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Petrological features of volcanic scoriae from the Southern part of the Cameroon Volcanic Line and their supplementary cementations application

Volcanic scoriae from the southern part of the Cameroon Volcanic Line (Limbé, Loum-Tombel, Yamba, Doupé, Njinkouo, Foumbot, Manjo-manengollé, Galim and Djoungo) were investigated in order to determine their chemical and mineralogical composition, to deduce their origin and to identify their natural characteristics which may be useful to the cement industry. The mineralogical composition was determined by X-ray Diffractometer (XRD); X-ray Fluorescence (XRF) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) instruments provided geochemical data. In order to establish the relationship between the natural characteristics of volcanic scoriae and the properties of cements, the amount of amorphous phases was determined by dissolution using sodium hydroxide solution and the pozzolanic activity by thermogravimetric analysis. Field observations show that these rocks are basalts, basanites, hawaiites and picrobasalts. Their mineralogical composition includes augite, olivine, plagioclase, enstatite, feldspars, ettringite, portlandite and Fe–Ti minerals. Overall, they are characterized by high MgO, Fe2O3, CaO, and TiO2 contents. The behavior of major and trace elements suggests that volcanic scoriae have an evolution dominated by partial melting. Besides, high chondrite normalized La/Yb (8–22), Tb/Yb (>1.9) and Dy/Yb (>2) values suggest that the melting corresponds to the garnet lherzolite stability field. The scoriae exhibit good pozzolanic reactivity after 28 days according to their considerable amount of amorphous phases, low CaO contents and their large specific surface area. According to ASTM C618 standard, the sum of SiO2, Al2O3 and Fe2O3 (SAI = 65.96–76.34 wt.%); LOI (-0.1–16.99 wt.%), and of CaO, Fe2O3 and MgO (CIM = 23.43–34.06 wt.%) suggest that those less weathered materials seem appropriate as an additive in cement manufacture. The suitable use of volcanic scoriae in the cement industry closely depends on the petrological features of amorphous phases.


