Special Issue "Geopolymers"

A special issue of Minerals (ISSN 2075-163X).

Deadline for manuscript submissions: closed (28 February 2018)

Special Issue Editor

Guest Editor
Prof. Dr. Thomas N. Kerestedjian

Department of Mineralogy and Mineral Resources, Geological Institute, Bulgarian Academy of Sciences, 24 Acad. Georgi Bonchev str., 1113 Sofia, Bulgaria
Website | E-Mail
Interests: mineralogy; ore geology; crystal growth; crystal structure; powder diffraction; crystal morphology; sector zoning; twining; environmental mineralogy; geochemistry; databases

Special Issue Information

Dear Colleagues,

The term “geopolymer” was introduced in the early 1970s by Joseph Davidovits, for inorganic polymeric materials, synthesized (by him) from natural (geo-) silicon and aluminium containing sources, reacted with alkaline media (solvent). Geopolymers consist of repeating siloxonate—(Na, K, Ca) (-Si-O-Si-O-) or sialate—(Na, K, Ca) (-Si-O-Al-O-) units (oligomers), polycondensed into typically ceramic, covalently bounded, non-crystalline (amorphous) 3D networks. Further research widened their definition by adding ferro-sialate and alunino-phosphate oligomers, as well as acidic (using phosphoric or humic acids as solvent) geopolymerization routes.

The scientific interest in this innovative class of materials is driven by three main factors:

  1. A series of features, making geopolymers applicable and even preferred for many industrial applications, including:

Geopolymer resins and binders

  • Fire-resistant materials, thermal insulation, foams
  • Low-energy ceramic tiles, refractory items, thermal shock refractories
  • High-tech resin systems, paints, binders and grouts
  • Bio-technologies (materials for medicinal applications)
  • Foundry industry (resins), tooling for the manufacture of organic fiber composites
  • Composites for infrastructures repair and strengthening, fire-resistant and heat-resistant high-tech carbon-fiber composites for aircraft interiors and automobiles
  • Radioactive and toxic waste containment

Geopolymer cements and concretes

  • Low-tech building materials (clay bricks)
  • Low-CO2 cements and concretes
  1. The possibility of employing in their synthesis a number of inorganic industrial waste products, like blast furnace slags, thermal power plant fly-ash, mine tailings, etc., some of which are abundantly available all over the world.
  2. Environment-friendly industrial production. The use of industrial waste can enormously enhance the resource efficiency of industrial branches generating such waste, like mining or metallurgy. On the other hand, the use of already-existing waste material can significantly diminish large waste dumps, directly improving the environmental status of affected areas.

The possible replacement (even partial) of ordinary cements and concretes by geopolymers (produced by carbon-free sources) is also a route to low-carbon production, diminishing the industrial tension on climate change.

Prof. Dr. Thomas N. Kerestedjian
Guest Editor

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Published Papers (8 papers)

