For this study we selected three (03) types of local sand to formulate the standardized sand. The sands studied come from different localities in Burkina Faso, namely Manga in the Centre-South, Dori in the North and Kaya in the Centre-North. The sand from Manga and Kaya was natural river sand, while that from Dori was dune sand. Manga sand is the local sand most widely used in construction in Burkina Faso. After on-site sampling, the sands were oven-dried at 105˚C to constant mass. The dry sand obtained was passed through a 2 mm sieve to collect the by-product for further study. Standardized sands formulated from a mixture of these sands were used to characterize a CEM II/B-M (P-L) 42.5 R cement from CIMBURKINA. The water used is potable tap water maintained at a temperature of 20˚C.

After sampling, the local sands were individually characterized from a physical point of view. Granular distribution was determined by particle size analysis in accordance with EN 933-3 [9] , bulk density in accordance with EN 1097-3 [10] , specific density and water absorption coefficient in accordance with EN 1097-6 [11] , and cleanliness by sand equivalent test in accordance with EN 933-8+A1 [12] .

The preparation of normal sand from these local sands was based essentially on the correction of local sand particle size distribution. Different mixtures were prepared to obtain sands with a particle size curve in accordance with standard EN 196-1 [3] . Three (03) mixes were selected: two (02) are different sand mixes and one (01) is obtained by mixing precise proportions of different rejections of the same sand after sieving. It should be noted that Manga sand was used as the base sand in all the mixes selected.

The sands thus formulated were used to make normal mortars in accordance with standard EN 196-1 [3] , whose characteristics in the fresh (physical) and hardened (physical and mechanical) states were analyzed. In the hardened state, tests were carried out on 4 × 4 × 16 cm³ prismatic specimens made to standard EN 196-1 [3] and kept in a water basin in a room regulated at 20˚C ± 1˚C until the age of the test. Specimens are tested after 2 and 28 days of curing in accordance with EN 197-1 [2] .

Table 1 shows the results of the physical characterization of the selected sands compared with imported standardized sand. The standard sand has a bulk and absolute density of 1.70 g/cm³ and 2.63 g/cm³ respectively. Its absorption coefficient is 0.2%. It is a clean sand with a sand equivalent of 93%. In terms of bulk and absolute density, Manga sand is close to standardized sand, with values of 1.59 g/cm³ and 2.63 g/cm³ for bulk and absolute density respectively. The water absorption coefficient of 2.97%, on the other hand, is higher than that of standardized sand. In its natural state, Manga sand is less clean than standardized sand, with a sand equivalent of 81.64%. It was then washed and dried before use. The apparent and absolute densities of Dori sand are 1.35 g/cm³ and 2.63 g/cm³ respectively. It therefore has a lower bulk density than standardized sand, but the same absolute density. It is as clean as standardized sand, with a sand equivalent of 94.16%. Its absorption coefficient, however, is higher than that of standardized sand, at 1.92%. Kaya sand has a bulk density of 1.47 g/cm³ and an absolute density of 2.56 g/cm³. It is the sand with the highest clay content of the three (03) sands studied, with a sand equivalent of 71.91%. It was also washed before use. Its absorption coefficient is 2.30%. Based on the absorption coefficients of local sands, they appear to have a rather high level of intragranular porosity compared with standard sand.

The particle size distribution curves for the different sands are shown in Figure 1 . It can be seen that the curve for imported standardized sand fits perfectly into the grading range of standard NF EN 196-1 [3] . Its fineness modulus is 2.63, its grain size uniform and well graded, with a Cu uniformity coefficient of 0.47 and a Cc curvature coefficient of 1.37. Grain size analysis of the Manga sand shows that the coarse part of the sand between 0.8mm and 2mm fits into the spindle, but the fine part below 0.8mm lies above the upper limit of the spindle. Manga sand has a lower percentage of fine elements below 0.8mm than standard sand. It is therefore coarser than standardized sand, with a fineness modulus of 3.01. Its grain size is also uniform and well graded, with a Cu uniformity coefficient of 0.5 and a Cc curvature coefficient of 1.02. Dori sand is a dune sand, and the particle size distribution curve shows that it is a very fine sand with a grain size of 0/0.5, which does not at all fit into the spindle of standardized sand. Its modulus of fineness is 1.59 and its grain size is uniform and poorly graded, with uniformity coefficients Cu of 1.74 and curvature coefficients Cc of 0.98. Kaya sand has a grain size similar to that of Dori sand, except that its curve fits more or less well into the spindle between grain sizes 0.1 and 0.2mm, unlike that of Dori. Its modulus of fineness is 1.59, with a uniform, poorly graded grain size whose coefficient of uniformity Cu is 0.71 and that of curvature Cc is 0.99.

None of the local sands therefore have the characteristics of a standardized sand. Based on the results of the particle size analyses, we have made corrections to obtain sand mixtures whose curves fit within the particle size range of standardized sand.

