The aluminosilicate material used in this work is a clay taken in the department of Bouenza (Republic of Congo), in the district of Mouyondzi (Figure 1) [4]. Moutou and all have shown that kaolinite, illite and quartz constitute the mineral phases of this clay. Kaolinite is the most dominant clay species. Table 1 gives the results of the chemical oxide analysis. Observation of this table shows that

SiO₂ is the oxide with the highest percentage followed by Al₂O₃. The SiO₂/Al₂O₃ ratio gives 2.9, making Mouyondzi clay favorable for the development of geopolymers.

The sample was taken from a depth of one meter, dried in the open air on the bench in the laboratory, crushed using a porcelain mortar, sieved at 96 μm then stored in plastic jars to prevent any contamination. This powdered sample referenced Mou was heated to 600˚C, 700˚C and 800˚C, with a heating rate of 1˚/min. The calcined clay is stored in waterproof plastic pots, and referenced respectively Mou6, Mou7 and Mou8.

The transformation of kaolinite into metakaolinite begins very slowly from 450˚C, it continues up to 800˚C [4], hence the choice of processing temperatures of 600˚C, 700˚C and 800˚C.

The activator solution consisted of a 12 M sodium hydroxide solution and a sodium silicate solution. The soda solution was prepared by dissolving 99% by mass purity caustic soda in distilled water. The sodium silicate was composed of (mass%): SiO₂ (31.00), Na₂O (12.8) and H₂O (56.2). This prepared activator solution was prepared at room temperature stored for 24 hours before use to achieve equilibrium.

The geopolymer paste was formulated by mixing the calcined clay powder at different temperature (Mou6, Mou7 or Mou8) with the alkaline solution. The mixture was kneaded with an electric mixer for 10 minutes with a one minute pause after 5 minutes, to promote good diffusion throughout the mixture.

The resulting paste was poured into the 4 × 4 × 16 cm steel prism molds. The molds were vibrated after casting for 10 minutes to remove air bubbles trapped during the casting process. Demoulding was carried out after 24 hours and the freshly demolded test pieces were packaged in polyethylene. These prismatic specimens were used for compressive strength measurements after 28 days of age. Part of the debris from this measurement was used for SEM and another part was crushed and sieved at 50 µm for performing XRD and IR.

Table 2 shows the samples and their respective labels.

The characterization techniques of formulated geopolymers and calcined clay powders used in this work are:

1) DRX

X-ray diffraction was used for the determination of the mineral phases of Mou, Mou6, Mou7, Mou8, GMou6, GMou7 and GMou8. The measurements were carried out using a Bruker D8 type diffractometer in Bragg-Brentano geometry using CuKα radiation. The data was recorded in a domain between 10˚C and 70˚C with a step of 0.006˚C and a fixed divergence slit of 0.3˚C.

2) IRFT

Fourier transform infrared spectroscopy measurements were carried out on a FTIR Thermo is 10 spectrometer. The Mou, Mou6, Mou7, Mou8, GMou6, GMou7 and GMou8 samples (in the solid state and sol-gel) were characterized by Attenuated Total Reflection (ATR) by mounting an ATR unit in the sample compartment of the spectrometer. The spectra were recorded in a range between 500 cm⁻¹ and 4500 cm⁻¹ with a spectral resolution of 0.5 cm⁻¹.

3) MEB

The SEM was performed at the AARON KLUG CENTER for Imaging and

Analysis Electron Microscope Unit at the University of Cape Town in South Africa. The device used is FEI Nova NanoSEM 230 type. The Nova NanoSEM is a high resolution field emission SEM, combining low kV imaging and analysis capabilities with unique low vacuum performance.

The preparation of the specimens and the mechanical resistance were carried out in the physics laboratory of the Société Nouvelle des Ciments du Congo (SO.NO.C.C) in Congo Brazzaville.

The compressive strength of the samples of prismic geopolymer cement paste hardened after 28 days as a function of the calcination temperature of the clay fractions is shown in Figure 2.

It is observed that the compressive strength increases with the curing temperature of the clay. This increase is attributed to the increase in the amorphous phase in the aluminosilicate material [5] [6]. It is evident that when the dehydroxylation of kaolinite clays takes place, the structure of the resulting metakaolinite is in a state of disorder which increases with temperature up to 800˚C. The maximum of this disorder is observed at 800˚C. The increase in disorder leads to better characteristics of the geopolymer product [6]. Calcination of kaolinite clays above 800˚C decreases the tendency for disorder in the structure of metakolinite, giving rise to another crystalline phase [7].

The SEM images of geopolymer mortar GMou6, GMou7 and GMou8 are presented in Figure 3.

On these images, we observe undissolved particles associated with binding phases of amorphous aluminosilicate type characteristic of geopolymers [7].

In Figure 3(a) and Figure 3(b), the image appears as an assembly of aggregates with high porosity. Figure 3(c) and Figure 3(d) shows continuous structures with some undissolved particles, indicating low porosity compared to others.

These continuous structures are of amorphous aluminosilicate type material characteristic of a geopolymer [8]. The cracks observed are caused by the impact of the mechanical study.

In Figure 3(e) and Figure 3(f) there is a considerable decrease in undissolved particles, some of which are visible in Figure 3(e).

The SEM results indicate the formation of a new product with a structure different from that of metakaolin. These observations are in agreement with the results of the compressive strength because the strong presence of undissolved particles and the gathering in aggregates of GMou6 makes it less resistant than the other matrixes. GMou7 has a lower level of geopolymerization than GMou8 due to the fact that GMou7 carries more undissolved particles therefore not having reacted, so it is less resistant than GMou8.