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Alkali-activated materials were prepared from pond ash from the Darkhan city (Mongolia) thermal power station. This ash contains about 60 wt % X-ray amorphous material in addition to quartz, mullite, hematite and magnesioferrite, and presents significant storage problems since it is accumulating in large amounts and is a hazardous waste, containing 90-100 ppm of the heavy metals As, Pb and Cr, and about 800 ppm Sr. Alkali-activated materials synthesized from the as-received pond ash achieved compressive strengths of only 3.25 MPa. Reduction of the particle size by mechanical milling for up to 30 min progressively increases the compressive strength of the resulting alkali-activated geopolymer up to 15.4 MPa. Leaching tests indicate that the combination of milling and alkali treatment does not cause the release of the hazardous heavy metals from the product, making it suitable for construction applications. RESUMEN: Propiedades de geopolímeros preparados a partir de cenizas de estanque.Se prepararon materiales activados alcalinamente a partir de cenizas de charca de la central térmica de Darkhan (Mongolia). Esta ceniza contiene alrededor de un 60% en peso de material amorfo además de mullita, hematita, cuarzo y magnesioferrite. Presenta además importantes problemas de almacenamiento ya que se acumula en grandes cantidades y es un desecho peligroso, que contiene entre 90-100 ppm de metales pesados como Pb y Cr, y alrededor de 800 ppm de Sr. Los materiales sintetizados a partir de las cenizas de estanque presentaron resistencias a compresión de tan sólo 3,25 MPa. La reducción del tamaño de partícula por fresado mecánico hasta 30 min aumentó las resistencias mecánicas hasta valores de 15.4 MPa. Las pruebas de lixiviación indican que la combinación de molienda y activación alcalina no causan la liberación de los metales pesados peligrosos del producto, por lo que resulta adecuado para aplicaciones en construcción.PALABRAS CLAVE: Cenizas de estanque; Molienda; Cementos activados alcalinamente; Resistencias a compresión; Microstructura ORCID ID: J. Temuujin

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The effect of two different milling devices, namely attrition mill versus vibration mill, on the reactivity of fly ash was studied. High calcium fly ash from 4th Thermal power station of Ulaanbaatar (Mongolia) was used for the experiments. The raw and processed samples were characterized by XRD, SEM, Particle size distribution, BET, Blaine surface area and density measurements. The efficiency of 1 hour milling was evaluated with the Blaine surface area set to be more than 5000 cm2/g. The physical and chemical properties of the attrition milled fly ash changed not much compared to the vibration milled samples. For example the d50 particle size became reduced from 29 µm to 6 µm by attrition milling and in vibration milled fly ash it was reduced to 7 µm. The density increased from 2.44 g/cm3 of raw fly ash to 2.84 g/cm3 and 2.79 g/cm3 in attrition and vibration milled samples, respectively. Mechanical milling revealed not only a particle size reduction but also the formation of a denser microstructure. As a result the vibration milled fly ash showed a weaker interaction with the alkaline solution (8 M NaOH used here) compared to the attrition milled fly ash. Consequently, compressive strength of the binder prepared using the attrition milled fly ash was higher, 61 MPa, while for vibration milled fly ash it was 49 MPa. For comparison unmilled fly ash, it was 21 MPa.

Efflorescences formed on the surface of air (ambient) and elevated (70°C) temperatures cured high calcium fly ash based geopolymer type concretes have been characterized by the XRD, SEM-EDX and FTIR techniques. The mineralogical composition of the efflorescence depends on curing temperature. At ambient temperature the main phase of efflorescence consists of atmospheric carbonation product of sodium hydroxide such as thermonatrite (Na2(CO3)·H2O, PDF 8--448), while in the 70°C cured concrete it represents sodium calcium carbonates gaylussite (Na2Ca (CO3)2·5H2O, PDF 21-343). Mineralogical composition difference between the ambient and 70°C cured concrete’s efflolorescences is related to chemical reaction of alkaline liquid with fly ash constituents.