Phase and dimensional stability of volcanic ash-based phosphate inorganic polymers at elevated temperatures

Phosphate geopolymers are part of chemically bonded phosphate cements obtained from an aluminosilicate and phosphate solution. Their structure consisting of phosphate bonds makes them suitable for use as refractory material. This study deals with the influence of phosphoric acid concentration (6, 8 and 10 mol/L) on the stability of volcanic ash-based phosphate geopolymers exposed to 100, 600 and 1000 °C. The results reveal that the onset of crystallization is about 600 °C with the formation of aluminum phosphate (V) and tridymite, then crystallization of iron (III) phosphate (V) and hematite at 1000 °C. The degree of crystallization of these phases increases with phosphoric acid concentration. The geopolymers obtained with 8 mol/L of phosphoric acid showed the best thermal stability at 1000 °C in terms of compressive strength change. The maximum thermal linear shrinkage recorded was 3%. The major phases of all geopolymers remain stable up to 1000 °C, after which the melting of phases happens.TU Berlin, Open-Access-Mittel – 202

Rational utilization of volcanic ashes based on factors affecting their alkaline activation

WOS:000405154700006International audienceThe present study evaluates the criteria that affect the characteristics of both volcanic ashes and their alkaline activated products, hence their rational utilization. To that end, five samples of volcanic ash were examined chemically, mineralogically and physically whereas certain characteristics of their alkaline activated products were determined. In order to elucidate the relationship between the characteristics of volcanic ashes and the products of their alkaline activation, mechanical strength, Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) of geopolymer specimens aged 14 days were examined. Volcanic ashes with large amount of amorphous phase (18.2-42.5% by mass) led to geopolymers with compressive strengths of 3.1-12.6 MPa. As for volcanic ashes with small amount of amorphous phase (10.2-12.5% by mass), alkaline activation at 80 degrees C led to geopolymers with compressive strength of 1.0-4.1 MPa. The results obtained showed that volcanic ashes could be used either for geopolymer synthesis or as filler. The classification depended mostly on the amount and the SiO2/Al2O3 molar ratio of amorphous phase. (C) 2017 Elsevier B.V. All rights reserved

Role of washing process in the improvement of surface properties of porous geopolymers

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Pozzolanic activity of kaolins containing aluminum hydroxide

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Net-Shape Clay Ceramics with Glass Waste Additive

International audienceIn this paper, a glass powder from waste containers was mixed (10 - 40 wt.%) with a kaolinitic sandy clay from Cameroon to elaborate net-shape ceramics, fired at 1100°C. The sintering behavior was from dilatometry and thermo gravimetric analyses together with the characterization of porosity and flexural strength. The increase of glass to kaolinite ratio reduces the sintering shrinkage leading to a none-densification sintering when 40 wt.% of glass is added in the mixture. The volume variation during the whole firing process is from the individual volume variations during the quartz transformation, the structural reorganization of kaolinite and during sintering. Quartz size and relative quantity have a significant role on the first processes since it leads to either cohesive or un-cohesive behavior. But the glass quantity strongly controls the second and the third thermal processes because glass additions change the recrystallization processes, leading to the formation of dense clay-glass agglomerates distributed within the three dimensional quartz network

Additional file 4: of Red ceramics produced from mixtures of kaolinite clay and waste glass

Table S4. Chemical composition of waste glass V (LOI = Loss On Ignition)

Amorphous phase of volcanic ash and microstructure of cement product obtained from phosphoric acid activation

International audienceAlkaline activation of volcanic ash using Na2SiO3 as activator showed its limits because of their low reactivity, nevertheless, phosphoric acid activation using H3PO4 as activator can offer a great possibility of utilization of the latter aluminosilicate. The present study aims to determine the chemical composition of amorphous phase of volcanic ash and examine through the microstructure of the obtained cement-based phosphoric acid activation the relation between silicon and phosphorus atoms. Chemical composition of amorphous phase was determined by dissolution with NaOH and HCl. Cement fresh paste was subjected to isothermal calorimetry and temperature variation. Meanwhile, the hardened paste was characterized by X-ray diffraction, thermogravimetry analysis, scanning electron microscopy, energy dispersive spectrometry and Fourier transform infrared spectroscopy in order to study the microstructure. Also, the cement product was submitted to compressive strength test. The results obtained showed that, the amorphous phase of volcanic ash contain SiO2, Al2O3, Fe2O3, MgO and CaO oxides. This study allowed to understand the fact that phosphoric acid activation of volcanic ash at ambient temperature is an exothermic reaction and its described as a dissolution/precipitation process. Also, the obtained product was hydrated. Compressive strength of the cement product aged 1 and 28 days was respectively 20.6 and 30.5 MPa. Based on SEM/EDS analysis, silicon are partially replaced by phosphorus atoms and the product is made of –Si–O–Al–O–P–O–phospho-sialate network. Phosphoric acid activation is a promising route of valorisation of volcanic ash

Pozzolanic activity of kaolinite material rich in gibbsite calcined at low temperature and its effect on physical and mechanical properties of Portland cement mortars

Gibbsite in tropical kaolinite material greatly influences its reactivity as pozzolanic material. The aim of this research is to study the effect of gibbsite, Al(OH)3, (15.9 mass%) on the pozzolanic activity of a kaolinite material calcined at low temperature. To this effect, the as-received raw kaolinite material was calcined at 600°C and the output product was used to partially replace Portland cement by 0, 10 and 20 mass% to produce mortars. The calcined kaolinite material was firstly subjected to the modified Chapelle test and strength activity index in order to evaluate its pozzolanic activity and physical and mechanical properties of the as-produced specimens were assessed. The results show that the calcined kaolinite material presents a high pozzolanic activity (Chappelle test = 1665 mg of Ca(OH)2/g) compared to the reference un-calcined kaolinite material (535.0 mg of Ca(OH)2/g). After 28 days of curing, mortars obtained from partial replacement with 10 mass% of the calcined kaolinite material show higher compressive strength (54.5 MPa) compared to those obtained with 20 mass% (51.3 MPa). Indeed, these values are higher compared to those of mortars produced without replacement. In fact, aluminium compound promotes the formation of metastable hydrated phases (CAH10/C2AH8) at early age which temporally hinder cement hydration. Conversely, these phases are progressively transformed into stable hydrated phases of C3AH6 with time, thereby favoring the hardening of specimens. Thus, partial replacement of Portland Cement by 10 mass% of the calcined kaolinite material is suitable to obtain mortars endowed with enhanced compressive strength for construction purpose

Reaction sintering and microstructural evolution in metakaolin-metastable alumina composites

Fine needles of mullite grains were obtained successfully in a compact and low porous matrix using solid state sintering. We treated high-grade kaolin and sand-rich kaolin at 750 °C to amorphous metakaolins, and bauxite at 1,000 °C to metastable alumina. By designing a stochiometric composition of mullite, each amorphous metakaolin was added to metastable alumina. Fine grains of mullite with almost complete crystallization were obtained from 1,350 °C in a case of amorphous metakaolin from high-grade kaolin and at 1,550 °C in the other case where amorphous metakaolin is from sand-rich kaolin. The difference in the temperatures of mullitization was linked to the late dissolution of silica from the cristobalite and quartz phases which were still present in the sand-rich metakaolin sample at 1,350 °C. The use of metastable alumina and metakaolin instead of kaolin to design the mullite matrix allows the increase in number of mullite nucleation sites. This results to high densification and crystallization, fine grain size, and high mechanical properties of the final matrix. © 2014 Akadémiai Kiadó, Budapest, Hungary