AbstractIn Kenya soil is the basic house construction material for low-income groups. However, its lack of durability and low strength, are limiting factors to its use. The aim of the current research programme was to establish, for lime-stabilised murrain (from Kenya), the optimum lime content, compaction pressures and curing conditions which are necessary to achieve the minimum strength required by the Kenyan standard for stabilised soil building blocks.
A comparative study was also done on murram-cement and murram-lime-gypsum. Initially mixtures of murram containing different amounts of lime (i.e. 5, 10 and 15wt%) and murram containing different amounts of cement (i.e. 2, 4 and 6wt%) were compacted at 5N/mm2 and cured at 30°C and 100% relative humidity for periods ranging from 1 to 4 weeks. In the extended work mixtures of murram containing different amounts of lime and lime-gypsum (i.e. 5, 10 and 15wt% lime and 5wt% lime-1.5wt% gypsum, 1Owt% lime-3.0wt% gypsum and 15wt% ldjne-4. 5wt% gypsum) were compacted into cylinders at 1ON/mm2 and cured under the same conditions.The chemical and microstructural changes which occurred during curing were determined using thermal analysis, Xray powder diffraction and scanning electron-microscopy.
The strength of lime-stabilised murram increases with both increase in curing time and increase in lime content. Addition of up to 6wt% cement and compacting at 5N/mm pressure appears to be less effective with regard to strength development. Increasing the compaction pressure from 5N/mm2 to 1ON/mm2 increases the compressive strength of the cured murram-lime material by amounts ranging from 50 to 100%, although only the 15wt% lime addition material meets the Kenyan standard requirement for minimum strength (1.5N/mm2 ). Addition of small amounts of gypsum produces very significant increases in strength which meet the Kenyan standard requirement, however, this weakens the lime stabilised murram when exposed to wet-dry durability tests. Diluting murram with 30wt% sand enabled higher strengths to be achieved with the same lime content. Stabilising the murram with 10wt% cement and employing 1ON/mm2 pressure also enabled the minimum strength to be achieved, but the material disintegrates when soaked in water. In wet conditions therefore, gypsum addition is not recommended. Also cement stabilisation in the current work fails. Reaction of lime with murram results in the formation of calcium silicate-aluminate hydrate (C 2ASHg) and carbonate substituted calcium aluminate hydrate (C4ACo.5H12^ When gypsum is present the principal reaction * product is ettringite which is a calcium sulpho-aluminate hydrate (C3A3CSH32 ).
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