INTRODUCTION TO THE DEVELOPMENT OF A HYDRAULIC ROAD BINDER BASED ON MINERAL COAL BOTTOM ASH

1. Faculté Des Sciences Et Techniques Département de Géologie, Université Abdou Moumouni, Niamey, Niger. 2. Ecole Des Mines, de l’Industrie et de la Géologie (EMIG), Niger. 3. Laboratoire d’Analyse des Sols Et De Suivi Des Travaux Publics (LASTRAP), Niamey, Niger. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 23 December 2020 Final Accepted: 25 January 2021 Published: February 2021 The use of natural aggregates with suitable characteristics for road construction presents in some regions a real problem of availability or cost related to transport. However, the resources in place, which are naturally unusable, can be used after treatment with Hydraulic Road Binders (HRB). In this study, we propose to valorize the mineral Coal Bottom Ash (CBA), a residue from the combustion of the SONICHAR power station in Niger, as the main component of an HRB.After a physical, chemical and mineralogical characterizationof the coal bottom ash, several proportions of the CBA powder and a cement type CEM I 42.5 were studied. Compression tests were carried out at 07, 28, 56 and 90 days to determine the class of the binder corresponding to the mixture selected.The performance of the HRB thus obtained was evaluated on two types of weakly clay soils (IP <= 12, NF P 94-051), having an initially insufficient CBR. The 3% treatment of this product made the two aforementioned soils suitable for use as a base layer.

The use of natural aggregates with suitable characteristics for road construction presents in some regions a real problem of availability or cost related to transport. However, the resources in place, which are naturally unusable, can be used after treatment with Hydraulic Road Binders (HRB). In this study, we propose to valorize the mineral Coal Bottom Ash (CBA), a residue from the combustion of the SONICHAR power station in Niger, as the main component of an HRB.After a physical, chemical and mineralogical characterizationof the coal bottom ash, several proportions of the CBA powder and a cement type CEM I 42.5 were studied. Compression tests were carried out at 07, 28, 56 and 90 days to determine the class of the binder corresponding to the mixture selected.The performance of the HRB thus obtained was evaluated on two types of weakly clay soils (IP <= 12, NF P 94-051), having an initially insufficient CBR. The 3% treatment of this product made the two aforementioned soils suitable for use as a base layer.

…………………………………………………………………………………………………….... Introduction:-
The high cost of road construction works in developing countries remains a major issue in socioeconomic development. In some countries, this reality is linked not only to the almost total import of coating materials, but also to the distance from the quarries of natural aggregates with geotechnical characteristics suitable for the implementation of the different layers of pavement. In sub-Saharan Africa, even more than ten years ago, the average unit cost per kilometer of paved road less than 50 km already exceeded US $ 400,000 (Africon, 2008). However, this trend can be greatly improved through the use of materials in place or of deconstruction (involving a low cost of transport), originally unusable, but can be used after treatment with hydraulic road binders (HRB).
The objective of this study is to set up an HRB made from a majority component of mineral CBA associated with a cement of the CEM I 42.5 type. The coal bottom ash in question is the residue from the combustion of the coal collected at the SONICHAR power station in the north-eastern ofNiger.This, made up of 2 units of 18.8 megawatts, put into operation in 1981 and 1982 respectively, produces more than 75,000 m 3 CBA per year (Djibo, 2014). Currently, the main use of SONICHAR's CBA remains for backfilling the old quarry whose exploitation began in the 1980s.

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This operation was undertaken since 2010, and it follows the worsening of the landscape pollution of the mountains by the coal bottom ash as well as the agglomeration of their thin elements carried in the air coolers of the plant; the recovery in industrial quantities of this waste will then constitute a solution to an already thorny environmental problem.

General information on the CBA
Coal bottom ash is a grayish material with a heterogeneous character and very porous (67% porosity according to (Vinai and al., 2013)). It has a high amorphous phase and pozzolanic properties, making it a potential raw material for the cement industry. Its activity index is a function of the chemical composition, and therefore of the nature of the coal used. For example, TEFEREYRE CBA has a pozzolanic activity index of around 76% (Savadogo and al., 2015).
Physical Characteristics of the CBA: Table 1:-Some physical parameters of the coal bottom ash.
Chemical Characteristics of the CBA:

CBA of Sonichar Particle size distribution
The coal bottom ash comes in the form of granules with dimensions ranging from 0 to 25 mm or slightly larger. However, slices larger than 6.30 mm are very friable and can therefore be considered as an agglomeration of smaller grain sizes. In the various tests, we mainly used ground CBA powders, passing through an 80-micron sieve. The granulometric analysis opposite (Figure 1)

Chemical and mineralogical characteristics
The X-ray diffraction performed on the powder of the coal bottom ash showed that it contains many amorphous elements, with the composition: Mullite 75%, Quartz 24% and Hematite less than 1%. The X-Ray Fluorescence gave the chemical composition described in Table 4, with a percentage of Silica and Alumina greater than 80%, and a low level of CaO (0.34%).

Specific Gravity
Helium pycnometer measurements (NF EN ISO 8130-2 January 2011; FP-803-C-1) on the powder passing through the 80µm sieve gave average values of the order of 2.66 g/cm3, a little high compared to the usual values but lower than that of Mandal (Mandal and Sinha, 2017) which is 2.72 g/cm3.

The Cement
The cement used is CPA45/CEM I 42.5 according to the standard of NF P 15-301, manufactured by CIMTOGO.

Soils to be treated
For the analysis of the performance of the HRB, two laterites of different origins were used; the characteristics are described in the tables below.

Formulation of the binder
Based on the principle of producing a binder with the maximum amount of clinker and the minimum amount of cement, we have produced (following the procedure of the normal EN 196-1 mortar) the mortars whose compositions are described in the table below. Prismatic 4x4x16 specimens from these mortars are crushed at 07, 28; 56 and 90 days.

HRB Dosage
In order to determine the optimal dosage of the HRB chosen, soil R was studied successively by treatment with 1% cement, 2% HRB and 3% HRB. The objective is also to verify the impact of the coal bottom ash powder on the increase in the value of the CBR in addition to the cement input.

Soil Treatment
Soils A and B are mixed with the percentage of binder retained, the materials thus obtained are subjected to the Proctor test (NF P 94-093) then the CBR test (NF P94-078).

Class of binder
According to the NF P 15-108 standard, the class of hydraulic road binder is defined according to the value of the compression resistance at 56 days as shown in the table below In our study, the crushing results of the prismatic test tubes gave the values in Table 10 and Figure 3. The specifications in Table 9 classify the MC7030, MC6040 and MC5050 mixes as class 10, 20 and 30 binders respectively. The binder MC7030 with the highest proportion of the CBA will then be selected for the soil treatment.

Mix of the HRB with MC7030
The results in Table 11 show that the 3% treatment of MC7030 (containing 0.9% cement) gives more interesting CBR values than the 1% cement treatment especially at 98% OPM. This implies that the pozzolanic activity of the bottom ash has a positive impact on the increase of the CBR index.
Although the literature quite often recommends values of 2% to 3% binder for soil improvements, the study of 4 to 5% MC7030 could be advantageous in the case of very low bearing capacity soils.

Soil Treatment A and B at HRB MC7030
For soils A and B, Table 12 gives the new values of maximum dry density, optimum moisture contentand the CBR (at 95% of OPM after imbibition). Compared to Table 7, the CBR index increased more than 2-fold after improvement to the new HRB formulated. Dry density and optimal water content decreased slightly, suggesting the effects of quick-whitewash treatment in lasterinous gravelly (CEBTP 1984). The A and B soils thus treated become suitable for use in base layers.