Cölfen, Helmut
0000-0002-1148-0308 Berufsbeschreibung
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High Energy Absorption Nacre‐Like Calcium Silicate Hydrate (C‐S‐H) Composite Toward Elastic Cementitious Materials
The low toughness under the tension of cement and concrete materials has been a long‐standing issue for decades and it has become increasingly urgent to address in modern society due to the growing demand for the development of high‐performance and sustainable constructions. Manipulating calcium silicate hydrate (C‐S‐H), the main hydration product of Portland cement, which determines the mechanical properties of cementitious materials, is an attractive method for improving their toughness following a bottom‐up approach. Inspired by the microstructure of nacre, a high energy absorption C‐S‐H‐based composite with a highly ordered structure is fabricated by a designed ternary building block, in which exfoliated montmorillonite provides a template for the nucleation and growth of C‐S‐H generating the “brick”, and polyvinyl alcohol acts as a “mortar” binding all the building blocks together. With the hierarchical toughening strategy explored here, the obtained C‐S‐H composite achieves a remarkable energy absorption of 16.2 ± 2.6 MJ m −3 , which surprisingly outperforms the ultra‐high toughness cementitious materials by a factor of 20–60 and is even higher than that of natural nacre and other nacre‐like composites. These findings not only provide valuable insights into enhancing the toughness of cementitious materials but also open possibilities for broadening potential applications of C‐S‐H.
Biodegradable Mineral Plastics
Mineral plastics are a promising class of bio-inspired materials that offer exceptional properties, like self-heal ability, stretchability in the hydrogel state, and high hardness, toughness, transparency, and non-flammability in the dry state along with reversible transformation into the hydrogel by addition of water. This enables easy reshape-ability and recycling like the solubility in mild acids to subsequently form mineral plastics again by base addition. However, current mineral plastics rely on petrochemistry, are hardly biodegradable, and thus persistent in nature. This work presents the next generation of mineral plastics, which are bio-based and biodegradable, making them a promising, new class of polymers for the development of environmentally friendly materials. Physically cross-linked (poly)glutamic-acid (PGlu)-based mineral plastics are synthesized using various alcohol-water mixtures, metal ion ratios and molecular weights. The rheological properties are easily adjusted using these parameters. The general procedure involves addition of equimolar solution of CaCl2 to PGlu in equal volumes followed by addition of iPrOH (iPrOH:H2O = 1:1) under vigorous stirring conditions. The ready biodegradability of PGlu/CaFe mineral plastic is confirmed in this study where the elements N, Ca, and Fe present in it tend to act as additional nutrients, supporting the growth of microorganisms and consequently, promoting the biodegradation process.
The nucleation of C–S–H via prenucleation clusters
The nucleation and growth of calcium–silicate–hydrate (C–S–H) is of fundamental importance for the strength development and durability of the concrete. However, the nucleation process of C–S–H is still not fully understood. The present work investigates how C–S–H nucleates by analyzing the aqueous phase of hydrating tricalcium silicate (C3S) by applying inductively coupled plasma-optical emission spectroscopy as well as analytical ultracentrifugation. The results show that the C–S–H formation follows non-classical nucleation pathways associated with the formation of prenucleation clusters (PNCs) of two types. Those PNCs are detected with high accuracy and reproducibility and are two species of the 10 in total, from which the ions (with associated water molecules) are the majority of the species. The evaluation of the density and molar mass of the species shows that the PNCs are much larger than ions, but the nucleation of C–S–H starts with the formation of liquid precursor C–S–H (droplets) with low density and high water content. The growth of these C–S–H droplets is associated with a release of water molecules and a reduction in size. The study gives experimental data on the size, density, molecular mass, and shape and outlines possible aggregation processes of the detected species.
Localized Crystallization of Calcium Phosphates by Light‐Induced Processes
Medical treatment options for bones and teeth can be significantly enhanced by taking control over the crystallization of biomaterials like hydroxyapatite in the healing process. Light‐induced techniques are particularly interesting for this approach as they offer tremendous accuracy in spatial resolution. However, in the field of calcium phosphates, light‐induced crystallization has not been investigated so far. Here, proof of principle is established to successfully induce carbonate‐hydroxyapatite precipitation by light irradiation. Phosphoric acid is released by a photolabile molecule exclusively after irradiation, combining with calcium ions to form a calcium phosphate in the crystallization medium. 4‐Nitrophenylphosphate (4NPP) is established as the photolabile molecule and the system is optimized and fully characterized. A calcium phosphate is crystallized exclusively by irradiation in aqueous solution and identified as carbonate apatite. Control over the localization and stabilization of the carbonate apatite is achieved by a pulsed laser, triggering precipitation in calcium and 4NPP‐containing gel matrices. The results of this communication open up a wide range of new opportunities, both in the field of chemistry for more sophisticated reaction control in localized crystallization processes and in the field of medicine for enhanced treatment of calcium phosphate containing biomaterials.
