Low Calcium Fly Ash Geopolymer Research Articles

Stabilization of Soft Clay by a Low-Calcium Fly Ash Geopolymer

Stabilization of Soft Clay by a Low-Calcium Fly Ash Geopolymer

Calcium wastes as an additive for a low calcium fly ash geopolymer

A geopolymer is a low-carbon cement based on the utilization of waste ash in alkali-activated conditions. Coal fly ash is widely used as a source material for geopolymer synthesis since it contains a sufficient amount of reactive alumina and silica for geopolymerization. Geopolymer products are known to have beneficial fire resistance and mechanical properties. Class F or low-calcium fly ash (LCFA) is generally used as a primary aluminosilicate source; however, heat curing is required to complete the reaction and hardening process and achieve a strong composite. Furthermore, calcium additives are often required to improve the strength of LCFA geopolymers. This paper presents the potential of reusing calcium waste for this purpose. Three calcium wastes, namely calcium carbide residue (CCR), limestone waste, and waste cement (WC) slurry in powder form were used as additives and compared with the use of ordinary Portland cement (OPC). LCFA was replaced with the calcium additives at 20%. However, 20% CCR resulted in flash setting, hence 5% CCR was used instead. A durability test using 3% HCl solution was also performed. The results showed that the reactivity of calcium additives played an important role in strength development. In the calcium–aluminosilicate–alkali system, calcium silicate hydrate (CSH) and calcium aluminosilicate hydrate (CASH) were formed. The maximum strength of 21.9 MPa was obtained from the OPC/LCFA geopolymer, and 3% HCl solution had a deleterious effect on the strength. OPC and CCR were favorable reactive sources of calcium compounds to blend with LCFA. From the thermogravimetric results, lower thermal weight changes with higher strength gains were achieved. Low CaCO3 decomposition at 750 °C according to the TGA curves indicated the more formation of thermally stable CSH and high compressive strength of Ca/LCFA geopolymers.

Influence of Relative Humidity on Compressive Strength of Low-Calcium Fly Ash Geopolymer during Heat Curing

Influence of Relative Humidity on Compressive Strength of Low-Calcium Fly Ash Geopolymer during Heat Curing

Statistical analysis of sustainable geopolymer concrete

Geopolymer concrete is an environment-friendly material and is presently accepted as an alternative to conventional concrete. It utilizes industrial by-products like fly ash and slag to reduce CO2 emissions associated with cement production. Due to the variability of fly ash composition and concrete casting conditions, significant variability is observed in the performance of the concrete produced with such materials. In this study, a non-linear regression technique is used to develop predictive models for the compressive strength of high and low calcium fly ash geopolymer concrete. The criteria such as root mean squared error (RMSE), the statistical significance of model coefficients, and the R-squared values were adopted. The developed models were validated using plots comparing the experimental and predicted response and plots showing residuals. The dependency of the compressive strength on the significant parameters were highlighted using contour and adjusted response plots. From the contour plots, it was observed that, for geopolymer concrete sourced from the high calcium fly ash, compressive strength increases with the molarity of sodium hydroxide, the activator-to-binder ratio, and the curing temperature; while it decreases with the increase in coarse aggregate content. Similar contour plots, for the case of geopolymer concrete sourced from low calcium fly ash, exhibited improvement in compressive strength increases with the molarity of sodium hydroxide, the activator-to-binder ratio, and the ratio of Na2SiO3 –NaOH solutions; at the same time, it decreases with the increase in water-to-geopolymer solids ratio. For low calcium fly ash geopolymer concrete, the proposed model predicts that compressive strength of 90 MPa could be obtained for fly ash with oxide composition comprising a silica-to-alumina ratio of 4. The regression models developed in the present study will be helpful to develop a design mix for geopolymer concrete comprising either high calcium fly ash or low calcium fly ash.

Investigation on the Compressive Strength and Time of Setting of Low-Calcium Fly Ash Geopolymer Paste Using Response Surface Methodology.

