Influence of Humidity on the Nanostructure of the Cement Paste

Movement Inside the Concrete

Fig. 1 Water vapor sorption isotherm measured at a cement paste sample at 25° C using a TA Instruments VTI SA device.

Fig. 2 Automatic, dynamic water vapor and solvent sorption device DVS TA Instruments VTI SA with increased carrier gas flow to reach the mass equilibrium more quickly and reliably during changes in
humidity.

Fig. 3 Swelling and shrinkage of a cement paste sample depending on relative humidity. Note the hysteresis (as in the case of the sorption isotherm shown in Fig. 1).

Fig. 4 Pascal porosimeter. The vertical feed (right) during pressure-controlled degassing above the vapor saturation pressure enables porosimetric measurements on pre-wetted materials.

Fig. 5 Comparison of specific surfaces determined by small-angle neutron scattering measurements with the pore sizes measured on a pre-wetted cement paste sample in the mercury porosimetry analysis (CEM I, w/c=0.4, 28 days) (drying – desorption).

Fig. 6 Comparison of the coefficient of carbonation c with the
vapor diffusion resistance µ on a pre-wetted cement paste sample (CEM I, w/c=0.4, 28 days) (wetting – adsorption).

 Despite being the most popular building material worldwide, concrete is still not fully understood with respect to its physico-chemical and mechanical characteristics. Such an understanding becomes all the more important as both key safety parameters (such as tensile strength and resistance to earthquakes and strong winds) and the requirements in terms of durability and low environmental impact are continuously gaining in significance. Repair costs alone are so high that they are of relevance to the entire economy.

In the case of concrete, the length range of interest to researchers reaches from several kilometers to less than a nanometer in the microstructure. Concrete structures are often exposed to extreme environmental conditions. The microstructure of the cement paste (i.e. the binder contained in the concrete) determines almost all properties of the material. This cement paste forms a nanoporous system that is mainly composed of calcium silicate hydrates [1, 2] with nanopores with a radius of approx. 2 nm that are also contained in ultra-high performance concrete [3]. In standard concretes,...