Low Calcium Fly Research Articles

Microstructure and mechanical properties of FA/GGBS-based geopolymer

This study presents the microstructure and mechanical properties of geopolymer paste made from low-calcium fly ash (FA) and ground granulated blast-furnace slag (GGBS) through alkalination. The use of GGBS and FA is not only for sustainable construction but also for reducing the emission of CO2 due to the use of Portland cement. Different replacement ratios of GGBS to FA were used to determine the effect of GGBS presented to the compressive strength of geopolymer specimens. The alkaline activator solution used is a combination of sodium hydroxide (NaOH) 14 M and sodium silicate (Na2SiO3). A compressive strength test on cylindrical specimens (50 mm x 100 mm) at the ages of 7, 14, and 28 days was carried out. The results showed that the compressive strength increased with the increase of GGBS in the mixes (up to 100 MPa). Moreover, SEM-EDS, XRD, and TG-DTA characterization methods were conducted to investigate the microstructure, phase composition, and thermal stability of the geopolymer specimens respectively.

Generalized Fracture Toughness and Compressive Strength of Sustainable Concrete Including Low Calcium Fly Ash.

The paper presents the results of tests on the effect of the low calcium fly ash (LCFA) addition, in the amounts of: 0% (LCFA-00), 20% (LCFA-20) and 30% (LCFA-30) by weight of cement, on fracture processes in structural concretes. In the course of the experiments, compressive strength of concrete and fracture toughness for: I (tensile), II (in-plane shear) and III (anti-plane shear) models of cracking were measured. The tests determined the effect of age of concretes modified with LCFA on the analyzed parameters. The experiments were carried out after: 3, 7, 28, 90, 180 and 365 days of curing. Fracture toughness of concretes was determined in terms of the critical stress intensity factors: , , and then a generalized fracture toughness was specified. The obtained results are significant for the analysis of concrete structures subjected to complex loading. The properties of composites with the additive of LCFA depend on the age of the concrete tested. Mature concretes exhibit high fracture toughness at 20% additive of LCFA, while the additive of LCFA in the amount of 30% weight of cement has a beneficial effect on the parameters of concrete only after half a year of curing.

Enhancement of low-cement self-compacting concrete with dolomite powder

This study aims at exploring the benefits of using dolomite powder (DP) to improve the engineering performance of low-cement self-compacting concrete (SCC) with cementitious materials comprised of 50 wt% of ordinary Portland cement (OPC) and 50 wt% pozzolanic materials of ground granulated blast furnace slag (GGBFS/slag) and low calcium fly ash (FA). Experimental results showed that the DP addition to partially substitute for the pozzolanic materials had no impact on the setting properties of pastes with blended binder of slag and OPC while significantly improved those of the pastes with that of FA and OPC or ternary mixture of slag, FA, and OPC binder. In addition, the adjustment of DP addition to the above-mentioned three binders had successfully manufactured the self-compacting mortars (SCMs) with significant increase in the flowing ability and compressive strength, in which the DP addition of 30 wt% substituting for the total amount of pozzolanic materials resulted in the hardened mortars with highest compressive strengths after ages of 7 days. The improvement of compressive strengths of hardened SCMs can be obtained by the DP addition up to 50 wt%. The SCCs produced from the appropriate proportions of the resulting DP modified SCMs had the engineering properties mostly satisfying the technical requirements for practical concrete structures.

Dissolution of the glassy phase in low-calcium fly ash during alkaline activation

An experimental investigation on the dissolution of low-calcium fly ashes in an alkaline solution is presented. Fly ashes collected directly from different sources are used in the study. Changes in the elemental concentrations in the solution are determined using optical emission spectroscopy and the residue is characterised using X-ray-based techniques. The increase in the concentration of ions in the alkaline solution is primarily associated with the dissolution of the glassy phase and the crystalline content of fly ash is largely unaffected by the alkaline exposure. The increase in the concentration of Si in the solution relative to the concentration of Al is not influenced by the solution molarity or the temperature. The glass that contains Ca is more reactive and dissolves earlier than the aluminosilicate glass present within the fly ash. The X-ray diffraction pattern of the fly ash glassy phase is adequately fit using a single pseudo-Voigt (PV) peak. The peak position of the PV fit is invariant of the dissolution of the glassy phase and an intensity-based measure of the X-ray diffraction analysis intensity pattern with the PV fit provides an accurate estimate of the undissolved glassy content present in the fly ash.