Introduction
Pyroclasts, commonly known as tephra, are principally volcanic debris generated by cooling magma after explosive volcanic eruptions (Lemougna et al., 2018). Pyroclastic materials have a wide range of physical qualities and can range in their variable size (Brown and Calder, 2005). These materials proliferate in the world due to the activity of volcanic edifices. Scoriae overflow in Cameroon are found along the "Cameroon Volcanic Line (CVL)'', where they are already the object of artisanal and industrial uses. Many studies have been carried out on the scoriae from CVL. Some of these studies aimed at determining their formation processes in specific locality. Those used in this study come from nine localities: Limb e, Loum-Tombel, Yamba, Doup e, Njinkouo, Foumbot, Manjo-Manengoll e, Galim and Djoungo on the southern mainland of the Cameroon Volcanic Line (Figure 1). Galim and Djoungo scoriae have already been investigated (Djobo et al., 2014;Tchamdjou et al., 2017). Thus, it is known that the alkaline lavas, along the CVL differentiated into basalts and basanites, show similarities with Ocean Island Basalts (e.g., Tchuimegnie et al., 2015;Mbowou et al., 2020;Tchop et al., 2020). In the same vein, the geochemical features of rocks relate them to a mantle source (e.g., Tchamabe et al., 2014;Marzoli et al., 2015;Asaah et al., 2015;Ngwa et al., 2017;Dedzo et al., 2019Dedzo et al., , 2020Tiabou et al., 2019). Other studies on scoriae from the Cameroon Volcanic Line have been devoted to the industrial use of scoriae from a specific locality or two at most. This is precisely the case for Galim and Djoungo scoriae whose applications in the production of geopolymers are common (e.g., Ndigui et al., 2013;Djobo et al., 2014;Tchamdjou et al., 2017;Djon Li Ndjock et al., 2020). From the field observations, it emerges that the collected samples have various physical properties (color, shape and granulometry). However, the surface differentiation of volcanic scoriae originating from the same magma though different in colors and facies raises the debate on supergene processes. This study aims to determine the origin of volcanic scoriae in order to establish a relationship between their chemical composition and the possible uses in cement industry.  (Dumort, 1968). E.H. Ngono Onana et al. Heliyon 8 (2022) e08684 2. Geological setting The Cameroon Volcanic Line (CVL) is a N30 E alignment of oceanic and continental volcanic mountains, and anorogenic plutonic complexes, extending from Pagalù Island to Lake Chad (D eruelle et al., 2007). It is one of the few magmatic entities across oceanic and continental plates (Sato et al., 1990). The continental sector consists of a succession of large polygenetic volcanoes separated by monogenetic volcanic fields, where Hawaiian, Strombolian and moderate Vulcanian explosive phreatomagmatic eruptions engendered several short lava flows, pyroclastic cones and maar volcanoes (Sato et al., 1990;Nkouathio et al., 2008). These lavas range from basalts and basanites to more evolved felsic such as phonolites, rhyolites and trachytes (Sato et al., 1990;Marzoli et al., 2000;Kamgang et al., 2013;Asaah et al., 2015). The northern part of the CVL comprises the Mandara Mountains and large volcanic plateaus including Bui, Kapsiki and Ngaoundere made of alkaline basanites and basalts below phonolitic and trachytic flows (Marzoli et al., 1999;Nkouandou and Temdjim, 2011;Tiabou et al., 2019;Dedzo et al., 2019). Its southern continental part is marked by a succession of volcanic massifs (Mount Cameroon, Mount Rumpi, Mounts Manengouba, Mounts Bambouto and Mount Oku) ( Figure 1a).
The study area is limited by the longitudes 09 00 and 11 00 east and the latitudes 04 00 and 07 00 north (Figure 1b), in the Pan-African sector made up of syntectonic granitoids. According to Tamen et al. (2007), Mount Cameroon is one of the horsts of the Kumba. It is made up of basanitic, basaltic and hawaiitic lava flows, and cinder cones (Deruelle et al., 1991). This graben concentrates most of the monogenetic volcanoes along the CVL, and includes more than 100 Quaternary scoria cones with associated lava flows (Nkouathio et al., 2008). In the Bamoun plateau, lavas resting on the old Pan-African basement (granites, gneisses and gabbros) include basaltic and siliceous lavas (trachy-rhyolites, rhyolites and ignimbrites) (Moundi et al., 2007). All the rocks of the study area are predominantly basalts (recent basalts), trachytes and surrounding gneissic formations.

Analytical procedures
Samples were powdered at the University of Yaound e I (Cameroon). The mineralogy, using X-Ray Diffractometer (XRD), was done at the Geoscience Laboratories (Sudbury, Canada). The rocks were powdered with an agate mortar and pestle. The analytical instrument (PANalytical X'PERT PRO diffractometer) was equipped with a monochromator, and samples were analyzed with Co radiation at 40 kV and 45 mA. The X'Pert HighScore Plus software was used for the peak identification.
Major element concentrations were determined by X-Ray Fluorescence Spectrometry (XRF) using a PANalytical PW2400 spectrometer at the Geological Institute (ISTE) of the University of Lausanne (Switzerland). Major elements were obtained on fused lithium tetraborate glass discs after heating the samples to 1050 C in an oven in order to calculate the Loss On Ignition (LOI). The procedure for obtaining oxide weight percentages is described by Mbabi et al. (2020). Mg-number was obtained using the formulae (Mg# ¼ 100*MgO/(MgO þ FeO).
Trace element analyses of scoriae were performed using an ICP-MS instrument (Element, Finnigan MAT) at the Geoscience Laboratories (Sudbury, Canada). Powders were digested by acid attack in closed beakers for Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analysis for lithophile trace element concentrations. The analysis was carried out following the procedure of Doum et al. (2020).
Specific surface area, amount of reactive phase and pozzolanic activity were carried out exclusively on scoriae from Limb e and Galim sites by Djon Li Ndjock et al. (2017). The specific surface areas of powders were calculated from the Blaine model. The pozzolanic activity of volcanic ash was determined through DTG, analysis of dried slips was obtained from mixtures of Ca(OH) 2 and volcanic ash during 7 and 28 days (Mendoza and Tob on, 2013;Djon Li Ndjock et al., 2017). The amorphous phase content was determined by dissolution of sodium hydroxide solution according to ASTM C289-94 and French XP P18-594 standards (Djon Li Ndjock et al., 2017).

