Geopolymer Preparation Research Articles

Utilization of waste phosphogypsum in high-strength geopolymer concrete: Performance optimization and mechanistic exploration

Phosphogypsum (PG) is an industrial waste produced during the manufacture of phosphoric acid. In this study, dihydrate phosphogypsum (DPG), ground granulated blast furnace slag (GGBFS), fly ash (FA), Portland cement clinker (PCC), sulfate alumina cement clinker (SACC), water reducer, sand, and stone were combined to create a phosphogypsum-based geopolymer (PBG) concrete. Concrete mix design was carried out based on the principle of maximum packing density, and the effects of water reducer type, sand ratio, water-to-binder ratio, and DPG content on PBG concrete properties were investigated. Results indicated that compared to SiKa ViscoCrete 540P (SVC 540P) water reducer, Melflux PLUS 1087L (MP1087L) significantly enhanced the performance of PBG concrete. With a water-to-binder ratio of 0.34, a sand ratio of 32.2 %, and a PG:GGBFS:FA ratio of 5:4:1, the 28-day unconfined compressive strength (UCS) of the concrete exceeded 60 MPa. The concrete primarily consisted of phases such as dihydrate gypsum (CaSO4·2H2O), ettringite (Ca6(Al(OH)6)2(SO4)3(H2O)26), polymer (-Si-O-Al-), quartz (SiO2), albite (Na(AlSi3O8)) and Muscovite ((K,Na)Al2(Si,Al)4O10(OH)2). Using two mild alkalis as activators, PBG concrete can avoid the efflorescence often associated with geopolymer preparation and can cure at 20 °C. PBG concrete offers high strength, ease of preparation, and environmental benefits, providing a new avenue for the reuse of PG.

Use of pumice stone and silica fume as precursor material for the design of a geopolymer.

Geopolymers are alternative materials to cement because they require less energy in their production process; hence, they contribute to the reduction in CO 2 emissions. This study aims to evaluate the possibility of using industrial residues such as silica fume (SF) to improve the physical and mechanical properties of a pumice stone (PS)-based geopolymer. Through an experimental methodology, the process starts with the extraction, grinding, and sieving of the raw material to carry out the physical and chemical characterization of the resulting material, followed by the dosage of the geopolymer mixture considering the factors that influence the resistance mechanical strength. Finally, the physical and mechanical properties of the geopolymer were characterized. This research was carried out in four stages: characterization of the pumice stone, design of the geopolymer through laboratory tests, application according to the dosage of the concrete, and analysis of the data through a multi-criteria analysis. It was determined that the optimal percentage of SF replacement is 10%, which to improves the properties of the geopolymer allowing to reach a maximum resistance to compression and flexion of 14.10 MPa and 4.78 MPa respectively, showing that there is a direct relationship between the percentage of SF and the resistance. Geopolymer preparation involves the use of PS powder with a composition rich in silicon and aluminum. The factors influencing strength include the ratio of sodium silicate to sodium hydroxide, water content, temperature, curing time, molarity of sodium hydroxide, and binder ratio. The results showed an increase in the compression and flexural strength with 10% SF replacement. The geopolymer's maximum compressive strength indicates its non-structural use, but it can be improved by reducing the PS powder size.

New geopolymer preparation methods and its application by recycle of double-waste: ground granulated blast furnace slag and fly ash

ABSTRACT This article mainly studies new geopolymer preparation methods by recycle of double waste:ground granulated blast furnace slag (GGBFS) and fly ash as geopolymer precursor, through different mass ratio of GGBFS and fly ash, to prepare geopolymers by alkali activation, the samples is cured at ambient temperature and the mechanical test is carried out together with the analysis of XRD, SEM and EDS to research the mechanism of strengthening and geopolymerisation. After analysis we find that the sample prepared by 30% mass ratio of GGBFS have good initial strength and short setting time, as well as best mechanical performance, which is attributed to moderate Ca2+ introduction accelerates the speed of geopolymerisation and the reactants of N-A-S-H gel, C-A-S-H gel and hydrated C-S-H help to bridge the gaps between the different hydrated phases, unreacted particles and matrix, result in a homogeneous and compacted geopolymeric sample. When GGBFS mass ratio above 50%, the mechanical of samples is deteriorated due to too much hydrated calcium containing compounds are formed and covered on the unreacted GGBFS particles surface to block further geopolymerisation. The acid attack test and water resistance test are carried out to analysis the durability between 30% GGBFS introduced geopolymer and ordinary Portland cement (OPC). In additional, a field application in case of 30% GGBFS introduced geopolymer grouting repair material indicated desirable construction based on visual observation and ground-penetration radar scanning.