Study on the compressive strength of fly ash based geo polymer concrete

Introduction of the alternative materials for complete replacement of cement in ordinary concrete will play an important role to control greenhouse gas and its effect. The 100% replacement of binder with fly ash (in integration with potassium hydroxide (koh) and potassium silicate (k2sio3) solutions) in concrete gives a significant alternative to conventional cement concrete. This paper focuses on the effect of alkaline solutions koh and k2sio3 on strength properties of fly ash based geo polymer concrete (fgpc); compared the strength at different molarities of alkaline activator koh at different curing temperature. Fly ash based geo polymer concrete was produced from low calcium fly ash, triggered by addition of koh and k2sio3 solution and by assimilation of superplasticizer for suitable workability. The molarities of potassium hydroxide as 8m, 10m and 12m molarities were used at various curing temperatures such as 60°c, 70 °c and 80°c. Results showed that for given proportion to get maximum compressive strength the optimum molarity of alkaline solution is 12m and optimum curing temperature is 70 °c.

Properties of ambient cured blended alkali activated cement concrete

This paper presents results of the development and strength properties of ambient-cured alkali activated geopolymer concrete (GPC). The study looks at the strength properties, such as compressive strength, splitting tensile strength, and elastic modulus of such concretes and its dependency on various parameters. The parameters studied in this work are the type and proportions of pre-cursor materials, type of activator and their respective ratios and the curing time. Two types of pre-cursor material; low calcium fly ash (FA) and ground granulated blast furnace slag (GGBFS) were activated using different proportions of sodium silicate and sodium hydroxide solutions. The results indicate that ambient cured geopolymer concrete can be manufactured to match strength properties of ordinary Portland cement concrete (OPC). The strength properties of GPC are dependent on the type and ratio of activator and the proportion of GGBFS used. Increasing the percentage of GGBFS increased the compressive and tensile strengths, while reducing the setting time of the mix. The effect of GGBFS on strength was more pronounced in mixes that contained sodium silicate as activator solution. Unlike OPC, ambient-cured GPC containing sodium silicate gain most of their strength in the first 7 days and there is no change in strength thereafter. However, GPC mixes not containing sodium silicate only achieve a fraction of their strength at 7 days and extended curing is required for such concretes to gain full strength. The results also indicate that the elastic modulus values of GPC mixes without sodium silicate are comparable to OPC while mixes with sodium silicate have elastic modulus values much lower than ordinary concrete.

Compressive strength and microstructure of alkali-activated fly ash/slag binders at high temperature

This paper reports the results of the compressive strength and microstructure of various alkali-activated binders at elevated temperatures of 300 and 600 °C. The binders were prepared by alkali-activated low calcium fly ash/ground granulated blast-furnace slag at ratios of 100/0, 50/50, 10/90 and 0/100 wt.%. Specimens free of loading were heated to a pre-fixed temperature by keeping the furnace temperature constant until the specimens reached a steady state. Then the specimen was loaded to failure while hot. XRD, SEM and FTIR techniques were used to investigate the microstructural changes after the thermal exposure. The fly ash-based specimen shows an increase in strength at 600 °C. On the other hand, the slag-based specimen gives the worst high-temperature performance particularly at a temperature of 300 °C as compared to ordinary Portland cement binder. This contrasting behaviour of binders is due to their different binder formulation which gives rise to various phase transformations at elevated temperatures. The effects of these transformations on the compressive strength are discussed on the basis of experimental results.

Passivity of embedded reinforcement in carbonated low-calcium fly ash-based geopolymer concrete

This paper investigates the carbonation of two low-calcium fly ash-based geopolymer concretes to assess the effect of alkali concentration in the activator, and the carbon dioxide concentration on the pH drop and passivity of the reinforcement. Chemical adsorption of carbon dioxide at different concentrations into an aqueous NaOH solution, as representative of the pore solution, is studied to predict the distribution of carbonate species and pH drop. pH profiles were obtained during the exposure period. X-ray diffraction (XRD) was conducted to identify the carbonate phases. Half-cell potential and polarization resistance of reinforced concrete samples were monitored to assess the passivity of embedded reinforcement. The carbonated binders remained rather highly alkaline during the accelerated carbonation test which was in agreement with the predicted values. No sign of depassivation of reinforcement was observed, even for the lower strength grade concrete, during the long exposure time of 500 days to 1% carbon dioxide.