Major elements
The distribution of major elements is reported in  Figure 4 highlights a positive slope for SiO 2 , CaO, and MgO, and negative one for Al 2 O 3 , Fe 2 O 3 , TiO 2 , MnO and P 2 O 5 . According to several works carried out along the Cameroon Volcanic Line (Yamgouot et al., 2016;Mbowou et al., 2020;Tchop et al., 2020), the sum of SiO 2 , Al 2 O 3 and Fe 2 O 3 is up to 67.50 wt.% and the alkali contents attain 4.62 wt.% ( Table 2). The average contents of major elements are similar along the Cameroon Volcanic Line (Table 2).

Trace elements
Large ion lithophile elements (Sr, Ba and Rb) are the most abundant trace elements in volcanic scoriae, although Rb remains low in all samples (<65 ppm, Table 2). Strontium contents are higher in basanites than in basalts (Table 3) reflecting the cumulative character of plagioclase. Inversely, barium contents are low in basanites (av. ¼ 497 ppm) and high in basalts (av. ¼ 510 ppm). The least abundant LILE is Cesium, below 1.2 ppm (Table 3).
High field strength elements (Th, U, Zr, Hf, Mo, W and Ta) are lower than LILE. The most abundant elements are Zr (187-474 ppm) and Nb (38-112 ppm). The average contents of hafnium, Ta, Th, Mo, and U are below 10 ppm ( Table 3).
The ferromagnesian trace elements (Cr, V, Ni, Zn, Cu, Co and Sc) contents are higher than those of HFSE and lower than LILE. The highest  (Table 3). The comparative evaluation of the different elemental groups shows high LILE/HFSE exhibits by Ba/Nb ratio (5-8.72) and low HFSE/HFSE highlights by Zr/Nb ratio (2.64-4.85). The average contents of several trace elements are variable probably due to the nature of magma or the large number of samples (Tables 2 and 3).
Basanites are more enriched in REE than basalts (Table 4). Quantitative expressions of REE fractionation typically use (La/Yb) N ratio (Aubert et al., 2001;Yusoff et al., 2013). However, the depletion of HREE relative to LREE most likely indicates the presence of garnet in the source materials (Rollinson, 1993). The (La/Yb) N ratio varies between 8.34 and 21.93 and highlights the high REE fractionation particularly in basanites (Table 4). It might be due to presence of olivine, orthopyroxene and clinopyroxene. The Th/Ba, Th/Rb and Th/U data are similar to those obtained on oceanic basalts (Lee et al., 1994;Yamgouot et al., 2016). The LREE/HFSE ratio (e.g., La/Nb: 0.32-1.40) is high while that of LILE/L-REE (e.g., Ba/La ¼ 5.35-17.44) is low (Table 5).
Normalized REE with roughly subparallel patterns show (i) a decreasing slope from LREE to HREE; (ii) no Ce-anomaly (Ce/Ce*0 .98); and (iii) negative Eu anomaly (Eu/Eu*~0.44) (Table 4, Figure 5). The negative Eu anomaly might be due to the removal of feldspars through fractional crystallization (Duzgoren-Aydin and Aydin, 2009;Yusoff et al., 2013) or to the replacement of Eu by Sr in feldspars. Normalized-primitive mantle trace element patterns from other works (i.e., Yamgouot et al., 2016;Mbowou et al., 2020) are roughly parallel to those of this study. These materials would therefore be co-genetic and thus with similar features.

