Influence of new organic alkali activators on microstructure and strength of fly ash geopolymer

The fly ash geopolymer with improved mechanical properties was prepared by a new mixture alkali activator. In this paper, sodium tert-butanol, an organic strong alkali was used as an activator for preparing fly ash geopolymer to improve their mechanical properties. The effect of activator content and type on the macroscopic level of fly ash geopolymer was investigated experimentally by three types of activators: sodium tert-butanol, sodium silicate, and sodium tert-butoxide/sodium silicate mix activator. The microstructure of the fly ash geopolymer was characterized by ATR-FTIR, SEM-EDS, XRD, and Brunauer-Emmett-Teller (BET) physical adsorption method. The results showed that the new mixture alkali activator prepared by 5% sodium tert-butoxide and 10% sodium silicate improved the denseness and integrity of the microstructure of the fly ash geopolymer. Consequently, the mechanical properties of fly ash geopolymer are improved. The microscopic results demonstrated that the C–OH in tert-butanol after the hydrolysis of sodium tert-butoxide and Si–OH in the geopolymer can form C–O–Si bonds, forming a more complex three-dimensional network structure. This paper reveals the enhancement mechanism of organic alkali as activators for preparing fly ash geopolymer, and provides support for the subsequent development of organic strong alkali activators.     

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.     

Synthesis of nanometer enstatite from precursor gels prepared by the geopolymer technique

The geopolymer technique is one of the methods making inorganic polymers which can be prepared by polycondensation of silicic acids complexes containing foreign metal ions. Sodium silicate solutions prepared from Na₂O·SiO₂·9H₂O was applied as host solutions. Anhydrous silicate consisting of MgO and SiO₂ components were synthesized by heat treatment of gel precursors prepared by mixing sodium metasilicate solution with Mg–nitrate solution in various molar ratio. Amorphous and crystalline characters were discussed on the basics of DTA and XRD results. The XRD results showed that gels began to crystallize enstatite at about 900 °C.     

分选与磨细粉煤灰对水泥胶砂性能的影响

研究了分选与磨细粉煤灰的颗粒分布与形貌的差异及对水泥胶砂性能的影响。研究结果表明:当勃氏比表面积相近,磨细粉煤灰的中位粒径大于分选细粉煤灰,其圆珠状颗粒较少,表面较为粗糙。在相同水胶比的条件下,掺分选粗粉煤灰的水泥胶砂流动度及强度均低;分选粗粉煤灰磨细后,不仅减少了颗粒的粘连,增加了比表面积,而且提高了粉煤灰的反应活性和水泥胶砂流动度及强度,虽其水泥胶砂流动度仍小于掺分选细粉煤灰的水泥,3d水泥胶砂强度也略低,但其28d水泥胶砂强度略高于掺分选细粉煤灰的水泥;在相同水泥胶砂流动度的条件下,掺磨细粉煤灰配制的水泥胶砂3d强度低于掺分选细粉煤灰的水泥,但随着水化龄期的增长,其差距逐步缩小,至60d时可超过后者。     

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.     

掺加粉煤灰的砂浆和混凝土 抗压强度相关性研究*

为了解掺加粉煤灰的砂浆和混凝土抗压强度相关性,成型了不同粉煤灰掺量的砂浆和混凝土试样。在砂浆和混凝土试样标准养护到一定龄期后,基于《建筑砂浆基本性能试验方法标准》JGJ/T70—2009和《普通混凝土力学性能试验方法标准》GB/T50081—2002中方法进行抗压强度试验,发现：在标养不超过360 d、粉煤灰掺量0～40%范围内,粉煤灰掺量和龄期分别对砂浆和混凝土抗压强度的影响趋势基本一致,可以用成型砂浆试件代替混凝土试件,来找寻一定配比混凝土最大抗压强度所对应的粉煤灰掺量。     

Hydration reactions of cement combinations containing vitrified incinerator fly ash

One treatment option for municipal solid waste incinerator fly ash (IFA) is vitrification. The process yields a material containing reduced levels of trace metals relative to the original ash. The material is glassy and potentially suitable as a cement component in concrete. This paper examines the vitrification of an IFA and studies the hydration reactions of combinations of this vitrified material and Portland cement (PC). Isothermal conduction calorimetry, powder X-ray diffraction (XRD), thermogravimetry (TG) and scanning electron microscopy were employed to study the hydration reactions. As the levels of vitrified ash increase, the quantities of AFt phase produced decrease, whilst quantities of AFm phase increase, due to the reduced levels of sulfate in the vitrified ash. The levels of calcium silicate hydrate (CSH) gel (inferred from estimates of quantities of gel-bound water) remain constant at 28 days regardless of vitrified ash content, indicating that the material is contributing toward the formation of this product.     

