Compressive strength and microstructural analysis of fly ash/palm oil fuel ash based geopolymer mortar

This paper presents the effects and adaptability of palm oil fuel ash (POFA) as a replacement material in fly ash (FA) based geopolymer mortar from the aspect of microstructural and compressive strength. The geopolymers developed were synthesized with a combination of sodium hydroxide and sodium silicate as activator and POFA and FA as high silica–alumina resources. The development of compressive strength of POFA/FA based geopolymers was investigated using X-ray florescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and field emission scanning electron microscopy (FESEM). It was observed that the particle shapes and surface area of POFA and FA as well as chemical composition affects the density and compressive strength of the mortars. The increment in the percentages of POFA increased the silica/alumina (SiO₂/Al₂O₃) ratio and that resulted in reduction of the early compressive strength of the geopolymer and delayed the geopolymerization process.     

Compressive strength and microstructure of fly ash based geopolymer blended with silica fume under thermal cycle

This work aims to reveal the effects of silica fume on properties of fly ash based geopolymer under thermal cycles. Geopolymer specimens were prepared by alkali activation of fly ash, which was partially replaced by silica fume at levels ranging from 0% to 30% with an interval of 10%, by mass. Microstructure, residual strength and mass loss of fly ash based geopolymer blended with silica fume before and after exposed to 7, 28 and 56 heat-cooling thermal cycles at different target temperatures of 200 °C, 400 °C and 800 °C were assessed and compared. The experimental results reveal that silica fume addition enhances strength development in geopolymer. Under thermal cycles, the compressive strength of geopolymer decreases, and the compressive strength loss, as well as the mass loss, increases with increasing target temperature. The strength loss is the same regardless of silica fume content after thermal cycles. Microstructure analysis uncovers that pore structure of geopolymer degrades after thermal cycles. The pores of geopolymer are refined by the addition of silica fume. The incorporation of silica fume optimizes the microstructure and improves the thermal resistance of geopolymer. Silica fume increases the strength of the geopolymer and even though the strength loss is the same, the strength after heat cycle exposure is still good.     

Properties of geopolymer with seeded fly ash and rice husk bark ash

In the present work, compressive strength of inorganic polymers (geopolymers) produced of seeded fly ash and rice husk bark ash has been investigated. Different specimens made from a mixture of fly ash and rice husk bark ash in fine and coarse form were subjected to compressive strength tests at 7 and 28 days of curing. The curing regime was different: one set of the specimens were cured at room temperature until reaching to 7 and 28 days and the other sets were oven cured for 36 h at the range of 40-90 °C and then cured at room temperature until 7 and 28 days. The results indicate that in both 7 and 28 days regimes, the highest strengths are related to the specimens by SiO₂/Al₂O₃ ratio equals 2.99 cured at 80 °C. For these specimens, those contained finer fly ash particles show more compressive strength. Thermogravimetric analysis and Fourier transform infrared spectroscopy both also are in agreement with the obtained results from compressive strength tests. In addition, SEM micrographs of the specimens show that the finer the particle size of the utilized ashes, the denser the microstructure which confirms the results obtained by the strength tests.     

Use of OPC to improve setting and early strength properties of low calcium fly ash geopolymer concrete cured at room temperature

Most previous works on fly ash based geopolymer concrete focused on concretes subjected to heat curing. Development of geopolymer concrete that can set and harden at normal temperature will widen its application beyond precast concrete. This paper has focused on a study of fly ash based geopolymer concrete suitable for ambient curing condition. A small proportion of ordinary Portland cement (OPC) was added with low calcium fly ash to accelerate the curing of geopolymer concrete instead of using elevated heat. Samples were cured in room environment (about 23 °C and RH 65 ± 10%) until tested. Inclusion of OPC as little as 5% of total binder reduced the setting time to acceptable ranges and caused slight decrease of workability. The early-age compressive strength improved significantly with higher strength at the age of 28 days. Geopolymer microstructure showed considerable portion of calcium-rich aluminosilicate gel resulting from the addition of OPC.     

The effects of ground granulated blast-furnace slag blending with fly ash and activator content on the workability and strength properties of geopolymer concrete cured at ambient temperature

Inclusion of ground granulated blast-furnace slag (GGBFS) with class F fly-ash can have a significant effect on the setting and strength development of geopolymer binders when cured in ambient temperature. This paper evaluates the effect of different proportions of GGBFS and activator content on the workability and strength properties of fly ash based geopolymer concrete. In this study, GGBFS was added as 0%, 10% and 20% of the total binder with variable activator content (40% and 35%) and sodium silicate to sodium hydroxide ratio (1.5–2.5). Significant increase in strength and some decrease in the workability were observed in geopolymer concretes with higher GGBFS and lower sodium silicate to sodium hydroxide ratio in the mixtures. Similar to OPC concrete, development of tensile strength correlated well with the compressive strength of ambient-cured geopolymer concrete. The predictions of tensile strength from compressive strength of ambient-cured geopolymer concrete using the ACI 318 and AS 3600 codes tend to be similar to that for OPC concrete. The predictions are more conservative for heat-cured geopolymer concrete than for ambient-cured geopolymer concrete.     