Fractional crystallization
The evaluation of the fractional crystallization process is based on the behavior of major and trace elements. There are low Ni (90-226 ppm), Cr (133-498 ppm) and Co (33-57 ppm) in scoriae. They slightly overlap with those of primary magmas; Ni (300-400 ppm), Cr (300-500 ppm) and Co (50-70 ppm) (Frey et al., 1978). The Mg# values (29.67-47.75) are significantly lower than those of primary magmas (e.g. Mg# ¼ 68-72 for whole rock (Frey et al., 1978); Mg# ¼ 75-78 for peridotitic basalts (Tamura et al., 2014). Mg# values below 60 indicate an evolution of the materials since their source formation (Kelemen et al., 2007), thus suggesting its fractional crystallization (Zhang and Honaker, 2018). The    Nesbitt and Young (1984). Harmois (1988 differentiation process can be demonstrated by the variable ratios of some incompatible residual elements (e.g., La/Yb ¼ 1.28-31; Ce/Zr ¼ 0.16-0.60; and Th/Nb ¼ 0.06-0.16; Table 5) as these ratios are supposed to remain unchanged during fractional crystallization (e.g., Kamgang et al., 2010;Asaah et al., 2015). The diagrams Al 2 O 3 and Na 2 O vs. MgO highlight the fractionation of olivine ( Figure 4) and clinopyroxene as well as the positive trends of selected trace elements (Ni, Cr and Co) vs. MgO ( Figure 6). The behavior of Fe 2 O 3 , TiO 2 and MgO confirms the crystallization of Fe-Ti minerals during the evolution of the magma. Rubidium is the LILE which slightly varies with increasing MgO while Ba, Yb, Ta and La exhibit a negative correlation ( Figure 6). Moreover, the constant decrease in V, Ti, Zr and P 2 O 5 along the fractionation index reveals that the separation of Feminerals, Ti-minerals, zircon and apatite might control the variation of V, Ti, Zr and P (Ziem et al., 2018). The fractionation of apatite and Fe-Mg minerals such as olivine and clinopyroxene constitute the first stage of differentiation. The values of Ba/La (6.72-12.04, Table 5) associated to those of Y/Nb (0.28-0.69) indicate the last stage of the differentiation process also marked by alkali feldspar crystallization.
The gradual increase in Al 2 O 3 during nearly all the differentiation indicates other phases and not only plagioclase during fractional crystallization processes. Moreover, Sr/Ba and Rb/Sr ratios increase with SiO 2 , ruling out the fractionation of plagioclase and K-feldspars. The positive correlation between CaO and MgO confirms the presence of high amounts of amorphous phases.
According to Langmuir et al. (1978), the diagrams of the ratio between two strongly incompatible elements vs. the amount of an element or the ratio of two other incompatible elements reveal the processes that affect the petrogenesis of igneous rocks. To assess the mantle source properties of volcanic scoriae, elemental ratios of REE such as La/Sm and Tb/Yb were used. Fractional crystallization and crustal contamination processes have little effect on REE. The correlation of La/Sm vs. La and Tb/Yb vs. Yb (Figure 7a, b) indicate the role of partial melting during fractional crystallization. Moreover, these diagrams underline the probability of source heterogeneity in the genesis of volcanic scoriae.

