Pumice & Pumicite
in Dhamar Governorate
Prepared by /Anas Saad Al-Ghobari
1. Introduction:
Because of the growing importance of raw materials for the building industry in the Yemen Republic (Y.R.), the only occurrence of pumice already known in the country, in the area east of Dhamar and 95 km south-east of Sana'a, was explored in February/ March 1977 by the Yemen Minerals and Petroleum Authority and the German Geological Advisory Group in the Yemen Republic (Figure 1). The total area explored covers approx. 10.000 m2 and bounded as follows:
- West: Sana’a – Taiz road: from Sana’a between kilometre marks 80 and 115.
- North: Route via the villages Balad- Hyd al Abyad – Garasha – Hauwarwar – area immediately to the north of the Dj. Spill.
- East: Area immediately to the east of the Dj. Spill. - South: Road Dhamar
– Rada and wadi Shalala.
In this area, gravel- sized pumice and pumicite occur mostly in the neighborhood of the two volcanoes Dj. Lissi and Dj. Spill, approx. 15 km east of Dhamar.
Extensive occurrences of pumice in the reconnaissance area are referred to by DVOJEGLAZOV, MARTYNCV&SIMANVSKIJ (1966). Parts of the reconnaissance area around the Dj. Lissi were the subject of hydro- geological investigations in connection with the British Mountain Plane and Wadi Rima Project ( 1974- 1976 ).
Formation of pumice and pumicite: Certain volcanoes- e.g. the Dj. Lissi and Dj. Spill east of Dhamar – discharge a magma rich in SiO2 and aluminum silicate. Occasionally, the magma is dammed up inside the volcano during a period of eruption and an explosion subsequently occurs of the congested gas and Lava masses . during this process gas-rich magma is flung out of the volcano . it then becomes degasified, solidifies in the air and is finally deposited around the volcano in the form of very porous up to sand- sized pumicite ( volcanic ash ) and gravel-sized pumice.
Economic significance of pumice:
Pumicite:
Pumicite can have the properties of pozzuolana and is then a particularly valuable and much sought – after raw material. When finely ground and mixed with Portland cement, pozzuolanas develop workability- characteristics like those of ordinary commercial Portland cement average 10 to 35 % and can be even higher for very high- grade pozzuolanas.
For the cement and building industries, therefore, there are significant economic advantages in the production and use of pozzuolana cement:
- considerable reduction in the capital costs of erecting a pozzuolana cement works compared with a Portland cement works, for a given output.
- Lower production costs, especially owing to comparatively low energy consumption in pozzuolana cement production.
- Lower current consumption of cement, since pozzuolana cement can be used instead of any kind of ordinary commercial Portland cement.
- Possibility of ordinary the selling price.
Technologically, pozzuolana cement has the following advantages over Portland cement:
- lower sensitivity to alkalis. - Higher resistance to chemical attack.
- Higher impermeability.
- Lower heat of hydration.
- Improved workability of concrete containing cement due to reduction of phase segregation of mix component.
Drawbacks in using pozzuolana cement, compared with Portland cement, are especially:
- little stiffening in an early stage of setting
- higher water cement ratio
- greater shrinkage on drying
However, the advantage of using pozzuolana cement instead of Portland cement are the greater by far, if the optimum mix proportions are achieved through technological tests for the particular applications.
On this assumption, pozzuolana cement is generally used for large structures which are in direct with aggressive water-e.g. dam structures and harbour installations. It is used also for building construction, canal structures, reservoirs, concrete gutters and drains and concrete pipes.
Pumice:
Because of the high porosity in pumice, which can be up to 85% this material is generally used as an aggregate in the production of lightweight concrete. Compared with specifically heavier aggregates there are, in particular, savings in costs in the use of pumice concrete in building construction:
- Savings in the costs of design calculations, building materials and foundations up to approx. 25% of bare construction costs
- Low transport costs.
- Large block sizes and hence.
- Savings in mortar and labour time.