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Research

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Open AccessArticle Chemical Stability and Leaching Behavior of One-Part Geopolymer from Soil and Coal Fly Ash Mixtures
Minerals 2018, 8(9), 411; https://doi.org/10.3390/min8090411
Received: 15 July 2018 / Revised: 12 September 2018 / Accepted: 13 September 2018 / Published: 18 September 2018
Cited by 2 | PDF Full-text (5471 KB) | HTML Full-text | XML Full-text
Abstract
Aluminosilicate minerals have become an important resource for an emerging sustainable material for construction known as geopolymer. Geopolymer, an alkali-activated material, is becoming an attractive alternative to Portland cement because of its lower carbon footprint and embodied energy. However, the synthesis process requires [...] Read more.
Aluminosilicate minerals have become an important resource for an emerging sustainable material for construction known as geopolymer. Geopolymer, an alkali-activated material, is becoming an attractive alternative to Portland cement because of its lower carbon footprint and embodied energy. However, the synthesis process requires typically a two-part system for alkali activation wherein the solid geopolymer precursor is mixed with aqueous alkali solutions. These alkali activators are corrosive and may be difficult to handle in the field-scale application. In this study, a one-part geopolymer in which coal fly ash was mixed with solid alkali activators such as sodium hydroxide and sodium silicate to form a powdery cementitious binder was developed. This binder mixed with soil only requires water to form the soil-fly ash (SO-CFA) geopolymer cement, which can be used as stabilized soil for backfill/foundation. This geopolymer product was then evaluated for chemical stability by immersing the material with 5% by weight of sulfuric acid solution for 28 days. Indication suggests that the geopolymer exhibited high resistance against acid attack with an observed increase of unconfined compressive strength even when the immersion time in acidic solution was increased to 56 days. The mineralogical phase, microstructure, and morphology of the material were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), respectively. Results not only confirmed the formation of gypsum due to acid attack but also indicated the dissolution of anorthite and albite that may have caused the microstructure to be composed of sodium aluminosilicate hydrate (N–A–S–H) and calcium (alumino) silicate hydrate (C(–A)–S–H) with poly(ferro-sialate-siloxo) and poly(ferro-sialate-disiloxo) networks. A column leaching test with deionized water was also performed on the soil-fly ash geopolymer to study the leachability of metals in the material. Results showed that arsenic exhibits higher mobility in the geopolymer as compared to that of cadmium, chromium, and lead. Full article
(This article belongs to the Special Issue Geopolymers)
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Open AccessArticle The Compressive Strength and Microstructure of Alkali-Activated Binary Cements Developed by Combining Ceramic Sanitaryware with Fly Ash or Blast Furnace Slag
Minerals 2018, 8(8), 337; https://doi.org/10.3390/min8080337
Received: 25 May 2018 / Revised: 2 August 2018 / Accepted: 2 August 2018 / Published: 5 August 2018
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Abstract
The properties of a binder developed by the alkali-activation of a single waste material can improve when it is blended with different industrial by-products. This research aimed to investigate the influence of blast furnace slag (BFS) and fly ash (FA) (0–50 wt %) [...] Read more.
The properties of a binder developed by the alkali-activation of a single waste material can improve when it is blended with different industrial by-products. This research aimed to investigate the influence of blast furnace slag (BFS) and fly ash (FA) (0–50 wt %) on the microstructure and compressive strength of alkali-activated ceramic sanitaryware (CSW). 4 wt % Ca(OH)2 was added to the CSW/FA blended samples and, given the high calcium content of BFS, the influence of BFS was analyzed with and without adding Ca(OH)2. Mortars were used to assess the compressive strength of the blended cements, and their microstructure was investigated in pastes by X-ray diffraction, thermogravimetry, and field emission scanning electron microscopy. All the samples were cured at 20 °C for 28 and 90 days and at 65 °C for 7 days. The results show that the partial replacement of CSW with BFS or FA allowed CSW to be activated at 20 °C. The CSW/BFS systems exhibited better mechanical properties than the CSW/FA blended mortars, so that maximum strength values of 54.3 MPa and 29.4 MPa were obtained in the samples prepared with 50 wt % BFS and FA, respectively, cured at 20 °C for 90 days. Full article
(This article belongs to the Special Issue Geopolymers)
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Open AccessArticle Property and Microstructure of Waterborne Self-Setting Geopolymer Coating: Optimization Effect of SiO2/Na2O Molar Ratio
Minerals 2018, 8(4), 162; https://doi.org/10.3390/min8040162
Received: 12 February 2018 / Revised: 31 March 2018 / Accepted: 13 April 2018 / Published: 17 April 2018
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Abstract
As a kind of coating material, the inorganic coating of alkali-activated metakaolin geopolymer cured at high temperature has been studied a lot for special applications. To our best knowledge, however, not much attention has been given to investigate the influence of SiO2 [...] Read more.
As a kind of coating material, the inorganic coating of alkali-activated metakaolin geopolymer cured at high temperature has been studied a lot for special applications. To our best knowledge, however, not much attention has been given to investigate the influence of SiO2/Na2O molar ratio on property of the geopolymer coating. This paper is, thus, dedicated to investigate the role of SiO2/Na2O molar ratio on property and microstructure of metakaolin-based geopolymer coating at ambient temperature. The effects on setting behavior, adhesive strength, shrinkage deformation and permeability are discussed. Multiple experiments were used to reveal microstructure changes of the geopolymer coating with different ratios of SiO2/Na2O, including Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscope (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The results indicated that the optimal ratio of SiO2/Na2O was 1.0 for good properties of adhesive strength, shrinkage and impermeability. In addition, it has been found that the setting time of geopolymer coating increased with SiO2/Na2O ratio which increased from 0.8 to 1.5. That agrees well with the other property and results of exothermal rate of alkali-activated metakaolin. As for the microstructural changes, the SiO2/Na2O ratio of 1.0 reduced pore size and porosity of the geopolymer coating and particularly increased volume percentage of pores with a size lower than 20 nm. Besides, FTIR results suggested that geopolymer prepared by the ratio of 1.0 was likely to produce more heterogeneous geopolymer due to a greater silicate structural reorganization. Full article
(This article belongs to the Special Issue Geopolymers)
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Open AccessArticle Experimental Evaluation of Efficient Si Dissolution from Perlite at Low Level Activator’s Concentration
Minerals 2018, 8(4), 160; https://doi.org/10.3390/min8040160
Received: 20 March 2018 / Revised: 29 March 2018 / Accepted: 30 March 2018 / Published: 14 April 2018
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Abstract
This paper deals with the Si dissolution of fine perlite in alkaline solutions for the determination of the SiO2/Na2O mass ratio in the aqueous phase of geopolymer slurries. In the present work, the effect of the main synthesis parameters [...] Read more.
