Behavior of low-calcium fly and bottom ash-based geopolymer concrete cured at ambient temperature

This paper presents the results of an experimental study on the behavior of fly ash-, bottom ash- and blended fly and bottom ash-based geopolymer concrete (GPC) cured at ambient temperature. A total of 10 bathes of GPC and a single batch of ordinary Portland cement concrete (OPC) were manufactured. The tests of compressive strength, elastic modulus, flexural strength, workability, drying shrinkage and absorption capacity were carried out to determine the properties of fresh concrete and mechanical and durability-related properties of hardened concrete. Test parameters included the mass ratio of fly ash-to-bottom ash, liquid alkaline-to-coal ash binder ratio, coal ash content and concrete type. The results indicate that the selected parameters significantly affect the microstructure and the behavior of GPCs. It is seen that bottom ash-based GPCs exhibited significantly lower geopolymerization than that of the fly ash-based GPCs, resulting in the inferior behavior of the former compared to the latter.     

The microstructure and durability of fly ash-based geopolymer concrete: A review

Fly ash-based geopolymer concrete (FABGC) is a type of environment-friendly building material that displays remarkable mechanical properties and durability. It has the potential for extensive application in the field of civil engineering. This paper considers the related research on the microstructure and durability of FABGC to systematically summarize the results on its alkali-activated reaction, pore structure, and interface characteristics. The degradation mechanisms of FABGC in various corrosive environments are analyzed, and the factors that affect its microstructure and durability are discussed. It is observed that aluminosilicate gel produced by the alkali-activated process of FABGC has an optimizing effect on the pore structure and interfacial transition zone. An effective development of the microstructure can improve the durability of FABGC to a certain extent. At present, there is no consensus on the research conclusions on the microstructure and durability of FABGC. Therefore, further research is required.     

Physical and mechanical properties of fly ash-based geopolymer with disposable medical mask reinforcement

A tremendous amount of the nonbiodegradable microplastic waste has been generated after the outbreak of COVID-19 by the widespread use of single-use personal protective equipment, especially disposable medical masks (DMMs). This has caused harm to the health and safety of human beings and various organisms. Finding a way to properly deal with these single-use medical wastes has become an urgent problem. In this paper, an innovative way was explored to use DMMs in geopolymer (GP). The physical properties, mechanical strength, and resistance to high temperatures (200–800°C) of the composites were investigated. The findings of the study revealed that DMMs had negligible influence on resistance to high temperatures, but showed a positive influence on enhancing the compressive and flexural strengths of GPs at ambient temperature. The optimum DMMs content was 0.4 wt%, at which the compressive and flexural strengths of the GP composites were enhanced by 5.8% and 22.68% compared with the pure GP, respectively. The same polypropylene (PP) fiber amount increased compressive and flexural strengths by 7.49% and 9.76%, respectively. This thus confirmed that DMMs can be sustainably utilized in green building materials, playing a role as PP fibers toughening and contributing to the effective management of waste plastics.     

Chloride-induced corrosion of reinforced concrete bridge decks

A closed-form solution is developed to predict the corrosion initiation time of reinforced concrete bridge decks using measured time varying surface chloride accumulations. The data base for the surface chlorides are core measurements at a shallow depth below the surface of 15 bridge decks in the snow belt region. The data base was collected during the bridges' biennial inspections over a period of 15 years. Regression analysis is used to represent the surface chlorides by an exponential variation with time. The time predicted to initiate corrosion is computed for different values of the effective diffusion coefficient and the concrete cover thickness. The results are compared to the constant surface accumulation model commonly used in the literature. As expected, the corrosion initiation based on constant chloride accumulation at the surface is faster (in some case by up to 100%) than the initiation time calculated from actual chloride concentration data. Such results are useful for the realistic estimation of the service lives of bridge decks and for scheduling bridge deck maintenance and rehabilitation programs.     

