ARE MINERALOGY AND GEOCHEMISTRY USEFUL IN ENGINEERING GEOLOGY?

With the increased demand for construction materials, Texas has experienced a shortage of quality aggregates for road bases and concrete in many regions. This has resulted in the widespread use of marginal materials that are more susceptible to premature failure. Following is a review of two case studies where traditional mineralogical and geochemical techniques were essential in identifying the cause of early failure.

The first example is an airport runway that experienced heave prior to being open to air traffic. Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA) and X-ray diffraction (XRD) were tools used in identifying the problem and how it could have easily been prevented. The heave was caused by formation of a mineral called ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O]. The addition of lime to stabilize expansive soils (smectite rich) in Texas creates the perfect environment for ettringite formation if there is a source of sulfate. “Fly ash” or more correctly flue gas desulferization by-products, in this case supplied the sulfate.

The second case study involves the use of SEM and EPMA, coupled with false-color imaging, to identify deleterious reactions in prestressed concrete beams. Many beams became heavily cracked while still in the manufacturers yard. Two hotly debated mechanisms that commonly receive the blame for this type of failure are Alkali-Aggregate Reaction (AAR) and Delayed Ettringite Formation (DEF). Cement from two different manufacturers was used, and siliceous river gravel from one locality was the source of the aggregate. All beams using cement from one manufacturer were heavily cracked while beams constructed with the other source of cement did not exhibit any exterior signs distress. Data will be presented that support failure due to DEF.

In conclusion, a good foundation in traditional geology classes like mineralogy and geochemistry will be essential in solving many engineering problems in the 21st century as supplies of high quality geologic building materials decrease.