Durability of compressed and cement-stabilised building blocks
Adequate shelter is a basic human need, yet about 80% of the urban population in developing countries still live in spontaneous settlements as they cannot afford the high cost of building materials. The compressed and stabilised block (CSB) has been identified as a low-cost material with the potential to redress the problem and reverse the shelter backlog. While its other properties are well understood, the durability of the material remains enigmatic. The principal objective of this research was therefore to investigate the durability of CSBs, especially as used in the humid tropics. The thesis examines the interplay between three main factors: constituent materials used (cement, soil, water); quality of block processing methods employed; and the effects of natural exposure conditions (physical, chemical, biological). Through a multi-pronged methodology involving literature reviews, laboratory experiments, petrographic analysis and an exposure condition survey, block properties and behaviour are rigorously investigated. The findings are presented under the two main division of the thesis: Part A and Part B. Part A introduces a review of the literature on the main theoretical concepts of durability and cement-soil stabilisation. It discusses various deterioration modes, and examines in more detail mechanisms of stabilisation using Ordinary Portland cement. Part A also identifies and highlights critical stages of the CSB production cycle, and recommends a strict adherence to proper testing and processing procedures. Part B presents the results of direct investigation methods used. Findings from the fieldwork confirmed that premature deterioration was widespread in exposed unrendered blocks, with defects exhibited mainly as surface erosion and cracking. Quality checks on site materials and practice established an urgent need for improvement through the provision of appropriate standards and codes. Laboratory experiments which compared the properties of traditional blocks (TDB) and blocks improved by the inclusion of micro silica (IPD), established that the latter significantly out-performed the former. A new quick predictive surface test, the slake durability test, which is more reliable and repeatable than existing tests, is proposed. The thesis concludes that it is possible to significantly raise the strength, improve the dimensional stability and wear resistance of CSBs to the extent that they can be safely used in unrendered walls in the humid tropics. This improvement is achieved via better intergranular bonding, reduction in voids and lowered absorption. Using the slake durability test, it is now tenable to freely discriminate, classify, and compare not only blocks but other like materials of any category and storage history as well. New quantitative durability gradings are recommended for future incorporation into CSB standards. The findings are likely to contribute to the widespread use of CSBs. The research, however, also raises a number of new questions which are listed for further work.