The clinker used of alite nature is produced on the industrial site of the LUKALA cement plant (HeidelberCement Group Cilu-RDC plant). These nodules are micronized in a laboratory ball mill by co-grinding with gypsum and mineral addition.

The cement used is a variant of ordinary Portland cement taken as a reference, the composite cement is obtained by substitutions of Clinker at progressive rates and co-ground with gypsum.

The sampling of this rock covers the different flows of 2002, 2010, 2021. According to the mapping of Figure 1; this rock, which is the subject of a pozzolanic study.

The mixing water used in the HeidelberCement Group Cilu-RDC laboratory is distilled water.

The fine sand of LUILA and the moderately coarse sand of NSIAMFUMU constituted the mixture of a siliceous sand conforming to the standard sand standard.

Table 1 of mortar formulations presents the proportions of clinker, basalt, gypsum, sand and water to be used to produce mortars with different rates of substitution of clinker by basalt powder. The table shows that the total amount of clinker and basalt remains constant (436.5 g), as well as that of gypsum (13.5 g), sand (1350 g) and water (225 ml). The table will lead to evaluate the effect of the substitution of clinker by basalt on the physical properties of mortars, such as setting time, compressive strength and consistency. The table suggests that the

use of basalt as a natural pozzolan could reduce the consumption of clinker and thus the environmental impact of cement production.

The Setting Test of the cement paste is followed in accordance with the requirements of the standard on a multi-position automatic prisometer [37] .

To evaluate the setting times (start and end) of the control and reformulated mortars with basalt at progressive substitution rates, the standard used leads to a formulation for the paste to be tested that are shown in Table 2.

1) Los Angeles

The aggregates were crushed in the 10 - 25 granular class according to the relevant standard whose elements are presented in Table 3.

The Los Angeles test is a method to measure the weight percentages of elements having a size smaller than 1.6 mm, which are produced by the fragmentation of 5 kg of the tested material after being subjected to the impacts of standardized balls in a cylinder for 500 rotations during 15 minutes. The classes are defined for the Los Angeles test (for specific applications) as follows: Class A: LA < 30; Class B: 30 < LA < 35; Class C: 35 < LA < 40; Class D: 40 < LA < 50.

2) Uniaxial compressive strength [39]

The compressive strength test, also called RC, is the uniaxial compressive stress measured at the complete failure of the studied material; this stress is divided by the area of the offered surface. The classification of the rock is made possible by Table 4.

3) Microscopy and Spectroscopy (Fluorescence/FTIR)

a) Optical Microscopy

Optical mineralogy allowed us to analyze the Nyiragongo rocks and their minerals according to their optical properties in thin section. The observation presented in (Figure 2) was performed with an optical microscope in polarized light analyzed (PLA) and in polarized light not analyzed (PLNA). Optical

microscopy has limitations in terms of resolution, subjectivity and time required to identify volcanic rocks, which are more heterogeneous and complex than plutonic rocks [42] . This work shows that optical microscopy is simple and fast, but limited by the resolution and preparation of the samples, to work sharply powder X-ray diffraction is precise and quantitative, but requires optimal experimental conditions and complex data analysis. However, powder X-ray diffraction is more suitable for the identification of crystalline phases, which may require other techniques as volcanic rocks cool quickly.

b) The FTIR

The analysis by Fourier transform infrared spectroscopy of the local standardized sand allows to verify the absence and to guarantee the non-contribution of amorphous silicas in the granular structure of cementitious matrices (mortars) in accordance with quality requirements. The importance of complementary techniques (XRF, XRD and FTIR) is necessary to analyze the crystalline and amorphous phases of silica in sand.

The Fourier transform Infrared Alpha Bruker spectrophotometer was used to identify the functional groups of the samples. The samples were ground and placed between 400 and 4000 cm⁻¹ wavelength range for measurement at Council for Scientific and industrial Research(CSIR) in South Africa/Pretoria. The transmission spectrum showed the crystalline phases and their corresponding functional groups.

c) X-ray Fluorescence Spectroscopy

The elemental composition of the samples was determined by X-ray fluorescence spectroscopy using two instruments: a wavelength dispersive spectrometer (WD-XRF) belonging to the cement manufacturer Heidelberg Cement Group Cilu-RDC and an energy dispersive spectrometer (ED-XRF) located at the CGEA/CRENK laboratory.