Predicting Relationship between Mechanical Properties of Low Calcium Fly Ash-Based Geopolymer Concrete

ABSTRACTThis paper presents the results of the experimental study carried out on geopolymer concrete with the inclusion of alccofine. Efforts have been made to achieve the required compressive strength at ambient temperature. In the present investigation alccofine, richer in silica and finer than fly ash, has been added up to 10% of fly ash content. In addition, geopolymer concrete with different fly ash contents has been produced. The results are encouraging, as required compressive strength even 40 MPa could be achieved at ambient temperature. Compressive and tensile strength were increased with increase in fly ash and alccofine content. It was observed that better geopolymerization occured in presence of alccofine, and strength continuously increased significantly up to 90%. Attempts were further made to relate the compressive strength with splitting tensile and flexural strengths in presence of alccofine. A relationship has been suggested which holds good for geopolymer concrete with and without alccofine at ambiently, and heat cured specimens.

Effect of Element Distribution on Strength in Fly Ash Geopolymers

This study evaluates the influence of the elemental distribution in the fly ash particles and their impact on phase formation and compressive strength of five low-calcium fly ash geopolymers. The degree of geopolymerization in each geopolymer system was assessed by FT-IR and solid state27Al MAS-NMR analysis. The corresponding pore volume changes were investigated by mercury intrusion porosimetry (MIP). The uniformity of the distribution of SiO2 and Al2O3 in the fly ash was observed to directly influence the dissolution of the amorphous surface layer in the initial geopolymerization process and control aluminosilicate gel precipitation and gel-phase creation. The results showed that the higher the uniformity of distribution (coupled with the stable conversion of aluminium from octahedral to tetrahedral coordination), the higher the aluminium amalgamation with silicates. The result of this is the production of a three-dimensional (3-D) polysialatesiloxo (Si-O-Al-O-Si) polymeric gel structure with high rigidity and stability, which in turn results in higher compressive strength. It was also observed that an increase of meso-porosity in geopolymer phase formation coupled with a cumulative pore volume below 1000 nm (3.937 x 10-5 in.) is a good indicator of the degree of geopolymerization.

Influence of production on the strength, density and water absorption of aerated geopolymer paste and mortar using Class F fly ash

In this paper, the factors influencing the properties of aerated geopolymer concrete using low calcium fly ash through sodium-based alkali activation at a range of curing temperature is discussed. The parameters and their influence are statistically studied using design of experiments. The initial rate of reaction in the aeration process is controlled by using the alkaline solution (viz., sodium hydroxide (NaOH) and sodium silicate (Na-Si)) which is cooled to a lower temperature. The conditioning of solution at lower temperature helped in achieving uniform mixing and to increase the placing time without hindering aeration. The factors influencing and their interaction effects on fresh density, dry density, compressive strength and water absorption were studied. Compressive strength of aerated geopolymer paste is 55% higher than mortar for the given range of parameters.

Fracture properties of concrete specimens made from alkali activated binders

The aim of this paper is to quantify crack initiation and other fracture properties – effective fracture toughness and specific fracture energy – of two types of concrete with an alkali activated binder. The beam specimens with a stress concentrator were tested in a three-point bending test after 28, 90, and 365 days of maturing. Records of fracture tests in the form of load versus deflection (P–d) diagrams were evaluated using effective crack model and work-of-fracture method and load versus mouth crack opening displacement (P–CMOD) diagrams were evaluated using the Double-K fracture model. The initiation of cracks during the fracture tests for all ages was also monitored by the acoustic emission method. The higher value of monitored mechanical fracture parameters of concrete with alkali activated blast furnace slag were achieved with substitution blast furnace slag by low calcium fly ash in comparison with substitution by cement kiln dust.

