calorimeter and beginning the measurement of heat flow. A paste was prepared with a water cement (W/C) ratio of 0.375. One part of the cement paste was molded in a cylindrical container and vibrated for 10 s and used for XRD and SEM analysis. Another powder was used directly after mixing with water for calorimetry to study the heat flow and hydration heat of cement.

XRF technique
For XRF analysis, a hXRF Niton XL3t980 analyzer (equipped with an Ag-Anode 50 kV X-ray tube and Silicon-Drift-Detector 8 mm spot was used. The raw data were plotted in spectra, where x-axes represent element-specific fluorescence energy (unit keV), and y-axes quantify counts of photons (unit cps) received by the detector. Detection is possible for most of the elements with atomic numbers ranging from 12 magnesium to 92 (uranium). 21 silicon-based standards socalled Certified Reference Material (CRM), filled in cups and covered with 4 μm polypropylene film were measured by a hXRF device specific mode (mining/mineral mode). The measured values were plotted using a trend line equation and the "fitting coefficient" R 2 (correlation coefficients) were determined. Afterward, a classification was made according to the quality of the regression line and the distribution of the data.

Calorimetry technique
To illustrate the evolution of heat during the hydration process of the cement paste, an Erlanger calorimeter was used. The measuring principle of the Erlangen calorimeter is based on a temperature difference between the reference sample and the material to be analyzed. This temperature difference is converted into an electrical voltage and measured by a digital multimeter. Data recording of the heat source is carried out via the data transfer program OMI. There are two possible applications for the calorimeter: the injection method and the ready-mixed mixture. In the injection process, the water of about 1.125 g is injected directly into the dry mix about 3 g and the full heat history can be recorded. In the readymixed mixture, the quantity of the cement paste is about 8 g with W/C = 0.375 at 20 ± 2°C. Sample of cement paste were casted into three different plastic crucible, with 20 mm in diameter and 15 mm in height for assessing the measurement of heat flow calorimetry. The Qcement is calculated from Qsample with the formula:

XRD technique
XRD measurements were performed using a D8 diffractometer equipped with Lynx-eye position sensitive detector (Brucker -AXS), with Cu Kα 1 ʎ Cu = 1.54056 Ǻ radiation operated at 40 kV and 40 mA, increment 0.013°2Ɵ, and a measuring time per step of 30 s. The diffraction patterns were collected in the 2 theta range from 7.5 to 90°. Qualitative analysis of the phase composition of the powders was conducted using the PDF-2 2007 release software and X'Pert High Score Plus.

SEM technique
The grain size, surface topology and pore diameter of the hydrated cement was determined using an XL30 series Philips scanning electron microscope XL30 ESEM FEG (Fa, FEE) equipped with an energy dispersive analyzer (EDS), EDAX, Fa Annatet with GENESIS-software, run at 10 kV and at a resolution of 500-7500. The SEM images on cement sample have been done after curing age 14 days.

XRD results
The results of the XRD analysis of calcium dialuminate refractory cement in powder form is presented in Figure 1 and hydrated form in Figure 2. It is observed from Figure 1 And calcium aluminum iron oxide Ca 3 ðAl; FeÞ 2 O 6 hydrates to calcium aluminum iron hydroxide Ca 3 Al 1:54 Fe 0:46 ðOHÞ 12 Gehlenite Ca 2 Al 2 SiO 7 does not hydrates and it is observed on XRD of hydrated cement. Authors Antonovica et al. [5] proposed that gehlenite C 2 AS (considerable amount in cement) does not hydrate [5]. Contrary to this, investigation by Niziurska et al. [1] suggested that gehlenite C 2 AS reacts slowly with formation of stratlingite hydrated calcium aluminosilicate C 2 ASH 8 and we know that stratlingite is a stable 10 hydrate of importance in terms of late strength of alumina cement. Another way, Singh and Majumdar [2] reported that in the presence of granulated blasfurnace slag (ggbs) on CA 2 or a mixture of CA 2 + CA will form stratlingite (C 2 ASH 8 ) on hydration at 40°C within 3 days. This formation of stratlingite, which is hydrated calcium aluminosilicate. The absence of stratlingite on the hydrated cement made ( Figure 2) is explained by the low reactive of gehlenite Ca 2 Al 2 SiO 7 in the cement powder.

