Properties of lime stabilised steam-cured blocks for masonry

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 35, June 2002, pp Properties of lime stabilised steam-cured blocks for masonry B. V. Venkatarama Reddy and S. R. Hubli Department of Civil Engineering, Indian Institute of Science, Bangalore , India SCIENTIFIC REPORTS Paper received: March 15, 2001; Paper accepted: October 19, 2001 A B S T R A C T The paper addresses certain issues pertaining to the technology of lime-stabilised steam-cured blocks used for masonry construction. Properties of lime-stabilised steam-cured blocks using expansive soils and tank bed soils have been examined. Influence of parameters like steam curing period, lime content and fly ash content on wet strength of blocks is studied. Steam curing of lime stabilised blocks at 80 C for about 20 hours at atmospheric pressure leads to considerably higher strengths when compared with curing under wet cloth at ambient temperatures. Clay-fly ash fractions of the mix control the optimum lime content yielding maximum strength. Long-term strength behaviour of steam-cured blocks has been monitored. The results indicate a favourable limeclay ratio for stable long-term strength. A small-scale steam cured block production system has been designed and implemented to construct a load bearing masonry structure, thus demonstrating the potential of steam-cured block as a material for masonry construction. R É S U M É L article traite de certains problèmes liés à la technologie des blocs stabilisés à la chaux et curés à la vapeur, que l on utilise en maçonnerie. On a examiné les propriétés de ces blocs en utilisant des terres expansives et des terres issues de réservoirs. L influence des paramètres tels que le temps de maturation, le volume de chaux, le volume de cendres volantes sur la résistance de blocs face à l humidité a été étudiée. La maturation par la vapeur des blocs stabilisés à la chaux à 80 C pendant environ 20 heures en pression atmosphérique conduit à des résistances considérablement plus fortes que pour la maturation obtenue à l aide de tissus humides à températures ambiantes. Des fractions d argile et de cendres volantes du mélange contrôlent le volume optimal de chaux produisant une résistance maximale. Le comportement relatif à la résistance à long terme des blocs curés à la vapeur a été enregistré. Les résultats indiquent un rapport chaux-argile allant dans le sens d une résistance à long terme stable. Un système de production de blocs curés à la vapeur à une échelle réduite a été élaboré et mis en place afin de construire un ouvrage de maçonnerie avec élément porteur, démontrant ainsi le potentiel d un bloc curé à la vapeur comme matériau de construction. 1. INTRODUCTION Stabilised mud blocks are being used for the masonry construction in India and elsewhere [1-3]. These are produced using local soils and stabilisers such as lime and cement. Soil and stabiliser mixture, at an optimum moisture content is pressed into a dense block using a machine, cured and then used for construction of masonry walls. Investigations of Venkatarama Reddy [4], Venkatarama Reddy and Jagadish [5], Lunt [6], Walker [7], Fitzmaurice [8] throw more light on the production and properties of stabilised mud blocks. Sandy soils containing predominantly non-expansive clay minerals are ideally suited for cement stabilised soil blocks. Soils containing expansive clay minerals and also high clay soils are difficult to stabilise using cement alone as a stabiliser. Generally expansive soils and high clay soils are associated with high swelling and shrinkage characteristics. Lime is essential to stabilise such soils. At ambient temperatures (20-30 C), lime stabilised soil blocks require longer duration of curing (> 1 month) for satisfactory strength development. Curing at elevated temperatures can accelerate limeclay reactions. Steam-curing of lime-stabilised blocks for a short period ( ~ 10 hours) at 80 C, can lead to satisfactory strength development [9]. Studies of Mateos and Davidson [10], Mateos [11], Ladd et al. [12], Ranganatham and Pandyan [13] and Venkatarama Reddy and Jagadish [14], give more information on basic research on lime-clay reactions cured at elevated temperatures. The present investi /02 RILEM 293

