Comparative study of silica fume and sodium silicate as replacement of active reactive silica in Bauxite residue based geopolymer mortar

Konduru H., Karthiyaini S.

This work describes the synthesis of geopolymers, a family of amorphous alumino-silicates, employing Silica fume and bauxite waste as precursor materials in an alkali-activated polycondensation reaction and the investigation of their properties. The purpose of the study is to determine the feasibility of producing one-part alkali-activated geopolymer for use in cast-in-place construction. Following calcination of the bauxite residue at 800 °C, different amounts of solid activator Na2O (10 %, 15 %, and 20 %) were added. Furthermore, Silica fume content ranging between 10 % and 40 % was utilised at intervals of 10 %, in lieu of alkali-thermally treated bauxite residue. The primary objective of the study is to evaluate the fresh properties throughout the first 28 days of geopolymer formation, including consistency, flowability, heat evolution, initial setting time (IST), and final setting time (FST). Scanning electron microscopy (SEM) images are used in conjunction with the 28-day average compressive strength to demonstrate the solidification of one-part geopolymers. Furthermore, these properties are affected by adding Silica fume at regular intervals between 10 % and 40 % as a replacement of the alkali-thermally treated Bauxite residue. The 28-day average compressive strength, with a maximum value of 19 MPa indicating successful geopolymer formation, supports the solidification of one-part geopolymers.

Jwaida Z., Dulaimi A., Mashaan N., Othuman Mydin M.A.

Researchers have been driven to investigate sustainable alternatives to cement production, such as geopolymers, due to the impact of global warming and climate change resulting from greenhouse gas emissions. Currently, they are exploring different methods and waste materials to enhance the mechanical and physical properties of geopolymer and expand its application range. This review paper offers a thorough analysis of the utilization of various waste materials in geopolymer manufacturing and shows the creative contribution of this research to the development of environmentally friendly cement substitutes. The article covers the properties, durability, and practical applications of geopolymer composites made from various waste binders. It includes a microstructure and chemical analysis. The research findings indicate that geopolymers are an effective cementitious binder substitute for cement in various applications. Additionally, the ecological and carbon footprint analysis highlights the sustainability of geopolymers compared to cement.

Du P., Zhang Y., Long Y., Xing L.

Abstract To investigate the feasibility of preparing CaO–SiO2–Al2O3 inorganic fibers with melting-separated red mud, the properties of the melting-separated red mud were analyzed by X-ray fluorescence, X-ray diffraction, and differential thermal-thermogravimetric analyses. The composition of the melting-separated red mud satisfied the requirements for the composition of inorganic fibers. During the melting of the melting-separated red mud, tetrahedral skeleton fracture reactions occurred at 1,234°C, anionic group reverse binding occurred at 1,250°C, and there was no other obvious reaction peak during the whole melting process, which lasted for 51 min. The minimum suitable fiber forming temperature of the melting-separated red mud melt was 1,433°C, which was 83°C greater than its crystallization temperature, 1,350°C. Within this temperature range, the activation energy of particle movement in the melt was 1008.65 kJ·mol−1, and the melt exhibited good fluidity. Considering the temperature distribution corresponding to the melting properties of the melting-separated red mud, CaO–SiO2–Al2O3 inorganic fibers could be prepared when the melting-separated red mud was subjected to component reconstruction by increasing the silicon content, reducing the aluminum content, and adding a moderate amount of calcium. Quartz sand and light burnt dolomite were used as modifying agents and inorganic fibers were prepared under laboratory conditions. The fibers prepared from the modified melting-separated red mud by adding different amounts of melting-separated red mud had smooth surfaces and were arranged in a crossed manner at the macroscopic level. Their color was grayish-white, and small quantities of slag balls were doped inside the fibers. With an increase in the amount of melting-separated red mud from 50 to 100%, the average fiber diameter increased from 5.5 to 8.0 μm, and their slag ball content increased from 2.9 to 6.0%. Overall, under laboratory conditions, when the amount of melting-separated red mud added was 50%, dolomite was 22.5% and quartz sand was 27.5%, the performance of the fiber was the best.

