Herbert Pöllmann

Bio: Herbert Pöllmann is an academic researcher from Martin Luther University of Halle-Wittenberg. The author has contributed to research in topics: Cement & Ettringite. The author has an hindex of 15, co-authored 105 publications receiving 919 citations. Previous affiliations of Herbert Pöllmann include Wittenberg University.

Active iron-rich belite sulfoaluminate cements: clinkering and hydration.

Abstract: Ordinary Portland cement (OPC) is an environmentally contentious material, as for every ton of OPC produced, on average, 0.97 tons of CO2 are released. Conversely, belite sulfoaluminate (BSA) cements are promising eco-friendly building materials, as their production may deplete CO2 emissions up to 35% (compared to OPC). However, the hydration rate of belite is slow. Here, we report the clinkering of iron-rich BSA materials, their activation with B2O3, and establishing a methodology to measure their improved reactivities. Nonactivated BSA clinker contained only β belite phase, 52 wt %. Meanwhile, BSA clinkers activated with 1 and 2 wt % of B2O3 contained 28 wt % of β and 25 wt % of α′H; and 54 wt % of α′H phase, respectively. Therefore, activation of BSA has been proved as α′H-belite is stabilized. The hydration of the cements has been studied by laboratory and synchrotron X-ray powder diffraction (using Rietveld method and chemical constraints), calorimetry, and environmental scanning electron microscopy....

Comprehensive analysis of soiling and cementation processes on PV modules in Qatar

Abstract: Soiling of PV modules decreases their energy yield by blocking sunlight and is a serious problem in regions with high aerosol concentrations and little rain such as arid and semi-arid climates. Due to many influencing factors, which are often site-dependent, the complex processes of soiling are not fully understood. In order to approach this goal, detailed knowledge and analysis of the various influencing parameters at a microscopic level are necessary. In this study we present results of outdoor experiments to reveal the physical mechanisms of soiling-layer formation on exposed glass surfaces. The experiments were conducted at a PV test site in Doha, Qatar, and included sample cooling, heating and cleaning. Detailed microstructural investigations of the soiling layers were performed to study cementation of dust particles. To achieve a better understanding of the underlying processes, we provide an analysis of environmental parameters (relative humidity, ambient and module temperatures, wind speed) with special focus on dew formation, as well as dust characteristics (chemical composition, mineralogy, size distribution), which influence cementation and optical losses. In addition, we compare results of different methods for soiling-rate determination including light microscopy, spectroscopy, gravimetry and PV power analysis to determine the relation between micro-scale soiling measurements and actual PV energy losses. The results show that in Qatar, dew formation frequently occurs and leads to particle cementation by needle-shaped crystals of the clay mineral palygorskite, which directly precipitates at the glass surface. It is shown that preventing dew formation by sample heating inhibits cementation and can significantly reduce dust accumulation. Based on these findings, new mitigation approaches for PV soiling are proposed.

Geomechanical and geochemical effects on sandstones caused by the reaction with supercritical CO 2 : an experimental approach to in situ conditions in deep geological reservoirs

Abstract: The geochemical and geomechanical behaviour of reservoir rocks from deep saline aquifers during the injection and geological storage of CO2 is studied in laboratory experiments. A combination of geochemical and geomechanical studies was carried out on various sandstones from the North German Basin. After the mineralogical, geochemical and petrophysical characterization, a set of sandstone samples was exposed to supercritical (sc)CO2 and brine for 2–4 weeks in an autoclave system. One sample was mineralogically and geochemically characterised and then loaded in a triaxial cell under in situ pressure and temperature conditions to study the changes of the geomechanical rock properties. After treatment in the autoclaves, geochemical alterations mainly in the carbonate, but also in the sheet silicate cements as well as in single minerals of the sandstones were observed, affecting the rocks granular structure. In addition to partial solution effects during the geochemical experiments, small grains of secondary carbonate and other mineral precipitations were observed within the pore space of the treated sandstones. Results of additional geomechanical experiments with untreated samples show that the rock strength is influenced by the saturation degree, the confining pressure, the pore fluid pressure and temperature. The exposure to pure scCO2 in the autoclave system induces reduced strength parameters, modified elastic deformation behaviour and changes of the effective porosity in comparison to untreated sandstone samples. Experimental results show that the volume of pore fluid fluxing into the pore space of the sandstones clearly depends on the saturation level of the sample.

Calcium Aluminate Cements - Raw Materials, Differences, Hydration and Properties

Abstract: High alumina cement was used widely in the UK after World War I, expressing its higher content of aluminum oxide in comparison to Portland cement. Several descriptions of investigations on calcium aluminate cements appeared, starting around 1850, with a first patent field in 1888 (Scrivener and Capmas, in Hewlett 1998). More widely known is the work of Bied (1909, 1926) filing a patent in 1909 for the fabrication of cement using bauxite or some similar aluminum or iron-rich material, with low SiO2-contents and limestone. In 1918, the trade name Ciment Lafarge Fondue was used for the first time. Meanwhile in the USA, Spackman (1908, 1910a,b) developed cementitious material marketed under the name of Alca natural cements. Several patents were applied and granted (Bates 1921). A description of non-Portland cements was given by Muzhen et al. (1992). The reason for looking into alternative cement materials was to develop cements with improved stability against sulfate corrosion. Nowadays, calcium aluminate cements are used specifically for their distinct properties (Brown and Cassel 1977), some of which are presented in Table 1. Calcium aluminate cements do have special applications and are therefore widely used despite the fact that worldwide fabrication is by no means comparable to OPCs (Hohl et al. 1936; Garces et al. 1997; George 1976, 1980a,b, 1983, 1990, 1997; George and Montgomery 1992; George and Racher 1996; Gartner et al. 2002). Scrivener and Taylor (1990) and Scrivener et al. (1997a,b) described calcium aluminate cements and their use and microstructural developments. The use for experimental purposes was described by Auer et al. (1995). Thermal analyses for thermogravimetry of CAC-fraction and formation was discussed by Chudak et al. (1982, 1987). The …

