Fly Ash-based Binders Research Articles

Reuse of soil-like material solidified by a biomass fly ash-based binder as engineering backfill material and its performance evaluation

The reuse of soil-like material (SLM) obtained from landfill mining as engineering backfill materials contributes to sustainable waste management. In this paper, a new biomass fly ash-based binder (BB) containing biomass fly ash (BFA), carbide slag (CS), and phosphogypsum (PG) is designed to solidify the SLMs. The mechanical properties, permeability, microstructure, and physicochemical characteristics of the BB-solidified SLM are comprehensively characterized. The optimum proportion of ternary BBs consisting of 80% BFA, 15% CS, and 5% PG was determined through tests on paste samples. The different landfill depths of SLMs show significant variability in solidification/stabilization (S/S) effects due to their different physicochemical properties. The mechanical and permeability properties of BB-solidified SLM improve with the increasing BB content and the age of solidification. Microstructural and phase analyses indicated the formation of significant amounts of ettringite crystals and C-(A)-S-H gels were generated by the pozzolanic reaction, forming a stable and dense microstructure. The variations in pH and conductivity were correlated with the development of mechanical properties. Moreover, the organic matter content and leached heavy metal content of SLM2 and SLM3 after S/S treatment did not exceed the specified limits. This study can provide theoretical guidance for the resource utilization of the stabilized/solidified SLMs.

Influence of heat treatment on the mechanical properties of alkali-activated fly ash-based binders with marble dust substitution

Marble waste contains a high level of calcium, which is obtained from the cutting process in marble production. The properties of geopolymer binders are influenced not only by the amount of alkali activators, their ratio, the molarity used, and the Si and Al content of the mineral additives used in the mixture, but also by the duration of the heat treatment and the heat treatment temperature. This study aimed to produce alkali-activated geopolymer binders based on fly ash and marble dust. Alkali-activated geopolymer pastes were made both with heat treatment for 24h at 70°C and without heat treatment at (23±2) °C. Structural analysis by optical microscopy revealed, in the case of heat-treated samples, the formation of pores with an average size of 1÷1.5 mm, much larger than in the case of samples without heat treatment.

Microstructural behaviour of quarry fines stabilised with fly ash-based binder

Stabilised quarry fines have been explored as sustainable pavement construction materials. This study investigates the viability of such materials for applications in pavement structures, contributing to current knowledge on mechanical performance from the perspective of mineralogy, microstructure, and chemical composition of the material. This study uses fly ash-based material as stabiliser, promoting circular economy considering the availability of fly ash worldwide by 2030. Laboratory tests were performed including unconfined compressive strength (UCS), mineralogical analysis, and microstructure investigations by scanning electronic microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). Using different binders, Calcium was found to be the element with most variability in mass, and the UCS increased proportionally as the ratio of CaO/SiO2 or CaO/(SiO2+Al2O2) increased. As curing time accumulates to 28 days, more cementitious reaction products were generated and observed, and the void to solid ratio reduced in some samples, possibly leading to improvements in the UCS.

Infrared spectra experimental analyses on alkali-activated fly ash-based binders

One of the most used characterization techniques in the field of alkaline activated cements studies is infrared spectroscopy. Its prominence lies in that it allows characterizing mixtures during the alkaline activation by providing information about the vibrations of the chemical bonds in the molecular units, both of amorphous and crystalline products. This research paper is aimed at examining the influence of the concentration of calcium hydroxide (CH), sodium hydroxide (SH), temperature and curing time on the structure of alkaline activated cements, based on coal fly ash, from the deconvolution of the infrared spectrum between 4000 and 400 cm−1.9 mixtures were analyzed by infrared spectroscopy at 3 and 28 days after curing, based on a surface’s response experimental design by varying the amount of SH (5.17–10.83 M), CH (2.93–17.07% ash’s wt.) and the curing temperature (25, 35 and 45 °C). The results show significant variations in the frequency and area of the deconvolved bands in the functional groups: O-H (2600–3800 cm−1), C-O (1580 and 1350 cm−1) and T-O (T: Si (Al), 1300–400 cm -1). Such variations are due to the reorganization of the forming elements (present in the ash) network and modifiers (present in CH and SH) for the formation of cementing gels C-(A) -S-H (970 cm−1) and N-A-S-H (1009 cm−1).

Performance evaluation of limestone-calcined clay (LC2) combination as a cement substitute in concrete systems subjected to short-term heat curing

This paper evaluates the suitability of limestone-calcined clay (LC2) combination as a cement substitute material in concrete systems subjected to steam curing. Concrete with blends of portland cement and LC2 were subjected to short-term heat curing and compared against concrete with plain portland cement and fly ash-based binder (at 30% replacement) for two different concrete systems (a) 400 kg/m3 binder content and 0.40 w/b, and (b) 360 kg/m3 binder content and 0.45 w/b. Several parameters were considered to evaluate the potential of LC2 in such applications, which include - improvements in the 1-day compressive strength, strength gain at later ages, and resistance to chloride-ion penetration. Furthermore, the pore structure, phase assemblage and SEM micrographs were used to assess the alteration in the microstructure at 1-day to elucidate the microstructural development due to steam curing regime.The results show that concrete prepared with LC2 could achieve 1-day compressive strength equal to OPC concrete, which was not achieved with fly ash mixes. The durability performance was similar in both normal and steam-cured concretes across all the binder systems considered in the study. Concrete prepared with LC2 showed significant improvement in 1-day strength due to short-term heat curing. Correspondingly, there was a significant reduction in the porosity and critical pore diameter at 1-day for LC2 systems subjected to steam curing, while fly ash mixes showed almost negligible change in pore structure. LC2 systems could produce a dense microstructure at 1-day, despite the absence of ettringite and carboaluminate phases at elevated curing temperature. The study put forward the beneficial use of LC2 in precast fabrication units with steam-curing requirement for improved early-age strength and durability performance.

