5/ Country case studies on the production of selected indigenous cementitious materials have been provided in annex I.
85. There is a wide variety of indigenous building material which can be promoted in developing countries to have an impact on the precarious situation in the building materials sector. In general, the type of indigenous building material which can be promoted will vary from one country to the next. However, it is possible to focus attention only on those indigenous materials that are likely to have a remarkable impact on the building materials sector and above all, those that are of significance to almost all developing countries. Indigenous cementitious materials fit into this category for at least two related reasons. In the first instance, unlike some other innovative indigenous building materials, there is demonstrated know-how on the production and use of indigenous cementitious materials. Secondly, by virtue of the wide variety of raw materials which can be used in producing indigenous cementitious materials, it is likely that most countries possess one or any other type of raw material for the promotion of such an indigenous building material.
86. In developing countries, consumption trends for Portland cement have shown a rapid increase over the past 20 years, but the rate of import has been equally significant. For example, imports of cement rose from 54 per cent of total consumption in 1970 to 80 per cent in 1979. In monetary terms, the negative trade balance for the cement industry alone grew from $900 million in 1970 to over $1.7 billion in 1979.6/ Domestic production of Portland cement, for the same period, increased in all developing regions, notably showing a 64 per cent increase in Africa and a 13.6 per cent increase in Asia. In some particular cases, developing countries have become net exporters of cement.
6/ United Nations, Yearbook of International Trade Statistics, 1979.
87. Despite the trends of rising imports and expansion of domestic production, Portland cement is still not available in sufficient quantities and, in any case, is only available at costs which are prohibitive to the bulk of the population. The main reasons for this situation is that foreign-exchange constraints have limited quantities and kept costs high for imported products, and, because domestic production is generally dependent on imported inputs, the same limitations apply to the local product. Unfortunately, in most developing countries, the only binder used in the construction sector is Portland cement, so that its limited supply and high cost have meant that investments in construction are either withheld, diverted or abandoned. Even though Portland cement is not a low-cost material, its application as the sole binding material is popular with the low-income population, to the extent that it has become a crucial factor in the inadequate delivery of shelter to the low-income population.
88. Portland cement has a clearly defined role as a binder in fulfilling the functional requirements for high-strength applications. However, in actual construction practice, Portland cement has become a ubiquitous material, largely as a result of its wrong application in construction. Portland cement is predominantly used in low-strength applications, for foundation concrete, plasters, mortars and soil stabilization. The wrong application of cement in this manner is not only unnecessarily costly but, more important, technically defective.
89. For example, Portland cement, as a binder in the preparation of mortars, produces a mortar which is too harsh, of low workability and, sometimes, stronger than the blocks to be bonded. Portland cement is often used in preparing concrete for ordinary strip foundations, to support single-storey structures such as farm sheds and basic rural dwellings. Again, Portland cement is frequently used as a stabilizer in the preparation of soil blocks, yet it is the wrong choice for certain soils such as clayey soils or black cotton soils.
90. The degree to which Portland cement is wrongly applied in construction has reached alarming proportions, and it is estimated that only 20 per cent of world-wide use of cement requires the strength of Portland cement.7/ In some countries, this situation has come about because there is hardly any alternative binder to Portland cement, but, in a few countries, there are low-strength binders which can be used in place of Portland cement at low cost and with good performance, yet the predominant choice of the market is cement. However, in almost every developing country there are opportunities, based on known and established technologies, to produce binders or cementitious materials which, even though not exact substitutes for Portland cement, can serve the same function in construction and, most of all, can be produced within the resource limitations of developing countries, using indigenous factors of production. As explained below, indigenous cementitious materials can be produced and used in a variety of ways, thus offering individual developing countries a vast range of options to improve the availability of binders to suit their particular conditions.
7/ UNIDO, Optimum scale production in developing countries, Sectoral Studies Series, No. 12 (Vienna, 1984)
91. Before the advent of Portland cement, lime was the main binding agent used for construction, and, despite its-decline in popularity, its use as a binder is still valid. Portland cement and lime have one basic common property when mixed with water - they have the tendency of setting and hardening so that they are subsequently strong and impervious to water. Lime has a slower setting rate than cement, and the final strength is lower, but lime is perfectly adequate for most low-strength applications, such as mortars, plasters, foundation concretes and building blocks. In addition, lime has additional qualities, such as good workability, ability to accept movement without cracking, water retentivity and resistance to water penetration, thus making it more suitable than Portland cement for masonry work.
92. The raw materials for manufacturing lime are, normally, naturally occurring substances, such as limestone, dolomite, sea shells or coral (see annex II for an overview of raw materials for lime production in Africa). In most countries, information on available raw materials for lime manufacture has been restricted to large-scale deposits, particularly high-quality limestone suitable for cement production. However, for the purpose of low-strength binders, there are vast opportunities in terms of raw materials, including a multitude of small deposits. Lime can also be obtained as a residual product in the manufacture of sugar, paper and acetylene. Thus, the variety of sources from which lime can be obtained offers an opportunity to promote the material in several countries and in different parts of a country.
