Portland cement is the most common type of cement used in construction applications, but it is an expensive binder due to the high cost of production associated with the high energy requirements of the manufacturing process itself  . Raw materials such as limestone and clay are grinded and heated in a kiln at 1400˚C - 1450˚C to form predominantly clinker, which is then finely ground together with additives such as gypsum to obtain Portland cement    . Also, the heating of the raw material produces important CO2 emission in the atmosphere. Therefore, to reduce the CO2 emission and the cost of binder, other cheap inorganic materials with cementitious properties such as natural pozzolans, waste products from industrial plants and silica fume can be used as a partial replacement for Portland cement                .
The volcanic tuffs of Mako are produced during volcanic eruption dated from about 2.3 - 1.95 Ga       . In this paper, the potential use of these volcanic tuffs as a natural raw material in the production of pozzolanic cement is investigated. In Senegal the production of Portland cement is very expensive associated with the important CO2 emission. To provide all people to access to the cement it is very important to test new raw material. In this study, the physical, chemical and mineralogical characteristics of the volcanic tuffs are first examined, and then the compressive strength of Portland cement is compared with pozzolanic cement to determine the pozzolanic activity of these volcanic tuffs.
2. Geological Context
The volcanic tuffs of Senegal-Oriental in Mako area dating from Birimian (about 2.3 - 1.95 Ga)  -         . They are descripted as pyroclastic rocks which are produced during a submarine explosive volcano (Figure 1).
Figure 1. Geological context of the volcanic tuffs. We can see.
A cross section oriented ENE-WSW (Figure 2) shows volcanic tuffs of 50m of thick, over bedded by metasediment and lava flow. All these formations are metamorphosed under green schist-facies conditions        . The volcanic tuffs are exposed along the road on several kilometres (Figures 1-3).
Figure 2. Geological profile on Figure 1 showing the context of taking place of the volcanic tuffs.
Figure 3. Outcrop of the volcanic tuffs on the side of the road. The volcanic tuffs are stripped to build the road.
3. Material and Method
Clinker, gypsum and volcanic tuffs are the main components used in this study.
Clinker or Portland Cement Clinker obtained by burning calcareous and clayey materials.
Gypsum is added to clinker as a set regulator. It is hydrated calcium sulphate in chemical form and plays a very important role in controlling the rate of hardening of the cement.
Portland cement (PC)
Portland cement obtained by pulverizing gypsum (5%) and clinker (95%) (Figure 4).
Figure 4. Percentage of constituents of the cements. PC: Portland cement, PZ: Pozzolanic cement.
Samples of volcanic tuffs are obtained from a deposit located in Mako (Figure 3), the Kedougou district of Senegal-Oriental (Senegal). Representative samples amounting to a total of 100 kg were collected from.
Pozzolanic cements (PZ)
Four categories of cements are manufactured according to formulations defined on Figure 4.
Cements are obtained grinding clinker, gypsum and volcanic tuffs.
3.2.1. Chemical Analysis
Chemical analysis is carried out on clinker, gypsum, volcanic tuffs and cements to determine major elements composition. For each element a homogeneous laboratory sample is crushed and analysed according to  . The chemical analysis is an alkali fusion followed by a hydrochloric acid attack. Silica is quantified by gravimetry method. Oxides like SiO2, Al2O3, CaO, MgO, Fe2O3 and Al2O3 are quantified by complexometric titration. The loss on ignition (LOI) is determined by calcinations.
3.2.2. Physical Analysis
The volcanic tuffs are analysed with X-ray diffraction to determine minerals and amorphous or vitreous phase. Pozzolanic activity is proportional to the vitreous phase in the cement   .
The specific surface is measured only on the cements and the slightly crushed volcanic tuffs. The activity of a natural pozzolan, which is essentially determined by the reactive silica content, is also closely controlled by its specific surface area, chemical and mineralogical composition     .
Mechanical tests are the best method to evaluate the pozzolanic activity of cements. The tests are realized on standardized prismatic mortar bars (4 * 4 * 16 cm). The composition of mortar is:
− 1350 g of standardized European sand (CEN EN 196 1),
− 450 g of cement,
− 225 g of water.
The homogeneous mortars are conserved in mould for 24 hours in humid atmosphere.
After that, mortar bars are extract from mould and kept inside water at 20˚C.
Tests of compressive strength of mortar bars are realized at 2, 7, 14, 28 and 90 days after conservation inside water.
4. Result and Discuss
4.1. Specific Surface of Volcanic Tuffs
The specific surface of volcanic tuffs is 8895 cm2/g. This shows that the volcanic tuffs are very fine. According to Largent (1975), the potential pozzolanic activity of matter depends on a high specific surface.
4.2. Chemical Composition of Constituents
Insoluble residue (IR) is a non-cementing material which is present in Portland cement. This residue material affects the properties of cement, especially its compressive strength  . Insoluble residue is not measured in the gypsum and volcanic tuffs. But in these constituents the proportion of IR is very important.
In the volcanic tuffs the silica content is 61.63% (Table 1) greater than 45%. That gives them a potential pozzolanic activity   . So, these natural pozzolans content a high of SiO2 + Al2O3 (78.64%) but a low content of MgO and SO3, then it can exhibit a high pozzolanic activity  .
For the CaO, SiO2 and Al2O3 the position of volcanic tuffs of Mako on Ternary Keil-Rankin diagram (  ;  ) is between volcanic glass and ashes (Figure 5).
Figure 5. Position of Mako volcanic tuffs on Ternary Keil-Rankin diagram for the CaO-SiO2-Al2O3.
Table 1. Chemical composition of volcanic tuffs and gypsum.
IR: Insoluble residue. LOI: Loss on ignition.
The difference between the tenors in lime and silica of volcanic tuffs is 67.31%.