Comparative study on the preparation of phosphate-based geopolymers using different activators

Fly ash has the potential to be used as the precursor in the acid-activated geopolymer preparation, fly ash can react with phosphorus-containing activators to form geopolymers, but different types of activators present different activation effects on fly ash. In this study, common liquid phosphoric acid (PA), liquid aluminum dihydrogen phosphate (ADP), solid sodium dihydrogen phosphate (SDP) and solid potassium dihydrogen phosphate (PDP) are used as a single-component or composite activators and fly ash as raw material to prepare phosphate-based geopolymers (PGEOs). The effect of different activators on setting time, pH, electrical conductivity (EC) and compressive strength of PGEOs were assessed. Moreover, the activation mechanism of different activators on fly ash was revealed by using X-ray diffraction, Fourier transform infrared spectroscopy, Mercury Intrusion Porosimetry, X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy-Energy Dispersive Spectroscopy. The results showed that geopolymer prepared using ADP exhibited the highest compressive strength among the different activators, but combining ADP and PA would prolong setting time. However, the unreacted SDP and PDP particles were detected in geopolymers activated by PA+SDP and PA+SDP composite activator due to insufficient geopolymerization. Nonetheless, combining PDP or SDP with PA as composite activator is more effective than the single-component PDP or SDP activator in improving compressive strength. The pH values of the geopolymers increased with the curing time while EC showed a decreasing trend which was independent of the activator type. AlPO4 phase was found in all samples, and Al-containing phases are the main strength-given component of PGEOs.

Use of pumice stone and silica fume as precursor material for the design of a geopolymer

Background Geopolymers are alternative materials to cement because they require less energy in their production process; hence, they contribute to the reduction in CO2 emissions. This study aims to evaluate the possibility of using industrial residues such as silica fume (SF) to improve the physical and mechanical properties of a pumice stone (PS)-based geopolymer. Methods Through an experimental methodology, the process starts with the extraction, grinding, and sieving of the raw material to carry out the physical and chemical characterization of the resulting material, followed by the dosage of the geopolymer mixture considering the factors that influence the resistance mechanical strength. Finally, the physical and mechanical properties of the geopolymer were characterized. This research was carried out in four stages: characterization of the pumice stone, design of the geopolymer through laboratory tests, application according to the dosage of the concrete, and analysis of the data through a multi-criteria analysis. Results It was determined that the optimal percentage of SF replacement is 10%, which to improves the properties of the geopolymer allowing to reach a maximum resistance to compression and flexion of 14.10 MPa and 4.78 MPa respectively, showing that there is a direct relationship between the percentage of SF and the resistance. Conclusions Geopolymer preparation involves the use of PS powder with a composition rich in silicon and aluminum. The factors influencing strength include the ratio of sodium silicate to sodium hydroxide, water content, temperature, curing time, molarity of sodium hydroxide, and binder ratio. The results showed an increase in the compression and flexural strength with 10% SF replacement. The geopolymer’s maximum compressive strength indicates its non-structural use, but it can be improved by reducing the PS powder size.

A novel investigation of co-processing porous geopolymer using glass fibres recycled from waste turbine blades

Abstract This work investigates the possibility of using recycled glass fibre (rGF) to co-process porous geopolymer. A new procedure was developed to incorporate rGF into geopolymer preparation and the content of rGF in porous geopolymer was varied to investigate its effect on thermal and mechanical properties. The rGF was co-processed through mixing with a combination of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solution to supplement raw materials required to make geopolymer but also provide fibres as potential structure reinforcement. Due to the enriched silica content obtained from rGF and leftover fibres, a nearly 80 % strength increase (from 0.092 MPa to 0.165 MPa) was obtained without compromising its low thermal conductivity. This opens up a potential route to re-using rGF in the development of improved porous geopolymer for thermal insulation applications.