2 Materials and Structures/Matériaux et Constructions, Vol. 35, June 2002 gation deals with properties and production of lime-stabilised, steam-cured, compacted soil blocks using expansive soils and high clay soils. The scope of the present study is as follows: (a) Understanding the properties of lime stabilised steam-cured compacted soil blocks using expansive soils and high clay soils. (b) Monitoring long-term strength of lime stabilised steam cured blocks. (c) Developing small-scale production systems for lime stabilised steam-cured blocks. 2. MATERIALS USED Samples of one black cotton soil and three tank bed soils were used in these studies. Black cotton soil is a common term used in India to represent a very high swell-shrink potential soil found in semi-arid tracts of India. The clay mineral present in such soils is predominantly montmorillonite. Such soils are unsuitable for the production of burnt clay bricks. The quality of traditional burnt clay bricks produced in the black cotton soil Table 1 Properties of soil Soil Properties BCS TB1 TB2 TB3 Textural Composition Sand ( mm) 35.7% 23.8% 51.80% 17.00% Silt ( mm) 20.3% 31.8% 15.06% 40.75% Clay (< mm) 36.0% 44.4% 33.14% 41.75% Atterberg Limits Liquid Limit 53.1% 58.36% 41.80% 45.00% Plastic Limit 25.7% 31.05% 18.77% 24.00% Shrinkage Limit 9.2% 10.94% 12.48% 14.55% Plasticity Index 27.4% 27.31% 23.03% 21.00% USC Classification CH MH or OH CL CL Predominant Clay Montmorillonite Kaolinite Kaolinite and Kaolinite mineral Montmorillonite traces Chemical Properties ph Organic Matter 0.67% 2.32% 1.40% 1.26% zones of India is very poor. In southern Indian states, there are a large number of small surface water tanks for providing irrigation to small-scale agriculture. Generally area of such tanks range from hectares with some exceptions. These tanks generally get filled during rainy season from the run-off water of small catchments. Majority of these tanks have got filled up with finer soil particles (largely clay and silt fractions) due to soil erosion over a period of time. The local governments are promoting removal of the soil from such tanks in order to improve the storage capacity. The tank bed soils selected from three such local tanks have been used in these studies. Black cotton soil is designated as BCS and the tank bed soils as TB1, TB2 and TB3. Table 1 gives composition and other properties of these soils. The clay fraction ranges between 33% and 45%. All the three tank bed soils have predominantly kaolinite clay mineral while BCS soil has montmorillonite clay mineral. These soils do not possess non-clay minerals in the clay size fraction. All the soils are alkaline except TB3, whose ph is Tank bed soils have more organic matter than the BCS soil. Locally available lime (fresh calcium hydroxide) with 95% CaO was used in these studies. Fly ash from Raichur Thermal Power station (Karnataka state, India) with fines of 98%, < 75 µm was used. The chemical composition of the fly ash is given in Table 2. It has mainly silica and alumina with low CaO at 0.80%. All four soils have considerable amounts of clay size fractions. Studies of Venkatarama Reddy and Jagadish [5] have shown that durability characteristics of stabilised soil blocks are greatly influenced by the quantity and type of clay minerals present in the soil. Higher percentage of clay fraction in the soil demands larger quantity of stabiliser (lime/cement) for effective stabilisation. In the present investigation the clay content of the soils was suitably modified by diluting with appropriate quantity of sand leading to a more sandy mixture. Table 2 - Composition of fly ash Compound Percentage SiO Al 2 O TiO Fe 2 O MgO 0.5 CaO 0.8 K 2 O 0.8 Na 2 O 0.2 L.O.I (at 900 C) EXPERIMENTAL PROGRAMME Influence of steam-curing period, lime content and fly ash content on compressive strength of soil blocks was studied through laboratory experiments. Stabilised mud blocks used for the construction of masonry walls are normally bigger in size compared to conventional burnt clay bricks. Use of bigger size blocks in the laboratory studies involves handling of huge quantities of soil and stabilisers. Hence, influence of various parameters on the properties of lime stabilised steam cured blocks was studied using smaller cubes of size 76 mm, in the laboratory. The cubes were prepared by using a static compaction process similar to the one employed in the 294

3 Venkatarame Reddy, Hubli soil block making machines. 3.1 Preparation and testing of specimens Soil is oven dried at 60 C and then ground in a ballmill for 1 hour. Powdered soil, lime and fly ash are then interground for 30 minutes in the ball-mill. This mixture is immediately mixed with required quantity of sand, manually. Later, water is sprinkled on the thin layer of the mixture and thoroughly mixed by hand, such that there is a uniform distribution of moisture in the entire mix. Wetted lime-soil mixture is statically compacted into a 76mm size cube, using a metal mould and a hydraulic jack. Fresh bulk density of the cubes is kept constant at 20.2 kn/m 3. The moulding water content is controlled within a narrow range (10 to 12% by weight of the whole mix) such that the dry density of the cubes is about kn/m 3 with a variation of ± 2%. Dry density of the cubes can influence the strength of stabilised soil cube significantly and hence it should be kept constant. The moulding water content cannot be based on Proctor OMC, because the Proctor OMC varies with the soil composition leading to different dry densities. Also the energy supplied in a Proctor test is different compared to the energy supplied in static compaction