Synthesis of Fly Ash-Based Geopolymers: Effect of Calcite Addition and Mechanical Activation

Calcium wastes as an additive for a low calcium fly ash geopolymer

A geopolymer is a low-carbon cement based on the utilization of waste ash in alkali-activated conditions. Coal fly ash is widely used as a source material for geopolymer synthesis since it contains a sufficient amount of reactive alumina and silica for geopolymerization. Geopolymer products are known to have beneficial fire resistance and mechanical properties. Class F or low-calcium fly ash (LCFA) is generally used as a primary aluminosilicate source; however, heat curing is required to complete the reaction and hardening process and achieve a strong composite. Furthermore, calcium additives are often required to improve the strength of LCFA geopolymers. This paper presents the potential of reusing calcium waste for this purpose. Three calcium wastes, namely calcium carbide residue (CCR), limestone waste, and waste cement (WC) slurry in powder form were used as additives and compared with the use of ordinary Portland cement (OPC). LCFA was replaced with the calcium additives at 20%. However, 20% CCR resulted in flash setting, hence 5% CCR was used instead. A durability test using 3% HCl solution was also performed. The results showed that the reactivity of calcium additives played an important role in strength development. In the calcium-aluminosilicate-alkali system, calcium silicate hydrate (CSH) and calcium aluminosilicate hydrate (CASH) were formed. The maximum strength of 21.9 MPa was obtained from the OPC/LCFA geopolymer, and 3% HCl solution had a deleterious effect on the strength. OPC and CCR were favorable reactive sources of calcium compounds to blend with LCFA. From the thermogravimetric results, lower thermal weight changes with higher strength gains were achieved. Low CaCO3 decomposition at 750 °C according to the TGA curves indicated the more formation of thermally stable CSH and high compressive strength of Ca/LCFA geopolymers.

Co-disposal of municipal solid waste incineration bottom ash (MSWIBA) and steel slag (SS) to improve the geopolymer materials properties

In previous studies, municipal solid waste incineration bottom ash (MSWIBA) exhibited low compressive strength when made into geopolymer materials due to the lack of active Ca. The introduction of steel slag (SS) not only supplements MSWIBA with active Ca, but also enables further treatment of SS, an underutilized solid waste. In this study, mechanical properties, XRD, TGA, FTIR and MIP are the means to evaluate this binary geopolymer. The heavy metal leaching concentration of this geopolymer was used as a basis for assessing its environmental impact. The results show that the introduction of SS helps to improve the compressive strength of geopolymers. The introduction of SS supplements the active Ca and promotes the production of C-(A)-S-H gels. Increasing the alkali doping on this basis contributes to the dissolution of active substances in MSWIBA and SS and promotes the generation of silica-aluminate gels, which likewise contributes to the development of compressive strength of geopolymers. The activation of MSWIBA by alkali can be used as an aluminum removal process, which can reduce the volume of harmful pores in the geopolymer. The solidification efficiency of heavy metals after the introduction of SS can be>90%.

Microstructural and Mechanical Characteristics of Alkali-Activated Binders Composed of Milled Fly Ash and Granulated Blast Furnace Slag with µ-Limestone Addition

Concrete is the most used construction material, needing large quantities of Portland cement. Unfortunately, Ordinary Portland Cement production is one of the main generators of CO₂, which pollutes the atmosphere. Today, geopolymers are an emerging building material generated by the chemical activity of inorganic molecules without the Portland Cement addition. The most common alternative cementitious agents used in the cement industry are blast-furnace slag and fly ash. In the present work, the effect of 5 wt.% µ-limestone in mixtures of granulated blast-furnace slag and fly ash activated with sodium hydroxide (NaOH) at different concentrations was studied to evaluate the physical properties in the fresh and hardened states. The effect of µ-limestone was explored through XRD, SEM-EDS, atomic absorption, etc. The addition of µ-limestone increased the compressive strength reported values from 20 to 45 MPa at 28 days. It was found by atomic absorption that the CaCO₃ of the μ-limestone dissolved in NaOH, precipitating Ca(OH)₂ as the reaction product. SEM-EDS analysis showed a chemical interaction between C-A-S-H- and N-A-S-H-type gels with Ca(OH)₂, forming (N, C)A-S-H- and C-(N)-A-S-H-type gels, improving mechanical performance and microstructural properties. The addition of μ-limestone appeared like a promising and cheap alternative for enhancing the properties of low-molarity alkaline cement since it helped exceed the 20 MPa strength recommended by current regulations for conventional cement.