Durability of low‑calcium fly ash based geopolymer concrete culvert in a saline environment

This study reports the investigation of the durability of a low-calcium fly ash based geopolymer concrete (GPC) mix in a saline lake environment. Core specimens from the GPC and Ordinary Portland Cement (OPC) concrete structures underwent durability assessment including in-situ carbonation, chloride and sulphate ingress, microstructural characterization and porosity analysis. It was found that the particular GPC mix design studied was more vulnerable to carbonation and the deterioration effects due to chloride and sulphate ingress in GPC were higher than OPC concrete after 6 years of exposure. According to FT-IR and XRD analysis, there was no evidence of carbonation reaction products remaining in GPC. This indicates that the carbonation products dissolved when in contact with water, which caused high porosity of the concrete surface thereby facilitating more diffusion into the concrete. This study revealed that the development of a durable geopolymer concrete mix design for aggressive environments requires appropriate consideration of binder chemistry and preliminarily durability property testing to avoid premature deterioration.

SERVICEABILITY OF LOW CREEP FLY ASH GEOPOLYMER CONCRETE BEAMS

In reinforced concrete construction, deflection control is an important performance criterion for their serviceability. The aim of the research described in this paper is to assess the deformation of cracked reinforced geopolymer concrete beams under long term service loading. The geopolymer binder is Portland cement free, using 85% of low calcium fly ash, 15% of GGBFS (Ground Granulated Blast Furnace Slag) and a sodium silicate/sodium hydroxide based activator. Firstly, geopolymer concrete drying shrinkage and creep were measured. Different curing conditions at elevated temperature were used. All experimental results are compared to predictions made using the Eurocode 2. Secondly, geopolymer concrete beams were subjected to short time bending tests leading to concrete cracking (pre-cracking tests). Beams were then stored under sustained loading for a period of four months. Both deflection and cracks were monitored versus time. Results show that, providing an appropriate heat curing regime, geopolymer concrete creep is much lower than that observed for OPC concrete and predicted by the Eurocode 2. As a result, the time-dependent deflection of geopolymer concrete beams measured after 4 months under sustained loading was always significantly lower than that of traditional OPC concrete beams. All results are showing that the crack widths of geopolymer concrete beams are significantly smaller than those expected for OPC concrete beams according to fib model code 2010 for both short and long terms tests. It is concluded that low calcium fly ash-based geopolymer concrete is a promising option for precast applications.

Effect of phase and size characteristics of fly ash from power station on properties of bitumen composites

Fly ash from power stations is one of the effective fillers in bitumen binders due to its high specific surface area in origin state. This paper is focused on study of effect of phase and size characteristics of fly ashes on properties of bitumen binder. In this work a high-calcium (FA1) and low-calcium (FA2) fly ashes are used. According to chemical and mineral composition of FA1 is reactive in water medium because it contains hydraulic minerals. FA2 is hydraulically inert mainly consisting of aluminosilicate vitreous phase. FA1 is characterized by higher specific surface vs. FA2 according to Blain method data but lower specific surface measured with BET method. It can be associated with high porosity of FA2 that significantly influent on bitumen content in binding system. Lower packing density of FA2 particles leads to reducing of strength characteristics in bitumen composite. Low adsorption of bitumen to surface of FA1 particles initiates hydration process of reactive minerals as well as formation of ettringite phase that leads to extension of the binder system and structure decompaction. Bitumen composites based on FA2 with higher adsorptive capacity is characterized by more stable structure.

Simultaneous removal of aqueous Zn2+, Cu2+, Cd2+, and Pb2+ by zeolites synthesized from low-calcium and high-calcium fly ash

In this study, zeolites were synthesized from low-calcium (LCZ) and high-calcium (HCZ) fly ash, respectively. Subsequently, the zeolites were tested for their removal effectiveness for four aqueous cations, namely, Zn2+, Cu2+, Cd2+, and Pb2+, as a function of contact time, pH value, adsorbent dosage, and initial concentration of heavy metals. Both zeolites were characterized by X-ray diffraction, X-ray fluorescence spectrometry, scanning electron microscopy, specific surface area, and cation exchange capacity. The results show that HCZ mainly consists of an unnamed zeolite (Na6[AlSiO4]6·4H2O), whereas LCZ mainly consists of faujasite-type zeolite. The optimum sorption conditions were pH = 6.0; adsorbent dosage = 1.0 g·L-1; temperature = 25 °C; contact time = 100 min; and initial heavy metal concentration = 100 mg·L-1. The sorption kinetics of the four aqueous cations on both LCZ and HCZ followed the pseudo-second-order kinetic model, and the sorption isotherm data fitted well with the Langmuir isotherm model. For LCZ, the maximum adsorption capacities of Zn2+, Cu2+, Cd2+, and Pb2+ were 155.76, 197.86, 123.76, and 186.22 mg·g-1, respectively. For HCZ, the values were 154.08, 183.15, 118.91, and 191.94 mg·g-1, respectively. The zeolites were regenerated by NaCl solution (1 mol·L-1) and showed high removal efficiency. In conclusion, zeolites produced by fly ash are promising materials for removing Zn2+, Cu2+, Cd2+, and Pb2+ from wastewater.

