Characterization of natural pozzolan-based geopolymeric binders

Properties and characteristics of fly ash- or slag-based geopolymers have been extensively explored but comparatively less information is available for natural pozzolan-based geopolymers. The present work focuses on microstructural characteristics of natural pozzolan-based geopolymers activated by sodium hydroxide and a mixture of sodium hydroxide and sodium silicate. Synchrotron XRD and SEM-EDS studies combined with compressive strength tests successfully demonstrate the feasibility of the use of natural pozzolan for sustainable construction material. It is concluded that the geopolymers have sufficient strength as structural materials and matrices contain C–S–H like crystal as well as zeolites of hydroxysodalite and zeolite Y. Two zeolites of hydroxysodalite and zeolite Y are found as the main activation products in sodium hydroxide activation. Substitution with sodium silicate solution yields higher compressive strength and a denser microstructure with dominant activation products of C–S–H like crystal, zeolite Y, and phillipsite. It has been proposed that the crystal size of the activation products ranges from 10 nm to 1 μm. Different microstructural characteristics found herein provide a valuable information to develop natural pozzolan-based sustainable structural materials with improved properties.     

High strength concrete containing natural pozzolan and silica fume

Various combinations of a local natural pozzolan and silica fume were used to produce workable high to very high strength mortars and concretes with a compressive strength in the range of 69–110 MPa. The mixtures were tested for workability, density, compressive strength, splitting tensile strength, and modulus of elasticity. The results of this study suggest that certain natural pozzolan–silica fume combinations can improve the compressive and splitting tensile strengths, workability, and elastic modulus of concretes, more than natural pozzolan and silica fume alone. Furthermore, the use of silica fume at 15% of the weight of cement was able to produce relatively the highest strength increase in the presence of about 15% pozzolan than without pozzolan. This study recommends the use of natural pozzolan in combination with silica fume in the production of high strength concrete, and for providing technical and economical advantages in specific local uses in the concrete industry.     

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).     

High strength geopolymer binder based on waste-glass powder

This paper presents a study on the synthesis of geopolymers based on alkaline activation of waste-glass powder using aqueous solutions of sodium hydroxide and sodium silicate with different Na₂O contents as alkali activators. Three types of calcium aluminate cements were also incorporated into the dry binder at levels up to 24% by weight in order to modify the chemical composition of the geopolymer source materials. The prepared mortars were tested for workability, setting time, compressive strength, free-alkali content and tendency towards efflorescence formation. FTIR and SEM analyses were also performed to characterize the morphology and structure of the produced geopolymer. The optimized geopolymer mortar exhibited a remarkable maximum compressive strength of 87 MPa. The results showed that inclusion of calcium aluminate cements in the silica-rich waste-glass powder leads to release high amounts of reactive alumina into aluminosilicate gels, improving the geopolymerization reactions and resulting in the formation of a more cross-linked network that exhibits higher compressive strength. High alumina cement Secar 71 showed the greatest effect in strength enhancement due to the higher amount of reactive alumina releasing into the reaction medium. The findings demonstrate a new potential of value-added reuse application for waste-glass powder by adding a suitable amount of materials that are rich in reactive alumina.     

Influence of curing conditions on properties of high calcium fly ash geopolymer containing Portland cement as additive

This paper investigated the mechanical properties and microstructure of high calcium fly ash geopolymer containing ordinary Portland cement (OPC) as additive with different curing conditions. Fly ash (FA) was replaced with OPC at dosages of 0%, 5%, 10%, and 15% by weight of binders. Setting time and microstructure of geopolymer pastes, and flow, compressive strength, porosity and water absorption of geopolymer mortars were studied. Three curing methods viz., vapour-proof membrane curing, wet curing and temperature curing were used. The results showed that the use of OPC as additive improved the properties of high calcium fly ash geopolymer. The strength increased due to the formation of additional C–S–H and C–A–S–H gel. Curing methods also significantly affected the properties of geopolymers with OPC. Vapour-proof membrane curing and water curing resulted in additional OPC hydration and led to higher compressive strength. The temperature curing resulted in a high early compressive strength development.     

