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Chemical glass frosting processes are widely used to create visual attractive glass surfaces. A commonly used frosting bath mainly contains ammonium bifluoride (NH4HF2) mixed with hydrochloric acid (HCl). The frosting process consists of several baths. Firstly, the preliminary bath to clean the object. Secondly, the frosting bath which etches the rough light scattering structure into the glass surface. Finally, the washing baths to clean the frosted object. This is where the constituents of the preceding steps accumulate and have to be filtered from the sewage. In the present contribution, phosphoric acid (H3PO4) was used as a substitute for HCl to reduce the amount of ammonium (NH4+) and chloride (Cl−) dissolved in the waste water. In combination with magnesium carbonate (MgCO3), it allows the precipitation of ammonium within the sewage as ammonium magnesium phosphate (MgNH4PO4). However, a trivial replacement of HCl by H3PO4 within the frosting process causes extensive frosting errors, such as inhomogeneous size distributions of the structures or domains that are not fully covered by these structures. By modifying the preliminary bath composition, it was possible to improve the frosting result considerably. To determine the optimal composition of the preliminary bath, a semi-automatic evaluation method has been developed. This method renders the objective comparison of the resulting surface quality possible.
Superplasticizers are utilized both to improve the fluidity during the placement and to reduce the water content of concretes. Both effects have also an impact on the properties of the hardened concrete. As a side effect the presence of superplasticizers affects the strength development of concretes that is strongly retarded. This may lead to an ecomomical drawback of the concrete manufacturing. The present work is aimed at gaining insights on the causes of the retarding effect of superplasticizers on the hydration of Portland cement. In order to simplify the complex interactions occurring during the hydration of Portland cement the majority of the work focuses on the interaction of superplasticizer and tricalcium silicate (Ca3SiO5 or C3S, the main compound of Portland cement clinker). The tests are performed in three main parts accompanied by methods as for example isothermal conduction calorimetry, electrical conductivity, Electron Microscopy, ICP-OES, TOC, as well as Analytical Ultracentrifugation.
In the first main part and based on the interaction of cations and anionic charges of polymers, the interactions between calcium ions and superplasticizers are investigated. As a main effect calcium ions are complexed by the functional groups of the polymers (carboxy, sulfonic). Calcium ions may be both dissolved in the aqueous phase and a constitute of particle interfaces. Besides these effects it is furthermore shown that superplasticizers induce the formation of nanoscaled particles which are dispersed in the aqueous phase (cluster formation). Analogous to recent findings in the field of biomineralization, it is reasonable to assume that these nanoparticles influence the crystal growth by their assembly process.
Based on the assumption that superplasticizers hinder either or both dissolution and precipitation and by that retard the cement hydration, the impact on separate reactions is investigated. On experiments that address the solubility of C-S-H phases and portlandite, it is shown that complexation of calcium ions in the aqueous phase by functional groups of polymers increases the solubility of portlandite. Contrary, in case of C-S-H solubility the complexation of calcium ions in solution leads to decrease of the calcium ion concentration in the aqueous phase. These effects are explained by differences in adsorption of polymers on C-S-H phases and portlandite. It is proposed that adsorption is stronger on C-S-H phases compared to portlandite due to the increased specific surface area of C-S-H phases. Following that, it is claimed that before polymers are able to adsorb on C-S-H phases the functional groups must be screened by calcium ions in the aqueous phase. It is further shown that data regarding the impact of superplasticizers on the unconstrained dissolution rate of C3S does not provide a clear relation to the overall retarding effect occurring during the hydration of C3S. Both increased and decreased dissolution rate with respect to the reference sample are detected. If the complexation capability of the superplasticizers is considered then also a reduced dissolution rate of C3S is determined. Despite the fact that the global hydration process is accelerated, the addition of calcite leads to a slower dissolution rate. Thus, a hindered unconstrained dissolution of C3S as possibly cause for the retarding effect still remains open for discussion. In the last section of this part, the pure crystallization of hydrate phases (C-S-H phases, portlandite) is fathomed. Results clearly show that superplasticizers prolong the induction time and modify the rate of crystal growth during pure crystallization in particular due to the complexation of ions in solution. But this effect is insufficient to account for the overall retarding effect. Further important factors are the blocking of crystal growth faces by adsorbed polymers and the dispersion of nanoscaled particles which hinders their agglomeration in order to build up crystals.
