Phase separation of olivine in basalt aggregate induced by fatigue damage of ultra-high performance concrete

Abstract text (incl. figure legends and references)

Introduction

Ultra-high performance concrete (UHPC), a specialized form of concrete characterized by its high compressive strength of up to 200 N/mm², has garnered considerable interest since the first reports in the early 1990s.^[1] Today, it is used mostly in structures with high strength requirements such as bridges, high-rise buildings and wind turbine towers.

Objectives

Due to the high cement content of fine-grained UHPC, its production entails the emission of large amounts of CO₂. In an attempt to mitigate this effect, we try to introduce conventional coarse aggregate from suitable high-strength rocks such as basalt or granite into the formula. However, the average and maximum lifetime of specimens during cyclic loading tests significantly decreases compared to UHPC not containing any coarse aggregate. One of our aims is to investigate the origins of this phenomenon using electron microscopy methods.

Materials and methods

UHPC was fabricated with ordinary Portland cement (CEM I), a water-to-cement ratio (w/c) of 0.24, quartz sand (maximum grain size ca. 500 µm) and quartz powder (20 µm) as aggregate, nanoscale silica fume and a PCE superplasticizer. Additionally, up to 30 vol.-% of basalt gravel (grain size 2 to 8 mm) were used as coarse aggregate. After hardening for at least 56 days, cylindrical samples were subjected to cyclic uniaxial compressive loading between lower and upper limits of 5 and 80%, respectively, of the short-term compressive strength. We performed testing until failure or until a desired point on the lifetime curve was reached. From whole specimens or suitable fragments, samples were extracted via mechanical as well as focused ion beam (FIB) methods and investigated using SEM, TEM and EDS spectral imaging.

Results

The basalt we used is a volcanic rock comprised of phenograins with a size of ca. 100 to 200 µm of mostly pyroxene (augite and jadeite), olivine and titanomagnetite embedded in a feldspar groundmass.

Figure 1: Large-area EDS map of basalt.

Additionally, we observe small phenograins of chloroapatite. Among olivine grains, most are affected by weathering in the shape of numerous cracks often permeating the entire grain. While this phenomenon has been known for decades,[2] we observe the conversion becoming more pronounced with progressing fatigue damage from cyclic loading. Cracks grow in width from 2 to 4 µm in the pristine state to up to 40 µm in post-failure specimens. Using EDS elemental mapping, we find a phase separation of olivine (magnesium iron silicate) into two or more separate compounds. Most cracks exhibit a distinct structure with an iron-rich outer region bordering the unaffected olivine mineral and a zigzag-shaped inner region rich in magnesium and silicon. In the very center of each crack, we observe enrichments of trace elements elements not commonly found in olivine such as aluminum and occasionally calcium, nickel or sulfur.

Figure 2: BSE-SEM image and EDS maps of phase separation in olivine.

Conclusion

We conclude that trace elements in olivine accumulate at interfaces. In the event of mechanical fatigue, inner friction occurs at these interfaces, resulting in a local rise in temperature that enables and promotes a phase separation of the mineral.

References

[1] D. H. Wang et al., Const. Build. Mater. 2015, 96, 368-377.

[2] W. A. Deer, R. A. Howie, J. Zussman, An Introduction to the Rock-Forming Minerals, 3rd edition, Longmans, London, 2013.