Adsorption efficiency and photocatalytic activity of fly ash-based geopolymer foam mortar

Fly ash-based geopolymer foam mortar (GFM) was used as an adsorbent material to methylene blue (MB) and also the dye removal material using the photocatalytic mechanism. The GFM, containing 50 wt% river sand aggregate, was prepared to have approximately 46% open porosity, pore size distribution between 0.01 and 3.5 mm, and water permeability of 0.2 cm/s. The variation of adsorption efficiency and adsorption capacity with the contact time of the GFM was first evaluated using various GFM dosages (10, 20, 50, 80, and 100 g/L). The adsorption efficiency at equilibrium (AE_e) was found to linearly increase, while adsorption capacity (q_ae) exponentially decayed, with an increase of loading dosages. The photocatalytic removal efficiency of ~100% was obtained with 50, 80, and 100 g/L GFM loading dosages, with a shorter time at higher dosages. The GFM could be reused, without regeneration, for 5 cycles. The AE_e and q_ae for each reused cycle did not noticeably change suggesting the reusability. The photocatalytic removal efficiency, however, was found to decrease with an increase of the reused cycle. After the 5^th cycle, the highest removal efficiency was reduced to ~70%. The attempts to treat the GFMs with hydrochloric (HCl) and phosphoric (H₃PO₄) acid to reduce the excess alkaline did not give satisfactory results as expected. The photocatalytic removal efficiency had subsided after the treatment with both acids.     

Improving compressive strength of fly ash-based geopolymer composites by basalt fibers addition

In this study, ASTM Class C fly ash used as an alumino-silicate source was activated by metal alkali and cured at low temperature. Basalt fibers which have excellent physical and mechanical properties were added to fly ash-based geopolymers for 10–30% solid content to act as a reinforced material, and its influence on the compressive strength of geopolymer composites has been investigated. XRD study of synthesized geopolymers showed an amorphous phase of geopolymeric gel in the 2θ region of 23°–38° including calcium-silicate-hydrate (C-S-H) phase, some crystalline phases of magnesioferrite, and un-reacted quartz. The microstructure investigation illustrated fly ash particles and basalt fibers were embedded in a dense alumino-silicate matrix, though there was some un-reacted phase occurred. The compressive strength of fly ash-based geopolymer matrix without basalt fibers added samples aged 28 days was 35 MPa which significantly increased 37% when the 10 wt%. basalt fibers were added. However, the addition of basalt fibers from 15 to 30 wt% has not shown a major improvement in compressive strength. In addition, it was found that the compressive strength was strong relevant to the Ca/Si ratio and the C-S-H phase in the geopolymer matrix as high compressive strength was found in the samples with high Ca/Si ratio. It is suggested that basalt fibers are one of the potential candidates as reinforcements for geopolymer composites development.     

粉煤灰基地质聚合物微球吸附水中铜（Ⅱ）的研究

以粉煤灰为原料,以氢氧化钾溶液为碱激发剂,将二者按照优化配比（氧化钾与氧化铝物质的量比为1.5、水与氧化钠物质的量比为18）混合均匀后,采用悬浮固化法制备粉煤灰基地质聚合物微球,将微球用于吸附含铜废水中的铜（Ⅱ）。通过X射线衍射（XRD）仪、比表面积与孔径分析仪、BT-99型水质分析仪对微球进行了表征,探究了吸附时间、微球用量、吸附温度、铜（Ⅱ）溶液pH、铜（Ⅱ）溶液质量浓度等因素对微球吸附铜（Ⅱ）的影响。结果表明,粉煤灰基地质聚合物微球较粉煤灰原料具有更大的孔径和比表面积,具有更好的对铜（Ⅱ）的吸附效果,在最优条件下[微球用量为0.20 g、溶液pH为5、铜（Ⅱ）初始质量浓度为100 mg/L、溶液体积为100 mL、吸附温度为40℃、吸附时间为24 h]微球对铜（Ⅱ）的吸附量为45.62 mg/g、去除率达到91.46%,吸附过程遵循准二级动力学方程。     

Preparation of NaP zeolite block from fly ash-based geopolymer via in situ hydrothermal method

