Fly Ash Geopolymer Mortars Research Articles

Very high strength Na2SiO3 and NaOH activated fly ash based geopolymer mortar

The paper presents the results of research of high-strength geo-polymer mortars, prepared from two types of siliceous fly ash. Sodium hydroxide and mixtures of sodium hydroxide and sodium silicate in the form of solutions were used as activators. The activator was added in amounts ranging from 6 to 15% NaOH in relation to the ash mass. After preparation, the mortars were cured at the temperature of 60ºC, 80ºC or 100ºC for various times, from 24 hours to 7 days. Increasing the addition of sodium or a higher curing temperature increase the flexural and compressive strength of the tested mortars. The best results - compressive and flexural strength of 100 MPa and 20 MPa, respectively, were obtained for mortar activated with 15% sodium hydroxide, cured at 100°C for 24 hours. Blended activator – sodium silicate and sodium hydroxide also allowed to obtain high strengths, but lower than sodium hydroxide alone. Extending the curing time at high temperature beyond 24 hours did not produce favourable results in most cases. Fly ash geopolymer mortar, activated with 15% NaOH by mass of ash, cured at 100°C for 24 hours, showed about 18% reduction in greenhouse gas emissions compared to common Portland cement mortar.

Comparison of the alkali-silica reactions of ferronickel slag aggregate in fly ash geopolymer and cement mortars

The use of reactive aggregates in ordinary Portland cement (OPC) concrete is limited due to the potential expansion of alkali-silica reaction (ASR). This study evaluates the ASR expansion of ferronickel slag (FNS) fine aggregate in fly ash geopolymer mortar by the accelerated mortar bar test (AMBT) method. While FNS was found to be reactive in OPC mortar, it was found non-reactive with fly ash geopolymer. The OPC mortar showed extensive expansion and strength reduction after exposure to AMBT condition, whereas the geopolymer specimens showed low expansion and strength increment. Scanning electron microscopy (SEM) images of the OPC mortar specimens after AMBT showed the presence of some ASR gel around aggregate surface. However, a relatively firm bonding was observed between aggregate and binder in the geopolymer mortar specimens. The EDS results showed that the aluminosilicate consumed free alkali in geopolymer binder matrix that reduced the ASR of FNS aggregate in geopolymer mortar.

PH dependent leaching characterization of major and trace elements from fly ash and metakaolin geopolymers

The leaching characteristics of Cl−, SO42−, Na, K, Al, Si, Ca, Mg, Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Se, Sr, Tl, V, Zn from fly ash and metakaolin geopolymer mortars were characterized in a broad pH range (1–14) in this work. Apart from revealing the partitioning of chemical species between the solid and aqueous phase, the acid neutralization property of both geopolymers were studied. In contrast with the virtually pH independent characteristic of chloride and sulfate, the leaching of the other elements showed pH dependent features, which can be further divided into amphoteric, cationic and oxyanionic patterns. Formation of montmorillonite and amorphous silica were detected by X-ray diffraction after the leaching. The leaching results and mineralogical measurement were coupled with geochemical modelling to identify the solubility controlling phases of Al, Si, Ca and Mg.

Enhanced Electrical Properties of Fly Ash Geopolymer Composites with Carbon Nanotubes

Carbon nanotubes (CNTs) are used for the application in concrete especially due to their excellent physical properties. In this study, CNTs were used as a conductive admixture to prepare composites with enhanced electrical properties that might be potentially used in smart concretes or structures. We assessed the changes in selected electrical properties of fly ash geopolymer mortars (conductivity, resistance, capacitance) depending on the concentration of CNTs that ranged from 0.05 to 0.20%. The most convenient CNTs concentration was discussed considering both the electrical and mechanical properties (compressive and flexural strength). Mercury intrusion porosimetry and scanning electron microscopy were used to observe the distribution of CNTs and porosity of the mortars.

Application of carbon black in conductive fly ash geopolymer mortars

Materials with enhanced electrical properties are widely studied concerning their future application in smart or self-sensing constructions. Carbon black is known as one of the inexpensive multipurpose admixtures used in polymeric materials. This study is focused on the application of carbon black in fly ash geopolymer mortar to enhance its conductivity and other electrical properties. Geopolymer samples were prepared with the carbon black content from 0.5 to 4.0% to evaluate their performance in selected electrical properties (conductivity, resistance, capacitance). Further investigations included its influence on the mechanical properties (compressive and flexural strength) and microstructure of the binder (mercury intrusion porosimetry, SEM). Despite the considerable improvement in conductivity, the amount of conductive filler over 2% was also associated with increased porosity and reduced compressive strengths.

