CARBONATION OF STABILISED SOIL-CEMENT AND SOIL-LIME MIXTURES --DEVELOPING COUNTRIES: HIGHWAY CONSTRUCTION. PROC. OF SEMINAR H HELD AT THE PTRC TRANSPORT & PLANNING SUMMER ANNUAL MEETING, BATH UNIV. ENGLAND 7-11 SEPTEMBER 1987, VOLUME P295

Carbonation can affect the strength of cement and lime-stabilised soils. Correct curing is therefore important especially in hot, dry climates. Soil stabilisation with cement or lime is used in many countries in the construction of road bases and sub-bases and for the treatment of subgrade soils. Recent studies have shown that the performance of stabilised layers can be affected by carbonation which inhibits the formation of cementitious products in soil-cement and soil-lime reactions. This paper examines the evidence for carbonation in road trials constructed in a hot, arid area in botswana and describes the laboratory study that followed using a clayey sand (a poor quality calcrete) from botswana to investigate the effect of different curing conditions on soil-cement and soil-lime mixtures. Humidity, temperature and carbon dioxide in the environment were varied and the affect on strength and properties of the stabilised soils were assessed by the unconfined compressive strength and plasticity tests. Different curing periods were examined and the extent of carbonation was identified by the use of phenolphthalein indicator. The results showed that carbonation to varying degrees and loss of strength occurred in all the strength-age relations except those that were cured under sealed conditions. Samples that were first allowed to harden also subsequently lost strength when exposed to carbon dioxide. It was also shown that changes in plasticity which occur in soil stabilisation and are an essential requirement in specifications for modified soils could also be reversed by carbonation. There remains a need to obtain more field evidence of carbonation and its relation to pavement performance, curing conditions and climatic environment. In addition a better understanding of the mechanism of carbonation is required especially of those reactions which involve cemented products.(a) for the covering abstract of the seminar see IRRD 816292.