Laterite is a residual ferruginous rock, commonly found in tropical regions and has close genetic association with bauxite. The term 'laterite' was originally used for highly ferruginous deposits first observed in Malabar Region of coastal Kerala and Dakshina Kannada and other parts of Karnataka. It is a highly weathered material, rich in secondary oxides of iron, aluminium or both. It is either hard or capable of hardening on exposure to moisture and drying.

Laterite and bauxite show a tendency to occur together. Aluminous laterites and ferruginous bauxites are quite common. The most common impurity in both is silica. Laterite gradually passes into bauxite with decrease in iron oxide and increase in aluminium oxide [6]. The laterite deposits may be described on the basis of the dominant extractable minerals in it: (i) aluminous laterite (bauxite), (ii) ferruginous laterite (iron ore), (iii) manganiferous laterite (manganese ore), (iv) nickeliferous laterite (nickel ore), and (v) chromiferous laterite (chrome ore). Laterite with Fe₂ O₃ :Al₂ O₃ ratio more than 1, and SiO₂ :Fe₂ O₃ ratio less than 1.33 is termed as ferruginous laterite while that having Fe₂ O₃ :Al₂ O₃ ratio less than 1 and SiO₂ :Al₂ O₃ ratio less than 1.33 is termed as aluminous laterite.

Laterite can be considered as polymetallic ore as it is not only the essential repository for aluminium, but also a source of iron, manganese, nickel, and chromium. Further, it is the home to trace several elements like gallium and vanadium which can be extracted as by-products [6].

The construction cost can also be considerably decreased by selecting local materials, including local soils for the production of construction materials [9]. Lime and Fly ash in combination can often be used successfully in stabilizing building blocks [1, 11]. LF stabilization is often appropriate for stabilizing building materials. Fly ash is a pozzolanic material; it has been successfully used with granular and fine grained materials to improve soil characteristics [7, 10].

Lime has been widely used either as a modifier for clayey soil or as a binder. When clayey soils with high plasticity are treated with lime, the plasticity index is decreased and soil becomes friable and easy to be pulverized, having less affinity with water. Lime also imports some binding action [6, 8].

The main objective of stabilization is to increase the strength or stability of soil and to reduce the construction cost by making best use of the locally available materials.

The main aim of this experimental investigation was to study the properties of stabilized bricks for the various percentage proportionate, such as, 75% laterite soil + 15% sand + 5% fly ash + 5% lime, 73% laterite soil + 15% sand + 5% fly ash + 7% lime, and 71% laterite soil + 15% sand + 5% fly ash + 9% lime.

2.1 Laterite Soil

The disturbed laterite soil was collected from the laterite quarries of Hosanagar, Chandragutti, Thyagarti, and Anandapura of Shimoga district, Karnataka. Figure 1 shows the surface texture of laterite blocks before and after dressing. The residue (disturbed) laterite soil available after dressing (Figure 2) of natural laterite building blocks was used, reddish brown in nature classified as A-2-7(0) using AASHTO soil classification system and GP by the United soil classification system. Figure 3 shows the production of laterite stabilized soil bricks. The physical and chemical properties of tested laterite soil are given in Tables 1 and 2.

Figure 1. Surface Texture of Laterite Blocks Before and After Dressing

Figure 2. Residue Laterite Soil available after Dressing and passed through 4.75 mm IS Sieve for Production

Figure 3. Production of Laterite Stabilized Soil Bricks, Curing and Testing

Table 1. Physical Properties of Laterite Soil

Table 2. Chemical Properties of Laterite Soil

2.2 Sand

Sand collected from the river bed of Tungabhadra near Airani, Ranebennur taluk was used, having fineness modulus of 2.96 and confirmed to grading zone-III as per IS: 383-1970 specification.

2.3 Fly Ash

Fly ash used was obtained from Grasim Industries, Harihar, Karnataka.

2.4 Lime

Powdered Lime Ca(OH)₂ /White powder was obtained directly from the manufacturers.

2.5 Water

Ordinary potable water free from organic content, turbidity and salts was used for mixing and curing throughout the investigation.

For the production of pressure moulded laterite brick, 4 kg of laterite soil was taken for each brick, based on the size of mould, soil was spread on clean and flat hard surface, to this known percentage of stabilizer (Sand + Lime + Fly ash) was added on weight basis followed by thorough hand mixing to get uniform mix, finally potable water (calculated as per optimum moisture content) was added to the dry mix by sprinkling, soil was mixed with water gently but quickly, care was taken to avoid formation of lumps in it during mixing. Entire mixing was carried out in an enclosed area and nearer to the compressing device, to avoid the evaporations of moisture content in the mix and to maintain the workability of wet mix. As soon as the wet mixture was prepared, the homogeneous mix was transported to compacting machine for pressing. The wet mix was placed in known size of the mould and compacted through hydraulic pressure. After compaction, the mould was taken out from the machine by ejection process. Further brick was lifted carefully from the mould base plate and placed in an open dry place. Since the bricks are not burnt and less initial handling strength, extra care was taken while lifting, placing and stacking of the bricks.

