4.5 Preliminary Incorporation of Reactions for Slag in CEMHYD3D

In version 3.0 of CEMHYD3D, hydration reactions for slag have been preliminary incorporated into the hydration programs. At this point, the simplest and most direct approach has been taken by assuming that the slag reacts with calcium hydroxide (pozzolanic-type reaction) to produce a single (mixed phase) hydration product. This product would be a mixture of a C-S-H-type gel with a lower Ca/Si ratio than conventional C-S-H, AFm, hydrotalcite, etc. [19]. The user must supply estimates of the specific gravity, molar volume, and Ca/Si and H₂O/Si molar ratios of this slag hydration product, in a datafile called slagchar.dat, as will be outlined in the Execution of the Three-Dimensional Cement Hydration Model section to follow. An estimate of the Ca/Si molar ratio of the slag hydration product may be obtained using SEM/X-ray analysis of well hydrated specimens [20, 21]. In the CEMHYD3D v3.0 model, for slags where the Mg/Al ratio is significantly less than the value of 2 that is commonly encountered in a hydrotalcite-type product, the extra aluminate in the slag participates in hydration reactions similar to those of the C₃A phase in portland cement (e.g., formation of hydrogarnet, ettringite, and AFm).

Because the slag hydration product is typically observed to form in close vicinity to the initial slag grains [22] (perhaps due to a low mobility of the Mg++ or other ions), in version 3.0 of CEMHYD3D, the slag grains are basically hydrated in place, with slag voxels that are in contact with water-filled capillary porosity being eligible for conversion to the slag hydration product. The appropriate volume stoichiometry is maintained for this reaction by creating extra voxels of the slag hydration product in the local neighborhood of the reacting voxel, as needed. In addition, unlike the hydration of portland cement where the chemical shrinkage and self-desiccation occurs globally with the largest remaining water-filled capillary pores in the three-dimensional microstructure emptying first [1], for slag hydration, chemical shrinkage and self-desiccation occurs locally in an attempt to mimic the real world observation of (empty) porous regions forming within the hydrating slag grain. Compositions of two different slags that have been used to validate this portion of the CEMHYD3D codes, along with the assumed stoichiometry of their slag hydration products, are summarized in Table 2. For modeling the slag reactions under variable temperature conditions, it is necessary for the user to input an activation energy for the slag reactions as well. While for a Type I ordinary portland cement the activation energy for the hydration reactions is nominally on the order of 40 kJ/mol [23], for slag, a default value on the order of 80 kJ/mol, based on the work of DeSchutter [24], is recommended when no specific data on slag reactivity vs. temperature is available.

Table 2: Compositions of slags (in mass percentages) used in preliminary modeling of slag reactions in CEMHYD3D v 3.0.