Particular macroshape structures of zeolite are required in numerous application fields to avoid secondary powder pollution. In this paper, a NaP zeolite block with high strength was successfully fabricated from fly ash-based geopolymer through in situ hydrothermal process. The results of SEM and XRD confirmed that the alkalinity, crystallization time and temperature of the hydrothermal system played an important role in controlling the morphology and crystallinity of final samples. Under the optimal hydrothermal conditions (100 °C, 2.0 mol/L NaOH solution, 24 h), the NaP zeolite block exhibited a distinct diamond ball-like shape and sharp edges with good crystallinity. In addition, the BET specific surface area, the average mesopores diameter and the compressive strength of NaP zeolite block were respectively 50.46 m²/g, 11.06 nm and 23.21 MPa.     

Bond strength between blended slag and Class F fly ash geopolymer concrete with steel reinforcement

In this paper, geopolymer concrete bond with both deformed and smooth reinforcing steel bars is investigated using the standard RILEM pull-out test. The geopolymer binder is composed of 85.2% of low calcium fly ash and 14.8% of ground granulated blast furnace slag (GGBFS). The tests were aimed to assess the development of the bond strength from 24 h to 28 days after casting, with different heat curing conditions. The results show that 48 h of heat curing at 80 °C is required in order to obtain similar or better performances to those of the reference 45 MPa OPC concrete. The 28-day bond strength and the overall bond stress–slip behaviour of the geopolymer concrete were similar to those previously reported for OPC-based concretes. Providing intensive heat curing, high early bond strength can be achieved showing that Class F fly ash geopolymer concrete is well suited for precast applications.     

Multifunctional fly ash-based GO/geopolymer composite membrane for efficient oil-water separation and dye degradation

Membrane fouling and separation materials with low cost and high efficiency are challenges for membrane separation technology in wastewater treatment. Superhydrophilic and underwater superoleophobic membranes show broad application prospects in oily wastewater treatment because of their high permeability, selectivity, and antifouling performance; however, they are generally ineffective for organic pollutant molecules. In this study, a novel graphene oxide (GO)/geopolymer composite membrane with superhydrophilic and underwater superoleophobic characteristics was prepared by dipping a mixed slurry of GO and fly ash-based geopolymer onto a stainless steel mesh via a facile self-assembly process. The results show that GO could adjust the hydrophilicity and water flux of composite membranes. The composite membrane containing 0.4 wt% GO (4GO/GCM) had the best hydrophilic, water flux of 1363 kg/(m²·h), and high separation efficiencies (≥98.2%) for oil-water mixtures and oil-in-water emulsions under gravity-driven. In addition, the 4GO/GCM sample exhibited excellent stability under harsh conditions, including hot water and strong acid, alkali, and salt solutions. Importantly, the sample derived from fly ash exhibited unique photocatalytic degradation performance for organic dye molecules under simulated solar-light irradiation. Thus, it is believed to this strategy has substantial potential for high-value utilization of fly ash and the sustainable treatment of oily and dye wastewater.     

Optimization of coal fly ash-based porous geopolymer synthesis and application for zinc removal from water

Coal fly ash-based porous geopolymer (CFAPG) is a potential adsorbent for heavy metal-contaminated water remediation and can also mitigate the accumulation of coal fly ash from thermal power plants. Production parameters influence the physicochemical properties (e.g., adsorption capacity) of CFAPG. Ten potential factors involved in the CFAPG production process were examined using a Plackett-Burman design (PBD) and then an orthogonal experimental design (OED). The results show the alkali activator modulus (MS), alkali-ash mass ratio (AA), foaming agent-ash mass ratio (FAR), and sodium dodecyl sulfate-ash mass ratio (SDSA) were the most important factors influencing the Zn adsorption capacity of CFAPG. Ternary plots confirm the interaction between these four factors, with the role of FAR being easily masked by other factors and MS being the least influenced by other factors. Furthermore, Zn adsorption on the CFAPG created with optimal parameters was best described by the Bi_Langmuir model, indicating two different sorption site classes on the surface of CFAPG with a total maximum Zn adsorption capacity of 13.42 mg g^?1. These results provide key parameters for the production of geopolymers as heavy metal adsorbents.     

