Hydration Model Description

Detailed descriptions of the NIST digital-image-based cement hydration model are available [3, 4]. In the model, two-dimensional (2-D) area or three-dimensional (3-D) volume is represented by a set of pixels each identified as a particular phase of concrete. For this exercise, relevant phases are anhydrous cement (assumed to be pure tricalcium silicate- C₃S), calcium hydroxide (CH), calcium silicate hydrate (C-S-H), porosity, aggregate, and mineral admixtures. Thus a cement particle is represented as a collection of contiguous pixels assigned to be C₃ S. In this manner, real cement particle shapes may be utilized, as well as circles and spheres, in generating simulated cement microstructures.

Hydration is simulated by operating on all the pixels present in the cement paste volume or area. The two reactions considered are the hydration of C₃S to form CH and C-S-H and the pozzolanic reaction between CH and silica (found in silica fume or fly ash) to form secondary or pozzolanic C-S-H. The assumed reaction stoichiometries are as follows:

C3 S  + 5.3H   C1.7SH 4.0 +  1.3CH	(1)
S + 1.7CH +  2.3H   C1.7SH 4.0	(2)

In terms of volumes or areas, each unit of dissolved C ₃S produces 1.7 units of C-S-H and 0.61 units of CH. Regarding the pozzolanic reaction, each unit volume of silica is capable of reacting with 2.08 units of CH to produce 4.6 units of pozzolanic C-S-H. The value 2.08, corresponding to the number of volume units of CH which may be consumed by one volume unit of silica fume, is called the pozzolanic reactivity factor throughout this exercise.

Hydration is executed in discrete cycles consisting of dissolution, diffusion, and reaction phases. During dissolution, all C ₃S pixels in contact with water-filled porosity are given a chance to dissolve and produce diffusing CH and C-S-H species. The dissolution probability is based on the amount of C₃S surface in contact with water. During diffusion, the C-S-H and CH diffusing species execute random walks within the available pore space until reaction occurs. C-S-H forms on the surfaces of the original cement particles or on previously formed solid C-S-H. CH forms crystals by a nucleation and growth mechanism within the pore space. Additionally, if amorphous silica is present, the CH diffusing species react at silica surfaces to form pozzolanic C-S-H. When all diffusing species generated from one dissolution have reacted, a hydration cycle is complete and a new dissolution is begun. By monitoring how much cement remains after any number of hydration cycles, the degree of hydration, , of the system can be determined. As in real systems, the hydration is ultimately self-limiting as the surfaces of the remaining cement become totally surrounded by hydration products, preventing further dissolution. However, typically 80-90% hydration can be achieved before this situation occurs.