An interface was developed between COMSOL Multiphysics finite element analysis software and (geo)chemical modeling platform, GEMS, for the reactive-transport modeling of (geo)chemical processes in variably saturated porous media. The two standalone software packages are managed from the interface that uses a non-iterative operator splitting technique to couple the transport (COMSOL) and reaction (GEMS) processes. The interface allows modeling media with complex chemistry (e.g. cement) using GEMS thermodynamic database formats. Benchmark comparisons show that the developed interface can be used to predict a variety of reactive-transport processes accurately. The full functionality of the interface was demonstrated to model transport processes, governed by extended Nernst-Plank equation, in Class H Portland cement samples in high pressure and temperature autoclaves simulating systems that are used to store captured carbon dioxide (CO 2) in geological reservoirs.

Premature damage has been observed at the joints in numerous concrete pavements where calcium chloride and magnesium chloride deicing salts have been used. This damage results from a reaction between the deicing salt and the calcium hydroxide (CH) in the hydrated cement paste. This reaction leads to the formation of an expansive product known as calcium oxychloride (CAOXY). The use of supplementary cementitious materials as a replacement for cement has been proposed to reduce the CH that is available in the mixture to react with the deicing salts. Reducing the CH can reduce the amount of CAOXY that forms. In this study, mixtures representative of paving concrete were made with cements and fly ashes from across the country. CH amounts were determined by using thermogravimetric analysis, and CAOXY amounts were determined by using low-temperature differential scanning calorimetry. Various replacement levels of fly ash were used to demonstrate that the main parameter that influences the amounts of CH and CAOXY that form is the replacement level of fly ash. This paper proposes that a prescriptive specification requiring 35% cement replacement by volume with fly ash would reduce the damage caused by CAOXY formation and further proposes a performance specification to limit the CAOXY formation to below 15 g/100 g paste.

A numerical investigation was conducted to test the hypothesis that the composition of the pore solution in mill scale crevices on carbon steel rebar surfaces in concrete might be different from that of the bulk concrete pore solution, and this difference may create the necessary conditions for the premature breakdown of the passive film. The modeling was performed using a non-linear transient finite element algorithm, which involved the solution of coupled extended Nernst-Planck and Poisson's equations in a domain that represented typical mill scale crevices on carbon steel rebar. The numerical simulations showed that the chemistry of the pore solution, in particular pH and Cl − /OH − , within mill scale crevices provided more favorable conditions for depassivation than the bulk concrete pore solution. Local acidification and increase in Cl − /OH − within crevices were observed in all simulations, albeit to different degrees. Crevice geometry has been found to be the most important parameter affecting local acidification and the increase in Cl − /OH − . Simulations supported the hypothesis that the chemical composition of the pore solution within the crevices differs from that of the bulk solution through a process similar to the suggested mechanism of typical crevice corrosion. Within the alkaline environment of concrete, carbon steel reinforcement is protected against corrosion by a passive film, but chlorides from deicing chemicals or marine salts might cause the loss of this film when their concentrations exceed threshold values. [1][2][3][4][5][6][7] Whether they are represented in terms of total or free chloride concentrations, or chloride-to-hydroxide concentration ratio, Cl − /OH − , reported values of chloride thresholds for carbon steel reinforcement in concrete cover a wide range and have a large degree of variability. 9,10The surface conditions of rebar, in particular, the presence of mill scale on the steel surface, is one of the potential causes of the reported uncertainty and variability in chloride thresholds. 8,[11][12][13][14][15] It has been widely reported that higher chloride thresholds were observed for rebars with modified surfaces (i.e., without mill scale) through sandblasting, polishing or pickling than those in as-received conditions with mill scale. 12,13,15,16 In some cases, it was observed that corrosion did not initiate in highly-polished rebar even after the specimens were exposed to chloride concentrations in excess of that is typically found in sea water.11 In addition, modifying the rebar surface leads to reduced variability and fluctuations in electrochemical measurements, which can be explained by the fact that modified surfaces are nearly uniform, whereas as-received surfaces are locally much more diverse and complex due to the presence of mill scale. 17 These results suggest that the variability associated with the reported chloride thresholds may be partially attributed to the variability in mill scale properties resulting from the variability in manufacturing.In a recent...

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