Optimizing and Characterizing Geopolymers from Ternary Blend of Philippine Coal Fly Ash, Coal Bottom Ash and Rice Hull Ash

Abstract

Geopolymers are inorganic polymers formed from the alkaline activation of amorphous alumino-silicate materials resulting in a three-dimensional polymeric network. As a class of materials, it is seen to have the potential of replacing ordinary Portland cement (OPC), which for more than a hundred years has been the binder of choice for structural and building applications. Geopolymers have emerged as a sustainable option vis-à-vis OPC for three reasons: (1) their technical properties are comparable if not better; (2) they can be produced from industrial wastes; and (3) within reasonable constraints, their production requires less energy and emits significantly less CO 2 . In the Philippines, the use of coal ash, as the alumina-and silica-rich geopolymer precursor, is being considered as one of the options for sustainable management of coal ash generation from coal-fired power plants. However, most geopolymer mixes (and the prevalent blended OPC) use only coal fly ash. The coal bottom ash, having very few applications, remains relegated to dumpsites. Rice hull ash, from biomass-fired plants, is another silica-rich geopolymer precursor material from another significantly produced waste in the country with only minimal utilization. In this study, geopolymer samples were formed from the mixture of coal ash, using both coal fly ash (CFA) and coal bottom ash (CBA), and rice hull ash (RHA). The raw materials used for the geopolymerization process were characterized using X-ray fluorescence spectroscopy (XRF) for elemental and X-ray diffraction (XRD) for mineralogical composition. The raw materials' thermal stability and loss on ignition (LOI) were determined using thermogravimetric analysis (TGA) and reactivity via dissolution tests and inductively-coupled plasma mass spectrometry (ICP) analysis. The mechanical, thermal and microstructural properties of the geopolymers formed were analyzed using compression tests, Fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Using a Scheffé-based mixture design, targeting applications with low thermal conductivity, light weight and moderate strength and allowing for a maximum of five percent by mass of rice hull ash in consideration of the waste utilization of all three components, it has been determined that an 85-10-5 by weight ratio of CFA-CBA-RHA activated with 80-20 by mass ratio of 12 M NaOH and sodium silicate (55% H 2 O, modulus = 3) produced geopolymers with a compressive strength of 18.5 MPa, a volumetric weight of 1660 kg/m 3 and a thermal conductivity of 0.457 W/m-˝C at 28-day curing when pre-cured at 80˝C for 24 h. For this study, the estimates of embodied energy and CO 2 were all below 1.7 MJ/kg and 0.12 kg CO 2 /kg, respectively.

Chemical Treatment of Waste Abaca for Natural Fiber-Reinforced Geopolymer Composite

“…For every 1 kg of fly ash–fiber mixture, 400 g of alkaline activator (well-mixed 80% water glass solution, 20% 12 M NaOH by weight) was added gradually while mixing using a laboratory motorized mixer. The mass ratio was chosen to achieve a geopolymer mix with considerable compressive strength and good workability [39]. Mixing was continued for about 15 min until the mixture appeared homogenous.…”

Rice Hulls as a Renewable Complex Material Resource

“…X-ray diffractograms of the rice hull ash discovered the presence of a low quantity of crystalline phase of cristobalite-SiO 2 in mainly amorphous materials. Geopolymers were formed from the rice hull ash and other amorphous alumino-silicate materials by alkaline activation [19]. The rice hull ash composition was found as SiO 2 70.1%; K 2 O 1.10% and CaO 0.19% by mass.…”

Application of Fly Ash as an Adsorbent for Removal of Air and Water Pollutants

“…Figure 7c,d shows morphologies after FA carbonization, which has the "cubic-like" structures of calcite ( Figure 7c) and "needle-like" structures of aragonite ( Figure 7d). In addition, FA particles are reported to be globular and irregular with high porosity [58], glassy cenospheres [59,60], spherical-shaped and flake-like particles [61,62], spherical particles of varying sizes and particles of unburned coal [59,63,64], and predominantly spherical in shape and consisting of solid spheres, cenospheres, irregular-shaped debris and porous unburnt carbon [15]. In Figure 8, the FA particles are mainly spherical, consisting of solid spheres, cenospheres, irregular fragments and porous unburned carbon.…”