K-Based Geopolymer from metakaolin: roles of K/Al ratio and water or steam Curing at different temperatures

Abstract

K-based geopolymer binder was prepared by reacting metakaolin with alkaline solutions having different potassium contents and by water curing at room temperature and 80 °C as well as steam curing at 150 and 180 °C. The phase formation, microstructure and Al and Si nearest neighbor environments were studied using XRD, TEM and 27 Al and 29 Si MAS NMR spectroscopy, respectively. The results revealed that amorphous alumino-silicates were predominant in geopolymer prepared by curing up to 28 days at room temperature or at 80 °C. The amorphous alumino-silicates persisted after hydrothermal treatment at 150 °C/48 hrs and even at 180 °C/30 hrs. However, the samples cured hydrothermally at 180 °C/48 hrs revealed formation of crystalline potassium aluminum silicate and chabazite phases. The Al nearest neighbor environments was not significantly affected by increasing the K/Al ratio up to 1.55 or by the curing temperatures. On the other hand, the geopolymer reaction appears to have increased when cured at 80 °C or steam cured at 150° and 180 °C and crystalline aluminosilicates resulted when the geopolymer sample was hydrothermally treated at 180 °C/48 hrs. RESUMEN: Geopolímeros de metakaolin basados en potasio: papel de la relación K/Al y del curado en agua o con vapor a diferentes temperaturas.Se prepararon geopolímeros por reacción de metacaolín con disoluciones alcalinas con diferentes contenidos de potasio. Se realizó un curado con agua a temperatura ambiente y a 80 °C, y con vapor de agua a 150 y 180 °C. La formación de las diferentes fases así como la microestructura y entornos del Al y Si se estudiaron mediante DRX, TEM y espectroscopia de RMN MAS de 27 Al y 29Si. Los aluminosilicatos amorfos fueron predominantes en aquellos geopolímeros sometidos a curado de hasta 28 días a temperatura ambiente o a 80 °C. Los aluminosilicatos amorfos persistieron tras el tratamiento hidrotermal a 150 °C/48 horas e incluso a 180 °C/30 h. Sin embargo, las muestras curadas hidrotermalmente a 180 °C/48 h revelaron la formación de fases cristalinas de silicatos de aluminio y potasio, así como de chabazita. Los entornos de Al no resultaron afectados significativamente por el aumento de la relación K/Al de hasta 1.55, ni por las temperaturas de curado. Por otro lado, la reacción de geopolimerización aumentó cuando el curado se realizó a 80 °C o con curado a vapor a 150 y 180 °C.PALABRAS CLAVE: Geopolímero basado en potasio; Curado con vapor; Distribución de tamaño de partículas; Resonancia Magnética Nuclear (RMN)

Waste glass as a precursor in alkaline activation: Chemical process and hydration products

“…The findings illustrated in Figure 2 infer that the curing conditions (relative humidity) had a likewise significant impact on strength development in the waste glass pastes activated by all the solutions used except KOH. Whilst a number of studies have analysed changes in alkaliactivated materials properties under different curing temperatures [7,[69][70][71][72], very few have been conducted on the effect of relative humidity on curing [73,74]. One possible explanation for the higher mechanical strength when the pastes were heat-cured at a lower relative humidity is the significant role played by the water (adsorbed, absorbed or both) present in the this materials and its subsequent evaporation in the generation of a product with fewer microcracks or micropores [73].…”

Effect of K/Al Molar Ratio on the Thermo-Mechanical Properties of Metakaolinite-Based Geopolymer Composites

“…For example, in the sample GB-0.55, the crystalline phase was visible only at a temperature equal to 1200 • C. On the contrary, in the sample GB-1.30, the crystalline phases were visible already at a temperature equal to 800 • C. The kalsilite content increased with increasing ratios of K/Al. Leucite and kalsilite are common crystalline phases for potassium geopolymer binders formed after heat exposure at temperatures ranging from 800 • C to 1300 • C [36,[38][39][40][41]. Crystalline impurities such as quartz (SiO 2 ), anatase (TiO 2 ), and muscovite (K(Al 4 Si 2 O 9 (OH) 3 )) were present in all geopolymer binders.…”

“…The molar ratio of K/Al also have a significant influence on the properties of the geopolymer, but so far, nearly no research has been carried out on discovering the effect of K/Al molar ratio on the thermo-mechanical properties of geopolymers. Tawfik et al [36] investigated the effect of the K/Al molar ratio between 0.9-1.55 and type of curing on the microstructure and phase formation of a metakaolinitebased geopolymer. They discovered that increasing the molar ratio of K/Al had no significant effect on phase composition of the geopolymer binder.…”

A Novel MK-based Geopolymer Composite Activated with Rice Husk Ash and KOH: Performance at High Temperature

“…The production of commercial silicate solutions (Na 2 SiO 3 ·nH 2 O and K 2 SiO 3 ·nH 2 O) involves a high energy demand (temperatures close to 1000 °C-1400 °C) and the emission of greenhouse gases (7,8). In recent years there has been great interest for the use of alternative sources of amorphous reactive silica for producing alkaline activators: rice husk ash (3,4,(9)(10)(11)(12)(13)(14)(15)(16)(17), silica fume (2,3,(18)(19)(20)(21) and waste glasses (1,(22)(23)(24)(25) have been used and have shown similar microstructure and mechanical performance to geopolymers produced with commercial soluble silicate solutions (3,9,18,26,27).…”