This paper deals with the Si dissolution of fine perlite in alkaline solutions for the determination of the SiO2/Na2O mass ratio in the aqueous phase of geopolymer slurries. In the present work, the effect of the main synthesis parameters such as NaOH concentration and curing temperature on the setting time of the paste were studied. The obtained results showed that the inorganic polymer pastes present fast hardening at low concentrations of NaOH solutions for both 70 and 90 °C. This observation was also identified by the Si dissolution study of perlite pastes as a function of different concentrations of NaOH solutions and different solid to liquid ratios of the slurries, under a constant temperature. The optimum synthesis conditions for geopolymer pastes proved to be a low initial NaOH concentration in the alkaline phase (2–4 M NaOH), where the fast hardening of the paste was attributed to the high SiO2/Na2O mass ratio, enhancing the polycondensation phenomena and promoting the geopolymerization process. Full article
(This article belongs to the Special Issue Geopolymers)
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Open AccessArticle The Role of Clay Swelling and Mineral Neoformation in the Stabilization of High Plasticity Soils Treated with the Fly Ash- and Metakaolin-Based Geopolymers
Minerals 2018, 8(4), 146; https://doi.org/10.3390/min8040146
Received: 23 February 2018 / Revised: 29 March 2018 / Accepted: 2 April 2018 / Published: 7 April 2018
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Abstract
In the southern U.S. states, expansive soils are frequently encountered, presenting an important hazard in geotechnical engineering. This research relies on mineralogical and geochemical clues to explain the swelling behavior of smectite-rich, high-plasticity soils, documented in a series of geomechanical swelling tests that [...] Read more.
In the southern U.S. states, expansive soils are frequently encountered, presenting an important hazard in geotechnical engineering. This research relies on mineralogical and geochemical clues to explain the swelling behavior of smectite-rich, high-plasticity soils, documented in a series of geomechanical swelling tests that were performed on the soils stabilized with the metakaolin (MKG) and fly ash (FAG) based geopolymers. These geopolymers were mixed with the soil at several concentration levels. The lowest swelling percentage was shown to correspond to the sample stabilized with 12% FAG and was attributed to the neoformation of calcium silicate hydrates that acted as a cementitious material, preventing the soil from expanding by occupying the pore space, thus binding the clay particles together. Conversely, the 12% MKG-stabilized soil exhibited enormous expansion, which was explained by montmorillonite swelling to the point that it gradually began to lose its structural periodicity. The relatively high abundance of the newly formed feldspathoids in MKG-treated samples is believed to have greatly contributed to the overall soil expansion. Finally, the cation exchange capacity tests showed that the percentage of Na+ and Ca2+, as well as the pH value, exercised strong control on the swelling behavior of smectitic soils. Full article
(This article belongs to the Special Issue Geopolymers)
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Open AccessArticle Hybrid Alkaline Cements: Bentonite-Opc Binders
Minerals 2018, 8(4), 137; https://doi.org/10.3390/min8040137
Received: 2 February 2018 / Revised: 23 March 2018 / Accepted: 27 March 2018 / Published: 29 March 2018
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Abstract
Moderately alkaline activators can be used to formulate cementitious binders with a high Supplemetary Cementitious Materials (SCMs) and a low portland cement content (hybrid alkaline cements). This study aimed to prepare hybrid alkaline cements containing large percentages of dehydroxylated bentonite (BT) and small [...] Read more.
Moderately alkaline activators can be used to formulate cementitious binders with a high Supplemetary Cementitious Materials (SCMs) and a low portland cement content (hybrid alkaline cements). This study aimed to prepare hybrid alkaline cements containing large percentages of dehydroxylated bentonite (BT) and small Portland cement (OPC) fractions, with 5% Na2SO4 as a solid alkaline activator. The hydration kinetics of the pastes hydrated in water in the presence and absence of the solid activator were assessed by isothermal conduction calorimetry, whilst the reaction products were characterised with X-Ray Powder Diffraction (XRD) and Fourier-transform Infrared Spectroscopy (FTIR). The presence of the alkaline activator hastened OPC and BT/OPC hydration: more heat of hydration was released, favouring greater initial bentonite reactivity. The portlandite forming during cement hydration reacted readily with the Na2SO4, raising medium alkalinity and enhancing bentonite dissolution and with it reaction product precipitation (primarily (N,C)-A-S-H-like gels that co-exist with C-S-H- or C-A-S-H-like gels). The presence of sulfate ions favoured the formation of AFm-like phases. Preceding aspects accelerated the hydration reactions, with the formation of more reaction product and matrix densification. As a result, the 28 days Na2SO4 activated systems developed greater mechanical strength than the water-hydrated systems, with the 60% BT/40% OPC blends exhibiting higher compressive strength than the 100% OPC pastes. Full article
(This article belongs to the Special Issue Geopolymers)
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Open AccessArticle Influence of Addition of Fluid Catalytic Cracking Residue (FCC) and the SiO2 Concentration in Alkali-Activated Ceramic Sanitary-Ware (CSW) Binders
Minerals 2018, 8(4), 123; https://doi.org/10.3390/min8040123
Received: 7 March 2018 / Revised: 15 March 2018 / Accepted: 17 March 2018 / Published: 21 March 2018
Cited by 1 | PDF Full-text (3975 KB) | HTML Full-text | XML Full-text
Abstract
Production of Portland cement requires a large volume of natural raw materials and releases huge amounts of CO2 to the atmosphere. Lower environmental impact alternatives focus on alkali-activated cements. In this paper, fluid catalytic cracking residue (FCC) was used to partially replace [...] Read more.
Production of Portland cement requires a large volume of natural raw materials and releases huge amounts of CO2 to the atmosphere. Lower environmental impact alternatives focus on alkali-activated cements. In this paper, fluid catalytic cracking residue (FCC) was used to partially replace (0 wt %–50 wt %) ceramic sanitaryware (CSW) in alkali-activated systems. Samples were activated with NaOH and sodium silicate solutions and were cured at 65 °C for 7 days and at 20 °C for 28 and 90 days. In order to increase CSW/FCC binders’ sustainability, the influence of reducing the silica concentration (from 7.28 mol·kg−1 up to 2.91 mol·kg−1) was analyzed. The microstructure of the developed binders was investigated in pastes by X-ray diffraction, thermo tests and field emission scanning electron microscopy analyses. Compressive strength evolution was assessed in mortars. The results showed a synergetic effect of the CSW/FCC combinations so that, under the studied conditions, mechanical properties significantly improved when combining both waste materials (up to 70 MPa were achieved in the mortars containing 50 wt % FCC cured at room temperature for 90 days). Addition of FCC allowed CSW to be activated at room temperature, which significantly broadens the field of applications of alkali-activated CSW binders. Full article
(This article belongs to the Special Issue Geopolymers)
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Review

