A mineral material, upon thermal treatment, undergoes chemical and structural modification. These include dehydration, dehydroxylation, decomposition, new phase formation. These mineralogical transformations are influenced by the chemical composition, the initial mineralogical composition; the particles size, the firing temperature, the heating rate and the soaking time   .
The phase transformation of clay containing calcite is subject of many reported studies  - . Usually, for clayey material with low iron impurities (Fe < 5%), about 50% SiO2, 15% Al2O3 and 10% carbonate, the new phases formed are gehlénite, wollastonite, anorthite, cordierite, larnite, périclase, akermamite, forstérite, spinell and monticellite.
In Central Africa Republic (CAR), carbonate, schist, quartzite, micashiste and amphibole are found in the neighborhood of Bangui    . The valorization of the carbonate could be achieved in clay-carbonate mixture for improved ceramic products making. Such a mixture could influence the properties of fired products or induced different heating pathway.
This study evaluates the influence of carbonate addition to kaolinitic clay from Bimbo in the elaboration of fired products. In particular, this work aim is to analyze the influence of carbonate addition to iron rich kaolinite clay, on phase transformations during thermal treatment and its influence on the products densification.
To this end, several technics, including X-ray diffraction, Thermal analysis, TGA-DTA; Dilatometry; Scanning electron microscope (SEM), X-ray fluorescence, are used to analyze fired bricks at temperature ranging from 900˚C to 1200˚C. Technological properties such as water adsorption, porosity and flexural strength were evaluated.
2. Materials and Methods
The clayey material used (BIM1) was collected from Bimbo, a locality of the Ombella-M'Poko division, situated at about 8 km in the South-west of Bangui. This deposit altitude is 340 m and the GPS coordinates are as follow: 04˚19'17" North latitude and 18˚31'57" East longitude. The dolomite (DO) was collected at Dogbe, a locality situated at about 100 km from Bangui with the following GPS coordinates 05˚05'N et 18˚33'E.
The chemical and the mineralogical composition of the clay are given in Table 1 . The clay material is iron rich with a tenor of 9%. The SiO2 and Al2O3 contents are respectively, 50% and 25%. In a previous work , the fired brick, at 1100˚C, of this sample exhibits technological properties of interest (flexural strength 15 MPa and a porosity of 26%).
In this study, the addition of dolomite in the formulation will be 8% (mass basis) as proposed by a study by , to ensure quality mechanical response. This formulation will be denoted M-8 although the study.
After ambient drying in the laboratory, the samples are crush and sieved over a 100 µm mesh for BIM1 and an 80 µm mesh for dolomite (DO). A suspension of the mixture is then prepared and manually agitated for 5 min. The paste is left for evaporation at 60˚C in an oven for 24 h; the temperature is raised to 105˚C,
Table 1. Chemical and mineralogical composition of BIM1.
for the paste to dry to constant weight. The dried cake is then crushed and sieved over a 100 µm mesh.
For the characterization of the dolomite, X-ray diffraction (XRD), infra-red and differential scanning calorimetry (DSC) are used. X-ray fluorescence on a PANalytical Zetium equipment is used for the chemical analysis. The dolomite material is fused in lithium metaborate (LiBO2) prior to the analysis. The loss on ignition (LOI) is access by weighing the sample heated between 105˚C and 1050˚C.
The XRD pattern is acquired on a BRUKER D8 Advance, using a copper radiation in 2θ range from a 5˚ to 60˚ using a step size of 0.02˚/min.
Infra-red spectra are registered on a Perkin Elmer equipment from 400 - 4000 cm−1. A KBr pellet containing about 15% of the sample is used for the acquisition in attenuated total reflectance mode.
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were obtained on a couple DCS-TG analyzer, SETARAM TG-DSC Labsys Evolution, from room temperature to 1200˚C at a heating rate of 5˚C/min. About 25 mg of powdered material is used. An empty alumina crucible is used as reference.
Dilatometric analysis is done using cylindrical test sample of 10 mm diameter on a SERETAM TMA SETSYS Evo equipment. The analysis is conducted from room temperature to 1200˚C. The test samples are obtained by pressing 2 g of powder using a SPECAC hydraulic press at 30 MPa.
Physical characterizations of fired products were done on parallelepiped samples for linear shrinkage and on cylindrical pastel for water adsorption and porosity. All the characterizations are done following the norm NF EN ISO 10545-3, 1997-12.
The flexural strength was acquired using a three point bending electrohydraulic press (M & O, type 11.50N˚21). The parallelepiped sample is submitted to a continuous progressive load up to rupture.
For the SEM analysis, the sample is coated with platinum in order to render their surface conductive electrons. The micrographs are obtained using FEI-QUANTA EG ESEM 450 equipment operating under at 15 kV.
3. Results and Discussion
3.1. Dolomitic (DO) Characterization
The oxides CaO and MgO are the main constituents of the dolomite materials (Table 2) with silica as associate components. The loss on ignition is high (43%) and closed to the values usually found in the literature   . From XRD analysis (Figure 1), the main mineral is dolomite associated to quartz and calcite.
