The molds for the cement paste specimens were made from 1.6 mm (nominally 1/16 in.) thick polytetrafluoroethylene (PTFE) sheets. For each mold, two pieces, each 150 mm x 160 mm, were cut from the PTFE sheet. A mechanical punch was used to create twenty-three 25 mm diameter holes through one of the pieces. The two pieces were then adhered to one another using a room temperature vulcanizing (RTV) adhesive, forming a mold for 23 cement paste disk specimens, each 1.6 mm thick. A mold was assembled for each initial 100 % RH curing period, plus one mold for the specimens remaining indefinitely in the 100 % RH environment.
Cement paste specimens having w/c mass ratios of 0.30, 0.40, and 0.50 were prepared from the Cement and Concrete Reference Laboratory (CCRL) proficiency sample portland cement number 116 [8] and distilled water. Table 1 shows the cement chemical composition. For each experiment at a given w/c ratio, the cement and water were equilibrated at 25 ºC, then combined in a resealable plastic bag and kneaded by hand, with the aid of a vibration table, for a period of five minutes. The mixed paste was spread over each PTFE mold using a small strip of PTFE as a trowel. The cast specimens were placed onto a shelf inside a sealed glass container, the bottom of which contained distilled water to maintain a 100 % RH environment. The glass container was located in an environmental chamber maintained at 25 ºC. To minimize the effect of bleeding, the 0.5 w/c samples were inverted every 15 min for the first hour, every 30 min for the second hour, and every hour for the next four hours.
| Table 1: Chemical composition of the Cement and Concrete Reference Laboratory (CCRL) proficiency sample portland cement number 116. | |
|---|---|
| Oxide | Mass % |
| Calcium | 64.96 |
| Silica | 20.57 |
| Aluminum | 5.40 |
| Sulfur | 2.91 |
| Iron | 1.99 |
| Magnesium | 1.28 |
| Free Lime | 0.99 |
| Potassium | 0.66 |
| Sodium | 0.12 |
For each w/c value, four initial 100 % RH curing periods were investigated: 6 h, 12 h, 3 d, and 7 d. Five PTFE molds were filled with cement paste at the same time, and were then placed into the glass container and exposed to 100 % RH at 25 ºC. When a mold was removed from the glass container, the two adhered PTFE sheets were pried apart to expose both sides of the 23 paste disks remaining in the perforated sheet. The PTFE sheet containing the 23 paste disks was then placed into a glove box having an atmosphere of 25 ºC and 90 % RH air and equipped with a fan for circulation. To prevent carbonation, the glove box was supplied with CO2-free air at a rate of three glove box volumes per day.
The humidity and temperature within the glove box were maintained using electronic feedback controllers with thermocouple temperature and semiconductor humidity sensors. The humidity sensor was standardized using a commercial two-pressure humidity generator based upon the device developed at NIST[9]. The electronic controllers maintained the temperature to within 0.5 ºC, and the relative humidity to within 0.5 %. However, when the 6 h and the 12 h specimens were first placed into the chamber, the relative humidity would increase, and subsequently return to 90 % RH after a few hours.
Periodically, specimens in the glove box were removed from the PTFE mold for measurements of DOH and freezable water content. The degree of hydration was determined by mass loss on ignition between 105 ºC and 950 ºC. The cement paste disks were first ground using a mortar and pestle, and then flushed with a volume of methyl alcohol approximately equal to five times the sample volume. The ground and flushed samples were placed into a 105 ºC vented oven overnight, and were subsequently heated to 950 ºC in a box furnace for at least three hours. The DOH was determined from the ultimate loss on ignition (0.235 gm H2O/gm cement) for a completely hydrated paste using the technique described by Powers [10].
A qualitative measure of freezable water was made using a differential scanning calorimeter (DSC). Samples having masses ranging from 50 mg to 90 mg were taken from the cast paste disks. A DSC scan consisted of three steps: equilibrate the sample at a temperature of -5 ºC, equilibrate the sample at a temperature of +5 ºC, and scan from +5 ºC to -60 ºC at a rate of -0.5 ºC per minute. (Equilibrating at -5 ºC ensured repeatable results.) This scan rate was chosen since it is reasonably slow [11], and the rate of cooling does not change the estimated volume of supercooled ice formation [12]. Also, this rate had the practical advantage of allowing for multiple measurements in a single day.
The DSC measurements were used to monitor the presence of water, and not as a means of performing cryoporometry. Quantitative cryoporometry relies on the melting scan [13], and the results can be used to estimate the pore size distribution using the Gibbs-Thomson equation [14]. Cryoporometry studies of cement paste have revealed a unimodal pore size distribution (a single wide "hump") over the entire freezing temperature range [15]. By comparison, during freezing the ice front penetrates the largest pores first, then the progressively smaller pores as the temperature decreases, analogous to mercury intrusion porosimetry. Therefore, any localized peaks that may appear during a DSC freezing scan must be due to reservoirs composed of relatively large pores that are completely surrounded by passages composed of smaller pores.