Galvanic Corrosion Study between Tensile-Stressed and Non-Stressed Carbon Steels in Simulated Concrete Pore Solution

The present study investigated the galvanic effect between tensile-stressed and non-stressed carbon steels, in addition to the influence of the tensile stress on the passivation and corrosion behavior of steel in a simulated concrete pore solution. Three different levels of tensile stress, ranging from elastic to plastic stress on the surface, were applied by adjusting the displacement of C-shape carbon steel rings. Different electrochemical measurements including the open circuit potential (OCP), the electrochemical impedance spectroscopy (EIS), the zero-resistance ammetry (ZRA), and the cyclic polarization were performed. Based on the results of EIS, the tensile stress degraded the resistance of the oxide film in moderate frequencies while enhancing the charge transfer resistance in low frequencies during passivation. As corrosion propagated, the stressed steel yielded a similar charge transfer resistance to or an even lower charge transfer resistance than the non-stressed steel, especially in the case of plastic tensile stress. The galvanic effect between the tensile-stressed and non-stressed steels increased the chloride threshold value of the tensile-stressed steel, although the susceptibility to pitting corrosion was exhibited after being corroded.

Experimental Study of Slag Changes during the Very Early Stages of Its Alkaline Activation

The very early stages of alkaline activation of slag control its rheology and setting, but also affect its hydration, which occurs later. Simultaneously, these parameters are dictated by the nature and dose of the alkaline activator. Therefore, we investigated and compared the changes in slag particles (SEM, BET, laser diffraction), as well as in the pore solution composition (ICP–OES), pH, and conductivity, of alkali-activated slag (AAS) pastes containing the three most common sodium activators (waterglass, hydroxide, and carbonate) and water during the first 24 h of its activation. To ensure the best possible comparability of the pastes, a fairly nontraditional mixture design was adopted, based on the same concentration of Na+ (4 mol/dm3) and the same volume fraction of slag in the paste (0.50). The results were correlated with the pastes’ hydration kinetics (isothermal calorimetry), structural build-up (oscillatory rheology), and setting times (Vicat). Great differences were observed in most of these properties, in the formation of hydration products, and in the composition of the pore solution for each activator. The results emphasize the role of the anionic groups in the activators and of the pH, which help predict the sample’s behavior based on its calorimetric curve, and offer data for further comparisons and for the modelling of AAS hydration for specific activators.

Pozzolanic Efficiency of Calcined Clays in Blended Cements with Focus on the Early Hydration

The fundamental knowledge about the reaction mechanism of calcined clays in cement and the mutual interaction is important for their assessment as supplementary cementitious material and the resulting concrete properties. In this study, the hydration of two cements differing in alkali content and with the addition of a highly reactive, aluminum-rich metakaolin and one calcined common clay with low kaolinite content was investigated during the first 48 hours. For this purpose, four established methods that describe the early hydration were used: isothermal calorimetry, thermogravimetry, in-situ X-ray diffraction and chemical analysis of pore solution. This so far unique combination of methods enabled the understanding of the complex binder (cement-calcined clay) hydration behavior. The results showed considerable differences depending on type of calcined clay, its chemical-mineralogical composition, fineness and especially towards its reaction mechanism with aluminate clinker phases controlled by the composition of pore solution. The impact of calcined clay on the early clinker hydration exceeds significantly physical effects only.

Effect of Pore water expression on solid composition of cement paste

Pore solution expression is the most commonly used method to obtain aqueous phase in cementitious material. However, the high pressure applied on the sample may affect solid phase composition. This paper presents an experimental study on the chemical and mineral composition of cement paste before and after the expression. Results indicate that a small part of the alkali contained in samples can be excluded during the pore solution expression, mainly depending on alkali concentration in the pore solution. Due to the expulsion of interlayer water in C-S-H under high pressure, the pore solution expression reduces bound water content measured by TGA. The portlandite content determined by TGA is not affected by pore solution expression, but it leads to an overestimation of portlandite in QXRD, because of the enhanced preferred orientation of the (001) plane under the applied high pressure. In addition, the size of portlandite crystal decreases to some extent due to the creep caused by pressure. The content of hemicarbonate phase is found to decrease slightly after pore solution expression, which may be aroused by the increased solubility under high pressure.