In the clinker the percentage of IR is 0.09% (Table 2). So, the measurements of free lime and C3S mineral are respectively 0.3 d 57.74%. This confirms the good quality of the clinker.
Volcanic tuffs are natural rocks of volcanic origin and composed of silica and alumina oxides but almost no lime. Therefore, they cannot develop hydraulic properties in the absence of hydrated lime.
Hydrated lime or material that can release it during its hydration (e.g. Portland cement) is then required to activate the natural pozzolans as a binding material  .
The activity of a natural pozzolan, which is essentially determined by the reactive silica content, is also closely controlled by its specific surface area, chemical and mineralogical composition  .
4.3. X-Ray Diffraction of Volcanic Tuffs
The result of X-Ray Diffraction shows the presence of quartz and other weathering minerals like kaolinite, muscovite, montmorillonite, muscovite (Figure 6). The quartz is very abundant. The hematite is also present. An important amorphous phase (A) gives hopes a potential pozzolanic activity by the volcanic tuffs. Because the amorphous phase could be potentially reactive with Ca(OH)2 of cement  . So, according to  , the pozzolanic activity of matter is proportional of the present a morphous phase. Therefore the amorphous phase in these volcanic tuffs could be altered due to these ancient ages.
4.4. Time of Grinding and Fineness of the Cements
The cements are made by referring to the Figure 4. Grinding of the cements is executed at the same granulometry (Figure 7). That’s why the time of grinding of the cements is inversely proportional to the percentage of addition of volcanic tuffs (Table 3). With the addition of 40% of volcanic tuff I, this time decreases of 50% comparatively to Portland cement.
4.5. Specific Surface of Cements
For the specific surface, the pozzolanic cements values higher than the one of Portland cements (Table 4). This fact shows the degree of fineness of the volcanic tuffs. The specific surface is proportional to the addition of volcanic tuffs.
Figure 6. X-ray diagram of volcanic tuff of Mako area.
Figure 7. Fineness of cements. Cements are grinded at the same granulometry to avoid the influence of the fineness on compression strength.
Table 2. Chemical composition of the clinker.
IR: Insoluble residue, LOI: Loss on ignition, CaOf: free lime.
Table 3. Time of grinding of the cements.
Table 4. Specific surface of cements.
4.6. Chemical Composition of Cements
Chemical constituents like SiO2, Fe2O3 and Al2O3 increase proportionately with the addition of volcanic tuffs in blended cements (Table 5). But sulphate and loss on ignition contents are respectively lower than 3.5% and 5% required for a blended cement  .
The measurement of the CaOf and C3S in the clinker is respectively 0.2% and 47.26%. Then potential reactive silica of volcanic tuffs could associate with CaOf of clinker. These pozzolanic reactions lead to the formation of additional hydrate minerals (C-S-H) with binding properties  .
4.7. Mechanical Performance of Blended Cements
Mechanical performances at 2, 7, 14, 28 and 90 days age of mortars are illustrated in Table 6 and Figure 8. Generally, the compressive strength of cements increases with time. It is closely controlled by the addition of volcanic tuffs to the PC. We can deduce that the hydration of blended cements continues until 90 days. Which may due to the presence of new hydrated minerals formed by reaction between reactive silica from volcanic tuffs and free lime from clinker.
A consistent reduction in the compressive strength and rate of strength development of mortars is observed as the amount of the volcanic tuffs in the blended cement increased (Table 6 and Figure 8). The compressive strength of all mortars is higher than 16 and 32 N/mm2 respectively at 7 and 28 days. Then, in reference on NF EN 197-1 these cements with addition of volcanic tuffs are pozzolanic cement belong to the class CEM IV/BP 32.5.
Silicate minerals including feldspar, mica, hornblende, pyroxene and quartz or olivine present in volcanic rocks can easily undergo alteration to form secondary mineral phases such as clays, zeolites, calcite and various amphiboles  . The contribution of these secondary minerals to the pozzolanic activity of the volcanic tuffs is demonstrated by compressive strength growth.
Generally natural pozzolans include increased workability, decreased permeability increased resistance to sulphate attack, improved resistance to thermal cracking and increased ultimate strength and durability of concrete    .
On that experience the volcanic tuffs are used directly without any pre-treatment. Therefore, it would be interesting to see the enhancing of the pozzolanic activity of these old volcanic tuffs pre-treating by thermal or chemical method. The
Figure 8. Compressive strength of cements.
Table 5. Chemical constituents of blended cements.
(IR: insoluble residue, LOI: Loss on ignition, CaOf: free lime).
Table 6. Compressive strength of cements (N/mm2).
thermal method consists in heating the material at 800˚C - 900˚C before the addition and grinding with the PC. The chemical method  is to activate the pozzolanic reaction with chemical activators such as Na2SO4 and CaCl2.
For the first time, the volcanic tuffs of Mako area in Senegal-Oriental are used to the pozzolanic activity. They are altered but they have physical and chemical characteristics of pozzolans. The time of grinding to obtain pozzolanic cements decreases with the addition of volcanic tuffs, significantly reducing the costs. So, the addition of volcanic tuffs could reduce the production of clinker, which may significantly reduce CO2 emissions in the atmosphere. The pozzolanic activity of the volcanic tuffs of Mako area has been shown by the compressive strength which increases until 90 days of conservation in water. In fact, the active silica from the volcanic tuffs reacts with the free lime of clinker to produce new hydrated minerals which participate in the increasing of the compressive strength.
For perspectives, the pozzolanic activity of the volcanic tuffs of Mako area could be improved by thermal method or chemical method using Na2SO4 or CaCl2.
The authors thank SOCOCIM Industry for allowing us to realize chemical and mechanical test of the raw materials and cements and its financial support.