New selective dissolution process to quantify reaction extent and product stability in metakaolin-based geopolymers

A selective dissolution process is developed that can quantify the amount of soluble material, geopolymer gel and remnant unreacted precursor in metakaolin-based geopolymer systems and determine the nanostructural features of the raw materials and geopolymer gel components. The susceptibility of alkalis leachability from the alkaline aluminosilicate hydrate-type gel (N-A-S-H) produced during the geopolymerization is not fully understood. This phenomenon led to deleterious processes from a microstructural, aesthetic and performance point of view. Geopolymers were synthesised using different contents and types of alkalis (M/Al = 0.50–0.83, where M represents Na or K), different contents of soluble silica in the activator (expressed as SiO2/M2O ratio of 1.0, 0.5 and 0.0), and curing temperatures (25 and 50 °C). The selective dissolution process is based on neutral dissolution at pH 7 to extract the soluble materials and acid dissolution using a strong acid at pH 0 to dissolve the geopolymer gel, which provides for the first time a method to quantify the (i) soluble material, (ii) geopolymer gel and (iii) unreacted material in geopolymers. The soluble material provides a reliable indication of the materials that can be removed from the geopolymers in a neutral pH environment and hence the potential for leaching and efflorescence, which is useful for durability prediction and service life. Quantification of remnant unreacted metakaolin determines the reactivity of the precursor and assesses the suitability of different synthesis conditions for varied applications. This work therefore provides a novel and widely applicable approach to determine the susceptibility of geopolymer materials to leaching

Extraction of tricalcium aluminate for research applications by selective dissolution of portland cement clinker

Tricalcium aluminate (C3A) is the most reactive phase in portland cement clinker. In the study of the C3A hydration process, the use of synthetic samples is often preferred over clinker-sourced samples due to the absence of traces of other elements in synthetic C3A. However, the reproduction of results from synthetic samples using clinker-sourced C3A samples is challenging due to the difficulty of complete extraction of aluminate phases from clinkers without damaging their structure. Salicylic and maleic acid/methanol solutions were used in the past to isolate C3A, but complete extraction has not been observed. Thus, it was necessary to modify the selective dissolution (SD) method. Possible processes were tested with white and ordinary portland clinkers. A modified process with maleic acid/methanol solution was developed and the samples were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy, compared with synthetic C3A samples. The modified selective dissolution process with maleic acid/methanol solution proposed in this work was effective in achieving complete isolation of aluminate-based phases, and can be used for further studies of the hydration process of clinker-sourced C3A samples to improve the understanding and quality of portland cement clinkers

Metakaolin-based geopolymers: Relation between formulation, physicochemical properties and efflorescence formation

The efflorescence formation in metakaolin-based geopolymers is assessed in this study to provide a better understanding of the effect of the synthesis parameters. Efflorescence formation depends on the physical and chemical properties of geopolymers as well as the environmental exposure conditions. In this study a set of fifteen geopolymers were synthesized using different formulation. An accelerated test of efflorescence development is presented, where the grade of degradation was evaluated by visual observation and correlated to leaching potential, physical properties and microstructural features. The use of soluble silicate in the activator provides a denser and a less permeable matrix. This makes the extraction of free alkalis to the surface more difficult, reducing the extent of alkali leaching and therefore efflorescence. The use of K+ is also effective to reduce visible efflorescence. The efflorescence formation is predicted by the properties of the gel formed which are dependent on the mix proportioning

Catfishes as prey items of Potamotrygonid stingrays in the Solimões and Negro rivers, Brazilian Amazon

In spite of the fact that catfish have rarely been reported as food items in the diets of freshwater stingrays, they are commonly observed in the stomach contents of species inhabiting the Solimões and Negro rivers in the Brazilian Amazon. The stomach contents of six rays from the Solimões River and 64 from the Negro River were analyzed, and catfishes (distributed among the families Callichthyidae, Cetopsidae, Doradidae and Loricariidae) were found in the stomach contents of four and 10 of these rays, respectively, comprising a frequency of occurrence of 20%. These data indicate a relevant participation of catfishes in the diets of potamotrygonid rays in the Amazon, and may reflect the regional diversity and abundance of Siluriformes in the region

Binary alkali-activated systems obtained by the valorisation of calcined kaolin sludge and bottom ash

This paper assesses the use and valorisation of two industrial wastes generated at a large scale, which are currently disposed in landfills, as raw materials to produce geopolymers. Specifically, a kaolinitic sludge from the mining industry (CKS), and bottom ash (BA) generated during coal combustion in a thermal power station, were used as aluminosilicate precursors in geopolymer synthesis. The geopolymers were synthesised at 50°C, with a sodium oxide/aluminium oxide (Na2O/Al2O3) molar ratio of 1.0, and different silica/aluminium oxide (SiO2/Al2O3) molar ratios adjusted by manipulating the content of the soluble silicate solution used as the activator. The mechanical strength and reaction products formed during the geopolymerisation process were assessed up to 90 days of curing. The use of CKS as the main component of the precursor blend provides a geopolymer with better mechanical properties due to its higher reactivity than BA. The content of soluble silicates in the alkali activator plays an important role during geopolymerisation, improving the mechanical properties due to the formation of a more reticulated and dense structure. The mortars show a compressive strength higher than 55 MPa after 28 days and low water absorption by capillarity. This elucidates the feasibility of valorising these industrial residues as precursors for geopolymer cements

Metakaolin-based geopolymers : efflorescence and its effect on microstructure and mechanical properties

Efflorescence in geopolymers results from mobility of excess alkali and consequent crystallization of alkali carbonates. Efflorescence potential of various geopolymers has been reported previously but the knowledge regarding the effect of efflorescence on the microstructure and mechanical properties of geopolymers remains limited. In this work, metakaolin-based geopolymers were exposed to air, partially immersed in water, and fully immersed, to simulate different processes involved in efflorescence formation. The mechanical properties were assessed by compressive, splitting tensile and flexural strengths, and linear deformation. The microstructural features were investigated by SEM, synchrotron XRD, multinuclear MAS NMR, MIP and synchrotron X-ray microtomography. Extensive efflorescence resulted in a reduction of mechanical strength and changes in the nanostructure and microstructure, which is different from observations for Portland cement-based materials, where efflorescence is usually regarded as a surface or aesthetic problem. The understanding of the relationship between efflorescence formation, the synthesis and exposure conditions provides important insight into the manufacturing and application conditions of geopolymer related materials

Thermodynamic modelling of cements clinkering process as a tool for optimising the proportioning of raw meals containing alternative materials

The data contained in this workbook has been used to create the following figures in the journal article: Costa, A. R. D., Coppe, M. V., Bielefeldt, W. V., Bernal, S. A., Black, L., Kirchheim, A. P. and Gonçalves, J. P, 2023. Thermodynamic modelling of cements clinkering process as a tool for optimising the proportioning of raw meals containing alternative materials. Scientific Reports. https://doi.org/10.1038/s41598-023-44078-

Understanding the Nano and Micro Structure of Concrete to Improve its Durability

New and advanced methodologies have been developed to characterize the nano and microstructure of cement paste and concrete exposed to aggressive environments. High resolution full field soft X-ray imaging in the water window is providing new insight into the cement hydration process on the nano scale. Hard X-ray microtomography results on ice inside cement paste and cracking caused by the alkali-silica reaction (ASR) enables 3dimensional structural identification. The potential of neutron diffraction to determine reactive aggregates by measuring their residual strains and preferred orientation is being explored