Experimental study on mix proportion and fresh properties of fly ash based geopolymer for 3D concrete printing

This paper presents the material design and fresh properties of geopolymer mortar developed for 3D concrete printing application. Unlike traditional casting, in 3D printing, extruded materials are deposited layer-by-layer to build complex architectural and structural components without the need of any formwork and human intervention. Extrudability, shape retention, buildability and thixotropic open time (TOT) are identified as critical early-age properties to characterize the 3D printable geopolymer material. Five different mix designs of geopolymer are tested in a systematic experimental approach to obtain a best printable mix and later it is used to print a 60-centimeter-tall freeform structure using a concrete gantry printer to validate the formulation.     

Effect of nano-silica on the chemical durability and mechanical performance of fly ash based geopolymer concrete

In this study, the effect of nano silica on the short term severe durability performance of fly ash based geopolymer concrete (GPC) specimens was investigated. Four types of GPC were produced with two types of low calcium fly ashes (FAI and FAII) with and without nano silica, and ordinary Portland cement concrete (OPC) concrete was also cast for reference. For the geopolymerization process, the alkaline activator has selected a mixture of sodium silicate solution (Na₂SiO₃) and sodium hydroxide solution (NaOH) with a ratio (Na₂SiO₃/ NaOH) of 2.5. Main objectives of the study were to investigate the effect of usability or replaceability of nano silica-based low calcium fly ash based geopolymer concretes instead of OPC concrete in structural applications and make a contribution to standardization process of the fly ash based geopolymer concrete. To achieve the goals, four types of geopolymer and OPC concretes were subjected to sulfuric acid (H₂SO₄), magnesium sulfate (MgSO₄) and seawater (NaCl) solutions with concentrations of 5%, 5%, and 3.5%, respectively. Visual appearances and weight changes of the concretes under chemical environments were utilized for durability aspects. Compressive, splitting tensile and flexural strength tests were also performed on specimens to evaluate the mechanical performance under chemical environments. Results indicated that FAGPC concretes showed superior performance than OPC concrete under chemical attacks due to low calcium content. Amongst the chemical environments, sulfuric acid (H₂SO₄) was found to be the most dangerous environment for all concrete types. In addition, nano silica (NS) addition to FAGPC specimens improved both durability and residual mechanical strength due to the lower porosity and more dense structure. The FAIIGPC specimens including nano silica showed the superior mechanical performance under chemical environment.     

Brazilian fly ash based inorganic polymers production using different alkali activator solutions

Inorganic polymers are a relatively new class of inorganic binders of elevated mechanical strength and chemical resistance which can be produced from various industrial byproducts containing alumina and silica such as fly ash, slag and kaolin. In this work, fly ash from a Brazilian thermoelectric power station at Capivari de Baixo, Santa Catarina, was used to produce the inorganic cement. Different alkaline activator solutions were used in order to study their influence on the properties of the produced inorganic polymers. The results demonstrated that this fly ash is a viable starting material for the production of inorganic binders with compression strength higher than 40 MPa. It has been observed that the compression strength increased with increasing curing time of up to 28 days. Phase analysis by X-ray diffraction analysis revealed the formation of new zeolithic phases during the polymerization process. The polymerization process was also confirmed by modifications of the binding structure, as indicated by altered and new absorption bands observed by infrared spectroscopy. Furthermore, it has been found that physical properties, such as density and porosity, not only varied for different activator solutions, but also with curing time.     

