The influence of brine chemistry on the workability, strength development and mineralogy of fly ash pastes

Abstract

The ever-increasing demand for energy due to industrialization and population growth necessitates the combustion of more coal , which results in enormous fly ash production. The scarcity of pot able water as well as high water consumption during mineral and coal processing leads to inevitable saline brine production. Both saline brines and fly ashes pose an environmental threat, where utilization of fly ashes is negligible. The management of industrial brines is a concern for inland industries because oceanic disposal is uneconomical . It is therefore imperative to develop an innovative waste disposal practice . Consequently, are search study was initiated to investigate the suitability of brines for mine back filling as fly ash pastes . The objective was to examine and understand the influence o f brine chemistry on the paste behaviour with a specific fly ash. The tests involved varying paste consistency, brine and fly ash characteristics . A redundant Fine Ash Dam (FAD) was studied to establish what transpires over the long- term when brines interact with fly ash. Results showed that a FAD possesses distinctive chemical , mineralogical and physical properties which r each equilibrium within 4 years . The presence of approximately 60% amorphous content and the intrinsic nature of some properties of weathered ash suggest its viability for utilisation as a pozzo lan . Identified secondary phases include ettringite (Ca6 [Al(OH)6 ] 2(SO4 ) 3 . 26H2O), pyrrhotite (Fe7S8 ) , analcime (NaAl S i 2O6 ·H2O), C-S-H gel, magnetite (Fe3O4 ) , Strätlingite (Ca2Al2SiO2(OH) 1 0 . 8H2O), Friede l ’s salt (Ca3.Al 2O6.CaCl2. 1 0H2 O ) , sillimanite (Al 2 S iO5 ) , periclase (MgO) , calcite (CaCO3 ) , gypsum (CaSO4 . 2H2O) and halite (NaCl ) . Experimental results demonstrated that both salinity and chemical composition of brines determine the paste behaviour. Optimum brine salinity exists between 40 - 60 g/lf or both synthetic and industrial brines, where synergistic advantage of individual constituents is realized in a typical brine. Conversely, acid solutions showed undesired paste behaviour hence should be excluded in paste preparation. Chlorides seemed to accelerate the strength development while the use of sulphate solutions to make paste will necessitate stringent monitoring. In contrast, it was discovered that both physical and chemical properties of fly ash, not just alkalinity determine its effectiveness in immobilizing prevalent inorganic species from brines in the paste. This doctoral thesis has demonstrated that co-disposal of brines with fly ash as paste if properly engineered will mitigate environmental pollution .