process employed in cube making. More detailed information on the static compaction of soils especially with reference to choosing OMC in compressed earth block production can be found in the study of Venkatarama Reddy and Jagadish [15]. The dry density chosen in the present study represents the dry density achieved in the field by employing manually operated machines for stabilised mud block production. Lime-soil cubes prepared in the manner described above are kept in a water-bath after 24 hours of casting and then steam cured at 80 C (atmospheric pressure) for the required period of time. After steam curing the cubes are dried inside the laboratory for 14 days. The cubes are soaked in water (mainly to ensure that the blocks are completely saturated) for 48 hours prior to testing. Compressive strength tests were carried out in a strain controlled testing machine at a strain rate of 1.25 mm per minute. The strength obtained in this manner is designated as wet compressive strength. Saturated condition of masonry walls (submergence due to flooding, temporary water stagnation due to drainage problems, wetting due to rain impact, etc.) in a building is the most critical state for strength and safety of the building. Hence, all the strength tests on steam cured blocks were performed at saturated state. 4. RESULTS AND DISCUSSION 4.1 Strength gain versus curing period Duration of steam curing controls the strength development in lime stabilised soils. Influence of steam curing period on compressive strength was studied for Fig. 1 Strength versus steam curing period for BCS soil blocks. Fig. 2 Strength versus steam curing period for TB3 soil blocks. blocks using BCS and TB3 soils. Proportion of soil and sand was kept at 1:2 ratio by weight. Percentage of lime is based on total weight of soil and sand. Figs. 1 and 2 show the variation in strength with the steam curing period for BCS and TB3 soils respectively. The results shown in these figures represent the mean of 3 values. The following observations can be made from the results shown in Figs. 1 and 2. 1) There is a sharp increase in wet compressive strength as curing period is increased from 6 to 24 hours, for cubes using both the soils irrespective of lime content. 2) At higher lime contents (> 10%) there is a 2 to 3 fold increase in strength as curing the period is increased from 6 to 24 hours for specimens using both the soils. 3) Beyond 24 hours of steam-curing the strength remains constant for all the cases, except for 6% lime. In case of 6% lime there is a steady increase in strength up 295

4 Materials and Structures/Matériaux et Constructions, Vol. 35, June 2002 Table 3 Comparison of strengths of steam-cured and moist-cured blocks Specimen size: 76mm cube Wet compressive strength (N/mm 2 ) Sl. Curing details BCS soil + Sand TB3 soil + Sand No. Lime content Lime content 6% 10% 14% 18% 6% 10% 14% hours steam-curing days wet-cloth-curing development will depend upon the quantity of clay present in the soil-sand-lime mixture. Excess lime will remain as a weak filler leading to strength reduction beyond an optimum limit. It may be beneficial to retain more clay in the soilsand mixture from the point of view of strength, provided the cost economics permits. Percentage of clay fraction decides the optimum lime content for block production from the considerations of strength. Higher percentage of lime leads to increased cost of the block. to 36 hours of curing for the blocks using both the soils. Wet compressive strength of the steam-cured specimens and the specimens cured at ambient temperatures ( ~ 30 C) under wet cloth is given in Table 3. The results of the table clearly indicate that steam-curing of blocks significantly increases wet compressive strength when compared to 28-day wet-cloth-curing. Steam-curing leads to 3 to 6 fold increase in strength when compared to 28-day wet-cloth-curing at ambient temperature. 4.2 Influence of lime content on strength Pozzolanic reaction between lime and clay minerals is mainly responsible for strength gain in steam-cured lime-stabilised soil blocks [16, 17]. Influence of lime content on strength of lime-stabilised steam-cured blocks is shown in Fig. 3 for blocks using BCS, TB1 and TB3 soils. Percentage of clay fraction of the soil-sand mixture is also given in Fig. 3. The results shown in this figure represent the mean of 3 values. The curves shown in the figure indicate that compressive strength increases with increase in lime content until 14% lime, beyond which the strength increase is very marginal for BCS and TB1 soils. Where as for TB3 soil there is a drop in strength beyond 14% lime. The quantity of lime consumed in lime-clay reactions, responsible for strength 4.3 Influence of fly ash content on strength Clay minerals are mild pozzolanic materials responsible for strength development in lime-stabilised soils. Addition of more reactive pozzolanic materials in small quantities could be beneficial for