Approximately 2.78 Mt of coal fly ash is produced in the Philippines, with a low utilization rate. Using fly ash-based geopolymer for construction will lessen the load sent to landfills and will result in lower GHG emissions compared to OPC. It is necessary to characterize the fly ash and optimize the geopolymer components to determine if it can replace OPC for in situ applications. The activator-to-precursor ratio, the water-to-solids ratio, and the sodium hydroxide-to-sodium silicate ratio were optimized using a randomized I-optimal design from the experimental results of 21 runs with five replicates, for a total of 105 specimens of 50 mm × 50 mm × 50 mm paste cubes. The engineering properties chosen as the optimization responses were the unconfined compressive strength (UCS), the initial setting time, and the final setting time. The samples were also ambient-cured with the outdoor temperature ranging from 30 °C to 35 °C and relative humidity of 50% ± 10% to simulate the on-site environment. Runs with high unconfined compressive strength (UCS) and short setting times were observed to have a low water-to-solids (W/S) ratio. All runs with a UCS greater than 20 MPa had a W/S ratio of 0.2, and the runs with the lowest UCS had a W/S of 0.4. The initial setting time for design mixes with a W/S ratio of 0.2 ranged from 8 to 105 min. Meanwhile, five out of seven design mixes with a W/S ratio of 0.4 took longer than 1440 min to set. Specimens with an alkali activator ratio (NaOH/WG) of 0.5 (1:2) and 0.4 (1:2.5) also had significantly lower setting times than those with an alkali activator ratio of 1. The RSM model was verified through confirmatory tests. The results of the confirmatory tests are agreeable, with deviations from the expected UCS ranging from 0 to 38.12%. The generated model is a reliable reference to estimate the UCS and setting time of low-calcium FA geopolymer paste for in situ applications.

Practical Prediction Models of Tensile Strength and Reinforcement-Concrete Bond Strength of Low-Calcium Fly Ash Geopolymer Concrete.

There have been a few attempts to develop prediction models of splitting tensile strength and reinforcement-concrete bond strength of FAGC (low-calcium fly ash geopolymer concrete), however, no model can be used as a design equation. Therefore, this paper aimed to provide practical prediction models. Using 115 test results for splitting tensile strength and 147 test results for bond strength from experiments and previous literature, considering the effect of size and shape on strength and structural factors on bond strength, this paper developed and verified updated prediction models and the 90% prediction intervals by regression analysis. The models can be used as design equations and applied for estimating the cracking behaviors and calculating the design anchorage length of reinforced FAGC beams. The strength models of PCC (Portland cement concrete) overestimate the splitting tensile strength and reinforcement-concrete bond strength of FAGC, so PCC’s models are not recommended as the design equations.

A mix design method of fly ash geopolymer concrete based on factors analysis

The effect of mix proportion of geopolymer concrete on compressive strength has been studied in detail in many previous studies, but there is a lack of a proposed mix design method. This research proposed a mix design method for low calcium fly ash geopolymer concrete (FAGC) by finding the key factors that affect the workability and compressive strength of FAGC. The experiment results showed that the workability is governed by the water content. The data from the experiment and the published literature indicated that the strength is independent of fly ash content and AS/FA ratio, and decreases linearly with the increase of the water to alkaline activator ratio, wherethe alkaline activator refers to Na2O and effective SiO2 from the alkaline solution. There is a limit for the SiO2/Na2O ratio beyond which additional SiO2 no longer improves the strength. Based on the factors, a new method was proposed to design the mix proportion of FAGC. Target compressive strengths ranging from 30 MPa to 80 MPa at 3 days and target slump of 150 mm were achieved with laboratory experiments, using the proposed mix design method and a new fly ash. Therefore, this research paper provided a novel mix design method for FAGC that allows for high strength and excellent workability at the same time.

