Synthesis of heat and ambient cured one-part geopolymer mixes with different grades of sodium silicate

The use of solid activators in the manufacture of geopolymer enhances its commercial viability as it aids the development of a one-part “just add water” geopolymer mixture, similar to the conventional Portland cement-based materials. This study is aimed to synthesize heat and ambient cured one-part geopolymer mixes. Appropriate combinations of low calcium (Class F) fly ash, slag and hydrated lime as the aluminosilicate source materials were activated by three different grades of sodium silicate and a combination of sodium silicate and sodium hydroxide powders. A conventional two-part geopolymer mix with the commonly used sodium hydroxide and sodium silicate solutions was also made for comparison. Effects of the type and amount of the solid activator, the amount of fly ash replacement with slag and hydrated lime and water content on short term mechanical properties of the heat cured one-part geopolymer mixtures including workability of the fresh mix, hardened density and compressive strength were evaluated. Subsequently, effects of ambient curing on the properties of the developed one-part geopolymer mixes were also investigated. Moderate to high compressive strength of over 37 MPa developed for the heat and ambient cured one-part geopolymer mixes. The 28-days compressive strengths of the ambient cured one-part geopolymer mixtures, regardless of the type of activator and geopolymer source materials, were comparable to those of the counterpart heat cured one-part geopolymer mixes. Such one-part geopolymer mixes could enhance the commercial viability and large-scale applications of the geopolymer in the construction industry.     

室温碱激发低钙粉煤灰地质聚合物配比试验研究

为得到室温下粉煤灰与碱激发剂质量比、水玻璃与氢氧化钠溶液质量比和氢氧化钠溶液摩尔浓度对粉煤灰地质聚合物力学性能的影响,以低钙粉煤灰为原料,制备了地质聚合物胶凝材料。采用正交试验方法,分析粉煤灰地质聚合物抗压强度,探讨碱激发剂配比对粉煤灰地质聚合物力学性能的影响,结合SEM、XRD和FTIR对试样进行表征,并对该材料的应力-应变曲线进行了研究。结果表明:粉煤灰地质聚合物的抗压强度随着激发剂掺量的减少而增大,水玻璃在激发剂中的比值与粉煤灰地质聚合物的抗压强度呈现正相关,其中粉煤灰与碱激发剂质量比为1.8,水玻璃与氢氧化钠溶液质量比为2.5且氢氧化钠溶液的浓度为10 mol/L时,120 d龄期的抗压强度可达51.98 MPa。对应力-应变曲线分析得出,在一定程度上,激发剂的掺入量对粉煤灰地质聚合物的破坏应变和弹性模量有较大影响。SEM、XRD和FTIR分析表明随着养护时间增长,胶凝材料体系内结构更致密,生成了更多的硅铝酸盐凝胶。     

富镁镍渣-粉煤灰基地质聚合物的制备与性能表征

以工业固体废弃物富镁镍渣和粉煤灰为原料,以水玻璃和NaOH为碱激发剂,制备了一系列富镁镍渣-粉煤灰基地质聚合物。研究了不同粉煤灰掺量对地质聚合物力学性能的影响,并测定地质聚合物的线性收缩和碱溶出,通过XRD、IR、DTA等手段对产物进行表征。结果表明:富镁镍渣-粉煤灰基地质聚合物的强度随粉煤灰的掺入先升高后降低,当掺量为30%(质量分数)时,地质聚合物的抗压强度可达最高值22.15 MPa,较镍渣基地质聚合物强度提高42.2%;XRD分析表明富镁镍渣中MgO以镁橄榄石相存在,而非游离态,故地质聚合物具有良好的体积安定性。     

Mechanical properties and microstructure evaluation of fly ash - Slag geopolymer foaming materials