increased strength characteristics of lime-stabilised blocks. Fly ash being an industrial waste product is a good source for pozzolana. Hence, addition of fly ash to soil blocks is considered in this study. Influence of fly ash content on compressive strength of lime-stabilised steam-cured blocks was examined. Block specimens prepared using BCS soil-sand-fly ash-lime mixtures were steam-cured for 24 hours and the results are shown in Fig. 4. The percentage of clay fraction in the BCS soil-sand mixture used for preparation of specimens is 10.3%. The results shown in Fig. 4 represent the mean of 3 values. The following observations can be made from the results of Fig Quantum of fly ash and lime present in the blocks has significant influence on strength. Strength increases significantly with increase in fly ash content. There is a doubling of strength as fly ash increases from 0 to 30% for 10% lime. Similarly the strength increase is 4 fold as the fly ash increases from 0 to 50% for 18% lime. The reaction between lime and pozzolana supplied by clay minerals and fly ash is responsible for strength gain. As Fig. 3 Influence of lime content on strength. Fig. 4 Influence of fly ash content on strength. 296

5 Venkatarame Reddy, Hubli the total quantity of pozzolana (fly ash + clay mineral) increases, strength also increases, as indicated by the 2 curves in Fig For 10% lime, optimum fly ash content is about 30%, beyond which there is a marginal variation in strength. Demand for lime increases with increase in availability of pozzolana (clay mineral + flay ash). Beyond optimum fly ash of 30%, the fly ash will not have sufficient lime for pozzolanic reaction and hence it will remain as an inert filler without contributing much to strength gain. In case of 18% lime the strength increases linearly with increase in fly ash upto 50%. Experiments could not be performed beyond 50% fly ash content, as the specimens developed cracks after 24 hours steamcuring, when fly ash content is more than 50%. 3. There is hardly any difference in strength for 10% and 18% lime upto 20% fly ash. Beyond 20% fly ash, there is wide gap in strength for 10 and 18% lime. There is sufficient lime available for pozzolanic reaction even with 10% lime upto a fly ash content of 20%. Beyond 20% fly ash, the demand for lime increases, hence higher lime content gives higher strengths. These experimental results demonstrate that manipulation of lime and fly ash quantities can yield desired strengths for the blocks. Quantum of lime and fly ash added can also contribute significantly to the cost of the block. Influence of steam curing period, lime content and fly ash content on strength of lime stabilised steam cured blocks was examined and the results were discussed. The following guidelines emerge from these discussions. 1. Considerable difference in strength between steam-cured and wet-cloth-cured blocks. Hence, it is essential to steam-cure the blocks for about hours for complete strength development. 2. Optimum lime content is approximately equal to that of clay fraction in the soil-sand mixture used for block production. Fly ash addition to soil-sand mixtures can alter the optimum lime content yielding maximum strength. 3. Addition of fly ash greatly improves the strength. Higher quantity fly ash demands higher percentage of lime for obtaining higher strengths. Fig. 5 Long-term strengths of steam-cured blocks using BCS soil. 5. LONG-TERM STRENGTHS Building materials should possess stable long-term strength. Long-term strengths of the steam-cured soil blocks were monitored for periods upto two years. Lime-stabilised soil blocks were steam-cured at 80 C for 24 hours and then air dried inside the laboratory for 30 days. These blocks were tested for compressive strength after soaking them in water for 48 hours. This strength has been designated as strength after 1 month of steamcuring. Another set of steam-cured blocks were stored inside the laboratory and then tested for wet compressive strength at the end of 6 months, 12 months and 24 months. Annual variations in outside ambient temperatures are 16 C to 35 C with a relative humidity of 50- Fig. 6 - Long-term strengths of steam-cured blocks using TB2 soil. 60%. Figs. 5 and 6 show the variations in long-term strengths for the lime-stabilised steam-cured blocks prepared using BCS and TB2 soils respectively. The strength values shown in the figure have been normalised by taking the ratio of long-term strength to strength after 1 month of steam curing. The results shown in these figures represent the mean of 3 values. These studies were planned to verify the variation in the block strength with time. The following observations 297