Deterioration of ambient-cured and heat-cured fly ash geopolymer concrete by high temperature exposure and prediction of its residual compressive strength

This paper presents an investigation on the performance of ambient-cured and heat-cured low-calcium fly ash geopolymer concrete after exposure to elevated temperatures. The concrete specimens were heated to 100, 200, 400, 600, 800 and 1000 °C at a heating rate of 5 °C/min. The effect of high temperature exposure on geopolymer concrete was studied by visual inspection, mass loss, extent of cracking, residual strength and microstructure investigation. The total length of cross-section cracks and surface cracks reached their peak values at 800 °C and then declined at 1000 °C. The results show that all the concrete specimens could be heated at 600 °C for 2 h without strength loss. Heat-cured geopolymer concrete specimens showed higher residual compressive strengths than the ambient-cured specimens for all the exposure temperatures. A crushing index of 7.7% could be regarded as the threshold value for coarse aggregate to maintain the initial compressive strength of concrete up to 600 °C. Thus, the microstructural damage as observed by SEM images, dehydration of geopolymer shown by thermogravimetric analysis and the reduced strength of coarse aggregate are considered as the contributing factors for strength losses at temperatures higher than 600 °C. Finally, two prediction equations are proposed which correlated well with the experimental results of this study and those reported in published literature.

Ageing coefficient for early age tensile creep of blended slag and low calcium fly ash geopolymer concrete

The ageing coefficient is required in the age-adjusted effective modulus method to assess the effect of creep in concrete structures subjected to varied stress history. In this paper, experiments were carried out to calibrate the ageing coefficient for early age tensile creep of blended slag and low calcium fly ash geopolymer concrete. The development of total strains of geopolymer concrete under sustained tension, including instantaneous strain, creep strain and shrinkage strain was monitored by using the dog-bone shaped specimens. The specimens were loaded at the age of 2, 3, 4, 7, 14, 21 days, respectively. The strains of the unloaded companion specimens were monitored as well. The development of the creep coefficient φ(t,τ0) for geopolymer concrete was calculated based on the experimental results. By using the step-by-step numerical analysis, the ageing coefficient for early age tensile creep of geopolymer concrete was assessed. An ageing coefficient of 0.8 is recommended for structural design. The comparison between calculated and measured tensile strains from the restrained concrete ring test shows the validity of the proposed value of the ageing coefficient.

Improving the rheological properties of alkali-activated geopolymers using non-aqueous fluids for well cementing and lost circulation control purposes

Typical alkali-activated low-calcium fly ash (AAF) geopolymer formulations are yet to deliver acceptable rheologies for applications such as oil and gas well cementing, where the cement slurry needs to be pumped, and few effective superplasticizers have been identified. The work reported here centers on the discovery that the rheological properties (mixability and pumpability) of AAFs can be significantly improved by incorporating an organic-based non-aqueous fluid (NAF). NAFs such as oil-based muds and synthetic-based muds are water/brine-in-oil invert emulsions that are widely used in the oil and gas industry for drilling and completion operations. Adding NAF can significantly improve the rheological properties of AAF geopolymers, to the point where the rheology of the mixture approaches that of a Portland cement slurry. Overcoming the poor pumpability issue of AAF opens up a large variety of well cementing applications for AAF slurries including primary cementation and plug cementing in severe lost circulation zones.To modify the gelation and setting characteristics multiple fly ash sources and activators were considered in this study. The alkali activators tested in this study were sodium hydroxide and sodium hydroxide with addition of liquid or solid form sodium silicates. Sodium hydroxide activated AAFs showed improved rheological properties with the addition of NAFs. The liquid sodium silicate activated AAFs resulted in unfavorable rheology readings, gel strength measurements and accelerated the pumping time in comparison to use of pure sodium hydroxide activator. The addition of NAF further accelerated the pumping time, increased the viscosity of the AAF, leading to an unpumpable slurry with a rapid setting behavior. In contrast, activation with solid sodium silicate did not negatively affect the rheological behavior of AAF in the presence or absence of NAF. The use of a solid activator showed an initial high consistency peak that broke down with continued shearing. This early high consistency behavior may be exploited to combat lost circulation problems on troublesome oil and gas wells.