Calorimetry studies of calcium dialuminate cement without lithium carbonate (Li 2 Co 3 ) admixture
The hydration of calcium dialuminate cement paste was studied through the injection method Figure 3 12 . Authors Antonovic et al. [5], mentioned a difficulties in describing the origin of heat flow between 5 and 12 h when he used a mixture of CA and CA 2 . In this context, we supposed that no heat flow have been observed between 0 and 15 h.
In Figure 3(b) the heat flow curve presents one peak at 17.05 h attributed to the hydration of calcium dialuminate in the cement paste.  The calculated value of the enthalpies of reaction for CA 2 is −498 J/g obtained from corresponding enthalpies of formation by Klaus et al. [3]. But the results of hydration heat obtained by Singh and Majumdar [2] is 192 J/g at 20°C with W/C = 0.4 is higher than the one obtained (Table 2) at 168 J/g at 20 ± 2°C. Contrary, the result obtained in Table 2 for the hydration heat, is higher than the one obtained by Niziurska et al. [1] at 140 J/g with W/C = 0.43 at 23°C. The lower or higher value of hydration heat depends to the particles size distribution, the impurities in the cement like gehlenite and calcium aluminum iron oxide.

3.3.
Calorimetry studies of calcium dialuminate cement with (Li 2 Co 3 ) up to 0.6wt.% 3.3.1. Lithium carbonate as the factor for reduction of heat of hydration The heat flow for calcium dialuminate cement is presented in Table 2 and Figure 4. From Table 2, the hydration heat for cement without Li 2 CO 3 is about 168 J/g and higher than the one with Li 2 CO 3 with an average heat of 123 J/g. Niziurska et al. [1] obtained in the case of hydration of CA 2 cement pastes with 0.3wt.% of Li 2 CO 3 the total amount of evolved heat was 80 J/g lower than the one obtained in Table 2. With the addition of lithium carbonate, the total amount of evolved heat is lower than the one without lithium carbonate. The presence of lithium carbonate admixture reduces the hydration heat of cement pastes.

Lithium carbonate as the factor for reduction of hydration time
It's presented on Table 2 and Figure 4 that the hydration mean peak time of cement with Li 2 CO 3 is about 32 min and without Li 2 CO 3 is about 17.05 h. When a little amount of 0.1wt.% of Li 2 CO 3 is added, the peak time is reduced to 43 min (0.72 h) and this can be further reduced to 24 min (0.40 h) at 0.6wt.% of Li 2 CO 3 . We can confirm that Li 2 CO 3 is a good accelerator of calcium dialuminate refractory cement. Lithium carbonate influences the hydration of high alumina cement through its rapid dissolution in solution, and substitution of Ca 2+ by Li + , lithium ion is a light and less dense element with atomic weight of 6.94 g/mol and density of 0.534 g/cm 3 , while Ca 2+ is a heavy element  with atomic weight of 40.08 g/mol and a higher density of 1.54 g/cm 3 . The substitution of Ca 2+ by Li + is the first reason for the mobility of lithium ion and rapid setting time of calcium dialuminate cement paste. The influence of lithium ion on the hydration process of calcium dialuminate cement was studied by Dittrich et al. [16]. Introduction of Li + ion leads to precipitation of LA 2 H 10 in the paste, a phase transition, which prevents the formation of an impermeable coating on CA 2 grains. Substitution of Ca 2+ ion by Li + ions leads to the crystallization of C 2 AH 8 , while free Li + cation forms LA 2 H 10 transition phase again. Another lithium ion from chloride have been studied by Rodger et al. [17] and Acuna Gutierrez et al. [18]; they reported that LiCl addition form small aggregates of the compound Li 2 Al 4 O 7 (H 2 O) 11 in the cement that may act as a nucleation site for the hydrated products thus accelerating the conversion process. Friedels salt (JCPDS 089-52-94) Ca 4 Al 2 O 6 Cl 2 (H 2 O) 10 formed by the reaction of CAC with chloride ions from LiCl solution, and through the intermediate stable cubic phase Ca 3 Al 2 ðOHÞ 12 is also present. Nygaard et al. [19] reported from Justnes et al. [20] the effects of the more common alkali carbonates (Li 2 CO 3 , Na 2 CO 3 and K 2 CO 3 ) on setting times and strength of cements. Valenti et al. [21] proposed that Na and K carbonate retarded the setting at lower dosages, but accelerate the setting time at dosages higher than 0.1wt.%; when Li 2 CO 3 acted as an accelerator at all concentrations studied. This inhibition of further hydration of grains coated by layer of rapidly growing reaction products and faster conversion of hydrates into hydrogarnets in the case of cement mortars with lithium carbonate leads to strength decrease after 24 hours compared to cement mortars without admixture as observed by Matusinovic [12] and Niziurska et al. [1].