The existing fly ash-slag foaming geopolymer materials generally have the shortcomings of low fly ash content and low porosity. It is urgent to develop geopolymer foaming materials with high fly ash content and high porosity. Using fly ash and slag as the main raw materials, geopolymer foaming materials were prepared by alkali activation. The effects of activator content and sodium silicate modulus on the macroscopic mechanical properties, pore structures and microstructures of geopolymer foaming materials were studied. The experimental results showed that when the activator content was 21% (wt.) and the modulus of sodium silicate was 1, the specimen exhibited the best performance. The compressive strength of the specimen reached 2.18 MPa at 28 d, the porosity was 63.07%, and the average pore sizes of macroscopic pores were 920 μm. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Scanning Electron Microscopy and Energy Dispersive Spectrometer (SEM-EDS) analysis showed that when the content of activator was 21% and the modulus of sodium silicate was 1, the reaction grade of the system was the highest, reached 55.12%, meanwhile the main product Sodium silicate hydrate (N-A-S-H) gel produced the largest amount. The fractal dimension calculations showed that the spatial complexity of a specimen with large pores was greater than that of a specimen with small pores. This study can provide a basis for the design of geopolymer foaming materials with high proportion of fly ash and high porosity.     

水玻璃-Na2CO3激发富镁镍渣-粉煤灰基地质聚合物的制备及性能

本文使用正交试验法,研究了富镁镍渣与粉煤灰的质量比、复合碱激发剂(水玻璃-Na₂CO₃)掺量及水胶比对富镁镍渣-粉煤灰基地质聚合物力学性能的影响,通过XRD、SEM、EDS及TG等测试方法对水化产物进行表征。结果表明,最优试样28 d抗压强度可达37.50 MPa。XRD结果显示,7 d与28 d的水化产物中含有水化硅酸钙凝胶,结合SEM、EDS分析可知,产物中还有菱沸石(N-A-S-H)与钠镁硅铝酸盐(N-M-A-S)无定形凝胶相,这些凝胶相是地质聚合物强度增加的主要原因。     

Fly ash-based boroaluminosilicate geopolymers: Experimental and molecular simulations

Geopolymers are cement-free construction materials which are produced by mixing an aluminosilicate source such as fly ash with an alkali activator. Despite their eco-friendly nature, geopolymers suffer the negative impact of the sodium silicate part of the alkali activator on the environment. The use of borax, one of the eco-friendly salts of boric acid, as an activator can potentially lead to the production of more environmentally-friendly geopolymer. However, a better understanding of their theoretical properties could be a milestone to produce new generations of geopolymers with high performance. A growing interest in the prediction of the macroscale properties of geopolymer compounds was the most compelling motivation for this study. Building upon this, the current study focused on both points to apply borax as a potential replacement for silicate-based activators and model all the experiments by molecular dynamics (MD) simulation. Substituting boron with aluminium in the molecular structures of geopolymer was the core idea of the simulation. Compressive strength, density and elastic modulus tests were conducted, and the results were compared with the MD simulation outcomes. Increasing the content of borax in the mixture led to a decrease in all of the properties, although the range of 10–30% of replacement eventuated in acceptable results. A fair agreement between simulation and experimental results was achieved through which the best fitting parameters for atomistic modelling of geopolymers were found.     

Synthesis and characteristics of fly ash and bottom ash based geopolymers–A comparative study

The research was carried out to develop geopolymers mortars and concrete from fly ash and bottom ash and compare the characteristics deriving from either of these products. The mortars were produced by mixing the ashes with sodium silicate and sodium hydroxide as activator solution. After curing and drying, the bulk density, apparent density and porosity, of geopolymer samples were evaluated. The microstructure, phase composition and thermal behavior of geopolymer samples were characterized by scanning electron microscopy, XRD and TGA-DTA analysis respectively. FTIR analysis revealed higher degree of reaction in bottom ash based geopolymer. Mechanical characterization shows, geopolymer processed from fly ash having a compressive strength 61.4 MPa and Young's modulus of 2.9 GPa, whereas bottom ash geopolymer shows a compressive strength up to 55.2 MPa and Young's modulus of 2.8 GPa. The mechanical characterization depicts that bottom ash geopolymers are almost equally viable as fly ash geopolymer. Thermal conductivity analysis reveals that fly ash geopolymer shows lower thermal conductivity of 0.58 W/mK compared to bottom ash geopolymer 0.85 W/mK.     