Investigating Various Factors Affecting the Long-Term Compressive Strength of Heat-Cured Fly Ash Geopolymer Concrete and the Use of Orthogonal Experimental Design Method

This work quantified the hierarchy of the influence of three common mixture design parameters on the compressive strength and the rate of strength increase over the long term of low-calcium fly ash geopolymer concrete (FAGC) through designing 16 mixtures by the orthogonal experimental design (OED) method. The parameters used in the study were liquid to fly ash (L/FA) ratio, sodium hydroxide concentration (SHC) and sodium silicate solution to sodium hydroxide solution (SS/SH) ratio. The L/FA ratio showed little effect on compressive strength when it was varied from 0.40 to 0.52. SHC showed the greatest influence on compressive strength with little impact on the rate of strength increase after the initial heat curing. Even though the SS/SH ratio showed a small effect on the initial compressive strength, it had a considerable influence on the rate of strength increase over the long term. It was found that the compressive strength at 480 days was positively related to the Na2O/SiO2 molar ratio when it was varied from 0.49 to 0.80 and the Si/Al molar ratio was increased up to 1.87. Analysis of the failure types of specimens demonstrated that compressive strength of FAGC was associated with the strength of the mortar–aggregate interface zone (MAIZ).

Setting time and compression strength of low-calcium fly ash geopolymer paste with non-calcined red soil substitution in ambient curing

This paper describes the research of the effects of non-calcined red soil substitution in low-calcium fly ash geopolymer paste in ambient curing. Mechanical characteristics observed were setting time and compression strength. The non-calcined red soil substitutions were varied i.e 0%, 5%, 25% and 50% of mass of fly ash. The alkaline solutions used were natrium silicate solution and natrium hydroxide solution 10 M with mass ratios 1:1 and 2:1. The results show that the higher the substitution of non-calcined red soil the longer the initial and final time setting of the geopolymer paste. Meanwhile, the higher the substitution of non-calcined red soil the lower the compression strength of the geopolymer paste. Furthermore, geopolymer paste with ratio of Na2SiO3 / NaOH = 2.0 will has longer initial and final setting than geopolymer paste with ratio of Na2SiO3 / NaOH = 1.0. Geopolymer paste with ratio of Na2SiO3 / NaOH = 2.0 will has higher compression strength than geopolymer paste with ratio of Na2SiO3 / NaOH = 1.0. All the relationships between the substitution of non-calcined red soil and setting time or compression strength of geopolymer paste can be drawn in exponential function.

Pervious low-calcium fly ash geopolymer concrete using gap-graded limestone coarse aggregates in ambient curing

This paper describes the research of pervious geopolymer concrete using limestone from Timor Island in Indonesia as coarse aggregate. No sand was used to make this pervious geopolymer concrete. Two types of crushed limestone aggregates used were coral limestone (organic limestone) and limestone (calcilutite). Abrasion losses of the aggregates using Los Angeles Machine test were 18.9% for limestone (calcilutite) and 34.98% for coral limestone. Those aggregates had many characteristics such as 10-20 mm of diameter length, angular shapes and apparent specific density 2.74-2.75. The mass ratios of aggregate to geopolymer paste were varied from 2.86 to 6.91. To make the geopolymer paste, it was used Class F or Low-Calcium Fly Ash from Bolok-Kupang Coal Fired Power Plant as a rich silica-alumina of precursor material. Meanwhile, the mass ratio of natrium silicate solution to natrium hydroxide solution 10 M was 1.0 and the mass ratio of fly ash to alkaline solution was 2.0. Ambient curing was applied to all specimens. Mechanical characteristics obtained at the age of 28 days of hardened pervious geopolymer concrete were: (1) compression strength, (2) splitting tensile strength, and (3) abrasion losses using Los Angeles Machine Test. Besides, density of hardened specimens were also obtained. In addition, the comparisons of mechanical properties of pervious geopolymer concretes using coral limestone and limestone (calcilutite) as coarse aggregate were also conducted. According to the results of this research, it is feasible to make pervious geopolymer concrete using Timor limestone as coarse aggregate due to its mechanical characteristics.

