Josef Tritthart

Chloride binding in cement II. The influence of the hydroxide concentration in the pore solution of hardened cement paste on chloride binding

Abstract The influence of various factors on the binding of chloride in cement and the chloride concentration of the pore solution was investigated. It was shown that the pore solutions of samples prepared under addition of chloride taken from the same cement and with identical content of total chloride exhibited different chloride concentrations only, if their hydroxide concentrations diverged. The fact that the residual chloride concentration depends on the composition of the pore solution in general, and on the hydroxide concentration in particular, was confirmed by tests with subsequent chloride addition. Chloride-free cement pastes, when stored in chloride solutions of the same concentration, had bound different quantities of chloride, if the hydroxide concentration of the storing solution (pH-value) was different.

Chloride binding in cement I. Investigations to determine the composition of porewater in hardened cement

The investigation of various methods to determine the composition of pore water in hardened cement paste showed that only by the pressing-out method correct results can be achieved. Up to the highest pressure exerted of approx. 3500 bar, the expression force hardly had any influence on the chloride and hydroxide concentration of the pore solution. Crushing of samples prior to expression should be avoided as even brief contacts of the pulverized samples with ambient air result in a decrease in hydroxide and an increase in the chloride concentration of the pore solution due to CO2 uptake.

Transport of a surface-applied corrosion inhibitor in cement paste and concrete

Surface-applied corrosion inhibitors are a kind of repair material and usually contain an aminoalcohol and a component forming a salt with the aminoalcohol. According to the manufacturers, this type of inhibitor penetrates very rapidly into concrete; however, the transport mechanisms have not been sufficiently investigated so far. The major part of the study therefore focused on the transport of the ingredients of an inhibitor in cement paste and concrete, which contained an aminoalcohol and a phosphorous compound. It has been shown that the latter forms an insoluble calcium salt in the environment of cement and precipitates quantitatively. It is thus unable to penetrate from the outside into the alkaline concrete zone and cannot develop its inhibiting effect there. The aminoalcohol, on the other hand, is not bound by cement, but remains largely dissolved in the pore liquid, thus providing optimal conditions for high mobility. The analysis of the transport mechanisms involved has revealed that diffusion in the dissolved state is by far the most efficient transport mechanism. While basically the transport of the aminoalcohol via the gaseous phase is possible, it does play an inferior role only. Surprisingly, the substance had hardly been absorbed by concrete by capillary suction, but at first remained close to the concrete surface.

Electrochemical removal of chloride from hardened cement paste

Abstract The rehabilitation method “electrochemical chloride removal” can only be applied successfully if the bound chlorides can be released to the pore water and transported out of the concrete relatively quickly. Previous investigations have shown that the free chloride ions in the pore solution relatively rapidly can diffuse out of the concrete whereas the bound chlorides are only released slowly. It is therefore a matter of interest to what extend an applied direct current will enhance the release of bound chloride. This question was investigated in a collaborative project between institutes from Austria, Denmark and Sweden. It was shown that under the influence of direct current large quantities of bound chloride are dissolved and transported out of specimens rapidly. This effect is believed to be due to an easy desorption of adsorbed chloride in the electrical field.

Effect of adsorbents on the leachability of cement bonded electroplating wastes

Abstract This study is aimed at the determination of pore solution composition of hardened cement pastes prepared with the addition of chromium-, cadmium- and nickel-salts. Due to the high pH-value of the pore water of cement paste, cadmium and nickel were transformed into insoluble hydroxides and precipitated practically quantitatively. Chromium, added as Na 2 Cr 2 O 7 .2 H 2 O, which is soluble in a highly alkaline medium, was partly bound. It is not clear yet, whether chromium was adsorbed and/or incorporated in hydration products. The use of clinker instead of cement caused a more intensive bond. Adsorbents (silica fume, fly ash and activated carbon), with the exception of fly ash did not improve this bonding.

Pore solution analysis of cement pastes and nanostructural investigations of hydrated C3S

Abstract Pore solution investigations of cement pastes which had been prepared with the addition of ethanolamine showed that the ethanolamine had not been bound by cement during hydration and remained more or less completely dissolved in the capillary water, which can (theoretically) be expressed. This suggests that no significant binding of ethanolamine had occurred, neither chemically nor by way of adsorption and that the physically bound water (gel water) could not act as a solvent for the ethanolamine. The latter seems to be in contrast to the frost theory according to which a part of the gel water is mobile [M. Setzer, Micro ice lens formation, in: M.J. Setzer (Ed.), Proceedings of the 3rd International Bolomey Workshop “Pore Solution in Hardened Cement Paste”, University of Essen, June 1998, AEDIFICATIO Publishers, Freiburg, 2000, pp. 89–112]. This shows that a better understanding of structural details in the nanometer range of hardened cement is necessary. Therefore, small-angle neutron scattering (SANS) experiments were performed to study this part of the structure using C3S. SANS studies allow a nondestructive description of statistically representative microstructures in the scale range from micrometer to nanometer. In contrast to the established methods for microstructural investigations like MIP or BET, nondried samples can be used. Moreover, the scattering signals can be analyzed in a variety of ways, and therefore, a more detailed insight can be provided into the very complex cement paste microstructure. In this study, the signals were evaluated with respect to the specific inner surface and the particle-size distribution in the investigated nanometer range up to ∼100 nm.

Changes in pore water composition and in total chloride content at different levels of cement paste plates under different storage conditions

Abstract Besides the total chloride content, the Cl/OH ratio is a very important factor for the assessment of corrosion risk of reinforcements in concrete. First the changes of the total chloride were investigated at different levels of cement paste plates during about 6 months of exposure to 3% NaCl-solution. Then the changes of the same parameter and of the concentration of chloride and hydroxide ions as well as of the Cl/OH ratio in the pore solution were determined during storage for one year: (i) under water, (ii) outdoors (unprotected) and (iii) wrapped in plastic. Under these storage conditions the total chloride content decreased only in the boundary areas but increased in the inner ones as did the Cl-concentration and the Cl/OH-ratio of the pore solution. The decrease of the Cl/OH-ratio was smallest in the samples stored outdoors but about equal in samples immersed in water and wrapped in plastic, although the Cl concentration in the pore solution of the water exposed samples had decreased much more than in those wrapped in plastic. However, the OH concentration was found to be much higher in the boundary zones of the samples which were protected against environmental influences by plastic than in the water exposed ones so that the Cl/OH ratio was about the same.