Chloride-induced filiform corrosion of organic-coated magnesium

In-situ scanning Kelvin probe (SKP) potentiometry is used to study the underfilm corrosion of organic-coated, commercial purity (CP) magnesium under conditions of high relative humidity. Following initiation by the application of aqueous hydrochloric acid (HCl) to a penetrative coating defect, underfilm corrosion is characterised by the onset of coalesced filiform-like features, which lengthen with time and propagate at a rate which is largely independent of the quantity of initiating HCl. The potentials of the advancing underfilm corrosion-front range from −1.35 to ca. −1.45 V vs. SHE, with corrosion potentials in the tail region largely similar to those of the intact coated Mg surface. The rate of filiform corrosion (FFC) advance is shown to be insensitive to the presence of oxygen, but highly dependent upon the relative humidity of the holding environment. It is proposed that the mechanism of propagation is not governed by differential aeration, but rather a differential “electrocatalytic activation” phenomenon involving anodic Mg dissolution at the leading edge of the corrosion front, galvanically coupling with hydrogen evolution on a cathodically activated corroded region behind. This proposed mechanism also accounts for observations of FFC upon immersion of CP-Mg in dilute chloride-containing electrolyte, where differential aeration would not be expected to exert an influence.     

Kinetic studies of cobalt ion removal from aqueous solutions using fly ash-based geopolymer and zeolite NaX as sorbents

This study explored the cobalt ions removal efficiency from aqueous solutions by the sorption process on geopolymer and zeolite NaX. The influence of concentration and temperature on the sorption process was examined. FTIR and SEM analyses were conducted to elucidate the structure of sorbents. An additional goal was to test the experimental kinetic data using several kinetic models. A kinetic study has shown that the best ?t is achieved when the Blanchard model was applied, suggesting that the sorption of cobalt ions on geopolymer and zeolite NaX is a second-order reaction.     

Bond strength evaluation of palm oil fuel ash-based geopolymer normal weight and lightweight concretes with steel reinforcement

This article presents a comparison of the bond behaviour between palm oil fuel ash (POFA)-derived geopolymer and conventional cement-based normal weight and lightweight concretes. A total of 16 variables were tested, which includes concrete cover (50 and 100 mm), bar diameter (12 and 16 mm) and types of concrete (POFA-based geopolymer normal/ lightweight concrete and cement-based normal/lightweight concrete). Results showed that the bond strength of cement-based concretes had higher critical bond stress and ultimate bond strength as well as lower slip at the ultimate bond strength compared to the corresponding POFA-based geopolymer concretes. The cement-based and geopolymer lightweight concrete specimens also exhibited greater bond strength than the normal weight concrete specimens. All of the concrete specimens generally exhibited similar bond stress-slip curves. Besides that, bond strength models proposed in the past predicted satisfactory match (difference of up to 35%) to the experimental ultimate bond strength values in the case of cement-based normal weight concrete and geopolymer concrete whereas a difference in the range of 16–138% was found for the case of lightweight concrete.     

Prediction of reinforcement corrosion using corrosion induced cracks width in corroded reinforced concrete beams

This paper studies the evolution of reinforcement corrosion in comparison to corrosion crack width in a highly corroded reinforced concrete beam. Cracking and corrosion maps of the beam were drawn and steel reinforcement was recovered from the beam to observe the corrosion pattern and to measure the loss of mass of steel reinforcement. Maximum steel cross-section loss of the main reinforcement and average steel cross-section loss between stirrups were plotted against the crack width. The experimental results were compared with existing models proposed by Rodriguez et al., Vidal et al. and Zhang et al. Time prediction models for a given opening threshold are also compared to experimental results. Steel cross-section loss for stirrups was also measured and was plotted against the crack width. It was observed that steel cross-section loss in the stirrups had no relationship with the crack width of longitudinal corrosion cracks.     