Comparative study of geopolymer and alkali activated slag concrete comprising waste foundry sand

Abstract Concrete industry and current construction practices are highly unsustainable from the standpoint of energy consumption and their high dependence on natural resources. Using alternative binders to conventional cement such as geopolymers and alkali activated slags (AAS) can be one alternative. Besides, reusing industrial by-product and waste materials in construction industry can also be emphasized. These measures can substantially lower the carbon emissions and embodied energy of concrete along with solving the problem of industrial waste disposal. The current study reports the influence of inclusion of waste foundry sand (WFS), a by-product from foundry industries, on strength, permeability and microstructure of low calcium fly ash geopolymer concrete (GPC) and ground granulated blast furnace slag (GGBS) based alkali activated slag (AAS) concrete, both cured in ambient conditions. The mix proportions for both the concretes were kept same. The natural sand was replaced by WFS in the range of 0% to 100% at an interval of 20%. Tests on hardened concretes, to study, compressive strength, split tensile strength, capillary suction, SEM and EDS analysis, were conducted to assess the strength, permeability and microstructure of both types of concretes. Addition of WFS lowered the workability of GPC and AAS concretes and the effect was abrupt beyond 40% WFS replacement level. Strength and sorptivity of both concretes improved upto 60% WFS replacement in GPC, whereas, upto 20% replacement in AAS concrete mixes, besides more than 45% of strength of reference concretes (0% WFS) was achievable at 100% replacement by WFS.

Creep and Drying Shrinkage of Different Fly Ash-Based Geopolymers

Fly ash geopolymer concrete is a sustainable green construction material that has outstanding mechanical performance and is a low-energy material with a low carbon footprint. In this study, a detailed investigation of the long-term creep and drying shrinkage of three different 100% fly ash geopolymer concretes was carried out up to 1 year of age. Two geopolymers, produced from Gladstone and Pt. Augusta fly ashes, achieved approximately 700 microstrain at the end of 1 year-equivalent to the total creep strain displayed by portland cement (PC) concrete. Moreover, both geopolymer concretes displayed a lower creep coefficient than PC concrete. Hence, AS 3600 or the CEB-FIP model could be conservatively used to predict creep coefficient for two geopolymers. However, the Tarong fly ash geopolymer concrete differed significantly from the other geopolymers and achieved approximately 1900 microstrain after 1 year. The drying shrinkage of Gladstone and Pt. Augusta geopolymer concretes at 1 year are 175 and 190 microstrain, respectively, while Tarong geopolymer and PC concrete achieved 615 and 475 microstrain, respectively. All the fly ash geopolymer concrete showed lower drying shrinkage than the maximum permitted value recommended by AS3600. Incorporation of calcium-aluminasilicate-hydrate (C-A-S-H) gel with the sodium-alumina-silicatehydrate (N-A-S-H) geopolymeric gel was seen to positively affect the packing density of the gel phase. The degree of uniformity and compactness of aluminosilicate gel matrix together with the macroporosity in the 50 nm to 1 µm range was identified as determining the long-term creep and drying shrinkage of the 100% low-calcium fly ash geopolymer concrete.

Correlations between Mechanical Properties of Low-Calcium Fly Ash Geopolymer Concretes

This paper reports the correlation between existing standards for concrete and the major mechanical properties of geopolymer concrete produced from a range of low-calcium, Class F fly ashes. The data were collected at set points over a 1-year period and include four different sources of fly ash used in the manufacturing of geopolymer concrete. New relationships between mechanical properties and compressive strength were derived using statistical regression analysis. The applicability of current relationships of portland cement (PC) concrete as specified in Australian standards and American Concrete Institute codes for geopolymer concrete have been critically examined. The results indicated that the flexural strength of geopolymer concrete is higher than those predicted using current design equations for PC concrete with a similar compressive strength. However, the splitting tensile strength of geopolymer concrete is comparable to that predicted using the current design equation for PC concrete with similar compressive strength. It was also observed that the ACI 363 R equation significantly overestimates the splitting tensile strength of geopolymer concrete. Similarly, AS 3600 overvalues the elastic modulus of geopolymer concrete. Both of these overestimations may be critical when compressive strength is below 40 MPa.