Behaviour of fly ash geopolymer binders under exposure to alkaline media

Alkaline agents are primary binder components of geopolymer systems. In the converse, however, the hardened geopolymer binders can be adversely affected under exposure to strong alkalis. This paper presents an investigation conducted to evaluate the effects of highly alkaline exposure environment on fly ash geopolymer binders. In the study, a combination of sodium silicate and 12 M NaOH solution was used as the alkali activator to prepare a low calcium (Class F) fly ash-based geopolymer binder. Fly ash geopolymer pastes or mortars were cast into 50 mm cubes and 25 × 25 × 285 mm prisms, then immersed in different media consisting of water or 1–3 M NaOH solutions maintained at varied temperatures of 23, 38, and 80 °C. The tests done include compressive strength, expansion, and weight changes. Analytical studies were done using X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectroscopy, scanning electron microscopy. Leaching analysis of storage solutions was also conducted. Results showed that alkali attack occurred when the concentrations of storage solutions exceeded 1 M NaOH. The severity of attack increased with increase in NaOH concentration and in storage temperature. It was found that the alkali attack causes damage to the polymeric structure of the binder, leading to release of Al and Si. A gel-like substance is formed, causing expansion and microcracking. New crystalline zeolite phases comprising phillipsite, zeolite-P and chabazite, also formed under exposure of the fly ash geopolymer binder to elevated alkali concentrations.

Insulating and fire-resistant behaviour of metakaolin and fly ash geopolymer mortars

To achieve the highest thermal insulation and the lowest carbon dioxide emissions, different geopolymer mortar types with low thermal conductivity, reasonable compressive strength and high fire resistance were investigated and compared with traditional cement mortar (TCM). Two different types of aluminosilicate materials, namely metakaolin (MK) and fly ash (FA), were used as source materials. Either natural river siliceous sand or expanded perlite (EP) was used as fine aggregate. Each source material and selected fine aggregate was mixed and activated with sodium silicate. After curing, density and thermal conductivity values were measured. The compressive strengths of the mortars before and after being subjected to 600°C for 2 h was determined. The results show that it is possible to use MK-based and FA-based geopolymer mortars without adding any foaming agent as insulation and fire-resistant materials. The different geopolymer mortar types exhibited ∼33·57–73·57% lower thermal conductivity and 83·87–115% higher compressive strength than the TCM. The MK-based geopolymer mortars proved to have higher fire resistance than FA-based ones, while the FA-based geopolymer mortars proved to have higher thermal insulation than the MK-based mortars. The use of EP as fine aggregate resulted in much higher thermal insulation and better fire resistance than siliceous sand.

Method for comprehensive mix design of fly ash geopolymer mortars

This paper presents the development of a new mix design method for fly ash geopolymer mortars activated using combined sodium silicate and sodium hydroxide solutions. For a specified value of compressive strength, specified value of workability, and known chemical properties of the sodium silicate solution to be used in a mixture, the proposed method generates mixture design parameters consisting of activator to fly ash ratio, sodium silicate to sodium hydroxide ratio, and molarity of sodium hydroxide. It was found that the actual compressive strengths and actual workability values obtained from tests were satisfactorily comparable to the designed values. The proposed method concept can be applied to generate relevant mix design charts for various types of geopolymer binders.

The effect of water binder ratio on strength development of class C fly ash geopolymer mortar prepared by dry geopolymer powder

The use of geopolymer binder as cement replacement material can reduce the amount of carbon dioxide gas produced during the Portland Cement manufacturing process. However, the main issue of geopolymer binder is in the mixing process of sodium silicate and NaOH which requires specialized knowledge and strict supervision. This paper reports the effect of water binder ratio on strength development of fly ash geopolymer mortar using dry geopolymer powder. Fly ash with high calcium content was used as primary material. The dry geopolymer powder was prepared by wet mixing method which was made by drying a mixture of NaOH solution and limestone for 24 hours. The variations of water to binder ratio were 0.30, 0.35, 0.40, 0.45, and 0.50. Strength properties were measured by compressive strength at the age of 7, 14 and 28 days. The results showed that the water binder ratio significantly affect the strength development of geopolymer mortar prepared by dry geopolymer powder. The water binder ratio of 0.40 gives the highest compressive strength of 10.3 MPa at 28 days. This suggests that the use of dry geopolymer powder on geopolymer mortar production can overcome the difficulties of geopolymer mortar mixing on site.