Properties of low-calcium fly ash based geopolymer concrete incorporating OPC as partial replacement of fly ash

Results of an experimental investigation on the compressive strength and permeation properties of fly ash based geopolymer concrete are presented in this paper. Fly ash was partially replaced with 0, 10, 20 & 30% of ordinary Portland cement (OPC). Tests were conducted for compressive strength and permeation properties such as water absorption, porosity, sorptivity and rapid chloride permeability up to the age of 365days. Findings from this investigation indicate that compressive strength increased with the increase in OPC content at all ages, with optimum being at 20%. The inclusion of OPC resulted in the reduction in water absorption, porosity, sorptivity, and chloride permeability of geopolymer concrete due to the improvement in microstructure caused by co-existence of hydrated products along with geo-polymeric alumina-silicate polymer structure. The results were also verified by the microstructural analysis, SEM, EDS, and XRD, which indicate better compacted and dense microstructure of geopolymer concrete with the addition of OPC.

Evaluation of sodium content and sodium hydroxide molarity on compressive strength of alkali activated low-calcium fly ash

Activation of low calcium fly ash is investigated using activating solutions of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The oxide ratio of Na2O relative to the total reactive silica in the activated mix provides consistent results in achieving the highest ultimate strength. The total reactive silica used for calculating the ratio consists of the reactive silica contributed by fly ash and the silica from the activating solution. There is an increase in the ultimate compressive strength on increasing the total sodium content relative to the total reactive silica content in the activated system. Increasing the sodium content beyond a certain limit does not provide additional gain in the ultimate compressive strength. The ratio of total reactive SiO2 to Na2O in the activated system equal to 4.72 is shown to provide the highest compressive strength and there is no further increase in the ultimate strength on increasing the sodium content. A N-A-S-H type gel with reaction products containing Si, Al and Na, is formed in the activated system. The ultimate strength achieved is directly related to the reaction product content in the system and is dependent on the extent of glassy phase dissolution from fly ash. The extent of glassy phase dissolution and the quantity of reaction product formed in the system increases with an increase in the molarity of NaOH, which also contributes to an increase in the sodium content in the activating solution. The decrease in the unreacted glassy phase content of fly ash is sensitive to temperature at a lower molarity of NaOH. The Al/Na ratio in the reaction product approaches a value of 0.9 on increasing the sodium content in the activated system. The Si/Al ratio in the reaction product varies within a range of 2.3–2.8.

Effectiveness of Ternary Blends in Mitigating ASR Demonstrated on the Modified Mortar-Bar Test

Abstract Supplementary cementitious materials (SCMs) are the effective means of improving concrete durability. Combining two SCMs with ordinary Portland cement helps overcome the probable drawbacks of using single SCM. Ternary cementitious systems have become popular in the construction industry, particularly for use in high-performance concrete. In this study, the effectiveness of ternary blends in controlling alkali–silica-reaction (ASR) deterioration was tested in the comparison of a modified mortar-bar method and ASTM C1260-14/C1567-13. Our experiment included the evaluation of 22 binary or ternary combinations of five cementitious materials (Portland cement, metakaolin, low-calcium fly ash, high-calcium fly ash, and slag) tested with two different reactive aggregates. In addition to standard ASTM C1260-14/C1567-13, the mortar mixes were assessed on a modified mortar-bar method of short bars (25 × 25 × 150 mm3) stored at 60°C and >95 % relative humidity. Ternary cementitious blends containing metakaolin and low-lime fly ash proved to be very effective at controlling ASR. In the proposed modified test, a pass/fail criterion of 0.03 % expansion over 60 days was found to be in agreement with the expansion at 14 days of ASTM C1260-14/C1567-13 at 0.10 %.

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.