The gel point is one of the relevant parameters for the characterization of materials that undergo a sol-to-gel-transition during their setting. A test method for its determination could be the Dynamic Mechanical Analysis, currently used in the field of polymer science. For the first time this measuring principle has been adapted to the group of liquid alkali silicates, thus verifying the possibility to describe their setting process. Three analytical methods for the determination of the gel point were studied with regard to their applicability and consistency for a compound alkali silicate system used as chemical hardener. K E Y W O R D Sbinders/binding, mechanical properties, sol-gel
Despite the excellent thermochemical properties of magnesia carbon bricks, these exhibit one weak characteristic during their use: their carbothermally induced wear. Carbon has a high affinity to oxygen, which leads to a reaction between magnesia and carbon, forming gaseous products at very low oxygen partial pressures in the surrounding atmosphere. When magnesia carbon material is furthermore applied at negative pressures, the precited carbothermic reduction processes effect an internal decomposition or even degradation of the bricks. Mostly, high‐purity magnesia varieties (MgO ≥ 96 wt%) are used for the production of magnesia carbon bricks because the low‐melting calcium silicate secondary phases in magnesia impair the high‐temperature resistance of these bricks. The fundamental question if and to which extent secondary phases react with carbon and which impact they have on the carbothermally induced wear of bricks has been unsolved so far. The following paper presents which influence the mineral secondary phases, monticellite, merwinite, and belite that are most commonly occurring in magnesia, have on the carbothermally induced wear. The respective studies were conducted by means of thermogravimetric and microstructural analyses. The results of these studies show that monticellite in the MgO–C microstructure brings about an increase in weight loss on account of carbothermic reduction processes. On the contrary, belite and merwinite in the MgO–C structure do not exhibit any negative impact on the thermal stability of the microstructure.
Liquid alkali silicates (waterglasses) are used as chemical binders for a wide range of refractory applications, the setting of which can be initiated by the addition of phosphate hardeners. The duration of the setting process is of special interest for an economic lining with short aggregate downtimes. Therefore, in the present work, the influences of two different types of phosphate (aluminium orthophosphate and boron orthophosphate) on the hardening mechanisms of waterglasses are investigated. Time-dependent measurements by dynamic mechanical analysis (DMA) are carried out to observe the setting process. Structural information of the hardened amorphous samples is obtained by means of nuclear magnetic resonance (NMR) spectroscopy. It is shown that the use of different phosphates leads to differences in the setting rate, caused by different modes of network formation. The resultant silicate networks incorporate the aluminium or boron species but differ in the connectivity of those units. In addition, the distribution following the well-known Qn notation of the silicate units is directly influenced by the phosphate type.
Bauxite is an important raw material for the production of refractories. The availability of refractory grade ore worldwide is limited, and high iron contents in particular reduce the quality of the material. For refractory applications, a maximum iron content of 2 % is acceptable. In this study, acid leaching with HCl is used to decrease the iron content in different nonrefractory grade raw bauxites. Computerized design of experiments and statistical methods are used to determine optimum process parameters and influencing factors for different bauxites individually. Compared to previously published studies, the applied approach makes it possible to process even very iron-rich bauxites (e.g., 31 % Fe 2 O 3 in calcined substance) and to lower their Fe 2 O 3 contents below the permitted 2 %. In addition, larger grain sizes (around 5.5 mm) can be used. Statistical planning and mathematical modeling also allow the prediction of the minimum achievable iron content within the investigated parameter ranges. For selected parameter combinations, the achievable Fe 2 O 3 content can be predicted relatively accurately without the requirement for practical testing of the corresponding experimental setup.
Aluminum phosphates are known as inorganic hardening agents for the setting of alkali silicate solutions, but only few studies have been published on the setting mechanism of potassium water glass. The solution behavior of two aluminum metaphosphates in alkaline environments were investigated photometrically determining the dissolved aluminum content. The crystalline phase composition of the hardened potassium silicate systems was determined by X‐ray diffraction. New insights into the setting mechanism were obtained concerning the structure of the aluminum metaphosphate and the SiO2/K2O ratio of three different potassium silicate solutions. With increasing pH value aluminum tetrametaphosphate reacts rapidly and forms crystalline potassium tetrametaphosphate dihydrate by an ion‐exchange‐reaction. In parallel, a depolymerization of the cyclic metaphosphate structure occurs leading to potassium dihydrogen phosphate as final fragmentation product. With aluminum hexametaphosphate no ion‐exchange reaction product was observed. Only potassium dihydrogen phosphate could be found in higher quantities compared to the reaction with aluminum tetrametaphosphate.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.