Investigations of Impurities of the Ottawa Standard Sand

The Ottawa Standard Sand is derived from the Middle Ordovician St. Peter Sandstone near Ottawa, Illinois and has been widely applied in experiments by geologists and engineers for several decades. Common tests include compressive strength, air content, tensile strength for hydraulic cement, and geochemical experiments. The sand also serves as a specification for graded sand, 20/30 sand, as well as masonry cement. Practical applications include building stone, abrasive, manufacturing of glass, and molding sand.

The primary reason for its use is due to the uniform grain size and the presumed uncontaminated nature of the grains that do not show visible signs of any clay, loam, iron compounds, or other foreign substances. Because the sand is typically regarded as a standard often detailed mineralogical analyses are not conducted prior to experiments. We examined a lab grade sample to determine what impurities existed.

Quartz grains constitute approximately 98% of the materials and approximately 1-2 % of the grains are chert fragments. Scanning electron microscopy (SEM) indicates a variety of microtopography on grain surfaces occurs. SEM investigation also shows a variety of trace impurities including carbon, iron, manganese, chromium, chlorine, and potassium. Cr-rich textures are consistent with mechanical abrasion with metal. Mn and Fe-rich textures are unidentified oxide minerals. The source of other impurities is unclear.

The occurrence of such impurities may be detrimental to the validity of any experiment that involves the Ottawa Standard Sand, however those involving aqueous geochemical experiments are most susceptible to error. This may explain unusual behavior of some experiments. For example K or Cl may be released influencing pH or cations may be absorbed by oxide minerals. Grain topography variations may influence tortuosity. This investigation shows the clear need to perform a comprehensive characterization of the Ottawa sand.