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Open AccessReview Fly Ash-Based Geopolymer Binder: A Future Construction Material
Minerals 2018, 8(7), 299; https://doi.org/10.3390/min8070299
Received: 15 May 2018 / Revised: 24 June 2018 / Accepted: 1 July 2018 / Published: 12 July 2018
Cited by 2 | PDF Full-text (13097 KB) | HTML Full-text | XML Full-text
Abstract
A large amount of waste coming out from industries has posed a great challenge in its disposal and effect on the environment. Particularly fly ash, coming out from thermal power plants, which contains aluminosilicate minerals and creates a lot of environmental problems. In [...] Read more.
A large amount of waste coming out from industries has posed a great challenge in its disposal and effect on the environment. Particularly fly ash, coming out from thermal power plants, which contains aluminosilicate minerals and creates a lot of environmental problems. In recent years, it has been found that geopolymer may give solutions to waste problems and environmental issues. Geopolymer is an inorganic polymer first introduced by Davidovits. Geopolymer concrete can be considered as an innovative and alternative material to traditional Portland cement concrete. Use of fly ash as a raw material minimizes the waste production of thermal power plants and protects the environment. Geopolymer concretes have high early strength and resistant to an aggressive atmosphere. Methods of preparation and characterization of fly ash-based geopolymers have been presented in this paper. The properties of geopolymer cement/mortar/concrete under different conditions have been highlighted. Fire resistance properties and 3D printing technology have also been discussed. Full article
(This article belongs to the Special Issue Geopolymers)
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