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close this bookJournal of the Network of African Countries on Local Building Materials and Technologies - Volume 1, Number 4 (HABITAT, 1991, 48 p.)
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Mauritius: Use of calcarenite blocks in housing construction in Rodrigues*

* By G.T.G. Mohamedbhai, Professor of Civil Engineering, University of Mauritius, C.W. Chan Chim Yuk and B.K. Baguant, Senior Lecturers in Civil Engineering, University of Mauritius.

This paper was presented to the Seminar on Local Materials for Housing, Third International Seminar of the African Network of Scientific and Technological Institutions (ANSTI), Civil Engineering Subnetwork, held at the University of Mauritius, March 1990. ANSTI is a UNESCO-sponsored network.

Abstract

Blocks quarried from deposits of calcarenite on the island of Rodrigues have been used in housing construction for decades. However, no technical study of the deposits, the quarrying techniques or the properties of the calcarenite has ever been carried out.

This paper describes the interim findings of a research project on the Rodrigues calcarenite. The deposits and the methods of quarrying are described, the problem of wastage from the quarries is highlighted, the use of the blocks in housing construction is surveyed and some test results on the properties of the calcarenite are reported.

Introduction

The island of Rodrigues (110 km2) is part of the State of Mauritius and is situated about 515 kilometres to the east of the island of Mauritius. In 1983 (1), the population of Rodrigues was about 33,000.

Rodrigues has no direct international air or sea connections and all goods imported into the island have to come through Mauritius. Thus, building materials such as cement, reinforcement steel, corrugated-iron sheets etc., which come through Mauritius, are very expensive. Aggregates are locally available: the local basalt is crushed to produce coarse aggregate and the local sand from beach quarries is used as fine aggregate. Cellular concrete blocks are produced on the island.

Rodrigues has small deposits of calcarenite and blocks cut from the material are known to have been used as walling material since the beginning of this century (2).

In 1983 there was a total of 6962 residential buildings in Rodrigues (1). Table 1 shows that only about 32 per cent of these buildings have blocks as walling material, the remainder having mostly corrugated-iron sheets. Unfortunately, there is no record to show what proportion of the buildings with solid walls are built with calcarenite blocks. Buildings which do not have block walls do not resist the cyclones which frequently attack Rodrigues. There is, therefore, an increasing tendency to use either calcarenite or concrete blocks as walling materials.

Table 1. Wall material used in residential buildings in Rodrigues (ref. 1)

Wall material,

Number of buildings

Iron sheets or tin

4 598

Calcarenite/concrete blocks

2 257

Vegetable material

63

Timber

37

Other

7

Total

6 962

A two-year project to study the calcarenite deposits in Rodrigues, to determine the properties of the material and to look into the various aspects of block production and use in housing construction, is currently being carried out at the University of Mauritius. This paper summarizes the interim findings during the first year of the project (3).

The calcarenite deposits

Figure 1 shows the deposits of calcarenite in Rodrigues. There are two well-defined deposits, one on the eastern coast of the island and the other, larger, in the south-western part. The calcarenite consists of grains of calcareous sand (mostly aragonite) which, after having been deposited by the action of sea or wind, have been cemented together with cementite. The eastern deposits have sand grains of a mean size of 300 mm whereas the southern deposits have a grain size of 400-800 mm.


Figure 1. Map of Rodrigues showing deposits of volcanic ash, calcarenite and coral sand

The depths of the deposits vary considerably. At the edge of the outcrop in Petite Butte (see figure 2) the calcarenite was found to be only 1-2 m deep, whereas at Pointe Cotton, the quarrying had reached a depth of 8 m, with calcarenite still visible in depth. It is believed that in some places the calcarenite deposits can be as deep as 30 m.


Figure 2. Sawing of a vertical block of calcarenite in Rodrigues using a one-operative woodcutter saw

Block quarrying

In October 1988, there were two active quarries in Rodrigues: Pointe Cotton on the east coast and Petite Butte in the south. The other quarries (Anse Quittor, Saint Frans, Les Graviers, Pointe Tasman) had all been abandoned. Quarrying is done manually and the method of quarrying depends on the quality of the calcarenite. At Pointe Cotton, where the material is hard, large horizontal blocks are first removed from the deposit and smaller blocks are then sawed from these blocks. Simple tools such as wedges, plugs, hammers, chisels, levers, quarry bars and carpenter’s saws are used.