In the last main part of the work, the previously gathered results are utilized in order to investigate hydration kinetics. During hydration, dissolution and precipitation occur in parallel. Thereby, special attention is laid on the ion composition of the aqueous phase of C3S pastes and suspensions in order to determine the rate limiting step. All in all it is concluded that the retarding effect of superplasticizers on the hydration of tricalcium silicate is based on the retardation of crystallization of hydrate phases (C-S-H phases and portlandite). Thereby, the two effects complexation of calcium ions on surfaces and stabilization of nanoscaled particles are of major importance. These mechanisms may partly be compensated by template performance and increase in solubility by complexation of ions in solution. The decreased dissolution rate of C3S by the presence of superplasticizers during the in parallel occuring hydration process can only be assessed indirectly by means of the development of the ion concentrations in the aqueous phase (reaction path). Whether this observation is the cause or the consequence within the dissolution-precipitation process and therefore accounts for the retarding effect remains a topic for further investigations.
Besides these results it is shown that superplasticizers can be associated chemically with inhibitors because they reduce the frequency factor to end the induction period. Because the activation energy is widely unaffected it is shown that the basic reaction mechanism sustain. Furthermore, a method was developed which permits for the first time the determination of ion concentrations in the aqueous phase of C3S pastes in-situ. It is shown that during the C3S hydration the ion concentration in the aqueous phase is developed correspondingly to the heat release rate (calorimetry). The method permits the differentiation of the acceleration period in three stages. It is emphasized that crystallization of the product phases of C3S hydration, namely C-S-H phases and portlandite, are responsible for the end of the induction period.
On the mechanisms of shrinkage reducing admixtures in self con-solidating mortars and concretes
(2010)
Self Consolidating Concrete – a dream has come true!(?) Self Consolidating Concrete (SCC) is mainly characterised by its special rheological properties. With-out any vibration this concrete can be placed and compacted under its own weight, without segrega-tion or bleeding. The use of such concrete can increase the productivity on construction sites and en-able the use of a higher degree of well distributed reinforcement for thin walled structural members. This new technology also reduces health risks since in contrast to the traditional handling of concrete, the emission of noise and vibration are substantially decreased. The specific mix design for self consolidating concretes was introduced around the 1980s in Japan. In comparison to normal vibrated concrete an increased paste volume enables a good distribution of aggregates within the paste matrix, minimising the influence of aggregates friction on the concrete flow property. The introduction of inert and/or pozzolanic additives as part of the paste provides the required excess paste volume without using disproportionally high amounts of plain cement. Due to further developments of concrete admixtures such as superplasticizers, the cement paste can gain self levelling properties without causing segregation of aggregates. Whereas SCC differs from normal vibrated concrete in its fresh attributes, it should reach similar properties in the hardened state. Due to the increased paste volume it usually shows higher shrinkage. Furthermore, owing to strength requirements, SCC is often produced at low water to cement ratios and hence may additionally suffer from autogenous shrinkage. This means that cracking caused by drying or autogenous shrinkage is a real risk for SCC and can compromise its durability as cracks may serve as ingression paths for gases and salts or might permit leaching. For the time being SCC still exhibits increased shrinkage and cracking probability and hence may be discarded in many practical applications. This can be overcome by a better understanding of those mechanisms and the ways to mitigate them. It is a target of this thesis to contribute to this. How to cope with increased shrinkage of SCC? In general, engineers are facing severe problems related to shrinkage and cracking. Even for normal and high performance concrete, containing moderate amounts of binder, a lot of effort was put on counteracting shrinkage and avoiding cracking. For the time being these efforts resulted in the knowledge of how to distribute cracks rather to avoid them. The most efficient way to decrease shrinkage turned out to be to decrease the cement content of concrete down to a minimum but still sufficient amount. For SCC this obviously seems to be contradictory with the requirement of a high paste volume. Indeed, the potential for shrinkage reduction is limited to some small range modifications in the mix design following two major concepts. The first one is the reduction of the required paste volume by optimising the aggregate grading curve. The second one involves high volume substitution of cement, preferentially using inert mineral additives. The optimization of grading curves is limited by several severe practical issues. Problems start with the availability of sufficiently fractionated aggregates. Usually attempts fail because of the enormous effort in composing application-optimized grading curves or mix designs. Due to durability reasons, the substitution rate for cement is limited depending on the application purpose and on environmental exposure of the hardened concrete. In the early 