A critical review on mechanochemical processing of fly ash and fly ash-derived materials

Fly ash (FA) is a solid, fine powder that constitutes a by-product obtained when coal, biomass, municipal solid waste or a mixture of these are combusted. This review article focuses on the mechanochemistry of coal fly ash (CFA), as well as highlights the issue of fly ash from municipal solid waste (MSW). In general, FA is regarded as a waste of public concern (since it contains hazardous components), which is primarily consumed in the construction industry, as well as in chemical synthesis and environmental engineering. However, the actual amount of FA recycled is still less than the amount produced, with the reuse rate of only up to 30 %. Due to its relatively low reactivity and heterogeneity, FA is commonly landfilled in huge quantities. Nevertheless, the physical and chemical properties of FA can be tailored, for example, by mechanical forces, ultimately leading to a higher value-added product. Currently, mechanochemistry (MC) is drawing attention in chemical synthesis, pollution remediation and waste management, especially as a possible solution for various drawbacks of conventional syntheses and processes. Mechanochemical processing of FA can be considered eco-friendly, inexpensive and efficient, in particular for processing tons of readily available fly ash already stored in ponds or landfills. With the aim of highlighting the hidden potential and facilitating the favorable use of FA, this article deals with FA as an environmentally challenging material, FA reactivity and recycling through mechanochemical processing, mechanochemical stabilization of heavy metals in FA, as well as up-to-date challenges for life cycle assessment (LCA) in evaluating FA-derived materials. Furthermore, all these full-potential aspects of FA mechanochemistry have not been addressed before, which is a valuable contribution to the existing literature.

Geopolymers Based on Mechanically Activated Fly Ash Blended with Dolomite

This study reports the effect of natural dolomite addition to fly ash and the mechanical activation of this blend on the geopolymerization process. Dolomite was replaced with fly ash at 1, 3, 5, and 10 wt.%. Geopolymers were synthesized at ambient temperature using NaOH solution as an alkaline agent. The geopolymerization process, reactivity of the raw material, compressive strength, and microstructure were studied using X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetry, and scanning electron microscopy. It was shown that blending fly ash with dolomite and mechanical activation improved the geopolymer strength, especially during the early age of curing. For geopolymers prepared using a 90% fly ash + 10% dolomite blend cured for 7 d, the strengths were 8.2-, 2.3-, and 1.4-fold higher than those for geopolymers prepared using 100% FA for 30 s, 180 s, and 400 s milling times, respectively. A simple method for evaluating the increments of mechanical activation, carbonate additives, and the synergistic effect in the increase in the compressive strength of the composite geopolymer is proposed.

Editorial for Special Issue: Alkali Activated Materials: Advances, Innovations, Future Trends

Alkali activated materials (AAMs), also named geopolymers or inorganic polymers, are materials that are produced when alkaline solutions react with precursors containing aluminosilicate phases [...]

Fly Ash-Based Geopolymer Building Materials for Green and Sustainable Development

This study reports on formulations and conditions for producing fly ash-based geopolymers with a view to showing that the compressive strength required for construction applications can be obtained without the addition of aggregates, sand, and/or cement. It was shown in a series of experiments constituting at least 73% fly ash that a compressive strength of up to 90 MPa can be obtained depending on the curing conditions. While high alkalinity resulted in stronger materials, the results showed about 40% savings in CO₂ emissions without using sand and cement. Such materials are suited for construction applications with minimal environmental impact.