93. Lime can be produced with different technologies covering a wide range of scales of production. In general, there are large-scale, capital-intensive production technologies and small-scale technologies, often labour-intensive or adopting rudimentary techniques in production. Small-scale technologies normally satisfy an output of 1-50 tons per day, and the demands for such small-scale technologies, particularly for outputs of less than 10 tons per day, can easily be met within the capacities of most developing countries.
94. The main capital items in small-scale lime manufacture are a kiln and a storage shed, both of which can be locally fabricated with hardly any imported inputs. Kilns vary in terms of their efficiency in burning the raw material, but basically there are two types of kilns at the small-scale level: batch kilns and continuous fired kilns. The type of kiln and the efficiency of burning influence the quality of the final product. Lime production is energy-intensive, and the process of transforming the raw material (limestone) to hydrated lime is achieved through the firing stage, so that energy is the single most important factor of production. The fuels commonly used in lime manufacture, at the small scale, are firewood, coal and residual oil, however, the available fuel determines the type of kiln that can be used.
95. Even though small-scale lime production is relatively labour-intensive, it still requires some basic skills. Some basic understanding of the raw material is desirable as the first step in quality control. The fabrication of the kiln is a skilled operation, and so is the firing process. Similarly, careless slaking, storage and packaging of the fired product can undermine the entire objective of quality production.
96. In general, pozzolanas are classified into two groups: natural and artificial. A pozzolana is a material which, on its own, is not cementitious but, with the addition of lime, reacts to form a material which sets and hardens. Thus, for the purpose of construction, a pozzolana is not an end in itself but, rather, a means of achieving the ultimate product - lime-pozzolana. Lime-pozzolana is a low-strength binder used in the same manner as lime, to prepare mixtures for mortars, plasters and building blocks and for soil stabilization. Normally, a mixture of one part of lime to two parts of pozzolana is adequate for lime-pozzolana binders, and, even if a ratio of 1:1 is applied, considerable savings of about 50 per cent of the available supply of lime is achieved. In this way, where pozzolana is obtained at a lower cost than lime, lime-pozzolana becomes an attractive material for low-cost construction.
97. Natural pozzolanas are basically of volcanic origin and are usually found in areas which have experienced volcanic activities. For example, in Africa, natural pozzolana deposits can be found in six countries -Burundi, Cameroon, Caper Verde, Ethiopia, Rwanda and the United Republic of Tanzania (see annex III for a brief description). Pozzolanas of this type occur either in a pulverized state or in the form of compact layers, and this, in turn, determines the production process which the pozzolana has to undergo before being mixed with lime to produce a binder.
98. Where volcanic tuff occurs as a naturally fine-grained material, it requires no preparation apart from ensuring that it is sufficiently dry prior to mixing with lime. Sun-drying is feasible, even though a small-scale, locally fabricated kiln can be used for this purpose. For example, the Arusha-Moshi area of the northern part of the United Republic of Tanzania is volcanic, and large deposits of fine-grained pozzolanas are widely available. These deposits which require no grinding after quarrying can be mixed with lime to prepare mortars, plasters and building blocks.
99. Where the natural pozzolana occurs in a coarse-grained form, it is desirable to dry the material, either in the sun or a kiln, and, thereafter, grind it in a ball-mill to the desired fineness, ready for mixing with lime. In some instances, the grinding of coarse-grained pozzolanas is restricted to the preparation of mortars and plasters, while the preparation of blocks is feasible without any grinding. For instance, in Lembang, Indonesia, unground coarse-grained pozzolana is mixed with 20 per cent lime and sufficient quantities of water to produce solid blocks for building construction.
100. Unlike natural pozzolanas, artificial pozzolanas are obtained only after the basic materials undergo some basic production processes. The raw materials from which artificial pozzolanas are obtained are extensive in scope, covering materials of geological origin and agricultural and industrial residues. However, the most common raw materials used for production of artificial pozzolanas are as follows:
(a) Clay products. Suitable clay deposits can be quarried, fired and ground into fine powder in a ball-mill, for use as a pozzolana. Basically, most soil groups containing the common clay minerals can be used for this purpose, but plastic clays, such as those used for pottery, are the most likely to produce good pozzolanas. The firing of the clay should be under controlled temperatures, and a locally fabricated kiln or incinerator can be used for this purpose. The desired temperature for firing is around 600°C. As an alternative to firing raw clays, pozzolanas can be produced by grinding bricks or tiles obtained as residual products in the production of fired-clay bricks and tiles. Here, the only equipment required is a ball-mill or a hammer-mill to grind the material. Sometimes, the pozzolana and the lime are mixed and ground together in the ball-mill.