The Hydraulic press paver block and brick making machine was used for the production of bricks. The bricks produced have a uniform dimension of 230 x 100 x 75 mm. A net load of 10 tons was applied over the cross sectional area of 23000 mm² of the brick. Since there was a constant compaction effort on all the bricks, there was less variation in strength characteristics of bricks that are produced and expected an advantage of this research work. Because of heavy compaction, the difficulty of brick ejection from the mould may occur due to generation of suction pressure at the corners of mould.

The curing method used in this investigation was “Hay curing”. The paddy straw was spread over bricks, such that it completely covers them. The bricks were cured by sprinkling water gently on straws, thrice a day. The curing was continued for 7, 14, 21, and 28 days based on testing.

The following tests were conducted on stabilized and compressed laterite soil bricks.

3.1 Dry Compression Test and Wet Compression Test

The bricks were dried in sunshine for 2 to 3 days, to remove moisture content inside the bricks completely for dry compression test. To conduct wet compression test, completely dried bricks were immersed in water for a duration of 24 hours, the bricks were removed from water and the wetted surface was wiped out cleanly for test. For each average result, five bricks per set were taken and the bricks were tested for compression as per IS: 3495- Part (1)- 1992 specification on each specimen. The test results were tabulated in Tables 3 to 8.

Table 3. Test Results of Stabilized and Compressed Laterite Soil Bricks (H5) produced from Combination of 75% laterite soil + 15% sand + 5% fly ash + 5% lime

Table 4. Test Results of Stabilized and Compressed Laterite Soil Bricks (H5) produced from Combination of 73% laterite soil + 15% sand + 5% fly ash + 7% lime

Table 5. Test Results of Stabilized and Compressed Laterite Soil Bricks (H5) produced from Combination of 71% laterite soil + 15% sand + 5% fly ash + 9% lime

Table 6. Average Dry Compressive Strength and Strength Ratios Test Results of Stabilized and Compressed Laterite Soil Bricks

Table 7. Average Wet Compressive Strength and Strength Ratios Test Results of Stabilized and Compressed Laterite Soil Bricks

Table 8. Comparitive Test Results of Burnt Bricks and Stabilized and Compressed Laterite Bricks

3.2 Absorption Test

The dry weight (W_dry ) of the bricks was taken and then it was immersed completely in clean portable water for 24 hours. After 24 hours, the bricks were removed from water and the wet surface was wiped out with a clean cloth and the weight was taken (W_wet ) as per IS: 3495-Part (2)-1992 specification. The absorption was given by the relation,

(1)

Five bricks per set were taken and the average of five results was considered as 'Water absorption' of the bricks. The results were tabulated in Tables 3 to 8.

The following tables, i.e. Tables 3 to 8 give the details of the experimental results.

Based on the experimental results and observations, the following discussions and conclusions were made in comparison with traditional (red clay) burnt bricks.

5.1 Observations

• It has been observed that increasing in fly ash and lime content and use of hydraulic press paver block and brick making machine for brick making process, density of stabilized and compressed laterite bricks increases, inturn it increases the compressive strength and reduces water absorption. Maximum dry (94.50%) and wet (85.54%) compressive strength can be obtained at the age of 7 days of curing period; further strength can be obtained at the age of 28 days of curing period.
• •It has been observed that from the test results of stabilized and compressed laterite bricks, the minimum of 4% fly ash and maximum of 7% fly ash content along with lime is sufficient in order to obtain required dry (6.73 N/mm² to 8.86 N/mm² ) and wet compressive strength (2.43 N/mm² to 5.34 N/mm² ) and resistance to water absorption values (7.53% to 4.32%).

5.2 Discussion

Stabilized and compressed laterite bricks prepared in this experimental investigation are more stable than the traditional burnt red clay bricks to resist higher compressive loads and absorb very less water. Thus, higher the compressive strength of the brick, higher will be the serviceability of the wall, thus increasing serviceability /durability of the structure in case of load bearing walls too. Further, laterite soil, being more acidic in nature, not suitable for the process of traditional brick making, stabilisation of such a soil with stabilizers like cement, lime, pozzolana will enable in preparing stabilized and compressed laterite bricks. Production of stabilized and compressed laterite bricks, which do not require any heat treatment process to gain required compressive strength. Finally, stabilized and compressed laterite bricks are more economical and environmental friendly construction building material compared to conventional red clay bricks and it decreases threat to the environment by deforestation and Global warming.

Based on the experimental results, the following conclusions were drawn.

• It can be concluded that the compressive strength shown by stabilized and compressed laterite bricks are nearly twice and in some cases more than twice the strength of red clay bricks, which satisfies requirements of IS: 3495-Part (1) - 1992 specification.
• It can be concluded that the percentage of water absorption shown by stabilized and compressed laterite bricks are less than 20%, which satisfies requirements of IS: 3495-Part (2) - 1992 specification.
• It can be concluded that stabilized and compressed laterite bricks are fall under class 7.50 to class 12.50 of bricks category according to IS:1077-1992 specification.
• It can be concluded that stabilized and compressed laterite bricks can be used as a substitute to tradational burnt red clay bricks for construction purpose.