Luhar I., Luhar S.

The discovery of an innovative category of inorganic geopolymer composites has generated extensive scientific attention and the kaleidoscopic development of their applications. The escalating concerns over global warming owing to emissions of carbon dioxide (CO2), a primary greenhouse gas, from the ordinary Portland cement industry, may hopefully be mitigated by the development of geopolymer construction composites with a lower carbon footprint. The current manuscript comprehensively reviews the rheological, strength and durability properties of geopolymer composites, along with shedding light on their recent key advancements viz., micro-structures, state-of-the-art applications such as the immobilization of toxic or radioactive wastes, digital geopolymer concrete, 3D-printed fly ash-based geopolymers, hot-pressed and foam geopolymers, etc. They have a crystal-clear role to play in offering a sustainable prospect to the construction industry, as part of the accessible toolkit of building materials—binders, cements, mortars, concretes, etc. Consequently, the present scientometric review manuscript is grist for the mill and aims to contribute as a single key note document assessing exhaustive research findings for establishing the viability of fly ash-based geopolymer composites as the most promising, durable, sustainable, affordable, user and eco-benevolent building materials for the future.

Qaidi S.M., Tayeh B.A., Isleem H.F., de Azevedo A.R., Ahmed H.U., Emad W.

The production of cement results in the emission of much carbon dioxide, which contributes to ‎ ‎undesirable environmental impacts such as climate change and global warming. These ‎phenomena ‎have rekindled interest in utilizing a variety of industrial waste products to ‎produce geopolymer ‎‎(GP) composites and alkali-activated (AA) binders in order to reduce the ‎usage of ordinary ‎Portland cement in building construction. Waste red mud (RM), also known as bauxite residue, is one of these ‎hazardous radioactive ‎waste materials that is formed as a by-product of Bayer's aluminum ‎manufacturing process. This ‎paper conducts a systematic review of the literature on the use of ‎RM and slag in the production ‎of red mud-slag geopolymer (RM-SGP). An overview of the ‎economic and environmental ‎impacts, physical and chemical properties, production, ‎distribution, classification, and potential applications ‎of RM are presented. ‎Besides, recent advancements in the usage of RM and slag for geopolymer ‎production are ‎described in terms of physical, mechanical, durability, and microstructure ‎ properties. Moreover, this ‎study attempts to chart a route toward a realistic valorization that ‎reflects both real and perceived ‎concerns, such as radioactivity, leaching, and the life cycle ‎assessment of red mud geopolymer (RM-GP). The potential ‎use of RM-SGP production indicates the need for further studies into ‎the mixture proportion ‎and combination of these two raw ingredients with other cementitious materials ‎leading to new ‎energy-saving and affordable building products and processes. Also, it is ‎recommended that ‎research efforts be directed ‎toward economic, life cycle, and environmental ‎assessments.‎

Park S., Yu J., Oh J.E., Pyo S.

This paper investigates the effect of silica fume on the mechanical properties of metakaolin-based geopolymers with different silicon-to-aluminum molar (S/A) ratios. Geopolymer has been extensively studied as an alternative to traditional cementitious material because of its low CO2 emissions. Previous studies revealed that the application of silica fume can improve the compressive strength of geopolymer, however, the optimum dosages are different. To examine the reason for the different optimum dosages of silica fume, this study prepares geopolymer specimens of which variables are the S/A ratio and silica fume dosage, and conducts compressive strength and initial setting time tests. To examine whether the strength degradation is caused by the expansion due to the added silica fume in geopolymer, the volume and dynamic modulus are also measured. The results show that a part of silica fume dissolves and changes the S/A ratio of geopolymer, and that a part of silica fume remains in the geopolymer matrix. These combined effects of silica fume result in an irregular compressive strength trend, and, thus, an optimum dosage of silica fume can vary depending on the S/A ratio. Furthermore, the volume expansion of geopolymer with silica fume is observed, however, no sign of damage on the compressive strength is found.

Yang X., Zhang Y., Lin C.

