Cement stabilized soil block: Economical and environment-friendly

house.jpg (19267 bytes)By Asitha Jayawardena
Houses are going up, and so is the demand for conventional building materials such as clay bricks and cement sand blocks. Attempts to fulfil this rising demand can adversely affect the environment. For example, large scale mechanized clay mining may lower the productivity of adjoining rice fields, lower the water level in the adjoining wells, cause health hazards such as mosquito breeding, and above all, result in barren land full of pits. Adding more burden to the environment, fire wood is used for burning of raw bricks.

On the other hand, short fall of these conventional materials has caused a rise in the market price, making it extremely difficult for people to construct houses. Since the cost of walls is around 20 to 40% of the total construction cost of a house, a significant saving is possible if an economical building material is used for the construction of walls. It is in this context the cement stabilized soil block comes to play the role of an economical, environment-friendly alternative to conventional building materials.

Wide application of the cement stabilized soil block has a short history in Sri Lanka, only running back to 1992; but its performance in the field is high and promising. Pioneered by Dr Asoka Perera, Department of Civil Engineering, Moratuwa University, this technology has been successfully employed to construct over 600 single storey houses and also several two storey houses in various parts of the island. Unfortunately, its host of benefits still remains unknown to prospective home builders and the authorities.

Recently, however, some light was shed on the cement stabilized soil block at the monthly Question Time session of the Society of Structural Engineers Sri Lanka held at the Institution of Engineers Sri Lanka. There, Dr (Mrs) Chintha Jayasinghe, Senior Lecturer, Institute of Technology University of Moratuwa, spoke on "Alternative building materials for loadbearing construction." Although the presentation consisted of two parts, i.e. cement stabilized soil blocks and a precast slab system, this article addresses the former only.

The cement stabilized soil block is mainly soil and water, with cement acting as the stabilizing agent. In the extensive experimental programme conducted at Moratuwa University over the years, blocks of two sizes have been developed. For loadbearing walls, the length of the block is 29cm, width 24cm and height 9cm. For partition walls (i.e. non-loadbearing walls), the corresponding figures are 29cm, 14cm and 9cm.

A desirable soil should consist of gravel, sand, silt and clay. An important factor that affects the strength of the finished block is the fines content of the soil, i.e. the silt and clay content. For soil blocks intended for the construction of loadbearing walls, the desirable range of the fines content is from 20 to 30%. Therefore, high fines soil should be modified to the desirable requirement by adding sand. The fines content of a soil can be determined by a simple jar test at the site itself.

The wet strength of the raw block and the durability of the finished block are enhanced by a process called "stabilization". It is three fold, namely physical, chemical and mechanical. Physical stabilization is done by sieving the soil using a 12mm mesh to remove the larger particles. Chemical stabilization is achieved by adding cement. The desirable cement content range from 2 to 10% ; however, for single storey houses, 2% is adequate. To the cement soil mix, water is then added until a uniform consistency is achieved. Then the mix is mechanically stabilized, i.e, compacted using a machine or manually. Finally, the raw blocks are stacked in a dry place and cured with water for two weeks.

For experimental studies conducted at Moratuwa University, both machine and manual compaction had been adopted, using a compaction ratio of 1.65. One such machine is the Auram Press 3000 imported from India. With three men working with the machine and another three for mixing and stacking, the output was 800 blocks per day at 8 working hours a day. However, to popularize this technology in the rural areas, manual compaction is more appropriate. Therefore, manual compaction was investigated through a work study. With two men working with one mould, the output was 160 blocks per day.

A standard code of practice is still not available for cement stabilized soil blocks. For quality control at the site, a penetrometer can be used on the raw block, and bending tester on the finished dry block to check its bending strength. Using 1:6 cement, sand mortar, wall panels were constructed with the soil blocks. The experimental results show that the compressive strength of both soil blocks and walls constructed with soil blocks is adequate for its use in loadbearing construction, even at the ground floor level of a two storey house.

The plaster applied was also a soil based one, with proportions 1:6:6 cement, sand and soil. However, instead of a plaster, a soil based paint with proportions 1:1:6 cement, lime and soil can be applied. This paint would provide adequate durability for the wall.

The option of soil blocks offers more than one way for savings. The block is basically soil. The desirable type (i.e. laterite) may be available at the site itself, saving the material, loading, transport and unloading cost of soil. Besides labour cost can be saved by adopting self-help approach, which is not difficult in a rural context. For single storey construction, a cement content of 2% is adequate if the fines content is between 20 to 35%. Thus use of cement, which is relatively expensive, can be minimized. Since plastering and painting can be avoided, the soil block wall with a soil based paint would further reduce the cost.

The soil block seems to be the ultimate environment-friendly building material. With soil comprising over 90%, this unburnt building material would, when discarded, blend with nature with minimum undesirable effect.

Recognizing the string of benefits it offers at national as well as individual level, the authorities should take necessary steps to popularize the cement stabilized soil block as an alternative building material for the construction of houses, especially in the rural parts of Sri Lanka.