Properties of metakaolin based geopolymer incorporating calcium carbonate

An alkaline solution, thermally activated kaolinite clay and a mineral additive (calcium carbonate) were mixed with the aim to elaborate a geopolymer material with physical and mechanical properties comparable to those of classical construction materials.The starting reagents were characterized by quantitative chemical analyses (XRF), mineralogical analyses (XRD), thermal gravimetric analyses (TGA), and grain size distribution measurements. The setting of the mixture (polymerization) was implemented by measuring the evolution of the viscosity as a function of time at different temperatures.The geopolymers were synthesized at a temperature of 40 °C. The investigation of the mechanical behavior reveals that these materials display acceptable characteristics: the flexural and compression strength are around 4.6 and 26 MPa respectively, for an added calcium carbonate over dry matter ration up to 12% by weight.The promising results exposed in this paper show that the geopolymer formulations can be adapted for applications in construction and civil engineering structures as an alternative to conventional materials.     

High-volume natural volcanic pozzolan and limestone powder as partial replacements for portland cement in self-compacting and sustainable concrete

A laboratory study demonstrates that high volume, 45% by mass replacement of portland cement (OPC) with 30% finely-ground basaltic ash from Saudi Arabia (NP) and 15% limestone powder (LS) produces concrete with good workability, high 28-day compressive strength (39 MPa), excellent one year strength (57 MPa), and very high resistance to chloride penetration. Conventional OPC is produced by intergrinding 95% portland clinker and 5% gypsum, and its clinker factor (CF) thus equals 0.95. With 30% NP and 15% LS portland clinker replacement, the CF of the blended ternary PC equals 0.52 so that 48% CO₂ emissions could be avoided, while enhancing strength development and durability in the resulting self-compacting concrete (SCC). Petrographic and scanning electron microscopy (SEM) investigations of the crushed NP and finely-ground NP in the concretes provide new insights into the heterogeneous fine-scale cementitious hydration products associated with basaltic ash-portland cement reactions.     

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.     

Blended cements containing high volume of natural zeolites: Properties, hydration and paste microstructure

In this study, properties and hydration characteristics as well as paste microstructure of blended cements containing 55% by weight zeolitic tuff composed mainly of clinoptilolite mineral were investigated. Free Ca(OH)₂ content, crystalline hydration products and decomposition of zeolite crystal structure, pore size distribution and microstructural architecture of hydrated cement pastes were examined. Superplasticizer requirement and compressive strength development of blended cement mortars were also determined. The blended cements containing high volume of natural zeolites were characterized with the following properties; (i) no free Ca(OH)₂ in hardened pastes at the end of 28 days of hydration, (ii) less proportion of the pores larger than 50 nm when compared to portland cement paste, (iii) complete decomposition of crystal structure of zeolite at the end of 28 days of hydration, (iv) presence of tetra calcium aluminate hydrate as a crystalline product of pozzolanic reaction, (v) more compatibility with the melamine-based superplasticizer when compared to the naphthalene based product, and (vi) similar 28 days compressive strength of mortars to that of reference portland cement.     

Hydration kinetics in hybrid binders: Early reaction stages

Activated blends of Portland cement and fly ash with a high ash content (>70%) are a new alternative to traditional OPCs. A number of papers have been published on C–S–H and N–A–S–H, the two gels that constitute the main cementitious products generated by the alkaline activation of these cements, and the elements that may be taken up into their structure. Very little is known about the kinetics of these systems, however, particularly during the early stages of the reaction. The present study used isothermal conduction calorimetry to explore hydration kinetics during the first 72 h in a cement containing 30% OPC and 70% fly ash. Two activating solutions were used: a mix of NaOH + Na₂SiO₃ and a Na₂CO₃ solution. The findings showed that hydration kinetics were substantially modified by the type of alkaline activator used, particularly with respect to the secondary phases generated. In both cases the main reaction product appeared to be a mix of C–A–S–H and (N,C)–A–S–H gels, whose proportions were clearly impacted by the type of activator used.     

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.     

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.     

