The use of cements of different fineness in soil-cement mixtures

Summary A study of the literature has shown that increasing the fineness of the cement in mortar and concrete mixes results in an increase in their strength at early ages. This paper describes tests to determine whether a similar effect is obtained with mixtures of soil and cement, so that if possible a reduction could be made in the proportion of cement used in soil stabilization by using a cement of greater fineness than the commercial varieties now employed. That fraction of a sample of ordinary Portland cement that passed the B.S. No. 300 sieve was added to a silty clay and test specimens were prepared. The 7-day unconfined compressive strength obtained was found to be 140% of the strength obtained when the whole cement was used. Taking the compressive strength at 7 days as the criterion, this suggests that if 10% of ordinary Portland cement was normally required to stabilize this soil, 7% of the fine-grained cement should prove equally satisfactory. In further tests, a cement clinker was ground to three different degrees of fineness in a laboratory ball mill, the coarsest being equivalent to ordinary Portland cement. The 7-day compressive strength obtained when the finest cement (specific surface = 5,800 cm2/g) was added to the silty clay was 125% of the strength obtained when the coarsest cement (specific surface = 2,900 cm2/g) was used. The corresponding increase for mixtures of sand and cement was 120%. Chemical analysis showed that the removal of the coarse grains in an ordinary Portland cement increased the proportion of sulphur trioxide present. In a third series of experiments, the compressive strength of a cement-stabilized silty clay obtained with a cement of given fineness was found to increase when the proportion of gypsum added to the cement was increased to a certain optimum value, but the increase was not as large as that obtained in the previous experiments by sieving cement. It is believed that fineness, sulphur trioxide content and possibly also the particle-size distribution can affect the strength obtained with a given cement, and further experiments are needed to determine the combination of fineness and sulphur trioxide content required to obtain a cement of maximum strength from clinker processed in a given way.