Geopolymer is a green non-metallic material with high strength and favorable properties in resistance to corrosion, fire, and high temperature, which makes it a potential substitute for Portland cement. The existing studies have primarily focused on the preparation of geopolymers using silico-alumina materials such as fly ash, red mud, metakaolin, volcanic ash, and blast furnace slag to develop geopolymers. This study explores the potential of using ultrafine calcined coal gangue and ground granulated blast furnace slag to develop a new geopolymer with the activation of a single activator (sodium hydroxide) or mixed activator (sodium hydroxide, liquid sodium silicate, and desulfurization gypsum). The setting time and strength of the geopolymers were investigated, followed by the mineral, functional groups, microstructure, and elements analyses using X-ray diffraction, Fourier transform infrared diffraction, scanning electron microscope, and energy dispersive spectrometer to elucidate the effect of different activators on geopolymers. The results showed that the optimum molarity of NaOH single activator was 2 mol/L, the initial setting time and final setting time were 37 min and 47 min, respectively, and the compressive and flexural strengths at 28 days were 23.2 MPa and 7.5 MPa. The optimal mixing ratio of the mixed activator was 6% desulfurization gypsum, 0.6 Na2SiO3 modulus, and 16% SS activator; the initial setting time and final setting time were 100 min and 325 min, respectively, and the compressive and flexural strengths at 28 days were 40.1 MPa and 7.8 MPa. The coal gangue geopolymers were mainly C–A–S–H, N–A-S-H, and C–N–A–S–H gels. The mixed activator tended to yield higher strengths than the single activator, the reason is that the hydration reaction was violent and produced more gels. Meanwhile, the relation between setting time and activator and the relation between strength and activator were also obtained, which provide theoretical support for predicting the setting time of coal gangue base polymer and the ratio of alkali activator for geopolymers with a certain strength.

Khanna R., Konyukhov Y., Zinoveev D., Jayasankar K., Burmistrov I., Kravchenko M., Mukherjee P.S.

Managing red mud (RM), a solid waste byproduct of the alumina recovery process, is a serious ecological and environmental issue. With ~150 million tons/year of RM being generated globally, nearly 4.6 billion tons of RM are presently stored in vast waste reserves. RM can be a valuable resource of metals, minor elements, and rare earth elements. The suitability of RM as a low-grade iron resource was assessed in this study. The utilization of RM as a material resource in several commercial, industrial operations was briefly reviewed. Key features of iron recovery techniques, such as magnetic separation, carbothermal reduction, smelting reduction, acid leaching, and hydrothermal techniques were presented. RMs from different parts of the globe including India, China, Greece, Italy, France, and Russia were examined for their iron recovery potential. Data on RM composition, iron recovery, techniques, and yields was presented. The composition range of RMs examined were: Fe2O3: 28.3–63.2 wt.%; Al2O3: 6.9–26.53 wt.%; SiO2: 2.3–22.0 wt.%; Na2O: 0.27–13.44 wt.%; CaO: 0.26–23.8 wt.%; Al2O3/SiO2: 0.3–4.6. Even with a high alumina content and high Al2O3/SiO2 ratios, it was possible to recover iron in all cases, showing the significant potential of RM as a secondary resource of low-grade iron.

Chen S., Wang K., Wei E., Muhammad Y., Yi M., Wei Y., Fujita T.