Sustainable utilization of pre-treated and nano fly ash powder for the development of durable geopolymer mortars

Developments in geopolymer construction are gaining more interest nowadays due to the elimination of cement and the consequent effects such as carbon dioxide emission, greenhouse effect, etc. Although the use of fly ash as a binder in the geopolymer system acts as a key solution for the major hazardous effects like land dumping, soil contamination, groundwater pollution, and respiratory diseases, the slow reactivity of the fly ash resulted in the considerable reduction in the strength. In this paper, a novel pretreatment method was employed on the fly ash binder in terms of thermal and mechanical means. Also, a cost-effective nano fly ash powder was synthesized and used as filler material on the geopolymer system. The efficiency of the fabricated geopolymer mortar was assessed by examining the workability, compressive strength, and resistance against chloride ion penetration. The geopolymer mortars with pre-treated fly ash exhibited a highly workable mix of 130% improved flow rate without adding any superplasticizer. Further, the addition of 1% nano fly ash, exhibited the highest compressive strength of 71.22 MPa, confirmed almost nil chloride ion permeability, and sustained 90% residual strength after immersing in the brine solution for 60 days which explored the development of sustainable and cost-effective geopolymer construction in the marine environment.     

Effects of NaOH concentrations on physical and electrical properties of high calcium fly ash geopolymer paste

The effects of sodium hydroxide (NaOH) concentration on setting time, compressive strength and electrical properties at the frequencies of 100 Hz–10 MHz of high calcium fly ash geopolymer pastes were investigated. Five NaOH concentrations (8, 10, 12, 15 and 18 molar) were studied. The liquid to ash ratio of 0.4, sodium silicate to sodium hydroxide ratio of 0.67 and low temperature curing at 40 °C were selected in making geopolymer pastes. The results showed that NaOH concentration had significant influence on the physical and electrical properties of geopolymer paste. The pastes with high NaOH concentrations showed increased setting time and compressive strength due to a high degree of geopolymerization as a result of the increased leaching of silica and alumina from fly ash. The dielectric constant and conductivity increased with NaOH concentration while tan δ decreased due to an increase in geopolymerization. At the frequency of 10³ Hz, the dielectric constants of all pastes were approximately 10⁴ S/cm and decreased with increased frequency. The relaxation peaks of tan δ reduced with an increase in NaOH concentration and ranged between 2.5 and 4.5. The AC conductivity behavior followed the universal power law and the values were in the range of 3.7 × 10⁻³–1.5 × 10⁻² at 10⁵–10⁶ Hz.     

Analysis of geopolymer concrete columns

Ordinary portland cement (OPC) has been traditionally used as the binding agent in concrete. However, it is also necessary to search for alternative low-emission binding agents for concrete to reduce the environmental impact caused by manufacturing of cement. Geopolymer, also known as inorganic polymer, is one such material that uses by-product material such as fly ash instead of cement. Recent research has shown that fly ash-based geopolymer concrete has suitable properties for its use as a construction material. Since the strength development mechanism of geopolymer is different from that of OPC binder, it is necessary to obtain a suitable constitutive model for geopolymer concrete to predict the load–deflection behaviour and strength of geopolymer concrete structural members. This article has investigated the suitability of using an existing stress–strain model originally proposed by Popovics for OPC concrete. It is found that the equation of Popovics can be used for geopolymer concrete with minor modification to the expression for the curve fitting factor, to better fit with the post-peak parts of the experimental stress–strain curves. The slightly modified set of stress–strain equations was then used in a non-linear analysis for reinforced concrete columns. A good correlation is achieved between the predicted and measured ultimate loads, load–deflection curves and deflected shapes for 12 slender test columns.     

Effect of granite flour proportion on the strength properties of geopolymer mortar based on fly ash

Given global trends and challenges, the development of binders for the production of geopolymer concretes has become a topical area of building science. The purpose of this study is to determine whether granite can replace traditional construction aggregate, such as river sand, during geopolymer production, as well as to demonstrate the effect of the proportion of granite flour on the strength properties of fly ash-based geopolymer mortar. A combination of granite flour, quartz sand, and fly ash in various proportions was used as an aluminosilicate precursor. The scope of the study includes density measurements, compressive and flexural strength tests, abrasion by the Boehme method, and microstructural observations. Based on the obtained results, it can be concluded that granite can be successfully used as a replacement for quartz sand during the production of fly ash-based geopolymers. Moreover, the addition of granite makes it possible to improve the strength properties of geopolymers, compared to a geopolymer composite containing quartz sand.     

