Particle Density of Volcanic Scoria Determined by Water Pycnometry

Particle density is widely used in Soil Science. Several classical textbooks define this concept as the density of the soil solid-matter, whereas most standard test methods define it as the density of a soil sample crushed and passed through a sieve with a 2-mm mesh (for agronomical purposes) or 4.75 mm (for civil engineering purposes). We hypothesize that both definitions are not equivalent for volcanic scoria, because these contain small vesicles apparently isolated from the atmosphere. To test this hypothesis, an experiment was performed with basaltic scoria (known as tezontle in Mexico) with sizes ranging from 2 to 4.75 mm. These scoriae were divided into five samples that were crushed and passed through different sieves. The samples were then carefully saturated with water and their density determined by water pycnometry. Particle density was found to be 2.55 g cm -3 for the undisturbed scoria (i.e., passing a 4.75-mm sieve), 2.61 g cm -3 for slightly crushed scoria (i.e., passing a 2-mm sieve) and higher than 2.79 g cm -3 for finely crushed scoria (i.e., passing a 74-μm sieve). This study shows that the concept of particle-density must be carefully considered for soils with a high content of volcanic scoria.