Herein, we report the application of sol-gel method for the preparation of a new type of highly active Al 2 O 3 -2SiO 2 geopolymer precursor powder. The newly synthesized geopolymer precursor powder was characterized by SEM, XRD, TG, FTIR and MAS NMR. Experimental results showed that all the geopolymer precursor powders prepared by this method exhibited amorphous crystal structure and FTIR absorption peaks were consistent to typical aluminosilicate. TG analysis showed that G1-0-35 prepared with pure water as the aluminum source solvent had higher weightlessness rate, and with the increase of water quantity, the layered structure appeared, the resonance peak of hexacoordination Al(VI) was enhanced and the coordination of silicon was changed. ICP test results showed that the G1-0-35 precursor powder with pure water as the aluminum source solvent dissolves more quickly in the NaOH, and the relative crystallinity was the highest (84%) after alkali activation. The changes in conductivity of different geopolymer precursor powders during the reaction at different temperatures and the changes in the concentration of aluminum and silicon ions with time in the geopolymer reaction were explored, which showed that the activity of geopolymers powders increased with increasing the amount of water. The geopolymer precursor powder prepared in this study are in line with the requirements of green production, and prompts great advantages in the study of geopolymer reaction mechanism. • Preparation of Al 2 O 3 -2SiO 2 /geopolymer powder by hydrolytic sol-gel method • The G1-0-35 powder prepared with pure water have higher activity in alkali solution. • Using conductivity to explain the mechanism of geopolymer reaction process • The preparation process has the advantages of short gelation time and high activity.

Yang X., Zhang Y., Li Z., Wang M.

Coal gangue-slag geopolymer is a kind of environment-friendly material with excellent engineering performance and is formed from coal gangue and slag after excitation by an alkaline activator. In this study, three kinds of coal gangue-slag geopolymer were activated by different activators, and the compressive and flexural strengths of water and sulphate solutions in the wetting-drying (W-D) cycles were compared. The microscopic mechanism was analyzed by the XRD, the FTIR and the SEM. The following conclusions are drawn: The influence of W-D cycles on flexural strength was greater than compressive strength. The water migration and the recombination of geopolymers lead to the change of colour, as well as the reduction of flexural strength and compressive strength of geopolymers. The SH geopolymer had excellent anti-erosion ability in terms of flexural strength, and the reason for this was the recombination and polymerization reaction of geopolymer being weaker than the SS and the SSG. The corrosion resistance of the SS was reflected in the compressive strength, because its geopolymerization reaction was fierce, which produced more Na-rich C–N–A–S–H, N–A–S–H and C–A–S–H gels. Therefore, the compressive strength could still reach more than 39 MPa after 150 cycles. Sulfate solution could effectively control the reduction of compressive strength of the SH and the SS geopolymers during W-D cycles. The SSG had the worst corrosion resistance.

Occhicone A., Vukčević M., Bosković I., Ferone C.

The aluminum Bayer production process is widespread all over the world. One of the waste products of the Bayer process is a basic aluminosilicate bauxite residue called red mud. The aluminosilicate nature of red mud makes it suitable as a precursor for alkali-activated materials. In this work, red mud was mixed with different percentages of blast furnace slag and then activated by sodium silicate solution at different SiO2/Na2O ratios. Obtained samples were characterized by chemical–physical analyses and compressive strength determination. Very high values of compressive strength, up to 50 MPa, even for high percentage of red mud in the raw mixture (70 wt.% of RM in powder mixture), were obtained. In particular, the higher compressive strength was measured for cubic samples containing 50 wt.% of RM, which showed a value above 70 MPa. The obtained mixtures were characterized by no or scarce environmental impact and could be used in the construction industry as an alternative to cementitious and ceramic materials.

Moro D., Fabbri R., Romano J., Ulian G., Calafato A., Solouki A., Sangiorgi C., Valdrè G.

The present research investigates the possibility to create a silt-waste reinforced composite through a NaOH-activated, metakaolin-based geopolymerization process. In this regard, we used thermal exo–endo analysis, X-ray diffraction (XRD), and oedometric mechanical tests to characterize the produced composites. In our experimental conditions, the tested material mixtures presented exothermic peaks with maximum temperatures of about 100 °C during the studied geopolymerization process. In general, the XRD analyses showed the formation of amorphous components and new mineral phases of hydrated sodalite, natrite, thermonatrite and trona. From oedometric tests, we observed a different behavior of vertical deformation related to pressure (at RT) for the various produced composites. The present work indicated that the proposed geopolymerization process to recycle silt-waste produced composite materials with various and extended mineralogy and chemical–physical properties, largely depending on both the precursors and the specific alkaline-activating solution. Thermal analysis, XRD, and oedometric mechanical tests proved to be fundamental to characterize and understand the behavior of the newly formed composite material.