Suitability of Sarawak and Gladstone fly ash to produce geopolymers: A physical,chemical, mechanical,mineralogical and microstructural analysis

Two types of fly ash sourced from Sarawak, Malaysia and Gladstone, Australia reflect differences in chemical compositions, mineral phase and particle size distributions. In this paper, the Sarawak fly ash was used to produce geopolymer in comparison to the well-developed Gladstone fly ash-based geopolymer. Characteristics of fly ash and mixtures proportions affecting compressive strength of the geopolymers were investigated. It is found that the variations of both fly ash types on particle size distributions, chemical compositions, morphology properties and amorphous phase correspond to the compressive strength. The results obtained show that after 7 days, geopolymer using Sarawak fly ash has lower compressive strength of about 55 MPa than geopolymer using Gladstone fly ash with strength of about 62 MPa. In comparison with Gladstone fly ash-based geopolymer, it showed that Sarawak fly ash-based geopolymer can be a potential construction material. Moreover, the production of Sarawak fly ash-based geopolymer aids to widen the application of Sarawak fly ash from being treated as industrial waste consequently discharging into the ash pond.     

Mechanical properties and prediction of fracture parameters of geopolymer/alkali-activated mortar modified with PVA fiber and nano-SiO2

Properties of fly ash (FA) and metakaolin (MK) based geopolymer/alkali-activated mortar modified with polyvinyl alcohol (PVA) fiber and nano-SiO₂, including workability, compressive strength, flexural performance, elastic modulus and fracture property were tested in this study. PVA fiber content varies from 0 to 1.2%. Nano-SiO₂ content is 0 and 1%. Adaptive neuro-fuzzy interfacial systems (ANFIS) method was used to establish the artificial intelligence (AI) model to predict the fracture parameters of geopolymer/alkali-activated mortars. The inputs of ANFIS models include PVA fiber content, nano-SiO₂ content, compressive strength, flexural strength, elastic modulus, critical crack mouth opening displacement, crack load and peak load. The outputs of ANFIS model include critical effective crack length, initiation fracture toughness, unstable fracture toughness, and fracture energy. Experiment results showed that PVA fiber addition enhanced the mechanical properties especially the compressive strength and fracture performance, but decreased the workability. 0.8%–1.0% was considered as the optimal content of PVA fiber. Addition of 1% nano-SiO₂ shows a slight improvement on both workability and mechanical properties of the mortar no matter how much fiber is added. Based on the ANFIS algorithm and 42 sets of experimental data, the trained models were proved to have high accuracy with root mean square error (RMSE) under 0.15, mean absolute error (MAE) under 0.01, and coefficient of determination (R²) over 0.85. The ANFIS model established in this study combined the fracture properties with the basic mechanical properties of geopolymer/alkali-activated composites, which can provide a new method to assess the fracture performance of geopolymer/alkali-activated mortars modified with PVA fiber and nano-SiO₂ in the future.     

Influence of fly ash fineness on strength, drying shrinkage and sulfate resistance of blended cement mortar

In this paper, the influence of fineness of fly ash on water demand and some of the properties of hardened mortar are examined. In addition to the original fly ash (OFA), five different fineness values of fly ash were obtained by sieving and by using an air separator. Two sieves, Nos. 200 and 325, were used to obtain two lots of graded fine fly ash. For the classification using air separator, the OFA was separated into fine, medium and coarse portions. The fly ash dosage of 40% by weight of binder was used throughout the experiment. From the tests, it was found that the compressive strength of mortar depended on the fineness of fly ash. The strength of mortar containing fine fly ash was better than that of OFA mortar at all ages with the very fine fly ash giving the highest strength. The use of all fly ashes resulted in significant improvement in drying shrinkage with the coarse fly ash showing the least improvement owing primarily to the high water to binder ratio (W/B) of the mix. Significant improvement of resistance to sulfate expansion was obtained for all fineness values except for the coarse fly ash where greater expansion was observed. The resistance to sulfuric acid attack was also improved with the incorporation of all fly ashes. In this case the coarse fly ash gave the best performance with the lowest rate of the weight loss owing probably to the better bonding of the coarse fly ash particles to the cement matrix and less hydration products. It is suggested that the fine fly ash is more reactive and its use resulted in a denser cement matrix and better mechanical properties of mortar.     