Utilisation of steel furnace slag coarse aggregate in a low calcium fly ash geopolymer concrete

This paper evaluates the performance of steel furnace slag (SFS) coarse aggregate in blended slag and low calcium fly ash geopolymer concrete (GPC). The geopolymer binder is composed of 90% of low calcium fly ash and 10% of ground granulated blast furnace slag (GGBFS). Mechanical and physical properties, shrinkage, and detailed microstructure analysis were carried out. The results showed that geopolymer concrete with SFS aggregate offered higher compressive strength, surface resistivity and pulse velocity than that of GPC with traditional aggregate. The shrinkage results showed no expansion or swelling due to delayed calcium oxide (CaO) hydration after 320 days. No traditional porous interfacial transition zone (ITZ) was detected using scanning electron microscopy, indicating a better bond between SFS aggregate and geopolymer matrix. Energy dispersive spectroscopy results further revealed calcium (Ca) diffusion at the vicinity of ITZ. Raman spectroscopy results showed no new crystalline phase formed due to Ca diffusion. X-ray fluorescence result showed Mg diffusion from SFS aggregate towards geopolymer matrix. The incorporation of Ca and Mg into the geopolymer structure and better bond between SFS aggregate and geopolymer matrix are the most likely reasons for the higher compressive strength observed in GPC with SFS aggregate.     

Effect of silica fume, metakaolin, and low-calcium fly ash on chemical resistance of concrete

Effects of aggressive chemical environments were evaluated on the mortars prepared with ordinary portland cement (OPC) and silica fume (SF)/metakaolin (MK)/low-calcium fly ash at various replacement levels. The natural adverse chemical environmental conditions were simulated using sulfuric acid, hydrochloric acid, nitric acid, acetic acid, phosphoric acid, and a mixture of sodium and magnesium sulfates. Chemical resistance information was used in conjunction with compressive strength measurements to propose realistic OPC/mineral admixture proportions.     

Two-step preparation of fly ash-based composites for microwave absorption

To develop a novel utilization avenue for fly ash (FA), the Co-loaded FA (CoFA) was constructed utilizing FA as raw material to acquire the enhanced microwave absorption (MA) performance. In this study, the CoFA composites were fabricated by a two-step method, including the construction of FA-based ceramic matrix and a subsequent loading of magnetic components. The results of XRD, SEM, and elemental mapping images revealed that Co particles generated from the carbothermal reduction were well dispersed over the interior and surface of matrix. Compared with pure FA, the as-prepared CoFA composites demonstrated the impressive MA performances, which were attributed to the good impedance matching, conduction loss, and interfacial polarization effect between the matrix and Co. When the annealing temperature kept at 700°C, the minimum reflection loss (RL_min) of as-prepared CoFA700 reached up to −40.5 dB and the broad absorption band was measured to be 4.7 GHz with a thickness of 2.0 mm, which was superior to pure FA. Our strategy might provide a new direction to the fabrication of high-efficient MA materials derived from FA.     

Modeling of concrete cracking due to corrosion process of reinforcement bars

The reinforcement corrosion in Reinforced Concrete (RC) is a major reason of degradation for structures and infrastructures throughout the world leading to their premature deterioration before design life was attained. The effects of corrosion of reinforcement are: (i) the reduction of the cross section of the bars, and (ii) the development of corrosion products leading to the appearance of cracks in the concrete cover and subsequent cover spalling. Due to their intrinsic complex nature, these issues require an interdisciplinary approach involving both material science and structural design knowledge also in terms on International and National codes that implemented the concept of durability and service life of structures.In this paper preliminary FEM analyses were performed in order to simulate pitting corrosion or general corrosion aimed to demonstrate the possibility to extend the results obtained for a cylindrical specimen, reinforced by a single bar, to more complex RC members in terms of geometry and reinforcement. Furthermore, a mechanical analytical model to evaluate the stresses in the concrete surrounding the reinforcement bars is proposed. In addition, a sophisticated model is presented to evaluate the non-linear development of stresses inside concrete and crack propagation when reinforcement bars start to corrode. The relationships between the cracking development (mechanical) and the reduction of the steel section (electrochemical) are provided. Finally, numerical findings reported in this paper were compared to experimental results available in the literature and satisfactory agreement was found.