Calibrating the Iowa Pore Index with Mercury Intrusion Porosimetry and Petrography, October 2017

The Iowa Pore Index (IPI) test is a fast, non-destructive, inexpensive, and environmentally friendly test used by several Midwestern state departments of transportation to determine the volume ratio of macropores to micropores in a coarse rock aggregate. When combined with x-ray diffraction and x-ray fluorescence, this method has been shown to be effective in predicting the performance of aggregates in portland cement concrete. The test has the potential to replace mercury porosimetry and be integrated into any petrophysical laboratory. This research aimed to understand the geological factors (depositional environment, facies, grain and pore types, texture, and paragenesis) responsible for the results of the IPI test. Samples of various geologic ages were collected around Iowa to represent different combinations of accepted and rejected porosity and clay contents. The pore index of each sample was calibrated quantitatively via helium and mercury porosimetry and qualitatively via thin section petrography. The findings show that even the most homogeneous sources have at least three different rock types. Petrographic analysis showed that limestones with a sparite matrix, peloidal grains, and a low matrix-to-allochem ratio (i.e., grainy) are better for road construction than limestones with a micrite matrix, skeletal grains, and a high matrix-to-allochem ratio (i.e., muddy). Dolostones with fine to coarse grains, crystal-supported euhedral to subhedral rhombs, and porous intercrystalline areas are more desirable than dolostones with very fine grains and a tightly interlocking crystal mosaic in anhedral form. Several linear models were developed to relate IPI to helium porosity. Limestones with a helium porosity less than ~7% and dolostones with a helium porosity greater than ~13% were found to be desirable for use in road construction. The critical range of pore-throat size was found to be between 0.02 and 0.1 µm. Coarse aggregates with modal pore throat sizes above this range were found to be desirable for use in road construction