Characterisation and properties of geopolymer composite part 1: Role of mullite reinforcement

Geopolymer-mullite composite was prepared using fly ash and mullite powders with sodium silicate and sodium hydroxide as alkaline activators. Mullite was used as a replacement to fly ash in the 20–60 wt% range. Sodium silicate to sodium hydroxide (12 M) ratio was 1:1 while the liquid to solid ratio was 0.6:1. X-ray diffraction (XRD) analysis revealed that the set of geopolymer specimens without mullite replacement (control) showed the co-existence of amorphous and crystalline phases of quartz, magnesioferrite (Fe₂MgO₄), Lazurite (Na_8.56 (Al₆Si₆O₂₄) (SO₄)_1.56 S_0.44)) and calcium silicate hydrate. With an increasing amount of mullite replacement, calcium silicate hydrate and magnesioferrite diminished while the new phase of phillipsite (K, Na)₂(Si,Al)₈O₁₆·4H₂O) emerged. Microstructural analysis revealed Si-rich mullite needles possibly occurred by recrystallization of the original mullite. This suggestion was also confirmed by the change of the crystallite size as analysed using an X-ray diffraction technique. The ambient compressive strength was found to increase from 58 ± 21 MPa for the control geopolymer to 72–76 MPa, with a much smaller uncertainty, for the geopolymer-mullite composite. Modulus of rupture (MOR) was found to improve significantly from 0.7 ± 0.3 MPa to 3.7 ± 0.5 MPa in the 20% replacement and further to 7.8 ± 1.3 and 8.1 ± 1.1 MPa in the 40% and 60% replacement respectively. Improvement of fire resistance was observed in the 40–60% replacement thermal shock resistance property, however, was unchanged in these geopolymer-mullite composite.     

粉煤灰-偏高岭土基地质聚合物胶凝材料的研究

研究了以粉煤灰替代偏高岭土为主要原料制备的地质聚合物胶凝材料的抗压强度,并用SEM观察其微观形貌。结果表明,用含有质量分数20%、40%、60%粉煤灰替代偏高岭土为原料制得的地质聚合物,其受压破坏面物相成分较杂乱,有较多的球状粉煤灰颗粒和裂缝;与单用偏高岭土作原料制备的地质聚合物相比,试样各龄期抗压强度值均不高。     

Development of fly ash and iron ore tailing based porous geopolymer for removal of Cu(II) from wastewater

This work aims to verify the feasibility of utilizing iron ore tailing (IOT) in porous geopolymer and intends to broaden the application of porous geopolymer in heavy metal removal aspect. Porous geopolymer was prepared using fly ash as resource material, which was partially replaced by IOT at level of 30%, by weight, with H₂O₂ as foaming agent and removal efficiency, adsorption affecting factors, adsorption isotherms and thermodynamics of Cu²⁺ by the developed porous geopolymer were investigated.The experimental results uncover that the porous amorphous geopolymer was successful synthesized with total porosity of 74.6%. The transformation of fly ash and IOT into foaming geopolymer leads to the formation of porous structure encouraging Cu²⁺ sorption. Batch sorption tests were carried out and geopolymer dosage, Cu²⁺ initial concentration, pH, contact time and temperature were the main concern. Both Langmuir and Freundlich models could explain the adsorption of Cu²⁺ on the porous geopolymer due to the high fitting coefficients. The uptake capacity reaches the highest value of 113.41 mg/g at 40 °C with pH value of 6.0. The thermodynamic parameters ΔHº, ΔSº and ΔGº suggests the spontaneous nature of Cu²⁺ adsorption on porous geopolymer and the endothermic behavior of sorption process.     

Characterisation and properties of geopolymer composites. Part 2: Role of cordierite-mullite reinforcement

Our previous research paper on geopolymer-mullite composites showed promising results on compressive strength and fire resistance. However, no improvement in thermal shock resistance was observed in the afore mentioned study. In this study, further attempts to improve thermal shock resistance of the geopolymer were explored. The research was performed by compositing a fly ash-based geopolymer with cordierite-mullite at 20, 40 and 60 wt% replacement. X-ray diffraction (XRD) of the cured geopolymer composite specimens showed the existence of cordierite, mullite, quartz, cancrinite and lazurite. It was found that compressive strength and strength retention after thermal exposure at 400 °C were improved in the geopolymer composite specimens, especially those with 20–40 wt% replacement. Upon further heating to 600 °C, all geopolymer specimens showed insignificant differences in compressive strength. Fire resistance was found to improve with increasing proportion of replacement contents.     