At the CGEA/CRENK laboratory, we used ED-XRF spectrometry (XEPOS III, Figure 3(a)) with FP-Pellets CGEA and TQ-Pellets Fast methods to measure elements (ions) in pellets. We calibrated the spectrometer with standards ISE870, ISE 890, ISE919, ISE961 and SOIL-7. The spectrometer had four secondary targets: Mo, Al₂O₃, Co and HOPG Crystal of Bragg. The principle was based on X-ray fluorescence of the sample under X-ray beam. We identified and quantified elements by their characteristic peaks in the energy spectrum. We used the Kα1 peak (3.313 Kev) of K and normalized intensities with respect to the scattering peak for calculation. The results are in the table with t-Student confidence interval at α = 0.95.

Two methods of sample preparation were used according to the nature of the rocks: fused beads, which reduce matrix and inter-element effects, were used for silicate rocks, while pressed pellets, which are more suitable for carbonate rocks with high loss on ignition, were chosen for the latter [43] - [48] . At the HeidelbergCement Group laboratory/DRC, we used WD-XRF spectrometry (Figure 3(b)). The preparation of the samples in fused beads (Glass beads) consisted of mixing 1 g of sample and 9 g of eutectic flux with composition: 66%-Li₂B₄O₇/32%-LiBO₂/0.5%-LiBr [49] [50] [51] , then melting them in a crucible made of Au-Pt alloy (5% - 95%). This method was applied to the samples of Nyiragongo rock, gypsum, clinker and sand. The clinker used was of alite type, characterized by a Lime Saturation Factor (LSF) [52] - [57] .

L S F ( a l i t i q u e ) = % C a O 2.8 % S i O 2 + 1.2 % A l 2 O 3 + 0.65 % F e 2 O 3 > 0.85

The LSF is a parameter that allows to control the ratio of the main silicates of

the clinker and to evaluate the amount of lime available to react with silica, iron-II and alumina, forming thus the bicalcic and tricalcic silicates, as well as the aluminates. A LSF higher than 85% favors the formation of clinkers rich in alite [61] . The mineralogical composition of the clinker can be estimated from the oxide results obtained by XRF, using empirical formulas based on the Bogue Model [62] . However, this estimation presents uncertainties related to the presence of impurities in the mineral phases and to the variability of the cooling of the clinker, which affects the kinetics of mineral formation [52] [63] [64] .

According empirical formulas based on the Bogue Model [62] , we can write:

[ C 3 S C 2 S C 3 A C 4 A F ] = [ 4.071 − 7.600 − 6.718 − 1.430 − 3.072 8.600 5.068 1.079 0 0 2.650 − 1.692 0 0 0 3.043 ] [ C a O S i O 2 A l 2 O 3 F e 2 O 3 ]

According to the New Taylor model [65] , we can write:

[ C 3 S C 2 S C 3 A C 4 A F C a S O 4 K 2 S O 4 N a 2 S O 4 ] = [ 4.634 − 8.897 − 7.301 − 1.362 − 3.245 2.758 4.192 − 3.718 10.342 5.509 0.911 2.604 − 2.213 − 3.364 0.118 − 0.371 3.665 − 3.950 − 0.082 0.070 0.106 − 0.023 − 0.056 − 0.886 5.621 0.016 − 0.014 − 0.021 − 0.030 0.095 0.161 − 0.091 1.721 − 1.463 − 2.223 0.052 − 0.151 − 0.122 0.018 − 0.036 1.881 0.047 − 0.005 0.005 − 0.078 0.078 0.003 − 0.003 2.287 ] [ C a O S i O 2 A l 2 O 3 F e 2 O 3 S O 3 K 2 O N a 2 O ]

Based on the New Taylor model without alkali sulfates [65] , we can write:

[ C 3 S C 2 S C 3 A C 4 A F C S ¯ ] = [ 4.745 − 9.113 − 7.469 − 2.237 − 3.324 − 3.844 10.579 5.750 1.734 2.692 0.071 − 0.308 3.668 − 4..063 − 0.049 0.028 − 0.159 − 0.949 5.566 − 0.020 0 0 0 0 1.700 ] [ C a O S i O 2 A l 2 O 3 F e 2 O 3 S O 3 ]

The classification of magmatic (volcanic) rock samples is based on the oxide contents obtained by XRF, which are reported in the TAS diagram [66] - [71] , which allows to distinguish the different types of rocks according to their chemical composition.

d) The approximate Mineralogy by CIPW

The Nyiragongo rock samples 2002, 2010 and 2021 were analyzed by powder fluorescence, moreover, the CIPW (Cross, Iddings, Pirsson, Washington) normative abundances method [72] [73] [74] [75] was applied to the magmatic (volcanic) rock samples to estimate their virtual mineral composition or normative mineralogy.