1980s Shrinkage Reducing Admixtures (SRA) were introduced to counteract drying shrinkage of concrete. The first publications explicitly dealing with SRA go back to Goto and Sato (Japan). They were published in 1983, which is also the time when the SCC concept was introduced. SRA modified concretes showed a substantial reduction of free drying shrinkage contributing to crack prevention or at least a significant decrease of crack width in situations of restrained drying shrinkage. Will shrinkage reducing admixtures contribute to a broader application of SCC? Within the last three decades performance tests on several types of concrete proved the efficiency of shrinkage reducing admixtures. So, at least in terms of shrinkage and cracking, concretes in general and SCC in particular can benefit from SRA application. But "One man's meat is another man's poison" and with respect to long term performance of SRA modified concretes there are still several issues to be clarified. One of these concerns the impact of SRAs on cement hydration. It is therefore an issue to know if changes in the hydrated phase composition, induced by SRA, result in undesired properties or decreased durability. Another issue is that the long term shrinkage reduction has to be evaluated. For example, one can wonder if SRA leaching may diminish or even eliminate long term shrinkage reduction and if the release of admixtures could be a severe environmental issue. It should also be noted that the basic mechanism or physical impact of SRA as well as its implementation in recent models for shrinkage of concrete is still being discussed. The present thesis tries to shed light on the role of SRA in self consolidating concrete focusing on the three questions outlined above: basic mechanisms of cement hydration, physical impact on shrinkage and the sustainability of SRA-application. Which contributions result from this study? Based on an extensive patent search, commercial SRAs could be identified to be synergistic mixtures of non-ionic surfactants and glycols. This turns out to be most important information for more than one reason and is the subject of chapter 4. An abundant literature focuses on properties of these non-ionic surfactants. Moreover, from this rich pool of information, the behaviour of SRAs and their interactions in cementitious systems were better understood through this thesis. For example, it could be anticipated how SRAs behave in strong electrolytes and how surface activity, i.e. surface tension, and interparticle forces might be affected. The synergy effect regarding enhanced performance induced by the presence of additional glycol in SRAs could be derived from the literature on the co-surfactant nature of glycols. Generally it now can be said that glycols ensure that the non-ionic surfactant is properly distributed onto the paste interfaces to efficiently reduce surface tension. In literature, the impact of organic matter on cement hydration was extensively studied for other admixtures like superplasticizer. From there, main impact factors related to the nature of these molecules could be identified. In addition, here again, the literature on non-ionic surfactants provides sufficient information to anticipate possible interactions of SRA with cement hydration based on the nature of non-ionic surfactants. All in all, the extensive study on the nature of non-ionic surfactants, presented in chapter 4, provides fundamental understanding of the behaviour of SRAs in cement paste. Taking a step further to relate this to the impact on drying and shrinkage required to review recent models for drying and shrinkage of cement paste as presented in chapter 3. There, it is shown that macroscopic thermodynamics of the open pore systems can be successfully applied to predict drying induced deformation, but that surface activity of SRA still has to be implemented to explain the shrinkage reduction it causes. Because of severe issues concerning the importance of capillary pressure on shrinkage, a new macroscopic thermodynamic model was derived in a way that meets requirements to properly incorporate surface activity of SRA. This is the subject of chapter 5. Based on theoretical considerations, in chapter 5 the broader impact of SRA on drying cementitious matter could be outlined. In a next step, cement paste was treated as a deformable, open drying pore system. Thereby, the drying phenomena of SRA modified mortars and concrete observed by other authors could be retrieved. This phenomenological consistency of the model constitutes an important contribution towards the understanding of SRA mechanisms. Another main contribution of this work came from introducing an artificial pore system, denominated the normcube. Using this model system, it could be shown how the evolution of interfacial area and its properties interact in presence of SRAs and how this impacts drying characteristics. In chapter 7, the surface activity of commercial SRAs in aqueous solution and synthetic pore solution was investigated. This shows how the electrolyte concentration of synthetic pore solution impacts the phase behaviour of SRA and conversely, how the presence of SRA impacts the aqueous electrolyte solution. Whilst electrolytes enhance self-aggregation of SRAs into micelles and liquid crystals, the presence of SRAs leads to precipitation of minerals as syngenite and mirabilite. Moreover, electrolyte solutions containing SRAs comprise limited miscibility or rather show miscibility gaps, where the liquid separates into isotropic micellar solutions and surfactant rich reverse micellar solutions. The investigation of surface activity and phase behaviour of SRA unravelled another important contribution. From macroscopic surface tension measurements, a relationship between