- Further advantages of pumice concrete are: - Low wall-thicknesses for multi-storey buildings.
- Good insulation against heat and cold.
- Fire resistance.
- No condensation on wall surfaces.
1. Possible significance of pumice in the Y.R. Pumicite and pumice have not yet been out to any economically significant use in the Y.R.
- pumicite is used in the district around the locality Rada roughcasting internal and external walls.
- No use of gravel-sized pumice is known.
In the larger towns of the Y.R. numerous small production units exist which produce concrete blocks of various sizes for building-walls and garden-walls, in a process with only little mechanization, from a mix of crushed stone or also residual detritus, with sand and Portland cement. Thus, the initial technical conditions and know-how already exist in the country for making pumice and pumicite, too, usable on a corresponding scale of small industrial production.
Pumicite could be worked together with the Portland cement from the cement works at Bajil, 40 km NE of Hodeidah, or imported Portland cement, into pozzuolana cement. The production of pozzuolana cement could be taken into consideration also in the planning of a new Portland cement works.
These possibilities were related to a demand cement in the Y.R. of 150.000 to 200.000 t annually. 50.000 to 100.000 t annually is already being production in the cement works at Bajil. Through an extension of capacity, the output of the works is to be increased to 150.000 t annually. So far, the cement supplies of the Y.R. have been supplemented by imports.
However, economic use of the pumice occurrences at Dhamar is greatly limited owing to the distance of approx. 120 km and 165km respectively, to the nearest important economic and development centers, Sana’a and Taiz. The use of these occurrences should therefore be made subject to fulfillment of the following conditions: - Proof of the existence of commercial reserves.
- Technological proof of the suitability of the raw materials for the production of pozzuolana cement and lightweight concrete. - Favourable demand- and cost- analyses.
- Willingness of the Yemen Government and building industry to use pumice raw materials. - Introduction of modern pumice – processing techniques.
2. Geographical location and accessibility of the pumice occurrences:
The explored occurrences of pumice and pumicite are located 95 km in a direct line south-east of Sana’a and 15 km in a direct line east of the locality of Dhamar (2.500 m a.s.-1). Dhamar lies on the asphalt Sana’a – Taiz road, 100 km away from Sana’a and 145 km away from Taiz (Fig.1).
The area containing pumice is part of the eastern peripheral zone of the Dhamar – plain, which lies on average 2400 m a.s.-1. The pumice occurrences are 20 km from Dhamar by road, the first part of which, approx. 14 km, is being asphalted during 1977/79. The remaining approach is by in unmetalled sandy track.
The most prominent elevations in the area of the pumice occurrences are the volcanoes Dj. Lissi ( 2870 m a.s.-1.) and the Dj. Spill (3190 m a.s.-1. ). On the 1:250.000 topographical map, Sana’a sheet, the co-ordinates of the tow mountain are as follows:
- Dj. Lissi MB 5008.
- Dj. Spill MB 6512.
3. Geological setting:
3.1. Structure of the reconnaissance area:
During the Holocene, in the Dhamar area volcanic activity tock place, which resulted in the formation of the volcanoes Dj. Lissi and Dj. Spill and was associated with the discharge of magma rich in SiO2 and aluminum silicate (GEUKENS 1966). Rock complexes of rhyodacite and trachyte, among other rocks, were formed, and also tuff-horizons and layers of gravel-sized pumice and pumicite. This acidic volcanicity was replaced in a later magmatic phase by a basic volcanicity with basaltic rocks (cf. photogeologic map).
Most of the explored pumice and pumicite are located in the area around the volcano Dj. Lissi. In the immediate vicinity of the volcano Dj. Spill, layers of gravel-sized pumice were found only between the villages Raul al Melh and Hauwarwar, approx. 10 km ENE of the Dj. Lissi.
In the plain between the Dj. Spill and the Dj. Lissi, conditions for exploration are unfavourable. Pumice could be observed in this area only as the excavated material from a well, approx.0.5 km west of the village Maram. The thickness and extent of the pumice3 could not be determined, within the scope of the reconnaissance.
