MICRO-PORE SYSTEMS: A KEY COMPONENT IN CHARACTERIZING CARBONATE FACIES USED IN CONCRETE AGGREGATE EXPLORATION AND PRODUCTION

Understanding micro-pore systems within carbonate facies is vital when choosing carbonate concrete aggregate. Aggregates will experience a variety of physical and chemical factors during their service lives. Pressure changes during winter season freezing can physically degrade the aggregate. Water chemistry will vary widely, due to the curing process, meteoric precipitation, and highway maintenance. For example, meteoric water will act as a weak carbonic acid and dissolve carbonate rock, while deicing salt containing sodium, calcium, and magnesium chlorides can chemically deteriorate the aggregate by encouraging dolomitization and dedolomitization reactions. It is therefore vital to utilize carbonate aggregate that can withstand winter season deterioration, while also providing the most chemical stability.

Microcrystals that host micropores in carbonate rocks have been correlated with the “secondary load” in the Iowa Pore Index Test, which itself correlates with poor service life. These crystals consist of primarily low-Mg calcite with variable morphologies suggesting complex diagenetic histories. The diagenetic pathways to form and alter these crystals remain unclear, and even the depositional mineralogy and texture of these microcrystals is controversial. Microcrystals can have different zones that represent crystal growth from specific water chemistries. Recognizing growth zones in macro-crystals is relatively straightforward. On the micron-scale, zonation is not only difficult to sample, but also difficult to see. Bulk analyses yield compositions that represent mixes between different diagenetic trends, obscuring the composition of specific zones.

Scanning electron microscopy (SEM) can image individual microcrystals and characterize porosity, permeability, and overall ability to withstand strain. Chemical zonation can be characterized using SEM-Electron Dispersive Spectroscopy. Careful consideration needs to be given to sample preparation, but obtaining chemical data in specific zones is possible. Using chemical zonation to understand the complex morphologies of these micro-crystals will assist in exploring for and producing from a carbonate deposit, ultimately allowing more accurate predictions of carbonate aggregate quality and more economic resource recovery.