An Evaluation of the Mechanical and Chemical Dispersion Methods for a Tropical Old Alluvium

Abstract Resulting from tropical weathering, the old alluvium in San Juan, Puerto Rico contains Fe-oxides, which cause the formation of aggregates through both cementation and aggregation, and hence, is not readily dispersible in water. Various methods of sample preparation were adopted to disperse this deposit, consisting of mechanical remolding, varying dispersant type and quantity, and drying. The resultant degree of dispersion was evaluated by particle size analysis. Results reveal that both mechanical and chemical dispersion is required in order to obtain a completely dispersed and stable suspension, and chemical dispersion alone is not effective in disintegrating the aggregates caused by cementation. Drying usually includes mechanical, physico-chemical, and chemical reactions and can either disperse a soil or produce more aggregates, depending on drying temperature and a soil's chemistry and composition. Air-drying disperses this soil more than oven-drying. This investigation also indirectly evaluated the applicability of the ASTM standard test method for particle size analysis to this deposit.