Fly Ash Porous Material using Geopolymerization Process for High Temperature Exposure

This paper presents the results of a study on the effect of temperature on geopolymers manufactured using pozzolanic materials (fly ash). In this paper, we report on our investigation of the performance of porous geopolymers made with fly ash after exposure to temperatures from 600 °C up to 1000 °C. The research methodology consisted of pozzolanic materials (fly ash) synthesized with a mixture of sodium hydroxide and sodium silicate solution as an alkaline activator. Foaming agent solution was added to geopolymer paste. The geopolymer paste samples were cured at 60 °C for one day and the geopolymers samples were sintered from 600 °C to 1000 °C to evaluate strength loss due to thermal damage. We also studied their phase formation and microstructure. The heated geopolymers samples were tested by compressive strength after three days. The results showed that the porous geopolymers exhibited strength increases after temperature exposure.     

粉煤灰地聚合物力学性能影响因素研究综述

粉煤灰地聚合物是以粉煤灰为硅铝质原料制备的,具有强度高、耐高温、耐腐蚀、有效固封金属离子等优点。但它固有的脆性以及需高温养护才能快速获得高强度的特点限制了其运用范围,而以纤维作为增强材料不仅可以提高粉煤灰地聚合物的强度,还可以改善其延性和韧性。本文主要从粉煤灰原料特性、碱激发剂、养护制度和增强材料四方面入手,重点阐述了粉煤灰粒径和化学组成,碱激发剂的种类、用量和模数,升温养护时间和初期养护温度对抗压强度的影响,以及纤维对粉煤灰地聚合物抗压强度和弯曲性能的影响。最后,根据现有的研究成果,对四种影响因素分别是如何影响粉煤灰地聚合物力学性能进行总结。     

Synthesis of volcanic ash-based geopolymer mortars by fusion method: Effects of adding metakaolin to fused volcanic ash

The present study aimed at improving the properties of geopolymer mortars obtained from volcanic ash as a source material. An alkali fusion process was used to promote the dissolution of Si and Al species from the volcanic ash and thus to enhance the reactivity of volcanic ash. Various amount of metakaolin (30%, 40%, 50% and 60% MK by weight) was used to consume the excess alkali needed for the fusion. The amount of amorphous phase was determined both in the volcanic ash and the fused volcanic ash and X-ray diffraction analysis was used to evaluate effect of the alkaline fusion method. Geopolymers were prepared by alkali activation of mixtures of powders of fused volcanic ash, various amount of metakaolin and river sand using a sodium silicate solution as activator. The geopolymer mortars were characterized by determination of setting time, linear shrinkage, scanning electron microscopy and compressive strength. The results of this study indicate that geopolymer mortars synthesized by the fusion method exhibit low setting time (7–15 min), low shrinkage (0–0.42%) and high compressive strength (41.5–68.8 MPa). This study showed that, by enhancing the reactivity of volcanic ash by alkali fusion and balancing the Na/Al ratio through the addition of metakaolin, all volcanic ashes can be recycled as an alternative source material for the production of geopolymers.     

激发剂对赤泥地聚物砂浆力学性能的影响

以热活化氧化铝赤泥为主要原料制备赤泥地聚物砂浆,对比研究了激发剂种类和掺量对赤泥地聚物力学性能的影响及其合成机理。结果表明,水玻璃、石灰-碱（质量比为2∶1）、石膏-碱（质量比为2∶1）均能改善氧化铝赤泥的反应活性,促进赤泥地聚物的合成,其中水玻璃（氧化硅与氧化钠物质的量比为1.5）的改性效果最为显著。水玻璃、石灰-碱、石膏-碱的最佳掺量分别为20%、7%、10%。水玻璃掺量为20%时,赤泥地聚物砂浆28 d抗压强度和28 d抗折强度分别为32.1 MPa和6.0 MPa。改变激发剂的种类和掺量,可以调整赤泥地聚物砂浆体系的碱度,改变铝硅酸结构的解聚和地聚物的缩聚过程,从而影响其力学性能。     

