The synthesis and characterization of geopolymers based on metakaolin and high LOI straw ash

Highlights

• The straw ash applied to the metakaolin-based geopolymers has been investigated.

• The 33 wt% straw-ash addition decreases the setting time from 720 min to 120 min.

• FTIR analyses have proved a formation of geopolymer bonds even after ash addition.

• The compressive strengths are negatively influenced by the straw ash addition.

• The straw ash incorporation reduces the polycyclic aromatic hydrocarbons extraction.

Abstract

Incompletely combusted biomass ash containing unburned carbon is very often complemented by polycyclic aromatic hydrocarbons (PAHs). The straw ash containing PAHs, marked as hazardous waste, has been used as a filler in metakaolin-based geopolymers with the purpose of studying the quality of its incorporation and its influence on the resulting properties. The obtained data have shown that although the mineralogical composition of geopolymers with straw ash added is almost identical, the results of compressive strength indicate some differences between the samples depending on the amount of ash added and the setting temperature. The pH measurement has confirmed a significant decrease in the original ash value (11.52) after ash addition into the geopolymer matrix. Fourier transform infrared spectroscopy (FTIR) has proven the formation of geopolymer bonds by alkaline activation even after ash addition. The inhibition of PAHs has been verified through their extraction and determination, with the results showing a significant reduction in the amount of the PAHs detected.

Introduction

Worldwide pressure on environmental protection attaches increasing importance to ecological behavior and to the reduction of the volume of the fossil fuels combusted [1], [2]. The main task is to maintain sustainable industrial development and find renewable energy resources [3].

One of the solutions, apart from wind, sunlight, sea waves and geothermal power, is the utilization of biomass for energy purposes, e.g. wood chips, straw or alternative types [1], [4]. The combustion of biomass or its co-combustion, co-pyrolysis or co-gasification with a coal or various waste materials, such as used tires, plastics and a large amount of corn straw, rice husk and nut shells, etc., have also been the focus of many laboratory studies all over the world [5], [6], [7].

The processing of biomass gives rise to new types of waste materials, whose chemical and mineralogical compositions as well as properties are dependent on the type of material and technology used.

One possibility, the use of biomass ash as a mineral fertilizer [8], [9], has been negatively affected by the increasing effectiveness of burning boilers which reduce the fertilizing efficiency of biomass ashes [10], [11].

Another possibility is the incorporation of biomass ashes by means of alkali-activated materials (AAMs), forming a new group of promising materials. The history of AAMs dates back to 1940, when Purdon’s work on the alkaline activation of slag appeared, followed by research conducted by Glukchovsky in 1959 [12]. In 1979, geopolymers, materials prepared by the alkaline activation of clay materials, were developed and named by Davidovits [12], [13]. Since then, there has been a great boom in this field of science [13], [14], [15], [16] and many laboratories have focused on alkali-activated materials (geopolymers) from different viewpoints, e.g. synthesis and related reactions [12], [13], [14], influencing factors [17], the resulting material properties [16], [18], possible source materials, additives and fillers [18], [19], waste or heavy-metal encapsulation or immobilization [20], [21], and their potential and real applications [14], [22], [23].

Generally, this type of material originates from the reaction of aluminosilicate precursors containing a tetra- or penta-coordinated aluminum ion ([4] Al³⁺ or [5] Al³⁺, respectively) with a solution of sodium, potassium or calcium silicates (alkaline activator), which results in solid polycondensed structures [12], [13], [14]. The resulting amorphous three-dimensional inorganic net is comprised of aluminum and silicon tetrahedra joined by bridging oxygen ions. The negative charge of the aluminum ion in tetrahedron is electrically balanced by the presence of a cation (Na⁺, K⁺, Ca²⁺) selected based on the alkaline activator used [12], [13].

The most widespread and tested aluminosilicate precursor is metakaolin [15], [24], followed by fly ash [25], slag from various sources [18], [24], biomass ashes [26], and various types of secondary or waste materials [19]. The main advantage of clay precursors is the possibility of exploiting unused or waste materials not only from the ceramic industry [27], [28].

The advantage of using alkali-activated aluminosilicate materials lies in their miscellaneous properties, including excellent mechanical properties, high strength [12], water insolubility, compact microstructure and low leachability [13]. These materials are also highly resistant to aggressive solutions [13], high temperatures (up to 1200 °C) and brisk temperature changes [25] and do not produce hazardous gaseous components during thermal exposure [13].

Some properties, such as mechanical properties, durability, porosity, microstructure, thermal and acoustic insulation and long-term stability, could be easily modified through a suitable choice of the matrix and/or aggregates used [14], [15], [17], [18]. A highly durable system based on alkaline activation is a very promising way of reusing different types of industrial by-products or waste materials [19], [21], [29], [30], [31], [32].

The utilization of biomass ashes in connection with geopolymers has been studied in many laboratories from different points of view. Potassium-rich biomass ashes, e.g. straw ash, are very often used as activators for the preparation of geopolymer materials [29]. It has been found that maize ash contains 30–32 wt% of K₂O, which can be used in mixture with water as an alkaline activator for the preparation of metakaolin-based geopolymers [29]. The mechanical properties of geopolymers can be positively influenced by increasing the Si/Al ratio through the addition of a rich SiO₂ source, e.g. rice husk ash [33], [34], sugar cane straw ash [35], and wheat straw [31]. In addition, rice husk ash has been successfully used in the preparation of a one-component geopolymer [26]. Previous research has also shown that the use of wood ash for alkaline activation is possible but problematic due to the very rapid solidification of the mixture [36]. However, all studies have been carried out on ashes that were not contaminated by dangerous substances.

The presented article describes a novel use of geopolymer materials for the incorporation of specific straw ash that contains poly-aromatic hydrocarbons (PAHs), which are carcinogenic [37], [38], [39]. For this reason, this ash must not be used as a mineral fertilizer and is marked and landfilled as hazardous waste. The ash in the original particle distribution is used as a filler in metakaolin-based geopolymers. The effect of the addition of ash on the geopolymer solids obtained and their resulting properties is studied by a leaching test and the measurement of mechanical properties. The formation of principal geopolymer bonding and the creation of amorphous Si-O-Al netting through the alkaline activation are investigated by Fourier transform infrared spectroscopy (FTIR). The inhibition of PAHs is specified through their extraction and determination.