excess surface concentration of SRA, bulk concentration of SRA and exposed interfacial area could be derived. Based on this, it is now possible to predict the actual surface tension of the pore fluid in the course of drying once the evolution of internal interfacial area is known. This is used later in this thesis to describe the specific drying and shrinkage behaviour of SRA modified pastes and mortars. Calorimetric studies on normal Portland cement and composite binders revealed that SRA alone show only minor impact on hydration kinetics. In presence of superplasticizer however the cement hydration can be significantly decelerated. The delaying impact of SRA could be related to a selective deceleration of silicate phase hydration. Moreover, it could be shown that portlandite precipitation in presence of SRA is changed, turning the compact habitus into more or less layered structures. Thereby, the specific surface increases, causing the amount of physically bound water to increase, which in turn reduces the maximum degree of hydration achievable for sealed systems. Extensive phase analysis shows that the hydrated phase composition of SRA modified binders re-mains almost unaffected. The appearance of a temporary mineral phase could be detected by environmental scanning electron microscopy. As could be shown for synthetic pore solutions, syngenite precipitates during early hydration stages and is later consumed in the course of aluminate hydration, i.e. when sulphates are depleted. Moreover, for some SRAs, the salting out phenomena supposed to be enhanced in strong electrolytes could also be shown to take place. The resulting organic precipitates could be identified by SEM-EDX in cement paste and by X-ray diffraction on solid residues of synthetic pore solution. The presence of SRAs could also be identified to impact microstructure of well cured cement paste. Based on nitrogen adsorption measurements and mercury intrusion porosimetry the amount of small pores is seen to increase with SRA dosage, whilst the overall porosity remains unchanged. The question regarding sustainability of SRA application is the subject of chapter 10. By means of leaching studies it could be shown that SRA can be leached significantly. The mechanism could be identified as a diffusion process and a range of effective diffusion coefficients could be estimated. Thereby, the leaching of SRA can now be estimated for real structural members. However, while the admixture can be leached to high extents in tank tests, the leaching rates in practical applications can be assumed to be low because of much reduced contact with water. This could be proven by quantifying admixture loss during long term drying and rewetting cycles. Despite a loss of admixture shrinkage reduction is hardly impacted. Moreover, the cyclic tests revealed that the total deformations in presence of SRA remain low due to a lower extent of irreversibly shrinkage deformations. Another important contribution towards the better understanding of the working mechanism of SRA for drying and shrinkage came from the same leaching tests. A significant fraction of SRA is found to be immobile and does not diffuse in leaching. This fraction of SRA is probably strongly associated to cement phases as the calcium-silicate-hydrates or portlandite. Based on these findings, it is now also possible to quantify the amount of admixture active at the interfaces. This means that, the evolution of surface tension in the course of drying can be approximated, which is a fundamental requirement for modeling shrinkage in presence of SRA. The last experimental chapter of this study focuses on the working mechanism and impact of SRA on drying and shrinkage. Based on the thermodynamics of the open deformable pore system introduced in chapter 5, energy balances are set up using desorption and shrinkage isotherms of actual samples. Information on distribution of SRA in the hydrated paste is used to estimate the actual surface tensions of the pore solution. In other words, this is the first time that the surface activity of the SRA in the course of the drying is fully accounted for. From the energy balances the evolution and properties of the internal interface are then obtained. This made it possible to explain why SRAs impact drying and shrinkage and in what specific range of relative humidity they are active. Summarising the findings of this thesis it can be said that the understanding of the impact of SRAs on hydration, drying and shrinkage was brought forward. Many of the new insights came from the careful investigation of the theory of non-ionic surfactants, something that the cement community had generally overlooked up to now.
Rice husk ash (RHA) is classified as a highly reactive pozzolan. It has a very high silica content similar to that of silica fume (SF). Using less-expensive and locally available RHA as a mineral admixture in concrete brings ample benefits to the costs, the technical properties of concrete as well as to the environment. An experimental study of the effect of RHA blending on workability, strength and durability of high performance fine-grained concrete (HPFGC) is presented. The results show that the addition of RHA to HPFGC improved significantly compressive strength, splitting tensile strength and chloride penetration resistance. Interestingly, the ratio of compressive strength to splitting tensile strength of HPFGC was lower than that of ordinary concrete, especially for the concrete made with 20 % RHA. Compressive strength and splitting tensile strength of HPFGC containing RHA was similar and slightly higher, respectively, than for HPFGC containing SF. Chloride penetration resistance of HPFGC containing 10–15 % RHA was comparable with that of HPFGC containing 10 % SF.