Crustal contamination
During the ascension of magma toward the earth's surface several processes occur such as crustal contamination which can affect its composition. Crustal contamination was highlighted on several materials from the Cameroon Volcanic Line (e.g., Marzoli et al., 1999;Kamgang et al., 2013;Tchuimegnie et al., 2015;Ziem et al., 2018;Mbowou et al., 2020). Major and trace elements were used to evaluate the assimilation of the crust. There is enrichment in SiO 2 and LILE (i.e., Rb, Ba and K) and depletion in HFSE (i.e., Nb, Ta and Ti) in continental crustal rocks (Rudnick and Gao, 2003). The studied scoriae are basalts, picrobasalts, hawaiites and basanites, whose SiO 2 values range from 39 to 49 wt.%, as well as other volcanic materials from the CVL (43.97-45.46; Table 2). Their mafic character indicates that they were not or less contaminated by siliceous materials during the magma ascent. Niobium is highly depleted in all continental crust rocks (Taylor and McLennan, 1985) and the La/Nb ratio was often used as an indicator of crustal contamination (Ziem et al., 2018). The magmas which have crossed the continental crust are identified by La/Nb > 1.5 and La/Ta > 22. These samples present La/Nb varying between 0.32 and 1.4, and La/Ta between 10.69 and 16.23 (Table 5), and suggest no contamination by the continental crust. The La/Ta value (>30) shows witness of a very low crustal contamination. Moreover, the Nb/Yb and Th/Yb ratios reveal that these materials derived from mid-ocean ridge basalts (MORB). The values of Zr/Nb (2.64-4.85) and Ba/Nb (5.06-8.72, Table 5) ratios are similar to those of Ocean Island Basalts (OIB, Sun and McDonough, 1989). The plot of samples in the MORB-OIB domain (Figure 8) also confirms the absence of crustal contamination. Rollinson (1993) considered the Th-enrichment and Nb-depletion as indicators of crustal contamination; this is not evident in volcanic scoriae patterns (Figure 9) which also reveal the absence of negative Nb-Ta spike. The positive Ba anomaly and negative K, Sr and Rb anomalies (Figure 9) might be linked to the presence of phlogopite and amphibole. These minerals being unstable at high temperatures in the upper mantle, the high K/Ba, K/Rb and Ba/Rb values indicate the effect of metasomatism. The Nb/Y vs. Rb/Y diagram ( Figure 10) allows to appreciate the effect of crustal assimilation compared to fractional crystallization. The high values of Rb/Y and Nb/Y along the fractional crystallization trend reflect the low impact of crustal contamination (Dedzo et al., 2020) which is also explained by Th/La and La/Nb ratios (0.04-0.30 and 0.32-1.40, respectively) lower than continental crust (Th/La ¼ 0.3; La/Nb ¼ 1.5).

Mantle source and melting
In general, poorly evolved alkaline basalts show relatively constant concentrations of transition elements of the first series (Co ¼ 60 AE 10     Villemant and Treuil, 1983). These concentrations result from partial melting of a peridotitic mantle source (Kamgang et al., 2013). The trace element modeling indicates (Figure 7b) that partial melting was the main differentiation process compared to fractional crystallization and crustal contamination.
The low values of Rb/Sr (0.01-0.07) could be reached by partial melting involving the typical granulite crustal rocks (Taylor and McLennan, 1985) confirming the Pan-African basement of the southern part of the CVL. Halliday et al. (1990) suggested that the partial melting of granulite rocks would produce magmas with very low Rb/Sr values ( Table 5). The low ratio values of incompatible elements, particularly LILE/HFSE (Rb/Nb ¼ 0.12-0.75; Rb/Zr ¼ 0.03-0.19; Th/Ta ¼ 1.01-2.88; Ba/Nb ¼ 5. 06-8.72), compared with the high values of the same ratios in the Pan-African basement rocks (Rb/Nb ¼ 1.66-6.31; Rb/Zr ¼ 0.30-1.02; Th/Ta ¼ 22.98-69.08; Ba/Nb ¼ 63.13-138.77; Kwekam et al., 2015) confirm the limited possibility that melting occurred in a crustal level. The high LREE/HREE values suggest the mantle-borne liquid might have been enriched in LREE. The major and trace elements help to highlight the origin of alkali basaltic magmas. This suggests that they derive from a garnet-bearing mantle source (Frey et al., 1978;Jung and Masberg, 1998). The high (La/Yb) N values might result from the high REE-fractionation (N ed elec et al., 1990;Shang et al., 2004). The (La/Yb) N ratio can state if partial melting occurred in garnet or spinel stability field. Farmer (2003) suggested that lavas with (La/Yb) N 5 and (La/Yb) N ! 5 derived from melting in spinel and a garnet lherzolite stability zone, respectively. High (La/Yb) N of the studied scoriae define a dominantly garnet-bearing mantle zone as the starting point of partial melting of their parent magmas (Figure 11a). High CaO/Al 2 O 3 and Zr/Y ratios (0.29-1.05 and 6.98-12.83, respectively, Tables 1 and 5) confirm these features. The presence of garnet would attest that partial melting occurred at a high pressure, above 3 Ga (Robinson and Wood, 1998) and at about 70-80 km depth (Tchuimegnie et al., 2015). The use of Tb/Yb vs La/Sm diagram (Figure 11b) confirms its specific depths beyond 90 km (Fan et al., 2004;Pearce, 2008). According to Wang et al. (2002), garnet-bearing mantle zone is delimited by (Tb/Yb) N > 1.9, in addition to (La/Yb) N (Figure 11a).