Analytical ultracentrifugation in colloid and polymer science : new possibilities and perspectives after 100 years
Analytical ultracentrifugation (AUC) is a classical polymer and colloid analysis technique invented by Theodor Svedberg 100 years ago. Modern hard- and software and powerful computers make it now possible to develop the methodology beyond what was possible with this technique before. This perspective aims to describe new possibilities, which extend the possibilities of AUC beyond the classical repertoire of the determination of distributions of sedimentation coefficient, particle size, and molar mass as well as stoichiometries and interaction constants of interacting systems. High-resolution simultaneous characterization of particle size and optical property distributions, investigation of nucleation by reaction in the AUC cell, characterization of particle interactions at a very high concentration, and characterization of complex fluids or osmotic pressures over large concentration ranges even crossing phase boundaries are among the discussed topics. They show that even after 100 years of successful application, AUC still has much yet unexplored potential in colloid and polymer science. Graphical Abstract This perspective paper spans from the days of invention of analytical ultracentrifugation to now including nonmainstream methodology and instrumentation, which has a huge potential for the future. This includes multiwavelength detectors, high-resolution particle size distributions, chemical reactions in the ultracentrifuge, high-concentration work, osmotic pressure distributions, and characterization of complex fluids.
Silica‐Functionalized Nanolimes for the Conservation of Stone Heritage
The relatively recent development of nanolimes (i.e., alcoholic dispersions of Ca(OH)2 nanoparticles) has paved the way for new approaches to the conservation of important art works. Despite their many benefits, nanolimes have shown limited reactivity, back-migration, poor penetration, and lack of proper bonding to silicate substrates. In this work a novel solvothermal synthesis process is presented by which extremely reactive nanostructured Ca(OH)2 particles are obtained using calcium ethoxide as the main precursor species. Moreover, it is demonstrated that this material can be easily functionalized with silica-gel derivatives under mild synthesis conditions, thereby preventing particle growth, increasing total specific surface area, enhancing reactivity, modifying colloidal behavior, and functioning as self-integrated coupling agents. Additionally, the formation of calcium silicate hydrate (CSH) nanocement is promoted by the presence of water, resulting in optimal bonding when applied to silicate substrates, as evidenced by the higher reinforcement effect produced on treated Prague sandstone specimens as compared to those consolidated with nonfunctionalized commercial nanolime. The functionalization of nanolimes is not only a promising strategy for the design of optimized consolidation treatments for the cultural heritage, but may also have important implications for the development of advanced nanomaterials for building, environmental, or biomedical applications.
The Structure, Preparation, Characterization, and Intercalation Mechanism of Layered Hydroxides Intercalated with Guest Anions
Since the intercalation of anions into layered hydroxides (LHs) has a great impact not only on their nucleation and growth but also on their structure, composition, and size, the intercalation chemistry of LHs has aroused the strong interest of researchers. However, the progress in the fundamental understanding of LHs intercalated with guest anions have not been paralleled by a concomitant development of the preparation and performance improvement of such materials. Considering the guidance of a timely in-depth review for scientists in this area, a systematic introduction about the development that is made on the above-mentioned issues is highly needed but yet missing so far. Herein, recent advances in understanding the chemical composition and structure of LHs intercalated with guest anions are systematically summarized. Meanwhile, typical and emerging bottom-up synthesis methods of LHs intercalated with anions are reviewed, and the potential impact of external reaction parameters on the intercalation of anions into LHs are discussed . Besides, different analytical characterization techniques employed in the examination of guest anion-intercalated LHs are deliberated upon. Finally, although progress is slow in exploring the intercalation mechanism, as many examples as possible are included in this review and inferred the possible intercalation mechanism.
Growth strategy for solution-phase growth of two-dimensional nanomaterials via a unified model
Two-dimensional (2D) materials prepared by a solution-phase growth route exhibit many unique properties and are promising for use in various fields. However, simple, rational and green fabrication of target materials remains challenging due to the lack of guiding principles. Here we propose a universal qualitative model for 2D materials grown for layered and non-layered crystal structures by a solution-phase growth route; both theoretical simulation and experimental results confirm the model’s validity. This model demonstrates that 2D growth can be controlled by only tuning the reaction concentration and temperature, and has been applied to fabricate more than 30 different 2D nanomaterials in water at room temperature and in the absence of additives. Furthermore, the model shows promise for optimizing the experimental design of numerous other 2D nanomaterials.