Next: Example Results Up: Main Previous: Cellular Automaton Simulations.

5. Relevant Experimental Techniques

Heat of Hydration

The heat of hydration of a cement can be assessed in a variety of ways: adiabatic or semi-adiabatic calorimetry, isothermal calorimetry, or using heat of solution techniques as indicated in the bibliographic database on this subject. At NIST, heat of hydration is generally determined using isothermal calorimetry in a multi-chambered microcalorimeter constructed at NIST [31]. A known mass of cement, along with several small stainless steel balls to facilitate mixing, are placed in a sealed calorimetric cell which is then equilibrated in the main calorimeter chamber. After a steady heat flux signal is obtained, the cell is removed, the appropriate mass of water (also thermally equilibrated to the calorimeter temperature) is quickly added using a syringe, and hand mixing performed (by shaking the cell) before restoring the cell to the calorimeter chamber. The voltage signals produced (proportional to heat flux) by the calorimeter cells are digitized using a PC-based high resolution A/D data acquisition system. Thus, during the initial hydration, data can be taken at 30 second intervals. Once the reactions slow down, data is typically acquired every 5 to 10 minutes over a period of about 7 days. At longer times, the signal of the calorimeter is very close to its background level, so that detection of the slow but ongoing hydration by heat release becomes unreliable.

In analyzing the heat release data, the initial exothermic "mixing" peak is ignored due to the necessity of removing the sample cell from the calorimeter chamber to assure adequate mixing. This could result in a difference in the cumulative heat released over a period of 7 days on the order of 10 kJ/kg or about 4%, as estimated from samples mixed in situ in the calorimeter. Due to mixing difficulties at low w/c ratios, calorimeter data is generally only acquired for specimens with w/c > 0.4.

Chemical Shrinkage

Chemical shrinkage, the volume reduction associated with the reaction between cement and water in hydrating cement paste, was first measured by Powers in 1935 [32]. Conventionally, this property can be measured using the technique described by Geiker [33], and more recently employed by Tazawa et al. [34]. Typically, to assess chemical shrinkage, a known mass of cement paste (on the order of 10 g) is placed in the bottom of a small glass jar (diameter of 2.5 cm and height of about 6 cm). After covering the cement paste with about 1 mL of water, the remainder of the jar is filled with an hydraulic oil. The jar is then sealed with a rubber stopper encasing a pipette graduated in 0.01 mL increments. The jar is then placed in a constant temperature water bath and the oil level monitored to the nearest 0.0025 mL over time. A control sample using only cement powder and oil (no water) is used to correct for minor room temperature fluctuations. By normalizing the change in volume by the mass of cement in the sample, the chemical shrinkage per gram of initial cement (mL/g cement) can be determined. Based on the estimated uncertainty of the mass (0.02 g) and volume (0.00072 mL) measurements, the maximum expanded uncertainty in the calculated chemical shrinkage is estimated to be 0.001 mL/g cement, assuming a coverage factor of 2 [35]. Typically, two replicates are run for each w/c ratio and cement of interest.

Next: Example Results Up: Main Previous: Cellular Automaton Simulations.