Comparison of long term performance between alkali activated slag and fly ash geopolymer concretes

This paper reports the comparison of engineering properties of alkali activated slag (AAS) and low calcium fly ash geopolymer (FAGP) concretes up to 540days. The results showed that the AAS concrete had higher compressive and tensile strength, elastic modulus and lower permeation characteristics than FAGP concrete in the initial 90days. However, a reduction in AAS concrete performance was observed between 90 and 540days, while an increase was noted in FAGP concrete over the same time period. The microscopy revealed that both reactions progressed beyond 90days with the slag–alkali producing excess C–S–H gel which was observed to increase the crack propagation and crack width at latter ages, attributed to the combined effect of disjoining pressure and self-desiccation. The fly ash geopolymerization also continued following an initial 24h heat curing resulting in a crack-free dense microstructure at 540days. Overall the discrepancy in microstructural development beyond 90days in the two concretes would explain the contradictory performance over the longer time frame.

Improving compressive strength of low calcium fly ash geopolymer concrete with alccofine

Geopolymer concrete is environmentally friendly and could be considered as a construction material to promote the sustainable development. In this paper fly ash based geopolymer concretes with different percentages of alccofine were made by mixing sodium hydroxide and sodium silicate as an alkaline activator and cured at ambient as well as heat environment in an electric oven at 90oC. Effects of various parameters such as the percentage of alccofine, curing temperature, a period of curing, fly ash content, was studied on compressive strength as well as workability of geopolymer concrete. The study concludes that the presence of alccofine improves the properties of geopolymer concrete during a fresh and hardened state of concrete. Geopolymer concrete in the presence of alccofine can be used for the general purpose of concrete as required compressive strength can be achieved even at ambient temperature. The 28 days compressive strength of 73 MPa, when cured at 90-degree Celsius, confirmed that it is also very suitable for precast concrete components.

Carbonation of a low-calcium fly ash geopolymer concrete

The carbonation resistance of a blended slag and low-calcium fly ash (FA) geopolymer concrete was investigated. The geopolymer binder studied was composed of 90% low-calcium FA and 10% ground granulated blast-furnace slag. The alkalinity of the pore solution plays a pivotal role in carbonation progression and subsequent corrosion initiation. pH profiles were measured to assess the pore solution alkalinity. Phenolphthalein indicator was used to measure the carbonation depth. X-ray diffraction (XRD) and quantification were carried out to identify and quantify the carbonation products. The obtained pH profiles illustrated a wider semi-carbonation zone in the geopolymer specimens, although the pH drop was insignificant in most cases. XRD analysis revealed that nahcolite mainly formed at 3% carbon dioxide concentration and led to a significant drop in pH values. The results further demonstrated that 1% accelerated carbonation replicated the natural carbonation process well, with only natron identified as a carbonation product. This work contributes to the assessment of the risk of carbonation-induced reinforcement corrosion in low-calcium FA geopolymer concrete.

Flexural strength and elastic modulus of ambient-cured blended low-calcium fly ash geopolymer concrete

Fly ash geopolymer is an emerging alternative binder with low environmental impact and potential to enhance sustainability of concrete construction. Most previous works examined the properties of fly ash-based geopolymer concrete (GPC) subjected to curing at elevated temperature. To extend the use of GPC in cast-in-situ applications, this paper investigated the properties of blended low-calcium fly ash geopolymer concrete cured in ambient condition. Geopolymer concretes were produced using low-calcium fly ash with a small percentage of additive such as ground granulated blast furnace slag (GGBFS), ordinary Portland cement (OPC) or hydrated lime to enhance early age properties. Samples were cured in room environment (18–23°C and 70±10% relative humidity) until tested. The results show that, density of hardened GPC mixtures is similar to that of normal-weight OPC concrete. Inclusion of additives enhanced the mechanical strengths significantly as compared to control concrete. For similar compressive strength, flexural strength of ambient cured GPC was higher than that of OPC concrete. Modulus of elasticity of ambient cured GPC tend to be lower than that of OPC concrete of similar grade. Prediction of elastic modulus by Standards and empirical equations for OPC concrete were found not conservative for GPC. Thus, an equation for conservative prediction of elastic modulus of GPC is proposed.