Interaction with environment
Thermodynamic modeling of the interaction with the environment
Sulfates present in groundwater can pose a serious threat to the durability of concrete structure. The interaction of cement with solutions containing sulfate leads to the formation of a reaction front within the porous material and to the precipitation of gypsum and ettringite near the surface. Further within the sample the depletion of portlandite is observed. Significant damage due to sulfate interaction can result in the structural breakdown of the concrete structures e.g. tunnel linings within a few years.
To test the “sulfate resistance” of a specific cement, mortar and concrete samples are generally submersed for a few weeks in concentrated Na2SO4-solutions and the mass and length gain is monitored as a function of time. High sulfate concentrations, as used in such tests, lead to the precipitation of gypsum, while lower sulfate concentrations, as present under field conditions, lead to no or only very little gypsum precipitation. The mortar samples show strongly differing expansion and mass gain depending on the reaction solution. Further factors influencing the expansion are the absence or presence of CO2 in the interacting solution, the amount of solution, the frequency of the exchange of the solutions, the sample geometry and the temperature. Considering all these factors it is not surprising that the tests carried out in different laboratories show large variations. In addition, the correlation of the test results with expansion and deterioration under field conditions is poorly understood.
Blending of Portland cement with slag (> 70%) or silica fume can increase the resistance against sulfate attack. An important factor is the finer microstructure and the reduced porosity often encountered in blended systems. The presence of limestone and or gypsum seems to further increase the resistance against sulfate attack.
In a number of different we investigate and still investigate the influence of the composition of the cements used and the influence of the interacting solutions (different cations and anions) on the composition of the solid and liquid phase and the respective expansion and deterioration of the mortar and concrete samples.
The combination of experimental techniques such as XRD, SEM/EDS with thermodynamic modelling enable us to understand the processes governing the interaction with the environemt on a chemical level.
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