Utilizing municipal solid waste incineration bottom ash and volcanic tuff to produce geopolymer materials

Large volumes of municipal solid waste incineration bottom ash (MSWIBA) are produced annually in China; however, their improper disposal causes serious ecological and environmental problems. This study investigated the feasibility of using MSWIBA and volcanic tuff to produce geopolymers. The effects of the MSWIBA content, silicon–sodium molar ratio, and alkali equivalent on the uniaxial compressive strength (UCS) and bulk density of the geopolymer samples were examined using single-factor and optimization tests. The test results showed that the UCS and bulk density of the geopolymer decreased as the MSWIBA content increased, whereas they increased as the alkali equivalent increased. As the silicon–sodium molar ratio increased, the UCS first increased and then decreased, and the change in bulk density was limited. In the final geopolymer preparation, the MSWIBA content was 17.85%, the silicon–sodium molar ratio was 1.71, and the alkali equivalent was 9.49%. According to X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, energy-dispersive spectrometry, and nuclear magnetic resonance analyses, the main phases of the geopolymer under the final preparation parameters were quartz, sanidine, calcite, orthoclase, gismondine, and illite. During the geological polymerization process, a large amount of CNASH gel was generated and attached to the surface of the particles or filled the pores between the particles, resulting in a pore size of 0.01–1 μm. The heavy metals in the geopolymers exhibited a good stabilization effect owing to the CNASH gel. Except for Cd, which posed a medium risk, the remaining heavy metals posed low or no risk.

Enhancing reactivity of granite waste powder toward geopolymer preparation by mechanical activation

Granite waste powder (GWP) is a solid waste produced during stone processing, and its accumulation has brought serious environmental and ecological problems. Due to its rich Si and Al components, GWP is a potential raw material for the preparation of geopolymers, but its low reactivity limits its utilization. This study aimed to enhance the reactivity of GWP toward geopolymer preparation by mechanical activation. During mechanical activation, the particle size of GWP decreased rapidly, and its internal pore volume gradually increased, which made its specific surface area continuously increase. The relative crystallinity of the main mineral phases in GWP decreases continuously with mechanical activation, and the aluminosilicate minerals (including albite, microcline, and biotite) are more easily amorphized than quartz. Leaching tests show that activated GWP can dissolve higher percentages of silicon and aluminum components in alkaline solutions. After 120 min of activation, the dissolution percentage of Si increased from 1.55% to 6.25%, and that of Al increased from 4.83% to 20.86%. The reaction heat result shows that the early geopolymerization involving activated GWP releases higher cumulative heat, which reflects a higher degree of reaction. Also, geopolymer incorporated with activated GWP, its compressive strength increased from 50.83 MPa to 78.94 MPa, and its flexural strength increased from 7.2 MPa to 10.0 MPa. These results confirmed that the reactivity of GWP can be significantly enhanced by mechanical activation. This technology may provide an efficient and promising method to improve the reactivity of GWP and promote its utilization in the preparation of geopolymers.

Synergistic utilization of industrial waste red mud and rice husk ash for eco-friendly geopolymer preparation: enhancing strength and mitigating hazardous leaching.

The prolonged stacking of substantial volumes of industrial waste red mud (RM) can have significantly hazardous effects on the environment. In order to address this critical problem, this study proposes the synergistic preparation of geopolymers utilizing RM in conjunction with another industrial waste, rice husk ash (RHA). Geopolymers with varying incorporation of RHA were prepared using sodium hydroxide and sodium silicate composite alkaline activator. The mechanical properties, microstructure, and environmental characteristics of geopolymers were investigated. The incorporation of RHA significantly enhanced the strength of RM-based geopolymers, with the highest strength of 25.40MPa achieved at 40% incorporation. According to XRD patterns and FTIR spectra, C-(A)-S-H and N-(A)-S-H were generated during the geopolymerization, thereby enhancing the strength of geopolymers. From SEM micrographs of geopolymers, it was evident that the geopolymer matrix was constituted by the encapsulation of unreacted inert particles of RM and residual fragments of RHA with C-(A)-S-H and N-(A)-S-H. The leaching levels of trace elements and heavy metals in geopolymers are both below the regulatory thresholds, thereby effectively mitigating the presence of hazardous substances in raw materials. These findings proved that the reuse of RM and RHA for the synergistic preparation of environmentally friendly geopolymers is a promising approach to address the issue of substantial RM stacking.

