Whilst investigating the use of cement stabilized minestone (discard from coal mining operations) as a structural element in a highway pavement, a number of soaking tests were conducted using a similar procedure to that described by the author. The results were broadly similar to those shown in his table 4 with soaked strengths (calculated as percentages of normally cured strengths), shown below in Table 1.

Observations made during these tests have provided an opportunity to re-appraise the philosophy of the experimental procedure.

The mechanism for strength changes described by the author is widely accepted and the comments relating to salt content are particularly useful, but the test procedure is not sufficiently investigative and merely reflects conventional wisdom with respect to water adsorption by clay soils and volume changes imposed by sulphate-cement reactions.

Water absorption by clay minerals is a function of mineral type and of previous stress history. Those clays which have been subject to diagenetic changes imposed by high overburden pressures react quite differently from geologically ‘young’ clays with respect to swelling characteristics as an inspection of e-σ′ curves will show. The strength reduction mechanism for soaked specimens of cement stabilized materials containing clay shale is probably related to the generation of excess pore water pressures caused by a combination of the applied load and the free water held in the voids of the material. Measurements of mass increase after 7 days soaking confirm water uptake but these are not balanced by contributory volume changes.

For the mixing moisture

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