SEM of hydrated calcium dialuminate cement with different amount of Li 2 Co 3
Scanning Electron Microscopic analysis of hydrated calcium dialuminate cement with and without lithium carbonate was performed. Figure 5 shows SEM image and corresponding EDS spectra of the hydrated calcium dialuminate cement without lithium carbonate. At this magnification (200 µm scale), two phases are observed: residual cement grain of CA 2 and a hydrated one in the interface of CA 2 cement grain. The surface has a sponge structure with the pores repartition up to 100 µm and open micropores between 10 and 0.1 µm and ultrapores on cement grain between 0.1 µm and 0.01 µm. In Figure 6, the SEM images show the topological repartition of particles in hydrated cement with lithium carbonate. The surface morphology reveals a lot pores and residual unreacted grains of CA 2 associated with hydrated phase. However, the surface of cement is more compact than the one without lithium carbonate. Figure 7 presents the surface of cement at high magnification (10 µm and 5 µm scales) on Secondary Electron SE (Figure7(a)) and Back Scattered Electron BSE (Figure 7(b)). The hydrated phase is seen a crosslink and contain many pores are also observed up to 5 µm in diameter. Cracks of length up to 10 µm are also observed. On BSE (Figure 7(b)), the white color and brown color are associated to titanium and iron elements respectively.    From (Figure 8), short needle-shaped crystal structures of C 3 AH 6 are developed on surface of the grain, when AH 3 forms a thin layer deposit mass on the surface. Newman et al. [22], from studying on SEM images of cement paste with W/C ratio of 0.4 observed that the C 3 AH 6 is finely dispersed in a matrix of hydrated alumina and presented like a cubic and had a morphology of compact equiaxed facetted crystals, where AH 3 is often poorly crystalline and deposited in formless masses.

