The analysis of water sector in Burkina Faso shows that 11% of urban population and 33% of rural population have no access to drinking water [1] . The principal reasons are the pollution of great part of surface and underground water. The pollutants are principally chemical (sulfate, nitrate, metals), physical (alteration of limpidity), microbiological and organic. Their origins are natural and anthropic. Chemical pollutants are generally present in the rock in salt form and dissolve in the running water at concentration higher than the standards of World Health Organization. The mining boom in our country is another reason of water pollution. The water resources of mining areas are polluted with cyanide and metals like arsenic, mercury. In addition, with population growth, the waters of the big cities are rich in lead, cadmium mainly from batteries, drain oils, fuels and garbage [2] . Besides, agricultural inputs (fertilizers and pesticides) participate significantly in water pollution.

To meet this situation, many polluted water treatment methods have been developed. Reverse osmosis, membrane techniques or electro-dialysis, bridged clays, and porous ceramic, are some examples of water treatment techniques [3] [4] [5] . Ceramic materials were hugely proposed for filtering polluted water. Thus mullite microfilters were synthesized and tested with outstanding results. Microfilter specificity resides in both its high micro-porosity and its mechanical strength [6] .

In this work, porous clay pots are synthesized using rice husks and raw clay materials from Burkina Faso. The first part of the work is devoted to the characterization of the basic and additive raw materials. The second part concerns the formulation of a pots and its mechanical, mineralogical, microstructural and chemical characterization.

The used raw clay was deposit in Namassa village, around 100 km in the north of Ouagadougou. Its geographic coordinates are 1˚48' West and 13˚05' North. This raw clay was used as a basic material on the elaboration of porous ceramic pots. The additive substance used material was rice husks. It is a major byproduct of rice milling process of local field of rice. It was used to create the pores in the ceramic bodies during the sintering.

In the rest of the paper, the raw clay and rice husks were abbreviated respectively NAM (from the Namassa village) and RHS.

Atterberg limits (liquid limit, plastic limit and plastic index) of the powder clay were carried out according to NF P 94-051 standard [7] .

Chemical analysis was performed with X-ray fluorescence wavelength dispersive technique with a Bruker TIGER S8. The samples were melted at 1250˚C with lithium tetraborate. Reference certificated materials were used for calibration.

Mineralogical composition was estimated by X-ray diffraction using a Bruker D5000 diffractometer operating at 40 kV - 40 mA and employing a graphite monochromatic CuKα radiation. The mineralogical composition was refined with Fourier Transform Infrared (FTIR) spectroscopy using a PERKIN ELMER A 100 with KBr as matrix.

Differential thermal analysis (DTA) and thermogravimetry (TG) analysis was carried out using SETARAM instrument operating at 10˚C/min from 20 to 1100˚C. Calcined alumina was taken as a reference.

Sintering behaviour of NAM was carried out with a dilatometer Setaram TMA-92 working with 5˚C/min as heating rate. The raw samples were pressed with a 20 kN force to form cylinders with 1 cm as green height. The measures were done in atmospheric air from 20˚C to 1100˚C.

Successive steps were used for the elaboration of specimen. The first step was the preparation of paste. The two crude samples were grounded until particle size less than 800 µm for NAM and less than 2 mm for RHS before their mixing. The different mixtures according to Table 1 were humidified and grounded during 15 min. The second step was the formatting of specimen. The obtained paste from the first step was pressed with 15 MPa using Magnolfi-Bigalli press. The used moulds were silt form according to Figure 1. Each specimen was made with 200 g of moistened powder. For the evaluation of flexural strength, parallelepiped forms were also done. The different specimens were dried at 105˚C for 24 hours before firing up to 1200˚C, 1300˚C or 1400˚C for 30 min in an NABERTHERM P330 electric furnace. The heating rate was 10˚C/min.

The fired pots were tested for unit weight (UW), shrinkage (S), water absorption (WA), flexural strength (σ) and filtration flow (F).

Unit weight was determined according to Equation (1). It corresponds to the ratio between the mass (m_s) and the volume (V_s) of the fired pots.

U W ( g / c m 3 ) = m s V s (1)

The shrinkage was determined by means of the Equation (2) with L_s and L_c respectively the height of green and fired specimens.

S ( % ) = L s − L c L s × 100 (2)

The water absorption is the relation of the mass of absorbed water to the mass of the fired pots. It is calculated with the Equation (3) with m_s and m_h respectively the mass of fired pots and the mass of fired pots after two (2) hours in boiling water [8] .

W A ( % ) = m h − m s m s × 100 (3)

The mechanical properties of specimens were measured by three points bending device with NANNETTI apparatus. The flexural strength σ was evaluated by the Equation (4) [9] .

σ = 3 F E 2 l e 2 (4)

where: F―applied force, E―distance between the two supports, l―the width of the bar, and e―its thickness.

The flow (F) of each fired pot was determined (Equation (5)). The same quantity of distilled water (V) was put on each specimen and the necessary time (t) for its filtration was determined. The test is repeated four times for each pot. After each test, the pot is rinsed and dried at 105˚C before the second test.

F ( cm 3 / s ) = V t (5)

Mineralogical evolution of fired pots was assessed using X-ray diffraction of the fired pot powder (<80 µm).

Microstructural evolution of fired pots was observed with scanning electron microscopy with apparatus type LEO 450 VP. A semi-quantitative chemical analysis using X-ray Energy Dispersive Spectrometer (EDS) was performed to complete the mineralogical characterization.

Atterberg limits of NAM are presented on the Table 2. According to Casagrande diagram [10] , Atterberg limits show that NAM is a clay with plasticity. The liquid limit of material permits to show the passage to liquid stage to plastic stage. This plasticity is interesting for the typesetting of the specimen. The feeble specific area shows that the sample must contain a clay without an interlayer. Table 3 presents the elementary chemical analysis of NAM. The main oxides are SiO₂ and Al₂O₃ and indicate that NAM is consisting principally by clay phases. The iron oxide Fe₂O₃ and potassium oxide K₂O are in relatively high quantity. Fe₂O₃ is ascribable to goethite and K₂O shows the presence of illite or muscovite phase in NAM. The loss on ignition (9.88% wt) is high and corroborates the predominance of clay phase in NAM. The analysis of X-ray diffraction pattern (Figure 2) according to ASTM (American Standards for Testing Materials) on open data base indicated that kaolinite, illite, montmorillonite, goethite and quartz are the main phases which compose NAM. Infrared spectra (Figure 3) corroborated the X-ray diffraction results. The characteristics covalent bonding and functional groups of kaolinite, illite, montmorillonite and quartz are presented on the infrared spectra. The band around 3600 cm⁻¹ is attributable to the bond Al-OH of kaolinite, and the band at 3620 cm⁻¹ is the bond Al-OH of illite and montmorillonite. The double band at 797 - 778 cm⁻¹ is ascribable to the bond Si-O of