From the oxide contents obtained by XRF and presented in Table 5, it is possible to calculate the LSF (Lime saturation Factor) and to estimate the proportions of the mineral phases of the clinker using the empirical formulas of BOGUE and TAYLOR.

LSF = 89%, confirms an alite clinker. Figure 4 shows the comparison of the proportions of the mineral phases of the clinker estimated by the empirical formulas of BOGUE and TAYLOR. It confirms that the clinker is rich in alite, as suggested by the LSF.

The empirical formulas of Bogue and Taylor give:

BOGUE: [ C 3 S C 2 S C 3 A C 4 A F ] C I L U = [ 45.989 28.5464 9.2351 12.4154 ]

The empirical formulas of TAYLOR also point to a clinker with a predominant Alite with alkaline sulfates around 0.4%c.

TAYLOR: [ C 3 S C 2 S C 3 A C 4 A F C S ¯ ] C I L U = [ 47.4844 29.9575 3.49954 15.20867 1.377 ]

TAYLOR: [ C 3 S C 2 S C 3 A C 4 A F C a S O 4 K 2 S O 4 N a 2 S O 4 ] C I L U = [ 52.76408 25.53679 5.63187 14.81596 0.56879 0.39051 0.43237 ]

The chemical composition of the Nyiragongo-2002, Nyiragongo-2010 and Nyiragongo-2021 rock, obtained by XRF, allows to classify it in the Total Alkali Silica diagram and to estimate its normative mineralogy with the CIPW software. The oxide contents are represented in the histograms of (Figure 5) and (Figure 6).

Figure 7 shows the trace element contents of the Nyiragongo-2002 basalt, obtained by XRF-Crenk, and compares them with those of the Noki granite, the Matadi metabasalt and the Mpozo syenite. The Nyiragongo-2002 basalt contains Cu, Zn, Pb, Mo, Sn, Ni, Sr, Nb, Ga and Rb. These elements could indicate the presence of andremeyerite BaFe²⁺ (Si₂O₇), with substitution of Ba by Sr or Zn. This mineral is often encountered in the basalts of Nyiragongo and Nyamulagira [76] . Figure 8 shows an example of a TAS diagram with the coordinates of the intersection points of the different classification zones.

To evaluate the mineralogical composition of the Nyiragongo rocks, we used a software that applies the CIPW (Cross, Iddings, Pirsson, Washington) model.

This model allows to determine the proportions of hypothetical minerals that could form a rock from its chemical composition. The software uses XRF (X-ray fluorescence) analyses to measure the oxide content of major and trace elements

in the rocks. Then, it solves a system of algebraic equations to calculate the percentages of normative minerals, such as quartz, feldspar, olivine, etc. Table 6 presents the results obtained by this method for the samples 2002, 2010 and 2021 of the Nyiragongo rocks.

According to Table 6 of CIPW, the rocks 2002, 2010 and 2021 contain minerals that could have an interest as a substitute for clinker, such as: Leucite (K₂Al₂Si₄O₁₂) and nepheline (Na₂Al₂Si₂O₈), which are aluminous silicates that can react with lime (CaO) to form hydrates of type C-S-H (calcium silicate hydrate) or C-A-S-H (calcium aluminate silicate hydrate), which are the main compounds responsible for the strength of cement. Wollastonite (CaSiO₃), which is a calcium silicate that can react with water to form C-S-H, or with tricalcium aluminate (C₃A) to form ettringite (C₆AS₃H₃₂), which is a compound contributing to the expansion of cement. Hematite (Fe₂O₃) and ilmenite (FeTiO₃), which are iron oxides that can react with lime (CaO) and alumina (Al₂O₃) to form C₄AF, which is a clinker phase having a dark color and a low reactivity. However, these rocks also contain minerals that could have a negative effect as a substitute for clinker, such as: Aegirine (NaFe³⁺Si₂O₆) and olivine ((Mg,Fe)₂SiO₄), which are ferromagnesian silicates that can be unstable in air and water, and release ferrous (Fe²⁺) or magnesian (Mg²⁺) ions that may cause undesirable secondary reactions, such as the formation of brucite (Mg(OH)₂) or magnetite (Fe₃O₄). Perovskite (CaTiO₃), which is a titanium oxide that can be inert or react with water to form rutile (TiO₂), which is a very hard and abrasive mineral.