Element
Niziurska et al. [1] studied the mixture of CA/CA 2 cement paste and observed a significant amount of carboaluminate C 4 ACH 11 and gibbsite AH 3 after 24 h for cement without Li 2 CO 3 , and when a 0.3wt.% of Li 2 CO 3 is added, carboaluminate and crystal of gibbsite are observed after 1 h of mixing with water.
Antonovic et al. [5] reported that irregular cubic crystals are completely or partially observed by a crystalline material. Crystalline gibbsite with diameter between 0.15 and 0.30 µm. When Acuna-Gutiérrez et al. [18] confirmed that water forms hydrated products with cement powder such as CAH 10 , C 2 AH 8 , C 3 AH 6 , and AH 3 . Although the temperature is higher than 30°C, AH 3 and C 3 AH 6 are formed and the possibility to accelerate with LiCl forms small aggregates of Li 2 Al 4 O 7 (H 2 O) 11 may act as nucleation site for the hydrated products accelerating the conversion process. In our studies, the hydration process with CA 2 with Li 2 CO 3 may be explained as follows.
Nucleation site constituting lithium aluminate hydrate Li 2 Al 4 O 7 (H 2 O) 11 is formed. This new site is surrounded by crystal of CAH 10 and AH 3 as seen from ( Figure 9) and corresponding EDS. This new phase assembles into flower like a polymeric structure (Figure 9). Such a small cluster of monomer crystals of CAH 10 in the correct arrangement can be transformed in to C 2 AH 8 and AH 3 , an intermediate stable cubic phase with formation of final stable phase C 3 AH 6 and AH 3 hydrated phases.
Niziurska et al. [1] stated that hydration of calcium monoaluminate with lithium carbonate significantly accelerates crystallization of C 2 AH 8 defined by a first hydration product of cement phases and rapid conversion into hydrogarnet. Contrary for this statement, the first hydrated phase of CA 2 obtained is CAH 10 from the polymeric flower (Figure 9). Figures 8 and 9 present CAH 10 crystal formed on the surface of CA 2 , the average diameter of CAH 10 prism is 0.3 µm, and C 3 AH 6 crystals present the average length reaching 9 µm; prevail irregular cubic crystal completely or partially covered by a crystalline material of about 4 µm (Figure 9). The X-ray microanalysis showed that elemental composition of this sample in molar percentage is Al 2 O 3 : 48.59%, CaO: 46.94% (H analysis was not performed with the spectrometer). The composition material is similar to CAH 10 .
Since some literature sources suggests that nitrates may form a complex salt with the aluminate phase in cement, it was initially assumed that the accelerating effect, was dependent on aluminate content of the cement. However, during the study, it was shown by X-ray diffraction that it was not possible to detect any reflection arising from the calcium-aluminate nitrate compound Ca 3 Al 2 O 3 Ca(NO 3 ) 2 (H 2 O) 10 .
The calculated surface charge of calcium dialuminate (CaAl 4 O 7 ) is σ þ σ À ¼ 0:16, for calcium aluminum iron oxide Ca 3 ðAl; FeÞ 2 O 6 is σ þ σ À ¼ 0:30 and for gehlenite Ca 2 Al 2 SiO 7 is σ þ σ À ¼ 0:18. Although the impurities in the cement increase the positive surface charge on the cement, the global surface charge remains negative for the cement and lower than the surface charge of CaAl 2 O 4 which is σ þ σ À ¼ 0:22. The strongest negative charge of calcium dialuminate cement improves their resistance to the corrosive effect of certain acids and industrial wastes. The resistance of corrosion down a pH 3.5-5 depends on the type of acid, temperature, length of exposure. It is mentioned that this type of cement can resist to alkali up to a pH of 12 with the exception of alkali hydroxide and give the possibilities to use cement for making concrete resistant to sulfate ground water and seawater.
3.5.2. Influence of additives/admixture on the hydration heat of calcium dialuminate cement Singh and Majumdar [1] studied different case of hydration heat of CA 2 cement without and with additives at 20°C and at 40°C (Table 3). They mentioned that the hydration heat of cement during 24 h at 20°C is 192 J/g, and at 40°C, the hydration heat is 359 J/g, and they observed that the evolved heat added to the system increased the speed of hydration of CA 2 products at 40°C than at 20°C. Singh and Majumdar used the granulated blast furnace slag (ggbs) as the addition to CA 2 cement in proportion 1:1 at 20°C. They obtained the hydration heat of 87 J/g less than half of the pure CA 2 at 20°C and at 40°C, the hydration heat with gbbs  gives 210 J/g and stratlingite C 2 ASH 8 was detected in both cases. The studies obtained from the addition of 20wt.% of CA in CA 2 at 20°C and 40°C with W/C = 0.4 give the total heat after 24 h at 118 J/g at 20°C and 338 J/g at 40°C lower than CA or CA 2 alone. The additives likes CA, gbbs can increase the speed of hydration of CA 2 products and reduce the hydration heat of cement. The work done by Niziurska et al. [2] obtained the amount of evolved heat with lithium carbonate admixture equal to about 80 J/g, while the amount heat during the hydration of cement paste without lithium carbonate was almost 240 J/g. They conclude that, the lower heat of hydration with lithium carbonate was due to the inhibition of further hydration of grains coated by layer of rapidly growing reaction products. It is then observed from Table 3 that the admixture like Li 2 CO 3 and additives like gbbs and CA reduce significantly the hydration heat of CA 2 cement. And, it is much more pronounced with Li 2 CO 3 than gbbs and CA, respectively.
The rapid setting time of CA 2 with Li 2 CO 3 admixture at 0.1wt% (Table 2) open the way of the research on biomedical applications for his high strength than the minimum required for bone cements according to the ASTMF451 standard. The formation of AH 3 phase in hydrated CA 2 sample is much more than in CA. This was expected as CA 2 contain more Al 2 O 3 , the strength of hydrated CA 2 is found to increase than CA.
This result improves the characteristics of cement for construction and biomedical applications.

Conclusions
A novel approach to understand the reactivity of calcium dialuminate CA 2 cement with lithium carbonate (Li 2 CO 3 ) admixture have been studied at low temperature 20 ± 2°C. A little amount of 0.1wt.% Li 2 CO 3 improve the reactivity of CA 2 by reducing the reaction peak time from 17 h to 43 min. The hydration heat is 168 J/g for the CA 2 cement without lithium carbonate and 123 J/g with 0.3wt.% of lithium carbonate. It was observed from SEM and EDS images that Li 2 CO 3 acted at a catalyst; a nucleation site of Li 2 Al 4 O 7 (H 2 O) 11 for hydrated products accelerating the conversion to CAH 10 and AH 3 which assemble to a polymeric « flower » like structure in the correct arrangement. The admixture like Li 2 CO 3 , reduces significantly the hydration heat of CA 2 cement. While the global surface charge of cement made with lithium carbonate admixture remains negative, thus improving the characteristics of the cement for construction and biomedical applications.