Weathering process
The weathering indexes (CIW, CIA and A-CN-K) have been used by several authors (e.g., Okewale and Coop, 2017, 2018, 2020Okewale, 2020) to quantify chemical weathering of volcanic materials. In this study, the CIW values (49-90%) are higher than the CIA values (42-88%), indicating the effects of chemical weathering. The CIA values reveal that the weathering is more extensive on basalts (51-75%) than on basanites (42-60%) and highest in hawaiites (88%, Table 1). The high La/Yb ratio also supports the degree of chemical weathering. Therefore, the samples still retain a detectable amount of the labile elements (Ca, Na, and minor K). The CIA predominantly tracks the feldspar dissolution and the concomitant release of Ca, Na and K relative to Aluminum, since Aluminum is typically retained within clay minerals or gibbsite (Babechuk et al., 2014). Gal and DP1 samples with the highest Al 2 O 3 content show the lowest NaO and CaO contents; inversely, the highest CaO contents (L2G, L2R and L2N samples) correspond to the lowest Al 2 O 3 contents (CaO/Al 2 O 3~0 .66; Table 1). The high values of index of alteration (CIA and CIW) confirm the intensity of weathering of feldspars, probably due to the mountain's tropical humid season and the former of slopes. This is favorable to the formation of clay minerals. The A-CNK ternary diagram (Nesbitt and Young, 1989) allows to consider the mafic mineral component in weathered rocks. Most samples plot in the CNK domain ( Figure 12) and suggest a greater relative mobility and more extensive leaching of Ca, Na and K compared to Mg (Babechuk et al., 2014). In the same diagram, samples plot near the A-CN edge, indicating an expected weathering trend. The volcanic scoriae are slightly weathered into clay minerals and Fe-minerals. On the ternary diagram, samples are closer to A apex which is the ideal field of clay minerals. The imprint of weathering processes on these materials guides various industrial demands. The use of materials containing reactive silica such as pozzolans in cement industry is linked to many economic, ecological and performance-related benefits. The accessibility and abundance of volcanic scoriae in CVL justify their uses in several domains. These materials show natural chemical properties favorable for artisanal uses. In the cement industry, they are requested for their pozzolanicity and the reduction of production costs as well as energy expenditure. Pozzolanic activity is an important parameter for the industrial recovery of the studied materials. It relates to the petrological features depending on their formation processes.