Use of 29Si and 27Al MAS NMR to study thermal activation of kaolinites from Brazilian Amazon kaolin wastes

Abstract: The study of thermal activation of kaolinite at various temperatures to produce highly reactive metakaolinite from Brazilian Amazon kaolin wastes was studied using 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR). The wastes used in the current study came from the Capim and Jari regions in northern Brazil. The mineralogical and chemical characterizations of Jari kaolin (KJ) waste were performed by XRD, FTIR, DTA–TGA, SEM and chemical analyses. The results indicate that the Jari and Capim kaolins (KC) wastes are composed primarily of kaolinite and that KJ is composed of a typical kaolinite with a low degree of structural order. The ideal temperature to produce highly reactive metakaolinite is 600 °C for KJ and 700 °C for KC, suggesting that the degree of structural order of kaolinite has a definite influence on its dehydroxylation temperature. These results were confirmed when zeolite NaA was synthesized from metakaolins produced at temperatures used in this work.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Thermodynamic Modelling of the Effect of Temperature on the Hydration and Porosity of Portland Cement

Abstract: The composition of the phase assemblage and pore solution of Portland cements hydrated between 0-60°C were modeled as a function of time and temperature. Results of thermodynamic modeling showed good agreement with experimental data gained at 5, 20, and 50°C. At 5 and 20°C, a similar phase assemblage was calculated to be present, while at ~50°C, thermodynamic calculations predicted conversion of ettringite and monocarbonate to monosulphate. Modeling showed that in Portland cements having an Al2O3/SO3 ratio of > 1.3 (bulk weight), above 50°C monosulphate and monocarbonate are present. In Portland cements containing less Al (Al2O3/SO3 < 1.3), above 50°C monosulphate and small amounts of ettringite are expected to persist. A good correlation between calculated porosity and measured compressive strength was observed.

Thermodynamic properties of Portland cement hydrates in the system CaO–Al2O3–SiO2–CaSO4–CaCO3–H2O

Abstract: A database is presented for commonly-encountered cement substances including C–S–H, Ca(OH)2, selected AFm, AFt and hydrogarnet compositions as well as solid solutions. The AFm compositions include stratlingite. The data were obtained for the most part from experiment and many of the predicted reactions were confirmed by focussed experiments. The temperature-dependence of the thermodynamic data for the above phases, determined partly from experiment and partly from thermodynamic estimations, are also tabulated in the range 1 °C to 99 °C. Relative to previous databases, sulfate AFm is shown to have a definite range of stability range at 25 °C thus removing long-standing doubts about its stability in normal hydrated cement pastes. Carbonate is shown to interact strongly with stabilisation of AFm across a broad range of temperatures and, at low temperatures, to substitute into AFt. The new database enables the ultimate hydrate mineralogy to be calculated from chemistry: most solid assemblages, the persistence of C–S–H apart, correspond closely to equilibrium. This realisation means that hydrate assemblages can be controlled. The development of a thermodynamic approach also enables a fresh look at how mineralogical changes occur in response to environmentally-conditioned reactions; several papers showing applications are cited.

Impact of admixtures on the hydration kinetics of Portland cement

Abstract: Most concrete produced today includes either chemical additions to the cement, chemical admixtures in the concrete, or both. These chemicals alter a number of properties of cementitious systems, including hydration behavior, and it has been long understood by practitioners that these systems can differ widely in response to such chemicals. In this paper the impact on hydration of several classes of chemicals is reviewed with an emphasis on the current understanding of interactions with cement chemistry. These include setting retarders, accelerators, and water reducing dispersants. The ability of the chemicals to alter the aluminate–sulfate balance of cementitious systems is discussed with a focus on the impact on silicate hydration. As a key example of this complex interaction, unusual behavior sometimes observed in systems containing high calcium fly ash is highlighted.

The geochemistry of acid mine drainage

Abstract: Mining and mineral processing generates large volumes of waste, including waste rock, mill tailings, and mineral refinery wastes. The oxidation of sulfide minerals in the materials can result in the release of acidic water containing high concentrations of dissolved metals. Recent studies have determined the mechanisms of abiotic sulfide-mineral oxidation. Within mine wastes, the oxidation of sulfide minerals is catalyzed by microorganisms. Molecular tools have been developed and applied to determine the activity and role of these organisms in sulfide-mineral-bearing systems. Novel tools have been developed for assessing the toxicity of mine-waste effluent. Dissolved constituents released by sulfide oxidation may be attenuated through the precipitation of secondary minerals, including metal sulfate, oxyhydroxide, and basic sulfate minerals. Geochemical models have been developed to provide improved predictions of the magnitude and duration of environmental concerns. Novel techniques have been developed to prevent and remediate environmental problems associated with these materials.