Synchronous recycling of multi-source solid wastes for low-carbon geopolymer preparation: Primary factors identification and feasibility assessment

Exploring efficient utilization technology for solid wastes is important for the sustainable development, and the geopolymer preparation is a potential strategy for the utilization of multi-source solid wastes. However, the complex components in wastes disturbed the actual compositions in raw materials, which limited the strengthening of geopolymer and simultaneous utilizing of multi-source solid wastes. This study aims to determine the primary factors and preparation feasibility of multi-source solid waste-based geopolymer (MSWG). In this study, the geopolymer with multi-source wastes showed significant advantages in mechanical property and emission/cost reduction than that of geopolymer with unitary waste. The mechanical strength of MSWG can reach 115.77 MPa, which increased by 481.76% and 20.88% than that of unitary fly ash-based geopolymer (19.90 MPa) and blast furnace slag-based geopolymer (95.77 MPa), respectively. Meanwhile, the silicon species, aluminum species, and their connectivity forms in different MSWG were characterized and quantitatively calculated by deconvolution, which provided a feasible method for identifying the primary factors (Si–O–Al bond, Q4(3Al), Q4(4Al), and [AlO4]-Q4(4Si)) for mechanical strength. Furthermore, the composite of wastes provided more significant advantage for geopolymer preparation in emission/cost reduction, and the carbon emissions (0.261–0.315 t-CO2/t) of MSWG were reduced by 62.55%–68.90% than that of ordinary Portland cement. These results demonstrated the potential of MSWG in preparation feasibility, strength enhancement, and emission/cost reduction, and provided a feasible and cost-effective method for the simultaneously resourceful treatment of different solid wastes.

Microwave-thermal-assisted curing method on geopolymer preparation from Panzhihua high-titanium slag by alkali activation

This paper emphasizes the feasibility of using microwave-thermal-assisted treatment to promote the rapid geopolymerzation and early strength improvement of Panzhihua high-titanium slag (HTS)-based geopolymers. The results reveal that the microwave radiation promotes the geopolymerzation reaction of the HTS during the alkaline activation reaction with a remarkable early compressive strength improvement of 623% only in 20 min. Since the dispersed CaTiO3 particles in the HTS serve as local thermal hot spots and nucleation elements, they will rapidly elevate the temperature in a short time and facilitate the hydration products growing to form a gel network structure. Moreover, due to more water evaporation and accelerating effects of the endothermic reaction in the dissolution-polycondensation stage, the optimum process of microwave-thermal-assisted treatment has further advanced the hydration process, which endowed the highest geopolymerization degree, the lowest residue water, the densest microstructures of geopolymeric network and a desirable mechanical performance (54 MPa) of the HTS-based geopolymer.

Enhancement of geopolymerisation reactivity and thermal resistance using mixed types of fly ash

Geopolymers prepared with class C fly ash (FA) often suffer from a flash geopolymerisation rate, making it difficult to work. Class F FA, however, may have low reactivity, so an admixture must be added to solve this problem. This research highlighted the use of mixed class C and class F FA as a precursor for geopolymer preparation to alleviate such problems. Optimisation of the precursor recipe and the solid-to-liquid ratio (S/L) was assessed. The highest compressive strength (30.54 MPa) and a suitable initial (136 min) and final (240 min) setting times were obtained with S/L = 2.00, and 50 : 50 proportions of class C to class F FA. The fire resistance test on a 10 mm thick geopolymer panel at 1000°C for 180 min showed a maximum temperature of ∼570–580°C on the reverse side without catastrophic failure, suggesting it can be applied for use in fire retardation.

Seawater used to Metakaolinite-based geopolymer preparation

The use of seawater (SW) to prepare geopolymers has attracted increasing attention, for its potential to save fresh water and reduce carbon dioxide emission. However, the ions in SW influence the geopolymers. This study investigated the effects of SW concentration on the microstructure and mechanical properties of metakaolinite (MK)-based geopolymers with different Si/Al molar ratios (1.3, 1.6, and 2.0). The compositions and structures of the obtained geopolymers were characterized via several techniques, including X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and mercury intrusion porosimetry.With an increasing Si/Al molar ratio, the high soluble silica content promoted geopolymer formation, further increasing the microstructure density and compressive strength of the geopolymer. The reaction between MK and alkaline solution with an Si/Al ratio of 1.3 resulted in the formation of a zeolite A-containing geopolymer, which exhibited a 28-day compressive strength of only 12.7 MPa. SW addition did not significantly influence the geopolymer microstructure and mechanical properties, but zeolite formation was inhibited with increasing SW concentration.In contrast, geopolymers with Si/Al ratios of 1.6 and 2.0 exhibited a dense microstructure, with 28-day compressive strengths of 35.0 and 57.5 MPa, respectively. The SW-mixed geopolymers exhibited a lower compressive strength than the pure water-mixed geopolymers. With increasing SW concentration (artificial SW), the geopolymer compressive strengths gradually decreased, but to a limited extent. The ions in SW altered the geopolymer composition, resulting in the formation of some salts. The formation of these salts negatively influenced the microstructure and pore size distribution of the geopolymer, thereby reducing its compressive strength. In addition, Cl- was distributed in the geopolymer matrix, but SO42- underwent precipitation at a high concentration of artificial SW.Overall, the results showed that the ions in SW reduced the compressive strength slightly by changing the microstructure and composition of the MK-based geopolymer. These findings suggest that SW-mixed MK-based geopolymers can be suitably used as building materials in the marine environment.