6 Materials and Structures/Matériaux et Constructions, Vol. 35, June 2002 can be made from the results shown in Figs. 5 and 6. 1) Lime-clay ratio and lime-fly ash ratio control the long-term strength behaviour of lime-stabilised steamcured blocks. Lower lime-clay ratio of less than 0.5 showed constant fall in strength with the age for both the soils. In case of BCS soil blocks the fall is 50% after 24 months, whereas for TB2 soil the reduction is about 15%. It should be noted that BCS soil has montmorillonite clay mineral and TB2 soil kaolinite mineral. The fall in strength may be attributed to lower lime content unable to fully stabilise the clay. Unstabilised clay soils undergo swelling and shrinkage with seasonal variations in ambient moisture content. The magnitude of swelling in clay soils depends on the type and amount of clay mineral. It follows the sequence montmorillonite > illite > kaolinite [18]. Free swell index tests reveal that montmorillonite has approximately 25 times higher swell potential than kaolinite [19]. The amount of shrinkage experienced by a clay soil is also a function of the type and amount of clay mineral. Montmorillonite clays by virtue of their high water content shrink more than the kaolinitic clays [18]. Given the high swell-shrink potential of montmorillonite clay, presence of any unstabilised montmorillonite clay in the stabilised blocks causes greater disruption of bonds formed due to lime-clay and lime-fly ash reactions, comparatively, the marginal swellshrink tendency of kaolinte clay leads to much lesser disruption of bonds formed by lime-clay reactions. This may explain the lower percentage reduction in strengths for TB2 soil blocks. 2) Lime-clay ratio of 0.85 and above leads to increase in strength initially upto a period of about 12 months and then stabilising at that level for both the soils. The strength increase with time can be attributed to prolonged lime-clay reactions at ambient moisture contents. Here, availability of free lime for the reactions is crucial. Optimum lime-clay ratio for blocks using BCS soil (montmorillonite clay) is 1.17 and for TB2 soil (kaolinte clay) Even with optimum lime-clay ratio the strength increase is more ( ~ 40%) for TB2 soil blocks when compared to BCS soil blocks ( ~ 25%). 3) In case of fly ash addition, there will be reactions between lime and fly ash as well as clay mineral. Fly ash has better pozzolanic reactivity than clay minerals, at the same time fly ash will not have swell shrink characteristics like clay minerals. Lime has to satiate the needs of both clay minerals and fly ash in order to develop bonds between particles of the mixture of sand and soil. Lime to clay + fly ash ratio values of the order of 0.3 show stable long-term strength. Higher lime to clay + fly ash ratios ( ~ 0.5) lead to higher strength increase with age. These studies show some trends in the long-term performance of steam-cured blocks. There is a need to investigate the behaviour of lime-stabilised blocks, especially micro-level changes in bonds with age. There are hardly any investigations on long-term behaviour of 298 lime-stabilised soils. With the limited tests performed in the present investigation, it can be concluded that there is a favourable lime-clay ratio for stable long-term strength of lime-stabilised block. This ratio is again dependent on type of clay mineral present in the soil. Lime-clay ratio of > 0.85 and lime-clay-fly ash ratio of > 0.30 show stable long-term strength. 6. SMALL-SCALE STEAM-CURED BLOCK PRODUCTION SYSTEMS Laboratory investigations on the properties of steamcured stabilised-soil blocks were discussed in the earlier sections of the paper. A small-scale steam-curing plant (1200 blocks capacity) was designed and implemented for producing steam-cured blocks for the construction of a 3-storey load bearing masonry structure. Steam-cured block production process involves 3 major activities: (1) preparing the uniform mixture of various ingredients, (2) pressing the mixture into a block and (3) steam-curing. A brief procedure adopted for the production of steam-cured blocks is as follows. TB3 soil and hydrated lime were mixed in a pan mixer. Lime-soil mixture along with cement, fly ash and sand was fed into a concrete mixer. Adequate water was sprayed and the mixing continued. After uniform mixing of all the ingredients, mixture was discharged and pressed into a dense block (dry density of 18 kn/m 3 ) of size 230 X 190 X 100 mm, using a manually operated block making machine. The blocks after 24 hours of casting, were charged into a steam chamber and steam-cured for 10 hours. Steam-cured blocks are discharged the next day and used for construction. Various operations involved in mixing of different ingredients (soil, lime, cement, fly ash, sand and water) are shown in Fig. 7. Fig. 8 shows the steam curing plant in operation. This plant consists of a hybrid boiler system having horizontal and vertical boilers. This system has been carefully designed to extract maximum heat during the burning of fuel to generate steam, yielding a thermal efficiency of > 60%. Typical temperature profile of a steam curing cycle is shown in Fig. 9. This figure Fig. 7 Block production in the field.

