IMAGING OF ASR GEL BY SOFT-X-RAY MICROSCOPY

The soft x-ray transmission microscope XM-1 was used to examine alkali silicate reaction (ASR) gel morphology in an experimental investigati on of the alkali-aggregate reaction (AAR). The XM-1 microscope is oper ated by the Center for X-ray Optics on beamline 6.1 of the Advanced Li ght Source, a third generation synchrotron radiation facility operated by the Ernest O. Lawrence Berkeley National Laboratory. The instrumen t is unique as samples can be observed wet, with high resolution (43 n m), over time, as chemical reactions proceed. Soft x-ray microscopy wa s used to examine the in situ reaction of ground ASR gel, obtained fro m a large dam, and solutions of sodium hydroxide, calcium hydroxide, a nd combined sodium and calcium hydroxide. From this investigation, it appears that ASR gel combines with alkalis present in pore solution to produce a reaction gel capable of swelling, while the reaction of the ASR gel in the presence of calcium hydroxide and no alkalis results i n the formation of a structure resembling C-S-H. It is theorized that the formation of C-S-H or a related compound will decrease the degree of swelling that would otherwise result from the formation of an alkal i-aggregate reaction product. The C-S-H-like structure may also contri bute strength. These hypotheses are currently under investigation. (C) 1998 Elsevier Science Ltd.