Effectiveness of fly ash on the restrained shrinkage cracking resistance of self-compacting concrete

This paper presents results from restrained shrinkage tests to assess the effectiveness of fly ash on cracking and relaxation behavior of Self Compacting Concrete (SCC). The effects of fly ash (FA) proportion, degree of restraint and curing regime are specifically addressed. The results show that curing condition and degree of restraint play a significant role on the effectiveness of FA on the cracking and relaxation behavior of SCC mixes. It was also found that addition of FA improves the cracking resistance and relaxation behavior of SCC relative to the control. The results further suggest that FA can replace cement by up to 50% for low degree of restraint, and up to 35% for high degree of restraint with significant improvement in cracking resistance, provided that appropriate moist curing is adopted. The normalized results presented may also prove to be an important tool in devising mixes best suited for onsite conditions.     

Effect of NaOH concentration on properties and microstructure of a novel reactive ultra-fine fly ash geopolymer

This study used reactive ultra-fine fly ash (RUFA) as the primary raw materials in the preparation of a novel RUFA geopolymer. Note that the solution to binder weight ratio was maintained at the same level by varying the concentration of the NaOH solution. Extensive analysis was conducted to characterize the flowability and mechanical properties. X-ray diffraction (XRD), Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS), Fourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP), thermogravimetric (TG), and zeta potential analyses were used to examine the microstructure of RUFA geopolymers. Increasing the concentration of NaOH also led to an increase in compressive strength. A high NaOH concentration of 12 mol/L resulted in compressive strength of 97.6 MPa at 28 days. Finally, increasing the concentration of NaOH increased the formation of the primary reaction geopolymerization product, N-A-S-H gel, resulting in a denser microstructure with lower porosity.     

Efficiency of fly ash in concrete

Earlier efforts towards an understanding of the efficiency of fly ash in concrete has led to the introduction of rational methods. Based on the results available on some of the more recent pulverised fuel ashes, the authors evaluated the efficiency of fly ash in concrete over a wide range of percentage replacements (15–75%). It was clearly shown that the overall efficiency of fly ash cannot be adequately predicted using a single efficiency factor at all percentages of replacements. The overall efficiency factor (k) has been evaluated at all percentages of replacements considering the general efficiency factor (k_e) and the percentage efficiency factor (k_p). This study resulted in a quantitative assessment of the behaviour of fly ash in concrete, especially for the 28 day compressive strength at different percentages of replacement.     

Compressive strength of HPC containing CNI and fly ash after long-term exposure to a marine environment

This study addressed the effect of calcium nitrite based corrosion inhibitor (CNI) and fly ash (FA) on the long-term compressive strength of high performance concrete (HPC). A 3³ full factorial design was developed to evaluate the influence of CNI at addition rates of 0, 12.5 and 25 L/m³ on the compressive strength of HPC manufactured with 8% silica fume blended cement in combination with 0%, 20% and 40% FA replacements and mixed at 0.29, 0.37 and 0.45 water to cementing materials ratios (w/cm). Standard 100 × 200 mm cylinders were prepared and tested for compressive strength at 28 days and 1 year. The 9-year old concrete specimens were obtained from small-scale reinforced concrete slabs that were exposed to a marine environment. Results indicate that the interaction of CNI and FA does not adversely affect the short and long term compressive strength of concrete. In fact, an enhancement on the compressive strength was observed in concretes containing such combination even after long-term exposure to a marine environment.     

Controlling ettringite formation in FBC fly ash geopolymer concrete

Fluidized bed coal combstion (FBC) is extensively used in small self-generation power plants. The fly ash obtained from this FBC process contains high quantity of calcium and sulfate compounds which hinders its use in the construction industry. In addition, its reactivity is low and additional source material or additive is, therefore, needed to increase the reaction. This research studied the use of Al(OH)₃ and high concentrations of NaOH to control ettringite formation in the FBC fly ash geopolymer. Two replacement levels of 2.5 wt.% and 5.0 wt.% of Al(OH)₃ and three NaOH concentrations of 10, 12 and 15 M were used in the study. Results indicated that the NaOH concentration affected the ettringite formation and strength of the FBC geopolymer. No ettringite was formed at high NaOH concentration of 15 M which helped the dissolution of calcium sulfate and formed the additional calcium hydroxide. The subsequent pozzolanic reaction led to strength gain of the geopolymer. For 15 M NaOH, the addition of 2.5 wt.% Al(OH)₃ promoted the reaction and formed a dense matrix of alumino silicate compound. Relatively high 7-day compressive strength of 30 MPa was obtained.     