The infrared spectra (Figure 2) show band characteristics of dolomite at 3417 - 3010 - 2900 - 2638 - 2530 - 1825 - 1440 - 1100 - 1035 - 878 et 730 cm−1  .
Table 2. Chemical analysis of the dolomitic material (DO).
d.l = detection limit; LOI: loss on ignition (here at 1050˚C).
Figure 1. XRD pattern of the dolomitic material.
Figure 2. Infra-red spectra of the dolomitic material.
Figure 3. TGA-DTA graph of the dolomitic material.
Si-O bonds are located at 1100 and 900 cm−1.
The dolomite thermal analysis graph (Figure 3) shows two endothermic peaks, on the DTG curve, that are related to the decomposition of dolomite in two stages. The percentage loss associated to this decomposition is estimated at about 42.57%, which is in accordance with the loss of ignition (43%) from the chemical analysis (Table 2).
3.2. Thermo-Dilatometric Characteristic of the Mixture (M-8)
The thermal analysis curves of the mixture (M-8) are presented in Figure 4. At T < 800˚C, the deshydroxylation of the kaolinite and the decomposition of the dolomite are registered. In this range the various mass changes are easily associable to each material as if it was alone. However, these changes occurred at lowered temperatures, compare to each material undergoing thermal decomposition alone. This difference is probably due to associate minerals in the mixture . At 573˚C, the conversion of quartz α to quartz β is observed through the endothermic peak on the DTA. The exothermic peak at 830˚C is associated to the formation of new mineral phase from metakoalinite and decomposition product of dolomite . At 950˚C, the reorganization of metakaolinite into a spinel phase is observed this spinel is converted to mullite around 1100˚C as shown by the exothermic peak at this temperature.
Dilatometric curve of BIM-1 and mixture M-8 are presented in Figure 5. The behavior of both systems is similar and characteristic of clayey materials  . On the M-8 curve a horizontal trace is registered from the beginning up to almost 500˚C. In this zone no transformations occur. Between 500˚C and 600˚C, a small shrinkage is registered. This change is associated with the dehydroxylation
Figure 4. DTA/TGA curve of the M-8 mixture.
Figure 5. Dilatometric curves of the clayey material BIM-1 and the mixture M-8.
of kaolinite which is converted to metakaolinite and the dehydroxylation of the Mg(OH)2 from the dolomitic material. The shrinkage between 900˚C and 1000˚C is associated to the structural reorganization of the metakaolinite into mullite. This conversion is generally associated to an increase densification  . From the graphs, the presence of the dolomite in the mixture induced an increase of the mass loss from 11.5% in BIM-1 alone to 11.9% in the mixture.
3.3. Phase Transformation during Thermal Treatment
This phase transformation was studied using XRD and the firing temperature is chosen based on the exothermic peaks in the DTA/TGA curves. The patterns are presented in Figure 6.
The various new-phase evolutions are illustrated in Table 3. As the firing
Figure 6. XRD patterns of firing products of M-8: (a) 920˚C (b) 1050˚C (c) 1150˚C (d) 1200˚C (e) 1250˚C.
Table 3. Phase evolution during thermal treatment.
temperature increase, the intensities of the main reflections of quartz (3.33 and 4.27 Å) are lowered while that of mullite (3.35 and 5.38 Å), cristobalite (4.03 Å) and hematite (2.69 and 2.51 Å) increase. Hematite is formed from the goethite transformation around 300˚C. Gehlenite and anorthite that appear at 1050˚C, with low intensities, disappear at 1200˚C. It is suggested that the low iron oxide and liquid phase, has favored the interaction of decomposition products to formed gehlenite and anorthite. The illite reflections are no longer observable after firing at 1050˚C. The disappearance of some phases is associated to their conversion into other phases, kaolinite and illite to form mullite; their coverage by the vitreous phase form (the case of quartz) or their transformation into vitreous phases (the case of anorthite) . The proposed phase transformations from the raw materials are illustrated by the following equations  :
Between 900 and 1000˚C,
when T ≥ 1050˚C,
Anorthite and gehlenite are formed by the reaction betwen metakaolinite (from kaolinite decomposition at about 500˚C) and lime (from dolomite decomposition arround 700˚C) .
For 1050˚C ≤ T ≤ 1200˚C
For 1150˚C ≤ T ≤ 1200˚C
3.4. Technological Properties of the Fired Products from M-8
The physical and mechanical characterizations of the fired products from M-8 are presented in Figure 7. The linear shrinkage increased for all the temperature (Figure 7(a)); this increase is rapid from 920˚C to 1100˚C and beyond 1100˚C, it is slowed. This shrinkage is associated to the transformation/rearrangement of the constituent to reach a more stable rearrangement of the particles . These transformations include constituent elimination (gas formation or water removal);
Figure 7. (a) Linear shrinkage; (b) water adsorption; (c) open porosity and (d) flexural strength of fired products from mixture M-8 at various temperatures.
structural reorganization and conversion into liquid or viscous phases. The rapid increase of the shrinkage is associated to the release of carbon dioxide from dolomite and decomposition of kaolinite, illite and goethite in the lowers firing temperature. Also, the presence of calcite influences the contraction of the product during the sintering . Above 1100˚C, almost all transformations/conversion are finished whish justified the low shrinkage beyond this temperature.