7 Venkatarame Reddy, Hubli (size: 230 X 190 X 100 mm) randomly collected from different batches is 6.89 MPa with a standard deviation of 0.75 MPa. Total thermal energy consumed in the block production including the energy content of cement and lime is 6.7 MJ per block, which is about 2/3 rd of that required for burnt clay bricks in the Indian conditions. 6.1 Uses of steam-cured soil-lime blocks The strength of the steam-cured block can be easily manipulated by varying the percentage of stabiliser and other additives. Wet compressive strengths in excess of 10 N/mm 2 (for blocks of size 230 X 190 X 100 mm) can be easily achieved in the small-scale production systems described above. Such blocks can be used for load bearing masonry structures in place of burnt clay bricks. Burnt clay bricks of size 230 X 107 X 70 mm are produced in small-scale plants (capacity: 50, ,000 bricks) in Fig. 8 Steam curing plant. shows that the entire stack of blocks is exposed to a temperature of > 80 C for more than 20 hours. Totally 18 runs were taken in the steam chamber, thus producing about 20,000 blocks of size 230 X 190 X 100 mm. These blocks were used in the construction of an office building shown in Fig. 10. The steam-cured blocks were prepared using TB3 soil and sand in the proportion of 1:2 by weight. Lime (8%), cement (2%) and fly ash (10%) were the other ingredients used for the blocks. The mean wet compressive strength of 16 blocks Fig. 9 Typical temperature profile in a steam curing cycle. Fig. 10 Load bearing masonry laboratory building using steam cured blocks. India and other developing countries. Bulk of the buildings (especially residential buildings) in India use load bearing masonry walls with a maximum of 3 storeys in height. In the Indian context (true elsewhere also) it is difficult to produce burnt clay bricks in the small-scale plants using expansive soils and high clay soils. Steam-cured blocks score over burnt clay bricks in terms of energy, use of expansive and high clay soils and small-scale production. These blocks can be used for masonry construction of following categories of buildings/constructions. a) Load bearing masonry walls of residential buildings (1-4 storeyed in height) b) Load bearing masonry walls of commercial/industrial buildings 299

8 Materials and Structures/Matériaux et Constructions, Vol. 35, June 2002 c) Infilled walls in multi-storey buildings and free standing boundary walls, etc. 7. CONCLUDING REMARKS Properties of lime stabilised blocks using soil-sandfly ash mixtures have been studied in some detail. Influence of curing period, lime content and fly ash content on the wet compressive strength and long-term strengths have been discussed. Investigation also demonstrates the small-scale steam-cured block production system in the field and its application to the construction of a load bearing masonry building. The following conclusions can be made from the results of the investigation. 1. Considerable difference exists in the strength of wet cloth (at ambient temperature) cured blocks and steamcured blocks. Steam-curing at 80 C has shown 3 to 6 fold increase in wet compressive strength when compared with 28 days wet cloth curing at ambient temperatures. 2. Compressive strength increases with the increase in steam-curing period irrespective of lime content and type of soil. The strength increase is sharp initially upto 24 hours of steam-curing and then the increase is marginal when higher lime contents were tried. Steam-curing period of hours will be sufficient to extract maximum possible strength of lime-stabilised soil blocks. 3. Optimum lime content for strength is a function of clay fraction of the soil-sand mixture used for block production. Beyond optimum lime content drop in strength can be noticed. Generally the percentage of clay fraction of the soil indicates the optimum lime content. Soil can be easily diluted by sand addition to adjust the clay and lime contents. 