Characterization of Volcano-Sedimentary Rocks and Related Scraps for Design of Sustainable Materials.

This work started as a joint academia and company research project with the aim of finding new applications for domestically sourced volcanic products and related waste (pumice, lapillus, zeolitic tuff and volcanic debris from Tessennano and Arlena quarry) by creating a database of secondary volcanic raw materials and their intrinsic characteristics to help industry replace virgin materials and enhance circularity. In this context, accurate chemical, mineralogical, morphological, granulometric and thermal characterizations were performed. Based on the results presented, it can be concluded that due to their lightness, these materials can be used in the design and preparation of lightweight aggregates for agronomic purposes or in the construction field. Furthermore, due to their aluminosilicate nature and amorphous fraction, pumice and lapillus can play the role of precursor or activator for geopolymer preparation. With its porous nature, zeolitic tuff can be exploited for flue gas treatment. Due to the presence of feldspathic phase (sanidine), these materials can be used in tile production as a fluxing component, and with their pozzolanic activity and calcium content, they have application in the binder field as supplementary cementitious material or as aggregates.

A thermo-mechano-chemical activation technique to use quartz rich marine clay for one-part geopolymer preparation

This research aims to prepare one-part geopolymer by using the quartz rich marine clay available in Singapore. The higher quartz and lower kaolinite mineral (∼29%) in marine clay is not ideal for a precursor material in geopolymerization. Hence, a novel thermo-mechano-chemical activation process is employed. The process of thermal activation (calcination at 850 ° C) transforms kaolinite into reactive metakaolin form. The mechanical activation by ball milling alters the surface group of quartz by forming non-bridging oxygen. The chemical activation process is promoted by adding alkali materials, such as NaOH, Na2CO3, and Ca(OH)2. XPS results inform the presence of higher non-bridging oxygen content in Na2CO3 and NaOH based mixtures than Ca(OH)2 based mixture. However, more reactive Q0(0Al) and Q1(1Al) states of 29Si are observed in Ca(OH)2 based mixture from NMR spectra. This resulted in higher compressive strength of Ca(OH)2 based one-part mixture. A temperature curing (at 80 ° C) is required to facilitate the geopolymerization process.

Recycling of steel slag as an alkali activator for blast furnace slag: geopolymer preparation and its application in composite cement

Recycling of steel slag as an alkali activator for blast furnace slag: geopolymer preparation and its application in composite cement

Activation Mechanism of Coal Gangue and Its Impact on the Properties of Geopolymers: A Review.

Coal gangue is one of the industrial solid wastes that may harm the human body through the ecosystem for a long time. Using coal gangue in geopolymer preparation can effectively reduce cement output and meet the sustainability requirements. In this paper, the physical and chemical characteristics, including the heavy metal content, of coal gangue from different producing areas are described. Then, the mechanism of physical activation (mechanical and thermal activation), chemical activation, and compound activation of coal gangue are illustrated. The machinability, as well as the mechanical, microscopic, and toxicity consolidation properties of geopolymers prepared from coal gangue, are summarized and analyzed. The results indicate that the coal gangue geopolymers can have higher mobility and mechanical strength than cement-based composites by adjusting high calcium element material, alkali activator content, Na2SiO3 modulus, and curing condition. After physical activation, coal gangue is used in geopolymer preparation with a chemical activator (alkali excitation agent), which effectively forms a three-dimensional silicon aluminate polymer network. The pore structure is dense, the physical fixation and chemical bonding are strengthened, and the solidification and adsorption of heavy metal ions are improved. Further, it can also be applied to solidifying radioactive waste, which is following the future development direction.

