Characterization of Pyroclastic Deposit from Three Different Areas within Foumbot Region (West-Cameroon): Comparative Studies of Their Effects as Pozzolanic Materials in Mortars and Cement Manufacture

Pyroclastics which are known natural pozzolanic materials due to amorphous contents, are present in several areas of the Mbepit Massif in West Cameroon. In this work natural pozzolan from three zones namely Pouoloum, Njimbouot and Nkouonja were characterized. A comparative study was then de-veloped to attest the effect of these pyroclastics as partial substitution in portland cement. The mixtures were made at different pozzolanic proportions (00%, 10%, 15%, 25% and 35%) of substitution of the cement. The compressive and flexibility strength was carried out at 7, 28 and 90 days on mortar specimens (4 × 4 × 16 cm 3 ). The results revealed apozzolanicactivity index of 81.99 %, 83.47% and 74.54% respectively for rock sample from Pouoloum (PCB), sample from Njimbouot (PCC) and sample from Nkouonja (PCN). After 90 days, for a substitution rate of 25% compressive strength are respectively 55.69 MPa, 60.4 36.99%, 24.84% for PCB, PCC and PCN respectively. These results also prove that Natural Pozzolan is interesting in the manufacture of composite cement CEM II, CEM IV in accordance with EN197-1 or can be added in mortar for buildings and sustainable environmental management.


Introduction
Pozzolans are a broad class of siliceous or siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties (Mehta, 1987). The quantification of the capacity of a pozzolan to react with calcium hydroxide and water is given by measuring its pozzolanic activity (Snellings et al., 2012). Pyroclastic rocks are blocks of solidified lava ejected during the eruption of a volcano and are classified as ashes, lapilli, bombs and blocks depending on their diameter (Heiken & Wohletz, 1985). The samples were collected in the three different areas because pozzolan coming from different areas may not necessarily react in the same manner. Also, there are some variations in grain size in the different pozzolans in these three areas.
The Mbepit massif in the Noun Division ( Figure 1) contains huge volcanic deposits of ash (diameter < 2 mm) and lapilli (2 mm > diameter > 64 mm), exploitable as pozzolanic materials, for mixtures in mortars and manufactures of cements. The pozzolan of Foumbot area comes from the volcanic activities. Those activities have resulted in materials with different textures and structures according to the specific condition of the emission of lava and eruptive styles. Studies reveal that pozzolanic activity of materials from different areas may not The natural pozzolans used as binder helps to protect the environment by reducing the consumption of portland cements. This also aids in reducing environmental pollution due to the manufacture of portland cement by partial substitution of clinker in portland cement with pozzolanic materials such as natural pozzolan (Mokhtaria et al., 2009), metakaolin, industrial waste such as silica fume, fly ash (Ayrinhac, 2005;Hosseini et al., 2011), blast furnace slag (Elke, 2012) and coal bottom ash (Cheriaf et al., 1999;Haldun & Mine, 2007). The pozzolanic materials will contribute to the improvement of the mechanical properties of concretes due to the development of pozzolanic activity and the formation of second-generation calcium silicate hydrates CSH II (Cheriaf et al., 1999;Haldun & Mine, 2007). The manufacture of portland cement produces very large quantities of CO 2 during the decarbonation of CaCO 3 . The cement industry emits 5% to 7% of the global CO 2 rate emitted into the atmosphere, with 0.9 tons of CO 2 emitted during the manufacture of one ton of cement (Emad et al., 2013). The responsiveness of a pozzolanic material depends on several parameters and its reactivity varies from one pozzolan to another. This work will help 1) appreciate the mechanical resistance of mortars containing natural pozzolan from the Foumbot area and 2) to compare the pozzolanicity of pyroclastic material from three different sites ( Figure 1) and 3) to appreciate their use in cement manufacture.

Materials and Methods
In this area, the basement of recent volcanic lavas is made up of gneiss and granite. The volcanic rocks are made up of basalts, rhyolites and colossal quantities of pyroclastic materials deposited in different areas (Tchokona, 2010).

Materials
Natural pozzolans were collected in three different areas, namely Pouoloum, Njimbouot and Nkouonja and the samples pseudo named as PCB, PCC and PCN respectively ( Figure 2). The pozzolanic effect depends on the fineness of the particles (Isaia et al., 2003) suggesting grinding as the most appropriate treatment method in the study of natural pozzolan. The duration of grinding and different sieving aided approximatively the same granulometry in the three pozzolans after grinding. Moreover, the parameters influencing the pozzolanic reactions are the nature of the active phases and their proportions, the SiO 2 content, the lime/pozzolan ratio in the mixture, the duration of the cure, the fineness of the Journal of Geoscience and Environment Protection According to the American Society for Testing and Materials (ASTM) C618 (Standard ASTM standard, C618, 2008), a material is pozzolanic if: • its chemical composition verified that SiO 2 + Al 2 O 3 + Fe 2 O 3 > 70%; • its glass content is such that (SiO 2 -CaO) > 34; • its activity index for a substitution rate of 25%, I 25 at 28 days of cure verified that 67% < I 25 < 100%. Ip = (Rp/R 0 ) × 100 R 0 = compressive strength of the control mortar with 100% cement; p = 25% of pozzolanic material; I = pozzolanic activity index; R p = compressive strength of mortar with p% of pozzolan replacement by weight in cement.

