Examining out-of-plane expansion of aggregate minerals in ion-irradiated concrete.

1 Introduction

Concrete consists primarily of the mineral quartz as coarse and fine aggregates to a weight of 50-60 %. After quartz, feldspar is generally contained in a weight quantity of 15-20 %. Quartz in its pristine state is the least soluble of the silicates, thereby providing concrete the highest possible chemical durability. Biological shielding concrete that surrounds nuclear reactor pressure vessels are exposed to neutron radiation over decades during the course of nuclear power plant (NPP) operation. Minerals that are subjected to radiation, whether it be neutrons, electrons, or ions, of sufficient fluence and energy, accumulate defects in their crystal lattices. Once the threshold concentration of defects has been reached, structural relaxation occurs, which is a volume expansive amorphization process. Of all the minerals examined for neutron radiation-induced volume expansion, quartz exhibits the highest, followed by feldspar with volume expansion maxima respectively being 17.8 and 7.7 % [1]. Irradiated quartz accumulates E’-center defects [2], which are essentially unpaired sp³ dangling bonds and chemically reactive in aqueous solution, particularly at the higher pH values typically prevailing in pore water in concrete. It is the objective of this research to investigate the increased dissolution rates of irradiated silicate minerals, particularly in the context of the alkali-silica reaction (ASR), which is the most significant degradation reaction known to occur in concrete.

2 Experimental

Polished sections of concrete partially masked with aluminium foil were subjected to Si-ion irradiation with a fluence of 5·1014 ions/cm2 at 300 keV under vacuum without cooling. Changes in the vertical profiles of the irradiated samples were examined by Vertical Scanning Interferometry (VSI) and Confocal Microscopy.

3 Results

The minerals making up the aggregate were identified by µ Raman spectroscopy to be α-quartz and potassium feldspar (microcline: KAlSi₃O₈), the latter always intergrown with quartz. Quartz (grain A) exhibited an out-of-plane expansion of ~ 80 nm. No observable difference in average height between radiated and non-radiated areas on the aggregate containing feldspar (grain B) could be ascertained. This is mainly attributable to the surface roughness of feldspar (> 1 µm), which is essentially out of range for interferometric techniques. Furthermore, the penetration depths estimated in the Kinchin-Pease calculation by SRIM [3] for Si-ion irradiation at 300 keV are 429 and 604 nm, respectively for α-quartz and microcline. As the damage to the structure occurs initially at the point where the ion stops, structural relaxation in the feldspar begins much further away from the surface than it does for quartz. In our case, it is likely that the expansion in the feldspar has not detectably reached the surface.

Literature:

[1] Le Pape, Y., Alsaid, F., Alain, B. and Giorla, B.: J. Adv. Conc. Technol. (2018) 16, 191-209.
[2] Douillard, L. and Duraud, J. P.: Nucl. Instrum. Methods Phys. Res. (1996) B16, 191-209.
[3] Ziegler, J. F., Ziegler, M. D. and Biersack, J. P.: Nucl. Instrum. Methods Phys. Res. (2010) B268, 1818-1823. (http://www.SRIM.org)