Coupled microwave hydrothermal dechlorination and geopolymer preparation for the solidification/stabilization of heavy metals and chlorine in municipal solid waste incineration fly ash

To improve the degradation efficiency of persistent organic pollutants (POPs) in municipal solid waste incineration fly ash (MSWIFA), as well as to overcome the difficulties of subsequent hydrothermal liquid and hydrothermal slag treatment, a two-step treatment strategy of microwave hydrothermal degradation coupled with geopolymer immobilization was proposed. Results showed that the optimal process parameters for microwave hydrothermal dechlorination were a temperature of 220 °C, a time of 1 h, and NaOH addition of 10 wt%. Microwaves accelerated the OH− mediated hydrolysis reactions and promoted the breaking of CCl bonds, leading to dechlorination. The compressive strength of the 20 % MSWIFA-based geopolymers reached 75.79 MPa, and the immobilization rate of the heavy metals (HMs) and Cl− surpassed 90 %. Alkaline environment provided by microwave hydrothermal promoted the formation of Ca(OH)2, which subsequently formed Friedel's salt (3CaO•Al2O3•CaCl2•10H2O) with Cl− in the geopolymer. The charge density difference and density of states (DOS) of Friedel's salt were analyzed by first-principles calculations, confirming that the existence of strong interactions between Ca-s, Al-p, O-p, and Cl-p states was the chemical mechanism of Cl− immobilization. The Friedel's salt and HMs were encapsulated by geopolymers with dense silica-alumina tetrahedral frameworks, achieving the solidification/stabilization (S/S) of HMs and Cl−. This work provided a new approach for the environmentally sound and resourceful treatment of MSWIFA.

Alkali-hydrothermal activation of mine tailings to prepare one-part geopolymer: Activation mechanism, workability, strength, and hydration reaction

Developing a low-cost and eco-friendly alternative to cement is of great significance for reducing CO2 emissions. Mine tailings (MTs) rich in Si and Al can be served as a promising precursor for geompolymer preparation only when the severe defect in the low reactivity is overcame. Alkali-hydrothermal activation technology was used to enhance the reactivity of MTs, and then activated tailings (ATs) and slag served as the precursors for the one-part geopolymer (OPG) preparation. Activation mechanism of MTs, workability, compressive strength, and hydration reactions of OPG samples were studied. The crystalline mineral phases in the MTs reacted with NaOH to form the amorphous sodium aluminosilicate, thus increasing the amorphization degree. High NaOH content favored the MTs activation, and the ATs with high NaOH content tended to create a higher initial pH and release more active species (Si and Al) for the slag hydration. The increase of NaOH content shortened the setting time, decreased the fluidity, and increased the strength of OPG samples. This was because the duration of the induction period was shortened and the hydration rate in the acceleration period of OPG samples was improved with increasing NaOH content, which generated large amounts of calcium–sodium aluminosilicate hydrate (C(N)-A-S-H) gel and yield the compact microstructure. However, excessive NaOH content resulted in a poor microstructure, and it is likely responsible for the reduction in strength.

The effect of different types of class F fly ashes on the mechanical properties of geopolymers cured at ambient environment

This research investigates the effect of the physicochemical characteristics of five cognate fly ashes on the mechanical properties of Class F fly ash-based geopolymers synthesized with the same mix proportion and cured at ambient conditions. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and low-field nuclear magnetic resonance were conducted to create the quantitative connection for geopolymers between micro-characteristics and macro-mechanical properties. With detailed experimental study, the research reveals that the difference in the inherent characteristics of fly ash such as specific gravity, median particle size, and amorphous phase content causes the formation of geopolymers with varied micro-characteristics and hence the discrepancy in the mechanical properties. The compressive strength increases with increasing geopolymers' amorphous phase content and decreasing porosity and Si/Al molar ratio of geopolymer binder, expressed with the reliable numerical relationships. Besides, a single and comprehensive reactivity index is proposed to assess the suitability of fly ashes for geopolymers. The results show that reactivity index of more than 20 (corresponding amorphous phase content, specific gravity, and median particle size of fly ash is >84%, >2.35, and <10 μm, respectively) leads to the high compressive strength of 60–80 MPa (28 d). Such a conclusion proves that high-temperature curing is not the prerequisite of activating the fly ash for geopolymer preparation. The approach of producing high-quality geopolymers under ambient conditions will contribute to the development of fly ash utilization and Class F fly ash-based geopolymers’ industrialization.