4. Addition of small quantities of industrial waste products like fly ash greatly improves the strength of lime-stabilised steam-cured blocks. Strength increases with increase in fly ash content. Maximum quantity of fly ash to be added will be dependent on lime content. Lime content of about 10-12% can accommodate about 30% fly ash, beyond which the strength increase is insignificant. Fly ash contents beyond 50% lead to development of cracks during steam curing process. 5. Lime-to-clay and lime-to-clay + fly ash ratios can indicate the status of long-term performance of steamcured blocks. Very low lime contents (lime-to-clay ratios of ~ 0.4) result in fall of strength with age. The test results indicate that there is a favourable lime-to-clay ( ~ 0.80) and lime-to-clay + fly ash ratio ( ~ 0.35) for stable long-term performance. 6. Steam-cured blocks can be produced in a smallscale production system. Adjusting the mix ratios can easily control the strength of such blocks. Such blocks can be energy efficient alternatives to burnt clay bricks. ACKNOWLEDGEMENTS implementation and running of small-scale steam curing system. Authors thank Prof. K. S. Jagadish for the help and support received throughout the course of these investigations. Department of Science & Technology (DST), Government of India funded this R & D work. Support from DST is gratefully acknowledged. REFERENCES [1] Jagadish, K. S., The progress of stabilised soil construction in India, Proceedings of National Seminar on Application of Stabilised Mud Blocks in Housing and Building, Bangalore, India, Nov. 1988, [2] Walker, P., Venkatarama Reddy, B. V., Mesbah, A. and Morel, J.-C., The case for compressed earth block construction, Proceedings of 6th International Seminar on Structural Masonry for Developing Countries, Bangalore, India, October 2000 (Allied Publishers Ltd.) [3] Houben, H. and Guillaud, H., Earth construction: a comprehensive guide (IT Publications, London, 1994). [4] Venkatarama Reddy, B. V., Studies on static soil compaction and compacted soil-cement blocks for walls, Ph. D. thesis, Dept. of Civil Engineering, Indian Institute of Science, Bangalore, India, (1991). [5] Venkatarama Reddy, B. V. and Jagadish, K. S., Influence of soil composition on the strength and durability of soil-cement blocks, The Indian Concrete Journal 69 (9) (1995) [6] Lunt, M. G., Stabilised soil blocks for building construction, Overseas Building Notes (184) (February 1980). [7] Walker, P., Strength, durability and shrinkage characteristics of cement stabilised soil blocks, Cement & Concrete Composites 17 (4) (1995) [8] Fitzmaurice, R., Manual on stabilised soil construction for housing, United Nations, New York, USA, (1958). [9] Venkatarama Reddy, B. V., and Lokras, S. S., Steam-cured stabilised soil blocks for masonry construction, Energy and Buildings 29 (1998) [10] Mateos, M. and Davidson, D. T., Steam hardening of lime, lime and fly ash and cement treated soil, for presentation at the 73 rd session of the IOWA Academy of Sciences, Indiana, IOWA, (April 1961), [11] Mateos, M., Soil lime research at IOWA State University, Journal of Soil Mechanics and Foundation Division, ASCE 90 (SM2) (1964) [12] Ladd, C. C., Moh, Z. C. and Lambe, T. W., Recent soil lime research at M. I. T. Highway Research Board Bulletin 262 (1968). [13] Ranganatham, B. V. and Pandyan, N. S., Strength gain in thermally cured lime stabilised clays, Proceedings of 4 th conference on Soil Mechanics, Budapest, 1971, [14] Venkatarama Reddy, B. V. and Jagadish, K. S., Pressed soillime blocks for building construction, Masonry International 3 (1984) [15] Venkatarama Reddy, B. V. and Jagadish, K. S., The static compaction of soils, Geotechnique 43 (2) (1993) [16] Herrin, M. and Mitchell, H., Lime soil mixtures, Highway Research Board, National Research Council, Washington D. C., U. S. A., Bulletin No. 304, (1961). [17] Thompson, M. R., Lime reactivity of Illinois soils, Journal of Soil Mechanics and Foundations Division, ASCE 92 (SM5) (1966) [18] Yong, R. N. and Warkentin, B. P., Soil properties and behaviour, (Elsevier Scientific Publishing Co., New York, 1975). [19] Sridharan, A., Rao, S. M. and Satyanarayana Murthy, N., A rapid method to identify clay type in soils by the free swell technique, ASTM Geotechnical Testing Journal 9 (4) (1986) The authors would like to acknowledge the invaluable help received from Prof. S. S. Lokras in designing, 300