The effect of organic matter on the setting of soil‐cement mixtures

Mixtures of soil, cement and water will harden sufficiently to be used in place of stone for road‐making. Some soil types, however, contain organic matter that prevents the mixture hardening. Experiments are described in which a range of organic compounds, of the types occurring in soil, were added to clean sand, which was subsequently mixed with cement and water. Compressive strength tests on specimens made from these mixtures showed that the organic compounds could be grouped according to their ‘activity’ in retarding the setting of cement; of the compounds considered, the most active were glucose and nucleic acid. Later experiments with a natural organic sandy soil showed that the retardation is probably due to an ‘active’ fraction of the organic matter, which affects the setting of cement by adsorbing calcium ions liberated during hydration. The ‘active’ material is produced in the surface soil, where it is associated with a low pH; it is subsequently leached into the subsoil, where it becomes adsorbed as a complex with iron in a zone of accumulation where the pH is higher. The retarding effect is overcome by the addition, with the cement, of calcium chloride, either in the form of super‐rapid‐hardening cement or as a separate additive.