The dry density of a silty sand compacted at temperatures below 0 °C can be improved by using calcium chloride as an additive. Compaction tests at −7 °C were used to determine the effect of low amounts of calcium chloride on the moisture–density relationship of the test soil. Tests at 20 °C were conducted to provide a frame of reference for comparing the results obtained at the below freezing temperature. The results from the experimental work are analyzed by modeling the pore fluid as a simple eutectic system using the concepts of phase equilibria. The amount of ice in the pore fluid is shown to have a significant effect on the dry density of a soil compacted at temperatures below 0 °C.