Figure 3. Cutting of smaller block of calcarenite in Rodrigues using a two-person woodcutter saw

At Petite Butte, in the south, the material is soft. Here, vertical blocks are sawed off the deposit and small blocks are then sawed from that block. The saws used, which are basically woodcutter saws, are either a one-operative or a two-person saw. At Pointe Cotton, the blocks produced are approximately 150 mm thick, 225 mm high and 375 mm long. At Petite Butte, the blocks are about 450 mm long, 300 mm high and either 150 mm or 100 mm thick. The average weight of a block is about 23 kg. It has been estimated that the annual production of calcarenite blocks is of the order of 25,000, a larger number of blocks being obtained from the southern quarry. As far as concrete blocks are concerned, some 180,000 blocks are manufactured annually in Rodrigues.

Wastage at the quarries

Quarrying and cutting the blocks inevitably leads to wastage in calcarenite, especially in the southern quarries. It has been estimated that the “quarry yield” (i.e., the volume of useful stone, in the form of blocks, to the volume of stone originally extracted) is of the order of 15-20 per cent at Petite Butte and 20-30 per cent at Pointe Cotton. The wastage could be reduced if some degree of mechanization were to be introduced and the techniques of quarrying improved.

The disposal of the existing calcarenite waste which has piled up over the years, especially at the abandoned quarries, as well as the waste produced annually (estimated at roughly 450 tons), remains a problem. The large pieces of calcarenite are already being used as undressed stones for wall construction. Two other possibilities exist: the crushing of the calcarenite for producing sand or coarse aggregate for use in concrete, or the burning of the calcarenite (which is almost pure calcium carbonate) for producing lime.

Lime production in Rodrigues poses a problem because of the shortage of fuel wood. An appropriate fuel (e.g., coal) could be imported through Mauritius, but this would considerably increase the cost of the lime. The other alternative would be to ship the calcarenite waste to Mauritius for producing lime there.

At present some 6000 tons of hydrated lime are produced annually in Mauritius, about 5000 of which are used in the sugar industry. The raw material used is corals but, for environmental reasons, removal of corals from lagoons is being increasingly restricted and the calcarenite waste would be an ideal alternative raw material. If 1000 tons of calcarenite waste were exported to Mauritius every year, it would meet about 15 per cent of the lime requirement of the sugar industry.

Use of calcarenite blocks in housing construction

In October 1988, the price of a calcarenite block was Mauritian rupees (Mau Rs) 4.00 ex-quarry and the cost of transport varied depending on the distance, but was normally between Mau Rs 1.00 and 2.50 per block. The price of a 150 mm concrete block was Mau Rs 6.40 ex-works, the price of transport being about the same.

Table 2 shows a comparison of unit surface area of 150 mm thick calcarenite and concrete blocks. The prices are as at October 1988 and transport costs have been included. It can be seen that the cost per unit wall area of the Petite Butte blocks is about half that of concrete blocks.

Table 2. Comparison of cost of blocks and unit surface area

Type of block

Surface area (cm2)

Cost of block (Mau Rs)

Cost per cm surface area (cents)

Pointe Cotton

844

5.50

0.65

Petite Butte

1350

5.50

0.41

Concrete

927

7.55

0.81

There is, thus, a clear demand for calcarenite blocks in Rodrigues which cannot be met by the present rate of production. It is, therefore, very common for residents to stockpile the blocks over a certain period, until they have acquired a sufficient number to start construction.

In housing construction, the blocks are used for load-bearing walls and are laid with mortar in exactly the same way as conventional concrete blocks. The calcarenite, especially that from the south-western deposit, turns greyish on exposure, and the walls are, therefore, always externally and internally plastered. However, it is necessary to brush and wash the calcarenite surface to remove all loose particles before plastering. It is estimated that between 600-800 blocks are required for the construction of a small house. Durability of the block is not a problem once the walls have been plastered.


Figure 4. A house in Rodrigues made with calcarenite blocks as walls. The roof is of reinforced concrete. The walls will receive external and internal rendering.

Properties of calcarenite

General

Thirty blocks from the Pointe Cotton quarry and 37 blocks from the Petite Butte quarry, as well as calcarenite waste from the disused quarries, were taken to the University of Mauritius where a comprehensive testing programme was undertaken. Only a summary of the results of some of the tests is reported in this paper, the other tests have either not yet been completed or their results have not yet been analysed.