The water adsorption decrease from 920˚C to 1100˚C (Figure 7(b)); same apply for the porosity (7c). These results are coherent with the decomposition of products such as illite to form vitreous phase is which the cristobalite from quartz transformations is dissolved . This is coherent with the observation from XRD on which cristobalite peak is observed as from 1050˚C and diminished when the temperature is increased as a result of its dissolution in a vitreous phase. The gran sintering and the inclusion of the vitreous phase in whole reduce the porosity as well.
The flexural strength (Figure 7(d)) shows an increase of the strength up to 1150˚C and this trend is reversed at 1200˚C. The increased of the strength is coherent with the porosity reduction as a result of increased densification . This densification is also coherent with the XRD results (Figure 6), where the main minerals, up to 1150˚C, are mullite cristobalite and quartz. These dense mineral induce and increase of the flexural strength. The formation of hematite crystallite, at higher temperature (1200˚C), cause de silico-aluminate phase to become more heterogeneous, which result in weakening of the products  and this justified the decrease of the flexural strength observed at 1200˚C. In addition, a formation of larger amount of glassy phase may also cause fragile product as from 1200˚C  .
3.5. SEM-EDS Analyses of the Fired Products
At 1150˚C, a large amount of glassy phase (EDS-A and B) is observed and inactive species such as periclase (EDS-C) are embedded within this glassy phase. The glassy phase is formed of Ca, Si, Al, Fe and Mg. Particle of MgO is from de decomposition of dolomite during thermal treatment. This elemental composition is also coherent with the detected minerals (Anorthite, gehlenite and mullite). The observation of mullite is not achieved probably due to a large amount of glassy phase.
At 1200˚C, new phases are clearly observable on the micrographs. The granular aggregates observable on the micrograph in Figure 9, are assigned primary mullite (EDS-D, F, I) . Needles of secondary mullite are also observable (EDS-G). From , it is shown that it is primary mullite that evolved to secondary
Figure 8. SEM micrographs of fired product at 1150˚C and EDS analyses ((a) and (b): amorphous phase; (c) periclase)
Figure 9. SEM micrographs of fired product at 1200˚C and EDS analyses on five points ((d), (f), (i) agragtes of granular mullite; (g) needles of mullet and (e) vitreous phase).
mullite; however when vitreous phase is formed both may be found embedded within the vitreous phase. The presence of iron favors the size increase of the mullite as suggested by . The global feature shows a large amount of glassy phase having a chemical composition (EDS-E), coherent with the identified minerals from XRD. The predominance of element Si in the EDS-E for instance is in agreement with the dissolution of quartz; mullite formation also agree with the predominance of Al and Si on EDS on points G, F and I. On the EDS of point D, the presence of Mg and Al may be associated to the formation of the spinel MgAl2O4, although non-detected from the XRD pattern.
The abundant glassy phase observable on the micrograph is coherent with the fragile structure revealed from flexural strength. Also, contribution of micro cracks (as point H in Figure 9) to the weakening is evidence from this image.
This paper was analyzing the effect of dolomite addition of fired product made from kaolinitic clay of Bangui (Central Africa Republic). The used of technics such as TGA-DTA, ERD and SEM was useful to identified the minerals phased that are formed during firing the mixture M-8. The global sintering behavior using dilatometric analysis is similar for both the clay and the clay-dolomite mixture. The densification of the clay or the mixture is marked at 1100˚C with the formation of mullite and cristobalite. The phase’s transformations during thermal treatment are influenced by the materials initial composition. The presence of illite and iron oxide in the clayey material induced liquid phase formation that is favorable to mullite and cristobalite development. A large amount of liquid phase formation, at a higher temperature (1200˚C), induced a fragile behavior that lowers the flexural strength. From SEM observations, the development of large amount of vitreous phase from the mixture is evidence. This formation indicates that the mixture can be used for the development of vitreous ceramic.
The present study, stand as a complementary work by Gonidanga et al. , revealed that mixture of dolomite-kaolinite incorporating 8% (by mass) of dolomite, lead to a large amount of vitreous phase formation that could be useful for the development of other type ceramic products from Bimbo clay. In particular, an optimization of the dolomite will be considered to better formulate mixture for targeted products.
The authors are thankful to:
· The French Ministry of external relation for financing this research through a grant from the Service d’Action Culturelle (SCAC) of the French embassy in CAR;
· The Institute of Research for Ceramic (IRCer) of the University of Limoges (France) is acknowledged for the laboratory facilities;
· Professor Phillipe Blanchart for his assistance in the designing of this work;
· Dr. J.A. Mbey for his assistance in the writing and proof reading of this paper.