Grain size correction is carried out experimentally by mixing Manga sand (coarse) with fine sand in increasing proportions, in order to obtain a grain size within the range specified in standard NF EN 196-1 [3] . After washing and drying the sand from Manga and Kaya, we were able to identify three sand mixtures, SM1, SM2 and SM3, whose particle size curves fall within the standardized sand spindle ( Figure 2 ). The SM1 mixture is made up of 80% Manga sand and 20% Dori sand. SM1 has a fineness modulus of 2.67 and uniformity and curvature coefficients of 0.34 and 1.20 respectively. The SM2 mixture is made up of 80% refusal and 20% passing through a 0.25 mm sieve of Manga sand. It has a fineness modulus of 2.72 and uniformity and curvature coefficients of 0.39 and 1.10 respectively. The SM3

mixture is made up of 77% Manga sand 0.25mm sieve refusal and 23% Kaya sand. It has a fineness modulus of 2.78 and uniformity and curvature coefficients of 0.42 and 1.21 respectively. All the sand mixes produced have a coarser particle size than standard sand. Their percentage of particle sizes between 0.5 and 2mm is higher than that of standardized sand. This is due to the fact that all the mixtures contain a majority of Manga sand, which is much coarser than standardized sand. Table 2 shows the composition of the various sand mixes.

The slump and fresh density results for the different mortars are shown in Figure 3 . Slump measurements show that mortars based on local sands have a much lower slump than the reference mortar based on standardized sand. From a value of 32.6 mm with standardized sand, SM1, SM2 and SM3 have values of 6.6 mm, 2.3 mm and 5.3 mm respectively. The low workability of mortars based on local sands is explained by their water absorption capacity, which is very high compared to that of standard sand. As a result, much of the mixing water is absorbed by these sands and can no longer be mobilized to fluidize the mix. According to Ruan et al. [13] , this low workability can also be explained by the fact that the local Manga sand used in all mixes is coarser than standardized sand. In their study, they showed that increasing the number of coarse particles in a sand increases water consumption. This explains why the SM2 sand, which consists solely of Manga sand, has the lowest slump.

We also note that mortars based on processed sands have lower densities than mortar with standardized sand. The values obtained are 1.57 g/cm³, 1.39 g/cm³, 1.31 g/cm³ and 1.51 g/cm³ respectively for SS, SM1, SM2 and SM3 sand. This drop in density could be explained on the one hand by the low bulk density of local sands, and on the other by the low workability of these mortars compared with

standardized sand mortar. This lower workability affects mortar placement in molds and reduces the compactness of cast mortars. The fact that local sands are coarser than standard sand could also explain this drop in density. Indeed, the fineness modulus of the sands are 2.63, 2.67, 2.72 and 2.78 respectively for SS, SM1, SM2 and SM3. Local sands would lack the fine particles needed for better grain stacking to achieve higher density.

The densities of the various mortars in the hardened state were determined after drying the specimens at 2 and 28 days of curing. The results are shown in Figure 4 . There was a slight increase in density of around 7% on average between 2 and 28 days for all mortars, attributable to further hydration of the cement between the two times [14] . The results also show that curing time has no influence on the density evolution of mortars with local sands compared to standardized sand. Indeed, mortar with standardized sand is always slightly denser than other mortars with local sand. Mortar density rises from 2.3 g/cm³, 2.2 g/cm³, 2.17 g/cm³ and 2.26 g/cm³ for 2 days’ curing to 2.37 g/cm³, 2.28 g/cm³, 2.26 g/cm³ and 2.3 g/cm³ for 28 days’ curing for SS, SM1, SM2 and SM3 sands respectively. The local sand mortars SM1, SM2 and SM3 have water-accessible porosities of 9.65%, 10.28% and 10% respectively, and are therefore higher than the reference mortar based on SS standard sand, which has a porosity of 9.59%. This may be explained by the fact that local sand mixtures are coarser than standard sand. Indeed, the literature shows that a better granular distribution favors mortar compaction in the fresh state, thus reducing porosity in the hardened state [13] . The difference in porosity could justify the slight drop in density observed between local sand mortars and the control mortar in Figure 4 .

The compressive strengths of mortar specimens at 2 and 28 days are shown in Figure 5 . Local sand-based mortars show lower compressive strengths than standardized sand mortar at 2 and 28 days curing. This may be explained by the higher porosity of these mortars compared to standard sand mortar [4] [15] . Indeed, the higher the porosity, the lower the compressive strength of the mortar. Mortar based on SM2 sand thus has the highest porosity and the lowest compressive strength.

At 2 days, the compressive strengths of SM1, SM2 and SM3 were 17.42 MPa, 14.68 MPa and 17.13 MPa respectively, compared with 21.12 MPa for standard sand. The low 2-day mechanical strength values achieved with local sands could be explained by the absorption of much of the mixing water by the grains of these sands, which can no longer be mobilized for cement hydration at a young age. At 28 days, the compressive strength of the mortars maintains the same trend as at 2 days, where the best value is obtained by the mortar based on SS standardized sand with 49.84 MPa. With local sand mixes SM1, SM2 and SM3, the compressive strengths obtained are 44.21 MPa, 37.32 MPa and 43.17MPa respectively. The SM1 and SM3 mixes achieved the expected strength for the type of cement used. Continued hydration between 2 and 28 days enabled M1 and M3 mortars to achieve sufficient densification and consequent improvement in strength. The SM2 sand-based mortar, on the other hand, gave a strength of 37.32 MPa, which is below the expected value. The coarse grading of the local sand used in this mix may explain this trend [13] .

The ratios between the compressive strengths of mortars based on local sands and those of standardized sand were determined (see Table 3 ). It can be seen that the SM1 and SM3 sand mixes give the best results, with ratios of over 80% of the strength obtained with standard sand. These ratios could be used as a correction factor, when SM1 and SM3 mixes are used to characterize cements. However, a repeatability test should be carried out to validate these correction factors, taking into account other cements.