CHAPTER 2. OVERVIEW OF MATERIALS AND EXPERIMENTAL TECHNIQUES
2.2. Experimental Techniques for Characterization of Microstructures
2.2.6. Solvent exchange method to stop the hydration
To characterize microstructures of cementitious materials, sample preparation procedures and storage is very important. The samples can be easily damaged during the procedures if inappropriate sample preparation and storage methods are taken. In addition, the reaction of cementitious materials is basically hydration, which means that if the water, contained in the cementitious materials, is not removed, further hydration steadily occurs. Thus, the water should be carefully removed with proper methods before characterizing cementitious materials.
Many different water removal techniques have been proposed for cementitious materials [76]:
oven-drying, vacuum-drying, D-drying, and freeze-drying. Oven-drying is one of the most convenient water removal methods, in which the specimens are just located in oven at below 105ιC until reaching constant mass. Although oven-drying is a very effective and easy method to eliminate evaporable water, it can cause micro-cracks in a specimen due to thermal expansion and contraction during the drying-weighting cycles, resulting in modification of microstructures of cementitious materials. In addition, some hydration products can be decomposed at high temperature at around 65ιC. However, the low temperature was also inappropriate for oven-drying because relatively low temperature significantly expands drying time.
Vacuum-drying is the method in which the specimen dries up under vacuum conditions.
During the procedure, capillary water turns into vapor and can be removed from the matrix of hardened cementitious materials. However, vacuum drying is only applicable for mature cementitious samples and can also induce micro-cracks in a hardened matrix.
D-drying is an abbreviation of dry-ice drying which is one of the most widely used methods
and is one kind of D-drying with rigorous conditions, which means that the sample is dried under vacuum conditions at -79ιC induced by a mixture of solid CO2 and alcohol. Under these conditions, all of the physically adsorbed water can be removed very slowly, taking about 14 days. D-drying is one of the most widely used methods because it is considered that the hardened matrix experiences little damage. However, Thomas et al. [85] reported that D-drying can remove some water contained in the gel and interlayer water.
Freeze-drying can be divided into direct freeze-drying and indirect freeze-drying. In direct freeze-drying, the specimens are directly immersed in liquid nitrogen or in a mixture of solid CO2 and methanol and then, thy dried in a vacuum desiccator for 24 hours. In indirect freeze-drying, the specimens are located in a glass vial submerged in liquid nitrogen for a few minutes and then they are also dried in a vacuum desiccator. In this method, the specimens do not directly contact the liquid nitrogen and need further treatments [86]. In general, freeze-drying can cause less damage to a hardened matrix than other drying methods. However, the freeze-drying methods have a limitation in which the specimens may not be completely dried if the microstructures of the hardened cement pastes are significantly dense.
The solvent exchange method is considered an alternative drying technique for cementitious materials because the method has been known to be mild to the microstructures of cementitious materials [76] if proper solvents are used. Some organic solvents such as acetone, ethanol, isopropyl alcohol (IPA), methanol, etc., have been used, frequently [76, 87]. In order to be used for hydration stop, the solvents should have small molecule size, low boiling temperature, miscible property with water, fast diffusion property in water, low surface tension and low vapor pressure [87].
The specimen size, solution-to-sample ratio, and solvent removal method are crucial factors affecting the microstructure of cementitious materials. Large size samples, generally, need very long soaking time. For example, Gran and Hansen [88] reported that about 3 weeks were needed for 95%
exchange of pore water by ethanol for a cylindrical sample with dimension 5.5mm ൈ 10mm.
Three weeks is too long a period for cementitious materials because the microstructure of cementitious materials can be highly modified during this time, especially at early ages.
In general, if the solution-to-sample ratio is low, the solvent should be renewed frequently and if the solution-to-sample ratio is high, the solvent renewal is not needed. Zhang and Scherer [87]
proposed that, for no renewal, the volume of the solvent should be approximately 300 times larger than that of the samples, assuming that the porosity of the samples is 0.3.
After hydration stop with a solvent exchange method, the solvent removal has been conducted using direct heating, vacuum drying, heating under vacuum, and so on. Beaudoin et al. [89], Zhang and Scherer [87] concluded that the use of IPA followed by vacuum drying for 1 day was the best known method to preserve the microstructures of cementitious materials with less damages of
specimens.
Throughout this research, IPA was used for the hydration stop of all hardened samples, followed by vacuum drying for 24 hours under a pressure of ~ 60 cmHg. The sample dimensions were chosen as 2 mm thickness sector and 5ൈ5ൈ5 mm cube for SEM and MIP measurements, respectively.