Chemical composition

Tests were performed on samples from various quarries to determine the silica (SiO2) content, the sulphate (SO4) content, the magnesium oxide (MgO) content and the calcium oxide (CaO) content. No trace of iron oxide (Fe2O3) or aluminium oxide (Al2O3) was found in any of the samples.

The results are shown in table 3. There is no significant difference in chemical composition of the calcarenite from one quarry to another. The observed variation in mechanical properties is, therefore, mainly due to the physical structure of the calcarenite. Also the calcarenite is about 95 per cent calcium carbonate and, therefore, suitable for use as a raw material in lime production.

Table 3. Chemical composition of calcarenite

Component

Percentage by weight of oven-dried sample


Pointe Cotton

Petite Butte

Anse Quittor

St Frans

Pointe Tasman

CaO

55.73

54.62

53.53

55.57

53.39

CO2

41.61

41.85

41.09

41.43

40.37

SiO2

0.11

0.06

0.86

0.39

0.12

SO4

0.99

0.98

1.11

1.05

1.07

MgO

0.64

1.33

0.66

0.68

0.70

Chemically bound H2O

0.70

0.50

0.69

1.06

1.86

Total

97.51

99.34

97.94

100.18

97.51

Physical properties

The apparent specific gravity, water absorption and porosity of the calcarenite were determined in accordance with IS 1124(4). Three or four tests were performed on the calcarenite from each quarry and the average results are given in table 4.

Table 4. Physical properties of calcarenite

Property

Quarry


Pointe Cotton

Petite Butte

Anse Quittor

Graviers

Apparent specific gravity

2.27

2.38

2.35

2.48

Water absorption (percentage)

13.7

31.7

29.0

19.2

Porosity (percentage)

31.1

75.4

68.2

47.6

It can be seen that, although, the values of specific gravity do not vary by much, there is considerable variation in water absorption and porosity. The Petite Butte calcarenite is nearly two and a half times more porous and absorbant than the Pointe Cotton calcarenite.

Strength tests

The tests, performed only on samples from Pointe Cotton and Petite Butte, were based on the methods described in IS 1121(5). Beams of length 400 mm and cross-section 100 x 100 mm were cut from the calcarenite blocks and their surfaces dressed. Six beams were prepared from each quarry, three in their air-dried condition and three saturated in water for 72 hours. The beams were tested to failure with the deposited layers in the horizontal position and using third-point loading over a span of 300 mm.

Two 100 mm cubes were then cut from the ends of each beam tested. The cubes from the saturated beams were saturated again and then tested in compression. The cubes from the air-dried beams were tested in their air-dried condition. One cube from each beam was tested with the bedding horizontal, the other with the bedding vertical.

The flexural and cube strength test results are given in table 5. The main conclusions to be drawn from the results are:

(a) The strength of the calcarenite in the saturated condition is lower than that in the air-dried condition;

(b) The strength when the bedding is horizontal is consistently less than when the bedding is vertical. (Interestingly enough, in Rodrigues, the blocks are always laid with the bedding vertical.);

(c) Whilst the strength of the calcarenite of Pointe Cotton is adequate, that of Petite Butte is quite low. The latter, therefore, should only be used for small, one-storey houses, or as non-load bearing blocks.

Table 5. Flexural and cube strength test results

Quarry

Flexural strength (MPa)

Cube strength (MPa)


Air-dried

Saturated

Air-dried

Saturated




H

V

H

V

Pointe Cotton

2.07

1.90

5.74

6.49

3.14

3.83

Petite Butte

0.85

0.52

1.60

1.87

1.06

1.79

H = bedding horizontal
V = bedding vertical

Acknowledgement

The research project described in this paper was funded by the International Development Research Centre (IDRC) of Canada, which is acknowledged.

References

1. Ministry of Economic Planning and Development, 1983 Housing and Population Census of Mauritius (Port Louis, Central Statistical Office, 1985).

2. Bertuchi, I.J., The Island of Rodriguez - a British Colony in the Mascarenhas Group (London, John Murray, 1923).

3. Mohamedbhai, G.T.G., Chan Chim Yuk, C.W., and Baguant, B.K., “Interim technical report on calcarenite (Mauritius) Project” (Port Louis, University of Mauritius, 1989).

4. Indian Standards Institution, Methods of Test for Determination of Water Absorption, Apparent Specific Gravity and Porosity of Natural Building Stones, IS: 1124 (1974).

5. Indian Standards Institution, Methods of Test for Determination of Strength Properties of Natural Building Stones, IS: 1121 (1974).