Cement pastes, mortars, and concretes may be prepared in two ways: A) dry potting and B) wet potting. Dry potting is used when the specimen has been dried before, when drying shrinkage-related cracking is not of concern, or when a rapid preparation is needed. Wet potting is used to prepare a polished section where the material has not been dried and therefore has not undergone any drying shrinkage. Cracks observed using this preparation may then be ascribed to physical or chemical processes acting upon the concrete, and not due to drying-related shrinkage.
Dry specimen potting involves taking a sawn section or block of material and drying the specimen at low temperature (less than 65 ° C). Removal of water is necessary as it can interfere with polymerization of the epoxy. The specimen is then placed in a container and surrounded by epoxy leaving a top surface exposed to the laboratory air, allowing the epoxy to be drawn into the microstructure by capillary suction. To speed the infiltration, the specimen may be completely immersed in epoxy, and a vacuum drawn to remove remaining air. Upon release of the vacuum, the epoxy is forced into the pore system. The epoxy is cured at low temperature (65 ° C), and then is ready for the cutting and polishing.
Wet specimen potting is a three-step process where the pore solution is replaced with alcohol (200 proof ethanol), the ethanol is replaced with a low-viscosity epoxy, and then the epoxy is cured. The slab and wafering saws are lubricated with propylene glycol or isopropyl alcohol to keep the specimen from drying when cutting. The cut section is then placed in a lidded jar filled with 200 proof ethanol for the alcohol - pore solution replacement stage. The use of a companion specimen allows one to gauge the time necessary for the alcohol - pore solution replacement. This companion specimen is usually a remnant from the specimen after trimming. This specimen is placed in a jar filled with ethanol dyed a deep red or blue using any alcohol-miscible dye. By splitting or sawing the companion specimen after a period of time, the depth of replacement is seen by the depth of dye coloration. When this front is equal to half the section thickness, the pore solution in the section has been replaced by alcohol. The section is then placed in a container with the low-viscosity epoxy. The time necessary for epoxy replacement of the alcohol is at least equal to that required for the first replacement stage. In our laboratory we provide about 1.5 times the pore solution - alcohol replacement time for that of the epoxy - alcohol replacement. Implicit in this method is that the thinner the section, the shorter time is required for each stage. The specimen is placed in a mold with fresh epoxy, which is then cured at low temperatures according to the manufacturer's directions. The specimen is now ready for the cutting and polishing stages.