Interest of chemical composition in cement production
The chemical composition is the first criterion that can orient the usefulness of volcanic scoriae in the cement industry. The chemical composition of pozzolans fluctuates between 45 and 65 wt.% for SiO 2 , 15 and 30 wt.% for Al 2 O 3 þ Fe 2 O 3 and reach 15 wt.% for CaO þ MgO þ alkali (Dron and Brivot, 1977). ASTM C618 (1993) recommends the following compositions for thermally activated or natural pozzolans used as additive in cement (class N): SiO 2 þ Al 2 O 3 þ Fe 2 O 3 : 70 % minimum; Loss On Ignition: 10% maximum. The results show that volcanic ashes from Limbe and Galim areas (Table 6) present similar chemical composition to that of this study, this complies with the specifications of ASTM C 618. According to Djon Li Ndjock et al. (2017), scoriae can be used for geopolymer synthesis or as filler. Ndigui et al. (2013) demonstrate that pozzolans from Djoungo and Foumbot areas are suitable for geopolymer due to their high alkali content (Table 6), which is significantly lower than that observed in the present study.

Interest of mineralogical composition in cement production
All scoriae from the CVL have alkali oxides (Na 2 O þ K 2 O, 4.28-4.58 wt.%) in their chemical make-up, indicating the existence of alkalifeldspars. Due to the abundance of scoria cones in many places of the world, various investigations of volcanic scoriae as a cement replacement have been conducted. In addition to the amorphous phases, plagioclase, olivine, clinopyroxene, illite, hematite and kaolinite are found in these materials (Khan and Alhozaimy, 2011;Rajaonarison et al., 2017). This mineralogical assemblage is similar to that of volcanic scoriae from Tombel Plain, Bamileke Plateau and Noun Plain (Mbowou et al., 2020). The high contents of alkali oxides (Na 2 O þ K 2 O) in the scoriae from CVL indicate the presence of alkali-feldspars. As reported by Djon Li Ndjock et al. (2017), the XRD patterns of volcanic scoriae from Limbe and Galim areas indicated the presence of a significant amount of amorphous phases.

Relation between amorphous phases and chemical composition
The glassy phase is a factor on which the reactivity of volcanic scoriae depends as well as the chemical composition. The glassy phase content of natural pozzolans is related to the content of SiO 2 and CaO in samples (Millet et al., 1977). The amorphous phases are more important when the difference between SiO 2 and CaO is greater than 34%. When the difference between SiO 2 and CaO content is below 34%, pozzolans would not be included in the glassy phase. The glassy phase was found to have a substantial role in the reactivity of the scoria sample (Ndigui et al., 2013). Volcanic scoriae have a glassy/amorphous structure with low SiO 2 and high Al 2 O 3 concentrations that react with lime or cement (Swamy, 1997). It is conceivable to assimilate scoriae from the CVL in the alumino-silicate materials (Wong et al., 2020). The Al 2 O 3 contents could influence the  (Blein et al., 2001) and (b) Tb/Yb vs. Yb diagrams (Fan et al., 2004). high reactivity of those materials (Poon et al., 2001). Another factor that affects the reactivity of pozzolans is their particular surface area. As a result, volcanic scoriae from the CVL have high specific surface area values (Table 6) which could impact the pozzolanic activity.