Enhancing the Strength of Volcanic Mud-based Class C Fly Ash Geopolymer Specimen by Limestone Inclusion

Volcanic mudflow disaster has brought negative impacts in the area around the mudflow. The contents of mud are predominantly silicate and aluminate, which are the primary constituent materials of fly ash-based geopolymer specimen. However, the main issue of fly ash-based geopolymer is the requirement of heat curing due to the low calcium in fly ash precursor. The aim of this research is to identify the effect of limestone inclusion on strength properties of volcanic mud-based class C fly ash geopolymer mortar. The volcanic mud was obtained from Sidoarjo mud in East Java Indonesia. The alkaline activator solution was prepared by mixing 10 Molar sodium hydroxide with sodium silicate with the ratio of 0.67. The compressive strength test was carried out at the age of 28 days. The specimens were cured at room temperature. The results show that the addition of 30% limestone in volcanic mud-based class C fly ash geopolymer specimen gives the highest strength at 28 days. It also improves the setting time rate due to the calcium addition. However, further addition of limestone tends to reduce the strength. This might attribute to the decrease of silicate and aluminate contents in geopolymer specimen along with the limestone addition.

Development of Geopolymer Mortar from Metakaolin Blended with Agricultural and Industrial Wastes

Geopolymer is a greener alternative cement produced from the reaction of pozzolans and strong alkali solutions. Generally, the cement industry is one of largest producers of CO2that caused global warming. For geopolymer mortar usage, Portland cement is not utilized at all. In this research, geopolymer mortars were prepared by mixing metakaolin, various wastes (fly ash, bagasse ash and rice husk ash) varied as 80:20, 50:50 and 20:80, 15M NaOH, Na2SiO3and sand. The influence of various parameters such as metakaolin to ashes ratios and pozzolans to alkali ratios on engineering properties of metakaolin blended wastes geopolymer mortar were studied. Compressive strength tests were carried out on 25 x 25 x 25 mm3cube geopolymer mortar specimens at 7, 14, 21, 28 and 91 air curing days. Physical and chemical properties were also investigated at the same times. The test results revealed that the highest compressive strength was 20% metakaolin - 80% fly ash geopolymer mortar. When the curing times increases, the compressive strength of geopolymer mortar also increases. The mixing of metakaolin and bagasse ash/rice husk ash presented lower compressive strength but higher water absorption and porosity. For FTIR results, Si-O, Al-O and Si-O-Na+were found. Moreover, the geopolymer mortar could easily plastered on the wall.

The Effect of Sodium Hydroxide Molarity on Strength Development of Non-Cement Class C Fly Ash Geopolymer Mortar

The use of fly ash as cement replacement material can overcome the environmental issues, especially the global warming problem caused by the greenhouse effect. This is attributed to the CO2 gas produced during the cement manufacturing process, which 1 ton of cement is equivalent to 1 ton CO2. However, the major problem of fly ash is the requirement of activators to activate the polymer reactions. The most common activator used in non-cement or geopolymer material is the combination of sodium hydroxide (NaOH) and sodium silicate. This study aims to identify the effect of NaOH molarity as activator on strength development of non-cement class C fly ash geopolymer mortar. The molarity variations of NaOH were 6 Molar (M), 8M, 10M, 12M, 14M and 15M. The compressive strength test was performed at the age of 3, 7 and 28 days in accordance with ASTM standard, and the specimens were cured at room temperature. The results show that the highest compressive strength was achieved by geopolymer mortar with a molarity of 12M. It exhibits a higher strength to that normal mortar at 28 days. However, the use of NaOH molarity more than 12M tends to decrease the strength of non-cement geopolymer mortar specimens.

