III. Microstructural Modeling

All microstructural modeling was conducted using version 3.0 of the CEMHYD3D software ( ftp://ftp.nist.gov/pub/bfrl/bentz/CEMHYD3D/version30).15 Starting microstructures, with a spatial resolution of 1 µm, were created to match the four different w/c values (0.25, 0.35, 0.435, and 0.45) used in the experiments. For each starting microstructure, isothermal hydration was conducted for the various curing conditions used in the experiments, generally consisting of two different hydration temperatures (20 ºC or 40 ºC) and two saturations (saturated or sealed curing). The main computer model outputs of interest for this study include the achieved degree of hydration and the percolation (fraction) of the capillary porosity as a function of time. For all results presented in this paper, a factor of 0.00027 h/(cycle*cycle) was selected to convert between model hydration cycles and real (experimental) time and an (Arrhenius-based) activation energy of 45.5 kJ/mol was used for the cement hydration reactions.⁵ This value of the activation energy implies that the cement hydration reactions for cement 152 are about 3.3 times faster at 40 ºC than they are at 20 ºC.

To further validate this choice of 0.00027 as the kinetic parameter for modeling the hydration of cement 152, two additional starting microstructures were created to simulate the results of the American Society for Testing and Materials (ASTM) C186 Heat of Hydration and the ASTM C109 Mortar Cube Compressive Strength tests,¹⁶ respectively, conducted in the CCRL proficiency sample testing program.¹⁰ In the first case, a starting microstructure with w/c = 0.4 was created and hydrated under sealed curing conditions at 23 ºC, in accordance with the ASTM C186 test method. ¹⁶ Using the same kinetic parameter given above, the CEMHYD3D model values for heat of hydration after 7 d and 28 d of hydration were determined for comparison to the experimentally determined values, as summarized in Table I. Excellent agreement was observed between the CEMHYD3D predictions and the measured values of heat of hydration for this particular cement, with the difference between model and experimental values being within 33 % of the standard deviation measured in the CCRL proficiency sample program.

In the second case, a w/c = 0.485 starting microstructure was created and hydrated under saturated conditions at 23 ºC, in accordance with the ASTM C109 test method.¹⁶ Compressive strengths were modeled using Power’s gel-space ratio,⁵ with an exponent of 2.6 and a strength prefactor of 99.3 MPa, to obtain agreement with the measured 3 d compressive strength value. The results are plotted in Figure 2, where it can be seen that the CEMHYD3D strength predictions for 7 d and 28 d (based on the 3 d measured value) lie within two standard deviations of the values measured in the CCRL proficiency sample program.