Ke X., Baki V.A.

This study investigates the suitability and potentials of using alkali-activated metakaolin geopolymers as sustainable low-cost thermochemical heat storage materials via water sorption and/or hydration reactions. Four different alkali-activated metakaolin geopolymer formulations were assessed. The cyclic water sorption capacity and moisture diffusion coefficients were assessed via two continuous water vapor sorption/desorption cycles using the dynamic water vapor sorption (DVS), proving the regeneration ability of geopolymers for sorption thermal energy storage. The thermochemical properties of geopolymers, including the dehydration enthalpy and activation energy, were determined. For the amorphous sodium aluminosilicate hydrate gel (N-A-S-H) with bulk Si/Al ratio of 1.5, the mass and volumetric energy storage capacity of 827.9 J/g hydrate sample and 350 kW h/m 3 were achieved, with a charging temperature of around 120 °C. The outcomes of this study suggest that alkali-activated metakaolin geopolymers have the potential to be used for both low-temperature water sorption thermal energy storage and medium temperature hydration/dehydration thermochemical energy storage. The energy storage performances of metakaolin geopolymers are closely related to their aluminosilicate framework structures and surface textural properties. The partial crystallisation of the amorphous N(K)-A-S-H gel, resulting in the formation of micropores, which does not seem to affect the maximal water uptake but increases the proportion of the chemically bound water. The presence of the amorphous mesoporous aluminosilicate gel N(K)-A-S-H gel in geopolymers leads to desorption hysteresis at relative humidity higher than 30 %. Further optimisation of the synthesis approach, aluminosilicate framework structures, micro and mesopore structures will be required to optimise their overall thermal energy storage performances. • Alkali-activated metakaolin geopolymers can be used for thermochemical heat storge. • Reversible dehydration and rehydration below 200 °C can be achieved by N -A- S -H gels. • Surface properties and gel structures control the energy storage performances. • Energy storage capacity of 350 kW h/m 3 was achieved by the amorphous N -A- S -H gel.

Mandal A.K.

In this present study, the development of a two-part geopolymer from waste red mud (RM) and silica fume (SF) by alkali activator is examined. The influence of silica fume addition, alkali concentration, curing duration are studied. The compressive strength of geopolymers has been achieved 0.8 MPa to 8 MPa with varying different compositions and synthesis parameters. The optimum RM/SF ratio is 60/40, the optimum solid/solution ratio is 1.8 gmL-1 or 2 gmL-1, and the optimum Na2SiO3/NaOH ratio is 0.5 day for 28 days curing time. The incorporation of iron in the geopolymer matrix contributes to geopolymerization. The study suggests that the produced geopolymer can be used as cementitious materials for making pavement and other valuable constructional materials. This procedure will be environment friendly and cheaper also.

Muraleedharan M., Nadir Y.

Geopolymer offers many advantages over OPC such as excellent mechanical strength, better durability, thermal resistance, and excellent stability in acidic and alkaline environments. Compared to other industrial by-products such as fly ash, slag, rice husk ash, silica fume, kaolin, etc., red mud (RM) from alumina production and granite waste powder (GW) from granite quarries are less utilized for geopolymer synthesis. This study presents a review of the research papers that utilize RM and GW to produce geopolymer binder and mortar. The parameters such as setting time and workability, influence of factors such as types of alkaline activator, concentration of alkaline activators, molar ratios of different chemical compounds, liquid to solid ratio, fine aggregate to binder ratio, curing conditions, addition of plasticizer and other additives on the mechanical and microstructural behaviour of RM and GW geopolymers are summarized in this paper. The durability and thermal behavior of RM geopolymers are also discussed. The potential usage of RM and GW for geopolymer synthesis points towards the requirement for further investigations on the mix proportioning and combinations of these two raw materials with other pozzolanic materials resulting in novel, energy-efficient, low-cost construction processes and products. • Review on geopolymer binder and mortar from red mud and granite waste powder is presented. • Mechanical properties and microstructure are elaborated. • Durability, thermal behaviour and environmental compatibility of red mud geopolymer are discussed.