硅酸钠模数对无机聚合物力学性能与微观结构的影响

用模数m=1.0、1.2、1.4和1.6的4种硅酸钠溶液作激发剂制备偏高岭土基无机聚合物,通过强度测试、红外分析(IR)、X线衍射(XRD)和扫描电镜(SEM)等方法考察激发剂模数对无机聚合物力学性能和微观结构的影响。结果表明:模数在1.0~1.6变化时,激发剂中硅氧四面体呈低聚合态;随养护时间延长,无机聚合物抗压强度和抗折强度提高,m=1.2的无机聚合物28 d抗压强度最高(74.6 MPa),抗折强度为11.2 MPa;4种无机聚合物主体相均呈非晶态,结构上由凝胶体和残留原料颗粒组成,其中,m=1.2时无机聚合物的显微结构最平整。     

Sodium silicate activated slag‐fly ash binders: Part I – Processing,microstructure, and mechanical properties

Alkali silicate activated slag and class F fly ash‐based binders are ambient curing, structural materials that are feasible replacements for ordinary Portland cement (OPC). They exhibit advantageous mechanical properties and less environmental impact than OPC. In this work, five sodium silicate activated slag‐fly ash binder mixtures were developed and their compressive and flexural strengths were studied as a function of curing temperature and time. It was found that the strongest mixture sets at ambient temperature and had a Weibull average flexural strength of 5.7 ± 1.5 MPa and Weibull average compressive strength of 60 ± 8 MPa at 28 days. While increasing the slag/fly ash ratio accelerated the strength development, the cure time was decreased due to the formation of calcium silicate hydrate (C–S–H), calcium aluminum silicate hydrate (C–A–S–H), and (Ca,Na) based geopolymer. The density, microstructure, and phase evolution of ambient‐cured, heat‐cured, and heat‐treated binders were studied using pycnometry, scanning electron microscopy, energy dispersive X‐ray spectroscopy (SEM‐EDS), and X‐ray diffraction (XRD). Heat‐cured binders were more dense than ambient‐cured binder. No new crystalline phases evolved through 28 days in ambient‐ or heat‐cured binders.     

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.     

地质聚合物激发剂及其激发原理

激发剂在制备地质聚合物的过程中发挥着重要的作用。首先概述了酸、碱和盐3类激发剂及其激发效果,通过介绍氢氧化钠、氢氧化钾和水玻璃等常用碱激发剂的激发效果,具体分析各类碱激发剂的作用差别。阐述了乙酸、盐酸、硫酸和磷酸等典型的酸激发剂的研究进展,以及硫酸盐、硅酸盐和铝酸盐等作为盐类激发剂的活化能力,在此基础上对3类激发剂的优缺点进行了总结分析。同时通过论述酸、碱和盐3类激发剂的激发原理,进一步表明了3类激发剂都具备激发地聚物原料活性的能力,都能够不同程度地加快地聚体系水化反应进程,显著提高地聚体系的强度。最后,对激发剂在地聚物领域的未来发展作出了展望。     

Resistance of geopolymer materials to acid attack

This article presents an investigation into durability of geopolymer materials manufactured using a class F fly ash (FA) and alkaline activators when exposed to 5% solutions of acetic and sulfuric acids. The main parameters studied were the evolution of weight, compressive strength, products of degradation and microstructural changes. The degradation was studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The performance of geopolymer materials when exposed to acid solutions was superior to ordinary Portland cement (OPC) paste. However, significant degradation of strength was observed in some geopolymer materials prepared with sodium silicate and with a mixture of sodium hydroxide and potassium hydroxide as activators. The deterioration observed was connected to depolymerisation of the aluminosilicate polymers in acidic media and formation of zeolites, which in some cases lead to a significant loss of strength. The best performance was observed in the geopolymer material prepared with sodium hydroxide and cured at elevated temperature, which was attributed to a more stable cross-linked aluminosilicate polymer structure formed in this material.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

地聚合物力学性能主要调控因素的研究综述

地聚合物作为一种新型绿色胶凝材料,影响其力学性能的因素复杂多样.综述了影响地聚合物力学性能的三大主要调控因素:硅铝质原料、碱激发剂和养护制度,并把每种调控因素对力学性能影响规律与原因,结合当前的研究现状得出结论.硅铝质原料主要对比了偏高岭土、粉煤灰和高炉矿渣;碱激发剂从种类的选取、碱当量以及模数进行了比较;养护制度以初期养护时,升高不同的温度和升温时间的影响做出了对比.最后,说明了当前有机聚合物改性地聚合物的一些研究现状.     

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。     

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.