Mineralogical and microstructural characterization of biomass ash binder

Abstract

While the incineration of biomass residues is gaining traction as a globally available source of renewable energy, the resulting ash is often landfilled, resulting in the disposal of what could otherwise be used in value-added products. This research focuses on the beneficial use of predominantly rice husk and sugarcane bagasse-based mixed biomass ashes, obtained from two paper mills in northern India. A cementitious binder was formulated from biomass ash, clay, and hydrated lime (70:20:10 by mass, respectively) using 2M NaOH solution at a liquid-to-solid mass ratio of 0.40. Compressive strength of the biomass ash binder increased linearly with compaction pressure, indicating the role of packing density. Between the two mixed biomass ashes used in this study, the one with higher amorphous content resulted in a binder with higher strength and denser reaction product. Multi-faceted characterization of the biomass ash binder indicated the presence of aluminum-substituted calcium silicate hydrate, mainly derived from the pozzolanic reaction.

Introduction

Biomass residues, rich in carbon due to their biogenic nature, have long been one of the most heavily utilized energy sources and are often consumed alongside coal and other fossil fuels as a feedstock for powering industry. Primary solid biofuels (i.e., plant matter used directly as fuel or converted into solid fuels) are responsible for approximately 9% of global energy production, while they play an even larger role in developing countries, contributing as much as 35% of total energy generation [1,2]. Though traditional uses of biomass, such as in-home burning for heat, are not expected to grow significantly, large-scale industrial biomass incineration for combined heat and power is predicted to triple by 2035, compared with 2008 levels [3]. This expansion is triggered, in part, by regulations touting biomass as a renewable source of energy and its combustion, a CO₂-neutral process [[4], [5], [6]]. While effective as a method of converting waste to energy, combustion often results in significant ash production due to the complex chemical makeup of many feedstocks. The incineration process generally does not consume inorganic constituents, which remain as ash along with a percentage of unburnt carbon, dependent on process temperature and efficiency [7,8].

Agricultural residues, a subset of biomass residue that includes straws, husks, and woods, have been studied by an array of disciplines due to their global prevalence and wide-ranging chemical compositions. The focus of soil and fuel scientists alike, agricultural residues have more recently received the attention of the cement industry and those interested in lower-emission alternatives. While the inorganic content of these residues is initially low, pre-combustion inorganic phases translate reliably to post-combustion ash composition. Therefore, materials such as rice husk ash (RHA) and sugarcane bagasse ash (SCBA) are gaining increasing attention due to their relatively high concentrations of silica (SiO₂), a compound vital to many industrial applications, including the production of cement and its lower energy alternatives, geopolymers and alkali-activated materials (AAMs) [[9], [10], [11]].

Consequently, the number of studies exploring the pozzolanic activity of biomass ashes has seen a steady uptick in recent years. These investigations have examined ash as a supplementary cementitious material, resulting in systems primarily composed of ordinary Portland cement (OPC) [12]. Alkali-activated and geopolymeric systems [[13], [14], [15], [16]] aim to decrease the use of conventional OPC, and have the potential to do so with significantly lower environmental impact than predominantly OPC systems [17]. With increasing interest in environmental sustainability and the reduction of greenhouse gas emissions, there has been a large push for the cement and concrete industries to reduce their carbon footprint, which, by some estimates, accounts for 5 to 7% of global annual CO₂ emissions [18,19]. The advent of these alternatives reflects and advances this development, especially through the beneficial use and up-cycling of industrial byproducts and waste materials such as biomass ashes.

While a number of studies have examined the chemical compositions of biomass residues and their ashes, few have done so with the expressed interest of their applicability to alkali-activated systems as we do in this study [20,21]. Biomass ashes have been found to be extraordinarily variable systems, with an array of physical and chemical properties often related to their original biological functions [7,22]. RHA and SCBA are of particular interest in this study due to their high silica content. With reported levels of SiO₂ in ranging from 65 to 95%, and global production exceeding 30 million metric tons annually [[23], [24], [25], [26]], they are prime candidates for use in both alkali-aluminosilicate and calcium silicate hydrate systems. Through their dissolution in basic media, ashes such as these have been shown to contribute much of their silicon to the formation of networked inorganic polymer and hydration products. The rate of this dissolution is highly dependent on the choice and concentration of solvent and is essential to the formation of strongly networked products [27].

Silica content alone, however, does not render these ashes useful. Combustion temperature of the original agricultural residues is an important determinant of ash reactivity, as it often directly impacts both the particle size and crystallinity of the resulting ashes [28,29]. For alkali-activated systems, high surface area, amorphous particles are known to be the most highly reactive. While high incineration temperatures can often result in smaller particles, they also tend to crystallize inorganic constituents, thereby diminishing particle reactivity [[28], [29], [30]]. Mechanical activation of ashes can also have a beneficial impact on reactivity, as activation directly increases particle surface area [31,32].

In this study, we explore the suitability of mixed biomass ash (composed predominantly of RHA and SCBA) as the primary component in a novel binding material. Motivated by challenges we have observed first hand in India, our focus has been the development of a product which simultaneously reduces the environmental impacts associated with this often-landfilled industrial byproduct and decreases the reliance on conventional topsoil-clay sourced bricks—a product which would have broader impact across the developing world. Collaboration with paper mills in northern India has allowed for the investigation of mixed-feedstock ashes in the pursuit of these goals.

Current work focuses on a detailed mineralogical and morphological characterization of both reactants and binder products resulting from alkali-activation of biomass ash. Earlier efforts by a subset of the authors aimed to establish a robust formulation capable of incorporating various ashes in a sustainable masonry product [33]. Where much of the literature has emphasized the use of coal fly ash as a precursor for AAMs, this paper examines the influence of physico-chemical characteristics of biomass ash on the evolution of mineralogy, microstructure, and properties of alkali-activated biomass ash binders. Furthermore, while some studies have examined the effects of ash pretreatment on reactivity, we make an effort to utilize as-received ashes in an attempt to minimize environmental burden and cost. The biomass ash used in this study exhibited similar compositional characteristics to those reported in previous studies [7,24]. By reacting industrially-sourced biomass ash, locally available clay, and hydrated lime in aqueous sodium hydroxide solution, this work exploits the pozzolanic nature of these precursors through alkali activation to further scientific understanding of a yet unexplored system.