Mechanical properties of fly ash geopolymer mortar reinforced with carbon fibers

Portland cement industry releases high amounts of CO2, so efforts have been made for alternative binders. Geopolymers are one of the highlighted elective binders. The aims of this study are to produce fly ash Geopolymer boards reinforced with carbon fibers and investigate the mechanical properties of several blends of fly ash Geopolymer mortar reinforced with carbon fibers (FGC). Carbon fibers (CF) were added at different percentages (WF) of 0.2%, 0.5%, 1%, and 1.5% by weight. The sodium hydroxide condensation of 8 molars, sodium silicate to sodium hydroxide weight ratio of 2.0, the alkaline liquid/binder proportion of 0.47 and the dry heat curing of 70°C for 24 hours were utilized as a piece of blends. Fresh and hardening state properties such as Flow capacity, flexural strength and compressive strength, and physical properties such as bulk density were determined from the 3-day FGC samples after the curing process. The results obtained from this study, show that the uses of CF were significant in improving the compressive and flexural strength due to the bonding characteristics of fibers with Geopolymer pastes. Then again, the used of CF results in slightly decrease of the bulk density. The optimal compressive and flexural strength outcomes were at weight percentage 1% of CF.

Characterization of the high-calcium fly ash geopolymer mortar with hot-weather curing systems for sustainable application

Fly ash geopolymers are an alumino-silicate material and thus enable the utilization of waste containing alumino-silicate effectively. Geopolymeric reaction occurs as a result of the activation of fly ash with alkali solutions. In Thailand, a large amount of high-calcium fly ash is available due to the use of low-grade lignite coal feedstock for pulverized coal combustion process and the calcium content becomes very high. In this study, heat curing at 35°C as a representative of a high ambient temperature (hot weather) and low cost was investigated. Curing at temperature of 65°C and room temperature of 25°C were also conducted to compare the results. Geopolymeric products were tested for compressive strength and characterized by XRD, IR, SEM and TGA techniques. The results showed that heat curing enhanced the geopolymerization resulting in the formation of SiOAl network product. Heat curings at 35°C and 65°C led to the formation of calcium silicate hydrate (C-S-H) and alumino-silicate (geopolymer bonding). Without heat curing, the product was predominantly C-S-H compound and the matrix was as strong as the heat-cured product. The immersion of samples in 3% sulfuric acid solution revealed that the performance of the heat-cured samples were better than those cured at room temperature. In addition, application of research results was to produce the geopolymer brick with outdoor heat exposure of 35°C. Pedestrian pathway was demonstrated.

Influence of duration of heat curing and extra rest period after heat curing on the strength and transport characteristic of alkali activated class F fly ash geopolymer mortar

In the study, the influence of heat curing duration and rest period after heat curing duration on the strength and transport characteristics of alkali activated fly ash (AAFA) mortar were investigated. A local class F type fly ash, CEN reference sand, sodium hydroxide and potable water were used in preparation of cement-less fresh geopolymer mortar mixture. Mixture ratios (in mass basis) were 3, 1, 0.29, and 0.1 for sand, fly ash, water and sodium hydroxide, respectively. Some samples were cured at 75°C temperature for 4h, 1, 2, 3 and 7days; then, they were tested after heat curing period. Additionally, some of the equivalent samples were kept in the laboratory at 23±2°C temperature with 50±5% relative humidity, until 28days after their initial heat curing period. Then, they were tested at 28days for combined curing conditions. The measured properties of AAFA mortar were unit weight, porosity, capillary water absorption, water absorption capacity, flexural-bending and split tension strength, compressive strengths and abrasion resistance. It was observed that the strengths of mortar were significantly increased with the increase in heat curing duration. A significant strength development was also observed after rest period. As a result of the study, high strength and abrasion resistant AAFA geopolymer mortar was produced. However, transport properties of AAFA geopolymer mortar found to be not as good as mechanical properties.

Optimization of lightweight GGBFS and FA geopolymer mortars by response surface method

In the first stage of the paper, the effect of binder content, curing temperature and curing time on the compressive strength of light-weight geopolymer mortar (LWGM) was investigated. The base materials used for LWGM are ground granulated blast furnace slag (GGBFS) and fly ash (FA). The main components of LWGM are lightweight pumice aggregate and alkali activated FA or GGBFS binder. Effectiveness of aforementioned parameters was examined in terms of variation of the compressive strengths of LWGMs. The experiments were performed on LWGM cubes under curing temperatures of 60, 80, 100 and 120°C with curing period of 2, 6, 8, 24, 48 and 72h. The alkaline activator is a mix of 12M NaOH solution with sodium silicate in ratio of 1:2.5. The ratio of alkaline solution to binder was taken as 0.50. 7 binder contents were taken between 650 to 1250kg/m3 with 100kg/m3 increment. Full factorial experimental program was adopted to observe compressive strength development of LWGMs. Therefore, 336 data samples were obtained. The second stage of the paper is to optimize those experimental parameters through response surface method. Test results indicate that the increment in the binder content increases the compressive strength of LWGM. The strength increases with the increase of curing temperature and curing period of LWGM. The experimental verification indicated a good agreement with optimized results.