Highly compacted, fine- to coarse-grained, breccious, brown to grey tuff, up to 30 m thick, form extensive horizons in the peripheral zones of the reconnaissance area. In the central area of reconnaissance they are exposed only near the village Garasha, 5 km NE of the Dj. Lissi. On the basis of the conditions of exposure there, the tuffs are older than the adjoining and overlying layers of pumice and pumicite. The tuffs are being quarried to obtain dimension stone. Basaltic Lava covers larges of the reconnaissance area and in some zones also overlies the pumice occurrences. Numerous basalt volcanic edifices are spread about.
Between the villages Waraqah and Hesn Gudyd, immediately to the north-west of the DJ. Lissi, there are 6 isolated hills which consist of lussatite (opal-CT: crystalline, metastable and highly disordered opal), and of lussatite-feldspar, which probably came into existence hydrothermally.
3.2. Distribution of the explored pumice sequences: In the reconnaissance area, 4 sub-areas containing varying pumice sequences were found. These 4 sub-areas are not correlateable with one another. During eruption and sedimentation of the pumice the processes obviously resulted in depositions of gravel-sized pumice and pumicite which differed from one another. The layers of pumice are generally flat or inclined at up to 10 ° in various directions. The 4 sub-areas can be described and localized as follows (cf. photogeol. Map; Fig.2):
Area I:
Covers the middle and lower, western slope area of the Dj. Lissi, with stratified and unstratified, mostly poorly-graded deposits of gravel-sized pumice, volcanic rock components, and pumicite.
Area II: This is a strip 7 km along and 1.8 km wide (12.6 km2 ), running SW-NE between the villages Hesn Gudyd and Waraqah, to the west and south-west of the Dj. Lissi. There is interbedding of stratified, well- to poorly-graded pumice and pumicite containing small, varying proportions of other rock constituents (obsidian, basalt, rock of the basement). There is also a similar sequence in places within a narrow zone north-east of the Dj. Lissi.
Area III: Forms a zone 5 km long 4 km wide (20 km 2 ) south-east of the Dj. Lissi. Light-grey to white pumicite predominates, containing little gravel-sized pumice and to well-rounded rock fragments of the basement (dia. Up to 20 cm).
Area IV: Occupies an area 2 km long and 1.2 km wide (2.4 km2 ), which lies between the villages Maram and Hauwarwar near Raul al Melh, NW of the Dj. Spill. Here a 6 m-thick horizon of layers of coarse-gravel-sized pumice beneath a 4 m-thick covering of firm tuff has been opened up.
4. Formation of the explored pumice occurrences: In areas I to IV (cf. Sect. 3.), a total of 15 pumice profiles were plotted (cf. photogeol. map):
4.1. Area I: Location: middle and lower, western-slope zone of the Dj. Lissi. Profile I, 1: south-west slope, lower part from the top downwards:
1. 11.0 m talus deposits of effusive rock fragments, up to 50 cm in diameter; small proportions of highly Porous pumice components; stratification can Still be observed in places.
2. 2.5 m porous effusive rock fragments (lapilli), max grain size approx. 3 cm; small proportions of Highly porous pumice fragments; obsidian occurs, representing approx. 10 %.
3. 1.0 m Effusive rock fragments, slightly porous, max. grain size approx. 1.5 cm; small proportion of Highly porous pumice fragments; obsidian occurs, representing less than 5%.
4. 0.5 m Formation as in stratum 3., max. grain size appr- ox. 0.5 cm
5. 1.2 m Formation as in stratum 3, max. grain size appr- ox. 1 cm
6. 1.0 m Formation as in stratum 3, max. grain size appr- ox. 3 cm
7. 4.0 m Formation as in stratum 3, max. grain size appr- ox. 0.5cm
ليست هناك تعليقات:
إرسال تعليق