Pozzolanic activity
The pozzolanic activity of the volcanic scoriae from Limbe and Galim were reported by Djon Li Ndjock et al. (2017). From this work, the formation of saponite 16 A-ferroan, calcium silicate hydrate and stratlingite are evidenced using XRD on samples age of 28 days (Djon Li Ndjock et al., 2017). According to Djon Li Ndjock et al. (2017), stratlingite is found only in sample which has higher content of amorphous alumina and the largest specific surface area but lower content of SiO 2 and CaO. Their thermogravimetric data (TGA) exhibits a mass loss around 100 C, associated to sulfur gases loss due to C-S-H functions and the mass loss associated to sulfur gases from C-A-S-H occur around 180-240 C. This could be explained by the fact that the amorphous phase in the sample contained calcium ions that hinder the consumption of calcium from portlandite. Ettringite, feldspars and plagioclase which are found in some of the scoriae investigated would have similar impact. As a result, the presence of a substantial proportion of portlandite in the mineral assemblage indicates that volcanic scoriae consume less Ca(OH) 2 . For the slips of sample with a lower glassy phase content, there was a significant decrease in the amount of Ca(OH) 2 (mass loss around 500 C) with age. It has a lower proportion of amorphous phases, a large specific surface area and high SiO 2 and CaO values. The minimum sum of 70% of SAI oxides (SiO 2 þ Al 2 O 3 þ Fe 2 O 3 ) is a key criterion of ASTM C618 (1993) that is typically adopted as the minimum requirement by other specifications (Lemougna et al., 2018). Saudi Arabian scoriae, for example, have SiO 2 values ranging from 41.14 to 46.5 wt.%; Al 2 O 3 and Fe 2 O 3 contents vary from 14.74 to 17.2 wt.% and 12.2 to 17.25 wt.%, respectively (Celik et al., 2014;Khan and Alhozaimy, 2011). Madagascar related materials have similar content (Rajaonarison et al., 2017). Similar findings were made on scoriae from the Cameroon Volcanic Line (Figure 13). The SAI oxide values for volcanic scoriae appropriate for the cement industry range from 68 to 79 wt.% (e.g., Tchamdjou et al., 2020;Mbowou et al., 2020;Djon Li Ndjock et al., 2020). The SAI oxide contents for the present study vary from 65.96 to 76.64 wt.%; most of them are up to 70 wt.%. However, the CIM (CaO þ Fe 2 O 3 þ MgO) contents vary between 26.75 to 35.74 wt.% (Table 2). According to Mbowou et al. (2020), samples with CIM above 29% are more prone to develop an amorphous phase that governs the reactivity of scoriae and seems appropriate as an additive in cement manufacture. Another criterion defined by the ASTM C618 (1993) standard is the LOI value. According to Al-Swaidani (2018), the average values of LOI from different countries range from 3.9 to 5.8 in Algeria, 0.9 to 4.9 in Saudi Arabia, and are 1.8 in Yemen and <9.3 in Cameroon. Dp1 and Gal samples with LOI >10, SAI<70 and CIM <29 would not be used as supplementary cementitious material. All these three criteria are favorable for MANR, MANJ, MANG, MDR, MDN, L2N,   Cox and Hawkesworth (1985).
L2R, YMB2 and NJK1 samples; other samples, meeting only two criteria could be processed in order to improve their CIM content or else find other applications.

Conclusion
In order to determine their petrological features and prospective industrial uses, petrological investigations on the scoriae from the southern continental region of CVL were carried out.
The studied samples are alkaline basalts, basanites, hawaiites and picrobasalts, and are characterized by their high MgO contents and affinity with OIB, which testify that the processes that contributed to the formation of rocks are fractional crystallization, metasomatism, and nearly inexistent crustal contamination. The geochemical features reveal that partial melting is the main process in the genesis of studied volcanic scoriae. The light REE abundance relative to heavy REE suggests melting in a garnet-bearing mantle.
The high values of SAI associated with those of alkali oxides and the high proportion of amorphous phases reveal that volcanic scoriae are suitable for geopolymer and pozzolanic binder.
The intensity of the chemical weathering is variable (moderate to high) along the Cameroon Volcanic Line. The less weathered samples can be used as supplementary cementitious materials. Figure 12. A-CN-K diagram (Nesbitt and Young, 1984) of scoriae from the CVL.    Sun and McDonough (1989), and the horizontal line according to Wang et al. (2002); b) (Tb/Yb) N vs. (La/Sm) N diagram for volcanic scoriae. Ratios are normalized to primitive mantle values from McDonough and Sun (1995). The discrimination dashed line separates two fields for melting of garnet-bearing peridotite and of spinel-bearing peridotite (Wang et al., 2002).
Feldspars, plagioclase and pyroxene influence the reactivity in a solution with Ca(OH) 2 . This reaction also depends on the high specific surface area.

Author contribution statement
Estelle Huguette Ngono Onana & Ndjigui Paul-D esir e: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Fuh C. Gentry: Performed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement
No data was used for the research described in the article.

Declaration of interests statement
The authors declare no conflict of interest.

Additional information
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