Activation Energy of Alkali-Silica Reactionin CFBC Fly Ash Geopolymer Mortars

The utilization of fly ashes produced by circulated fluidized bed combustion (CFBC) has been limited in construction application due to their inherent high sulfate and carbon contents although CFBC fly ash exhibits very good binding properties without requiring any supplementary activator. This study reports alkali silica reaction (ASR) behavior of CFBC fly ash geopolymer mortars in terms of activation energy using a modified ASTM C 1260/C 1567. Two different strengths of NaOH solution were used to test reactive and potentially reactive aggregates in the presence of CFBC fly ash. The other variables included a longer test period of 3 months and three different temperatures, namely 60°C, 70°C, and 80°C. It was observed that there was no significant expansion in CFBC fly ash based geopolymer mortar regardless of variation of temperature and alkalinity of test solution. Activation energy of CFBC fly ash geopolymer for ASR was higher than that of plain cement mortar irrespective of strength of NaOH solution.

Sorptivity and acid resistance of ambient-cured geopolymer mortars containing nano-silica

This study investigated the effects of nano-silica on flowability, strength development, sorptivity and acid resistance properties of fly ash geopolymer mortars cured at 20 °C. The changes in mass, compressive strength and microstructure of the specimens after immersion in acid solutions for different durations were determined. The microstructures were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis. It was found that addition of nano-silica in geopolymer mortars based on fly ash alone or fly ash blended with 15% GGBFS or 10% OPC improved the compactness of microstructure by reducing porosity. Thus, the nano-silica reduced sorptivity and increased compressive strength of the mixes. The average mass loss after 90 days of immersion in acid solutions reduced from 6.0% to 1.9% by addition of 2% nano-silica. Similarly, significant reduction in strength loss after immersion in acid solution was observed in the specimens by using nano-silica.

Evaluation of fly ash geopolymer mortar incorporating calcined wastepaper sludge

This study evaluates fresh and hardened properties of fly ash geopolymer mortars incorporating calcined wastepaper sludge cured under ambient conditions. The calcined sludge geopolymer mixtures were characterized by XRD, XRF, and SEM–EDS techniques. It was observed that addition of up to 40 wt% calcined sludge in geopolymer mortars at a fixed liquid/solid mass ratio of 0.2 reduced flow of mortars from 112 to 61%, while the corresponding setting time was shortened from 113 h to just under 1 h. At the maximum calcined sludge loading of 40 wt%, the compressive strength at 7 days was 29.0 MPa, a significant increase from 3.8 MPa for reference mortar samples with no addition of sludge. Correspondingly, a nearly fourfold reduction in 56-day drying shrinkage measured at 3600 microstrains was observed, compared to reference mortars. The plausible reaction mechanisms, in particular, apparent concurrent formation of C-A-S-H/N-(C)-A-S-H phases alongside geopolymer gel networks are discussed in detail.

Performance of Blended Fly Ash (FA) and Palm Oil Fuel Ash (POFA) Geopolymer Mortar in Acidic Peat Environment

This paper presents performance of blended geopolymer mortars prepared from fly ash (FA) and palm oil fuel ash (POFA). Both materials are used their Si and Al elements were activated by alkaline solution. The alkaline solution was prepared by mixing sodium silicate and sodium hydroxide. The optimum mix proportion of geopolymer mortar with FA:POFA mass ratio was 90:10. The ratio of sodium silicate solution to sodium hydroxide solution by mass was 2.5:1. The mass ratio of sand to blended ashes was 2.75:1. The mortar specimens were prepared using 5×5×5 cm cube and cured at room temperature (28oC) for 3 days before subsequently heat-cured at 110oC for 24 hours. The specimens were immersed in distilled water and peat water with pH 4-5 for 120 days. The compressive strength change, porosity, and sorptivity tests were taken. In general, the results shows there was a decrease in strength, an increase in porosity and sorptivity of the blended geopolymer mortars. Fourier Transform Infra Red (FTIR) test revealed that interaction of geopolymers mortar with the acidic peat water can also cause replacement of the exchangeable cations (Na, K) in polymers by hidrogen or hydronium ions. Formation of some new zeolitic phases in blended FA-POFA geopolymer mortar exposed to acidic peat water were observed.