John T. Brown

Imaging of ASR Gel by Soft X-ray Microscopy

Abstract The soft x-ray transmission microscope XM-1 was used to examine alkali-silicate reaction (ASR) gel morphology in an experimental investigation of the alkali-aggregate reaction (AAR). The XM-1 microscope is operated by the Center for X-ray Optics on beamline 6.1 of the Advanced Light Source, a third generation synchrotron radiation facility operated by the Ernest O. Lawrence Berkeley National Laboratory. The instrument is unique as samples can be observed wet, with high resolution (43 nm), over time, as chemical reactions proceed. Soft x-ray microscopy was used to examine the in situ reaction of ground ASR gel, obtained from a large dam, and solutions of sodium hydroxide, calcium hydroxide, and 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 in 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 alkali-aggregate reaction product. The C-S-H-like structure may also contribute strength. These hypotheses are currently under investigation.

X-Ray Microscopy in Berkeley

A new high resolution soft X-ray microscope (XM-1) has been used in a variety of applications. It is a conventional transmission microscope with a zone plate condenser and objective. A mutual indexing system incorporates state-of-the-art visible light microscopy and precise positioning of samples. XM-1 has a spatial resolution of 43 nm, as measured with a knife edge object, using the 10% to 90% intensities. It is used in collaboration with other groups to investigate variety of mostly biological samples. In our most extensive study, the life cycle of malaria parasites (plasmodium falciparum) in intact human red blood cel1s was mapped. Abnormalities in the parasites development with protease inhibitor treatments and membrane protein deficiencies have been investigated and were linked to parasite mortality. New structures in green alga (Chlamydomonas), uniquely visible with soft X-rays, have been confirmed and analyzed in unfixed samples. In addition XM-1 is used to map the morphological variation of genetically altered sperm cells. We also give a brief introduction of the history of X-ray microscopy

Investigation of alkali-silica reaction by transmission soft x-ray microscopy

The soft x-ray transmission microscope XM-1 was used to examine the alkali-silica reaction, a deleterious reaction that produces expansion in some concrete structures. Reactions of silica gel in alkaline solutions (pH⩾12.4) in the presence of pore solution cations (Na+, Ca++) and a chemical additive (CaCl2) were observed, and the morphology of the reaction products were examined to investigate the mechanisms of expansion and expansion control. From the investigation of the effect of pore solution cations, it was found that reactive silica combined with alkalis present in the pore solution to produce a reaction gel capable of swelling, while the reaction of silica in the presence of calcium ions alone resulted in the formation of a nonswelling product—calcium silicate hydrate. Reaction of the silica gel in the presence of CaCl2 also resulted in the formation of calcium silicate hydrate.