Performance evaluation of basalt fiber-reinforced geopolymer composites with various contents of nano CaCO3

High carbon footprint of cement production is the major drawback of plain cement concrete resulting in environmental pollution. Geopolymer composites paste can be effectively used as an alternative to Portland cement in the construction industry for a sustainable environment. The demand for high-performance composites and sustainable construction is increasing day by day. Therefore, the present experimental program has endeavored to investigate the mechanical performance of basalt fiber-reinforced fly ash-based geopolymer pastes with various contents of nano CaCO₃. The content of basalt fibers was fixed at 2% by weight for all specimens while the studied contents of nano CaCO₃ were 0%, 1%, 2%, and 3%, respectively. The compressive strength, compressive stress-strain response, flexural strength, bending stress-strain response, elastic modulus, toughness modulus, toughness indices, fracture toughness, impact strength, hardness, and microstructural analysis of all four geopolymer composite pastes with varying contents of nano CaCO₃ using scanning electron microscopy (SEM) were evaluated. The results revealed that the use of 3% nano CaCO₃ in basalt fiber-reinforced geopolymer paste presented the highest values of compressive strength and hardness while the use of 2% nano CaCO₃ showed the highest values of flexural strength, impact strength, and fracture toughness of composite paste. The SEM results indicated that the addition of nano CaCO₃ improved the microstructure and provided a denser geopolymer paste by refining the interfacial zones and accelerating the geopolymerization reaction.     

Mechanical and microstructural evolutions of fly ash/slag-based geopolymer at high temperatures: Effect of curing conditions

Designing a building material with excellent heat resistance is crucial for protection against catastrophic fires. Geopolymer materials have been investigated as they offer better heat resistance than traditional cement owing to their ceramic-like properties. Curing temperature and conditions are crucial factors that determine the properties of geopolymers, but their impacts on the heat resistance of geopolymers remain unclear. This study produced geopolymers from fly ash and ground granulated blast furnace slag by using sodium silicate and sodium hydroxide solutions as alkaline solutions. To examine the effect of curing conditions on the high-temperature performance of geopolymer, four different curing conditions, namely, heat curing (70 °C for 24 h), ambient curing (20 °C), water curing, and the combination of heat and water curing (70 °C for 24 h followed by water curing), were applied. At 28 d, the specimens were subjected to high temperatures (500 °C, 750 °C, and 950 °C), and their mechanical and microstructural evolutions were studied. The results revealed that the curing condition significantly affects the properties of the unexposed geopolymer; the effect on its high-temperature performance is insignificant. Furthermore, all the specimens could maintain adequate compressive strength after exposure to the maximum temperature of 950 °C, promising the use of geopolymer for structural applications.     

An experimental study on strength development of concrete containing fly ash and optimum usage of fly ash in concrete

This paper presents a laboratory study on the strength development of concrete containing fly ash and optimum use of fly ash in concrete. Fly ash was added according to the partial replacement method in mixtures. A total of 28 mixtures with different mix designs were prepared. 4 of them were prepared as control mixtures with 250, 300, 350, and 400 kg/m³ cement content in order to calculate the Bolomey and Feret coefficients (K_B, K_F). Four groups of mixtures were prepared, each group containing six mix designs and using the cement content of one of the control mixture as the base for the mix design. In each group 20% of the cement content of the control mixture was removed, resulting in starting mixtures with 200, 240, 280, and 320 kg/m³ cement content. Fly ash in the amount of approximately 15%, 25%, 33%, 42%, 50%, and 58% of the rest of the cement content was added as partial cement replacement. All specimens were moist cured for 28 and 180 days before compressive strength testing. The efficiency and the maximum content of fly ash that gives the maximum compressive strength were obtained by using Bolomey and Feret strength equations. Hence, the maximum amount of usable fly ash amount with the optimum efficiency was determined.This study showed that strength increases with increasing amount of fly ash up to an optimum value, beyond which strength starts to decrease with further addition of fly ash. The optimum value of fly ash for the four test groups is about 40% of cement. Fly ash/cement ratio is an important factor determining the efficiency of fly ash.