(b) Rice-husk-ash. Rice-husk is the residual product from milling rice. It often has no commercial value but, rather, poses a problem of disposal. The ash which results from burning rice husk is a pozzolana which reacts with lime and water to produce a binder suitable for low-strength masonry application. Normally, about 20 per cent of the volume of rice husk results in ash, and, because rice is grown in several countries, rice-husk-ash is potentially an important cementitious material. In Africa alone, there are about 40 countries where rice is grown, and, even though the quantity of output is not high enough in all the countries to justify commercial-scale production of rice-husk-ash, the potential that exists for promoting the material is encouraging (see annex IV for details on rice-husk availability). As a pozzolana, rice-husk-ash is produced under controlled temperatures of about 600°C in a kiln or incinerator. The incinerator for burning rice-husk can be locally fabricated, and, in countries where production has been commercialized, the scale of production if often as small as 1 ton per day. Apart from the incinerator, which can be locally built in bricks, the main capital item required for rice-husk-ash pozzolana manufacture is a ball-mill to grind the ash or ash and lime into a homogenous fine mix. In some countries, the ball-mill may have to be imported but, in a country such as India, it is readily available on the market.
(c) Fly-ash. Fly-ash is the residual product obtained when coal is fired and, thus, occurs as a waste product from coal-fired power stations. It is desirable for the fly-ash to be in a dry state prior to use. Often, fly-lash occurs in a coarse form and will have to be pulverized before mixing with lime to produce a binder, so that the main capital item required in preparing fly-ash pozzolanas is a ball-mill for pulverizing the ash to the desired fineness.
101. Blended cements are produced by mixing ordinary Portland cement with a low-cost cementitious material, notably, blast-furnace slag, lime or any of the popularly adopted pozzolanas. The principle behind blended cements is to obtain a binder which is nearly equal in strength to Portland cement but, at the same time, cheap in cost. Examples of blended cements are Portland-pozzolana, Portland-slag or Portland-lime pozzolana. There are cases where blended cements have been produced by replacing about 25 per cent of the volume of Portland cement with a pozzolana, and the resulting binder is recorded to have satisfied the same 28-day strength test as for normal Portland cement. Blended cements have an advantage over Portland cement in terms of workability and water resistance.
102. The production of blended cements is in two stages. First, the production of pozzolana and, secondly, the intergrinding of pozzolana or lime with Portland cement. The use of rice-husk-ash to produce blended cements has been gaining popularity over other types of pozzolana, and some demonstrations have indicated that up to 50 per cent of Portland cement can be replaced by rice-husk-ash, with only a marginal reduction in the strength of the resulting binder compared with normal strengths of Portland cement. The cost implications of blended cements could be very encouraging, as demonstrated in Rwanda8/ where pozzolana-lime-cement is estimated to be 50 per cent the cost of Portland cement.
8/ See annex I for details.
103. Unlike lime-pozzolana, the production technology for blended cements is relatively intricate. First, the production presupposes the availability of Portland cement; secondly, it is desirable to produce a finely ground pozzolana for the purpose of blending with the cement. However, the part of the operation which requires careful control is the intergrinding of the pozzolana or lime with the cement into a homogenous mixture, of uniform degree of fineness. For these reasons, blended cement manufacture is, in general, a capital-intensive process even though the capital-intensity per ton of output is still far less than Portland cement.
104. Gypsum in another naturally found mineral of widespread availability from which cementitious binders can be made, for use in mortars, plasters, concretes, blocks and prefabricated building products. The properties of gypsum plaster are rather different from those of either Portland cement or lime-based cements, and its suitability as a cement-substitute is, therefore, limited to particular uses. Unlike other cementing materials, gypsum plaster sets rapidly, even when setting retarders are added; it is somewhat porous and of rather lower strength than Portland-cement-based materials; and it is slightly soluble in water.
105. Its rapid setting makes it very suitable for internal renders; blocks can be made from it if they are cast immediately after mixing, and gypsum concrete can be made by pouring a fluid water/plaster mix into moulds already containing coarse aggregate. All of these uses must be restricted to internal applications or dry conditions, because of gypsums water-solubility. The most common use of gypsum is in the manufacture of thin reinforced sheets - fibrous plaster sheeting or plasterboard - using vegetable fibres, glass fibre or cardboard sheets as reinforcement. The outstanding fire resistance of gypsum plasters makes such sheets especially good as internal partitions.
106. As with lime-pozzolana binders, gypsum-based binders involve a very simple manufacturing process which can be carried out at small scale. The firing temperature, around 170°C, is much lower than that of lime, which means that gypsum-plaster manufacture is energy-saving an important advantage in areas where fuel is scarce. Two recent projects which made use of gypsum as a cementitious binder are those in Cape Verde Islands9/ and at Noumerate, Algeria, reported by GRET.10/ The physical qualities, low energy cost and adaptability to small-scale production are advantages of gypsum production.
9/ S. Cramer, Gypsum Production on Maio Island (Berlin, Institute for Geologie, 1979).
10/ Gypsum Plaster: Its Manufacture and Use in Building (Paris, GRET, 1982), translated for ITIS by B.M. Booth.