The influence of ambient pH on fly ash-based geopolymer

This study reports the comprehensive observation of the influence of ambient pH on the changes of fly ash-based geopolymer in an aqueous solution at various time periods up to 720 days. The aim of the study was to find a relationship between ambient pH, the change of composition, the structural changes and mechanical properties. The results of XRF, NMR, XRD showed that, Na can still leach into solutions of pH ≤ 13. The percentage of Na₂O decreased over time in solutions of pH ≤ 13, and the decreasing rate of the Na₂O percentage increased at low pH. The structural changes still proceeded for specimens in water, the number of Al-O-Si bonds increased over time. The cleavage Si-O-Si stopped, when specimens were immersed in the solution of pH = 1(HCl) due to the fast leaching of Na to solution and neutralization. In a high pH environment (NaOH), the Al-O-Si bond was more consistent than the Si-O-Si bond. The phase change was recorded only in the solution of pH = 14 with the small amount of Na-P1 zeolite. Even though the chemical composition and structure of specimens changed over time, the mechanical properties of the geopolymer were quite stable even when specimens were immersed in solutions of extreme pH (pH = 1, 2 or up to pH = 14).     

Strength properties of fly ash based controlled low strength materials

Controlled low strength material (CLSM) is a flowable mixture that can be used as a backfill material in place of compacted soils. Flowable fill requires no tamping or compaction to achieve its strength and typically has a load carrying capacity much higher than compacted soils, but it can still be excavated easily. The selection of CLSM type should be based on technical and economical considerations for specific applications. In this study, a mixture of high volume fly ash (FA), crushed limestone powder (filler) and a low percentage of pozzolana cement have been tried in different compositions. The amount of pozzolana cement was kept constant for all mixes as, 5% of fly ash weight. The amount of mixing water was chosen in order to provide optimum pumpability by determining the spreading ratio of CLSM mixtures using flow table method. The shear strength of the material is a measure of the materials ability to support imposed stresses on the material. The shear strength properties of CLSM mixtures have been investigated by a series of laboratory tests. The direct shear test procedure was applied for determining the strength parameters Phi (angle of shearing resistance) and C(h) (cohesion intercept) of the material. The test results indicated that CLSM mixtures have superior shear strength properties compared to compacted soils. Shear strength, cohesion intercept and angle of shearing resistance values of CLSM mixtures exceeded conventional soil materials' similar properties at 7 days. These parameters proved that CLSM mixtures are suitable materials for backfill applications.     

Effect of specimen size and shape on compressive strength of concrete containing fly ash: Application of genetic programming for design

The use of fly ash as a mineral admixture in the manufacture of concrete has received considerable attention in recent years. For this reason, several experimental studies are carried out by using fly ash at different proportions replacement of cement in concrete. In the present study, the models are developed in genetic programming for predicting the compressive strength values of cube (100 and 150 mm) and cylinder (100 × 200 and 150 × 300 mm) concrete containing fly ash at different proportions. The experimental data of different mixtures are obtained by searching 36 different literatures to predict these models. In the set of the models, the age of specimen, cement, water, sand, aggregate, superplasticizers, fly ash and CaO are entered as input parameters, while the compressive strength values of concrete containing fly ash are used as output parameter. The training, testing and validation set results of the explicit formulations obtained by the genetic programming models show that artificial intelligent methods have strong potential and can be applied for the prediction of the compressive strength of concrete containing fly ash with different specimen size and shape.     

Potential utilization of class C fly ash-based geopolymer in oil well cementing operations

The early age compressive strength development of class C fly ash-based geopolymers under high pressure and high temperatures of curing is considered as an alternative to well cements. Uniaxial compressive strength (UCS) results show how the curing temperature affects the early compressive strength development. As the temperature rises from 87 to 125 °C, a consecutive reaction seems to take place at the higher concentrations of NaOH, which decrease the compressive strength at the higher temperature. The taken scanning electron microscope (SEM) images show a change in the morphology of the samples at 125 °C with the higher concentrations of NaOH. Ultrasonic cement analyzers (UCA) were employed to investigate the instantaneous strength development of the geopolymeric slurries. As the common cement models were not able to assess the compressive strength development, the custom algorithm option in the UCA software was applied. The developed empirical correlations were not able to accurately estimate the sonic strength of the slurries remarkably at 125 °C. The rheological measurements of the prepared geopolymeric slurries showed a Newtonian like behavior.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.