Mixtures and Tests
Mixtures were made using water, sand, portland cement 52.5 with different mass substitution rate of cement by natural pozzolans. The mortars were molded into bars (4 × 4 × 16 cm 3 ). They were demolded after 24 hours and then kept under a b c Journal of Geoscience and Environment Protection water in favourable laboratory conditions. The compressive and flexural strength tests were carried out at 7, 28, and 90 days according to the European Standard methods of testing cement determination of strength (EN 196-1) (Standard EN 196-1, 2006).

Estimation of the Vitreous Phase by the Keyser Method
The standard Keyser method was done using 1% hydrofluoric acid solution for the dissolution of the amorphous silico-aluminates. 1g of pozzolanic material was introduced into 200 ml of 1% hydrofluoric acid solution and the solution was kept for 40 minutes and then filtered on an ashless filter paper. The residue was oven-dried at 105˚C for 24 hours. The amorphous phase of the material was given by the formula below: % amorphous = (m 1 − m 2 )/m 1 m 1 = mass of material before dissolution; m 2 = mass of the material after dissolution.

Results and Discussions
According to Table 1 The cement used is rich in CaO and SiO 2 ( Table 2).

Granulometric Results
The granulometric results show that the natural pyroclastics are mostly made up of volcanic ashes i.e. less than 2 mm ( Figure 4) and after grinding the average sizes were dominated by 10 µm (Figure 5).

Specific Surface Area
The specific surface area of the pozolans is about 2.5 m 2 /g for the three samples after grinding.

Keyser Results
The obtained results show that the amorphous phase is higher in PCC with 36.99% than PCB and PCN (Table 3(a)). These amorphous phases are responsible for the pozzolanic activities.

Mixtures
The table summarizes the various proportion of pozzolan that were successively utilized in the mixtures (Tables 3(b)-(d)).     value of 67.02 MPa at the same age. This is because at a young age (before 28 days) many pozzolan ash particles remained unattacked by calcium hydroxides as it is the case in coal bottom ash particle and in rice husk ash (Hwang et al., 2011).
Mortars made from pozzolan, despite their poor performance at a young age tend to catch up with time the reference mortar at 90 days (Table 4 and Table 5).

Pozzolanic Index Activity
The results reveal that for the three study pyroclastics, the pozzolanic activity index is greater or equal to 74.54% (Table 6).
According to Figure 8, the activity index values decrease with the increase of pozzolan content. The 03 pozzolans PCB, PCC and PCN activity indexes are 82%, 83.47% and 74.54% respectively. This shows that PCC is the most reactive pozzolan followed by PCB and PCN. According to ASTM Standard C 618 standard the pyroclastic materials (volcanic ashes and lapilli) of the study area are pozzolans. The results show that at a young age (28 days), the compressive strength decreases with the increase of the pozzolan substitution rate. This would be due to the low power consumption of lime Ca(OH) 2 by pozzolan at this age. At 10% of substitution rate, PCC sample compressive strength is 66.03 MPa closer to the reference moctar value of 67.02 MPa at 90 days compared to PCB (62.94 MPa) and PCN (59.8 MPa) (Table 7). Up to 35% of pozzolan substitution rate, the value of PCC (52.3 MPa) is greater than those of PCB (50.15) and PCN (46.31) at 90 days. No matter the substitution rates, the compressive strength of PCC, PCB, PCN increase with age. This is due to the pozzolanic activity of those pyroclastic materials which allow them to consume lime Ca(OH) 2 and CH and densify voids and pores in the mortar. This justifies the increase of compressive strength with age of the mortars. The hydration products fill the capillary pores and increase the resistance by refining these capillary pores and by transforming the large crystals of CH into CSH. Journal of Geoscience and Environment Protection

Conclusion
The results of the study reveal that the three pyroclastic materials are pozzolans according to the American ASTM C 618 classification, with the sum of major and that mortar mixtures with pozzolan will portray good mechanical properties at 90 days. Pozzolan PCC from Njimbouot is more reactive than the others and is expected to be best suited for cement manufacture. Composite cements manufactured from these rocks are environmentally sustainable as they will emit low CO 2 and is highly economically viable as the process requires less energy compare to Portland cement. The low production cost and the environment-friendly nature of the composite cement provide an affordable option for the construction industry.