Effect of porosity on the absorbed, reemitted and transmitted light by a geopolymer metakaolin base

In the present paper, the optical characterization of a geopolymer synthesized at three different temperatures (40, 60 and 90 °C) is described. The results were correlated to the porosity fraction in order to obtain a photoluminescent geopolymer.A two-flux model was employed to relate the fraction of light absorbed, remitted and transmitted by a representative layer of geopolymer. Porosity was measured by nitrogen adsorption and correlated with the optical properties to determine the fraction of UV/Vis light transmitted through the samples. It was found that the geopolymer synthesize temperature and the incident wavelength greatly affect the fraction of light transmitted by the samples.The UV/Vis spectrum was divided into three zones according to the observed behavior. In the first zone, the difference in transmission fraction was significant; in the second zone, this difference decreased and practically vanished at the third one.Considering the highest transmission fraction at 90 °C, a photoluminescent geopolymer with the strongest emitting peak at 469 nm was synthesized.     

Effect of fire exposure on cracking,spalling and residual strength of fly ash geopolymer concrete

Fly ash based geopolymer is an emerging alternative binder to cement for making concrete. The cracking, spalling and residual strength behaviours of geopolymer concrete were studied in order to understand its fire endurance, which is essential for its use as a building material. Fly ash based geopolymer and ordinary portland cement (OPC) concrete cylinder specimens were exposed to fires at different temperatures up to 1000 °C, with a heating rate of that given in the International Standards Organization (ISO) 834 standard. Compressive strength of the concretes varied in the range of 39–58 MPa. After the fire exposures, the geopolymer concrete specimens were found to suffer less damage in terms of cracking than the OPC concrete specimens. The OPC concrete cylinders suffered severe spalling for 800 and 1000 °C exposures, while there was no spalling in the geopolymer concrete specimens. The geopolymer concrete specimens generally retained higher strength than the OPC concrete specimens. The Scanning Electron Microscope (SEM) images of geopolymer concrete showed continued densification of the microstructure with the increase of fire temperature. The strength loss in the geopolymer concrete specimens was mainly because of the difference between the thermal expansions of geopolymer matrix and the aggregates.     

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.     

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.     

Effects of curing time and temperature on strength development of inorganic polymeric binder based on natural pozzolan

This paper reports the results of a study on the influence of curing conditions on compressive strength development in inorganic polymeric binder prepared from natural pozzolan. Three mixes with different chemical formulations were prepared and cured hydrothermally at different temperatures and times. In particular, the effect of a precuring at an atmosphere of more than 95% relative humidity at room temperature on compressive strength development before the application of heat was studied. Different curing regimes including hydrothermal treatment in steam-saturated atmosphere at different temperatures of 45, 65, 85 °C and for different time periods of 5, 10, 15, and 20 h after 1 and 7 days of precuring were applied. The mix exhibiting the maximum compressive strength after hydrothermal treatment was selected and cured in autoclave at temperatures of 125, 150, 180, and 210 °C for different time periods of 20, 30, 40, and 50 h for investigating the effects of higher times and temperatures of curing on strength development and also to determine the maximum achievable compressive strength. Results show that relatively long precuring in humid atmosphere is very beneficial for compressive strength development. The highest compressive strength achieved for three different regimes of curing including 28 days at an atmosphere of more than 95% relative humidity at 25 °C, 20 h hydrothermal treatment at 85 °C after 1 day precuring, and 20 h hydrothermal treatment at 85 °C after 7 days precuring were 37.5, 37.5, and 57.5 MPa, respectively. The maximum achievable compressive strength under autoclave curing at 210 °C for 30 h after 7 days of precuring was 108.7 MPa.     

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.     

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.     

Effects of light crude oil contamination on the physical and mechanical properties of geopolymer cement mortar

Fly ash and oil contaminated sand are considered as the two waste materials that may affect environment. This paper investigated the suitability of producing geopolymer cement mortar using oil contaminated sand. A comparison between physical and mechanical properties of mortar produced using geopolymer and Ordinary Portland Cement (OPC), in terms of porosity, hydration and compressive strength, was conducted. The results showed that heat curing can increase the compressive strength of geopolymer mortar up to 54% compared to ambient curing situation. The geopolymer mortar with 1% of light crude oil contamination yielded a 20% higher compressive strength than OPC mortar containing sand with a saturated surface dry condition. Furthermore, the formation of efflorescence decreased as the level of oil contamination decreased. Moreover, the heat curing method increased the kinetic energy and degree of reaction for geopolymer cement mortar, which cause an increment of the density of the pore system and improving the mechanical properties of the resulting composites. From the results of this study, it was demonstrated that geopolymer mortar has the potential of utilizing oil contaminated sand, and reducing its environmental impacts.