Two local sands were chosen and mixed to obtain a local standard sand, conforming to [36] . The XRF analyses show that the silica content of this sand is close to 98%, as required by the standard. Table 7 presents the chemical composition of these two sands in mass percentage of oxides.

In addition to the chemical composition, the particle size distribution of the sand must comply with the standard [36] . Figure 9 shows the particle size distribution curve of the local standard sand, which is in accordance with the standard.

Figure 10 shows the Fourier transform infrared (FTIR) spectrum of this sand, in the mid-infrared (MIR) range from 400 to 4000 cm⁻¹: it mainly shows the characteristic vibration bands of quartz (SiO₂), which are the symmetric stretching band Si-O-Si at 1080 cm⁻¹ and the asymmetric stretching band Si-O-Si at 800 cm⁻¹ [77] .

The sand of Nsiamfumu/Muanda was selected for its particle size distribution similar to that of the standard normalized sands defined by the standard [36] , and for its high content of crystalline silica. The presence of some impurities is also noted, such as the C-H stretching band at 2900 cm⁻¹, due to traces of organic

matter [78] . The functional group Si-O-Si is the main constituent of quartz, a very hard and abrasion-resistant mineral, which gives the sand its mechanical properties [78] [79] . This group is formed by the covalent bond between two silicon atoms and one oxygen atom, which forms an angle of 144˚ [80] . The length of the Si-O bond is 1.61 Å, and its strength is 452 kJ/mol [81] [82] [83] .

The group Si-O-Si can vibrate symmetrically or asymmetrically, depending on whether the two Si-O bonds lengthen or shorten in phase or out of phase. It can also vibrate in bending, that is, the Si-O-Si angle varies around its equilibrium value. These vibrations are detectable by infrared spectroscopy, because they induce a change in the dipole moment of the group [77] . The functional group O-H is present in water molecules, which can be adsorbed on the surface of the sand by electrostatic interactions or hydrogen bonds. This group is formed by the covalent bond between an oxygen atom and a hydrogen atom, which forms an angle of 104.5˚. The length of the O-H bond is 0.96 Å, and its strength is 467 kJ/mol [81] . The group O-H can vibrate in stretching, that is, the O-H distance varies around its equilibrium value. This vibration is detectable by infrared spectroscopy, because it induces a change in the dipole moment of the group [77] . The functional group C-H is present in organic molecules, which can be contaminants of the sand from natural or anthropogenic sources. This group is formed by the covalent bond between a carbon atom and a hydrogen atom, which forms an angle of 109.5˚ in alkanes, 120˚ in alkenes and 180˚ in alkynes. The length of the C-H bond varies from 1.09 Å to 1.06 Å, and its strength varies from 413 kJ/mol to 839 kJ/mol, depending on the type of hybridization of carbon [84] . The group C-H can vibrate in stretching, that is, the C-H distance varies around its equilibrium value. This vibration is detectable by infrared spectroscopy, because it induces a change in the dipole moment of the group [77] . Table 8 shows that the M-O-M group has a peak at 1097 cm⁻¹, the M-O group has a peak at 778 cm⁻¹ and the MO group has two peaks at 683 cm⁻¹ and 414 cm⁻¹. The peak at 683 cm⁻¹ corresponds to the symmetric vibration of the MO group, while the peak at 414 cm⁻¹ corresponds to the antisymmetric vibration of the same group.

The FTIR spectrum of the sand shows several peaks that correspond to the vibrations of the functional groups present in the material. These vibrations are due to the changes in the dipole moment of the chemical bonds when they are subjected to infrared radiation [85] . The peak at 1097 cm⁻¹ is attributed to the symmetric stretching vibration of the Si-O-Si bonds of quartz, which is the main constituent of sand. This vibration occurs when the two oxygen atoms bonded to the same silicon atom move away from or towards the silicon simultaneously [65] . The peak at 778 cm⁻¹ is attributed to the asymmetric stretching vibration of the Si-O-Si bonds of quartz. This vibration occurs when the two oxygen atoms bonded to the same silicon atom move in opposite directions with respect to the

silicon [77] . The peak at 683 cm⁻¹ is attributed to the bending vibration of the Si-O-Si bonds of quartz. This vibration occurs when the Si-O-Si angle varies around its equilibrium value of 144˚ [80] .