Effects of Accelerated Carbonation Testing and by-Product Allocation on the CO2-Sequestration-to-Emission Ratios of Fly Ash-Based Binder Systems

Carbonation of cementitious binders implies gradual capture of CO2 and significant compensation for the abundant cement-related CO2 emissions. Therefore, one should always look at the CO2-sequestration-to-emission ratio (CO2SP/EM). Here, this was done for High-Volume Fly Ash (HVFA) mortar (versus two commercial cement mortars). Regarding their CO2 sequestration potential, effects of accelerated testing (at 1–10% CO2) on as such estimated natural carbonation degrees and rates were studied. Production related CO2 emissions were evaluated using life cycle assessment with no/economic allocation for fly ash. Natural carbonation rates estimated from accelerated tests significantly underestimate actual natural carbonation rates (with 29–59% for HVFA mortar) while corresponding carbonation degrees are significantly overestimated (67–74% as opposed to the actual 58% for HVFA mortar). It is advised to stick with the more time-consuming natural tests. Even then, CO2SP/EM values can vary considerably depending on whether economic allocation coefficients (Ce) were considered. This approach imposes significant portions of the CO2 emissions of coal-fired electricity production onto fly ash originating from Germany, China, UK, US and Canada. Ce values of ≥0.50% lower the potential CO2SP/EM values up to a point that it seems no longer environmentally worthwhile to aim at high-volume replacement of Portland cement/clinker by fly ash.

Microstructural evolution and mechanical behaviour of alkali activated fly ash binder treated clay

This work focuses on the use of alkali activated fly ash-based binder to enhance engineering characteristics of soft clay-rich soils and as a substitute to standard stabilisers (e.g., lime or cement). Especially, it examines the microstructural evolution of a calcium-rich fly ash from coal combustion-based binder activated by a sodium-based alkaline solution. To this end, the processes generating the microstructure and the evolution of the pore network over time are investigated. A second point addressed by this study is how the presence of kaolin particles affects the microstructural features of the binder. The microstructure has therefore been investigated by considering the binder alone and the binder mixed with kaolin. The effects of microstructural evolution have been observed at macroscopic level by means of one-dimensional compression tests.The combination of completing techniques has been used including Optical microscopy, Scanning Electron Microscopy and Mercury Intrusion Porosimetry in order to gain an overview of the complex pore structure.Microstructural changes occur around calcium-containing phases derived from fly ash which are the reactive phases of the system. Namely, the dissolution of calcium-rich grains leads to the formation of new compounds that first cover the grain surfaces and then further grow into the available space. Furthermore, the evolution of the pore network over time is characterized by a progressive filling of capillary pores by new compounds while small nanometric pores are being formed and associated with the newly formed silicate-calcium chains. Similar tendencies are observed when the binder is mixed with the soil although the general porosity is lesser due to the filling of pores by small-sized kaolinite platelets. Experimental evidences at microscale level have been linked to the macroscopic behaviour of treated soil.

Investigation on the microstructure-related characteristics to elucidate performance of composite cement with limestone-calcined clay combination

This paper discusses the role of physical structure alterations on three binder types: plain portland cement, fly ash-based binder and calcined clay-limestone binder. The kinetics of physical structure development and the relevance in transport properties were distinguished using an interlinked parameter in concrete and paste. A systematic experimental investigation was carried out on a range of critical parameters such as strength development, resistivity development, transport characteristics and the time-dependent change in transport parameter. The role of microstructure in terms of the chemical composition of C-A-S-H and its physical states in the different systems is identified as the critical factor governing the development of microstructure. Chloride resistance was assessed by chloride migration experiments for a period of 4 years. The durability behaviours of the concrete with various binder were generalised using pore network parameters such as formation factor and tortuosity. A sensitivity analysis was used to dissect the contribution of the pore solution dilution and pore connectivity to the change in the pore network parameter.Based on the rise in macroscopic physical characteristics (i.e., formation factor here), a two-fold structure development mechanism to conceptualise microstructural evolution in cement composites is presented. Initially, capillary pore space reduces to a critical size range (i.e., 10–30 nm), which is followed by the densification of the physical state of the microstructure. At the point of densification, the pores become largely disconnected which leads to a dramatic increase in the formation factor. The binding matrix in calcined clay concretes reaches the critical pore size at an early age which leads to early densification of capillary pore space region in comparison to fly ash concretes.

Lithium slag and fly ash-based binder for cemented fine tailings backfill

This research work was an exploration of the feasibility of utilizing a lithium slag (LS) and fly ash (FA)-based binder for cemented fine tailings backfill (CFTB). Extensive experiments were conducted with different combinations of LS and ordinary Portland cement (OPC), along with FA as an additive. The unconfined compressive strength (UCS), micromorphology and slump values were analyzed. The results showed that (i) the LS and FA had a significant influence on the strength of binders. The OPC-LS-FA ratio of 2:1:1 appeared to be optimal with the highest strength and was referred as the LS and FA-based binder (LFB). (ii) The LFB significantly improved the UCS of the CFTB. The UCS values of CFTB specimens curing for 7,28 and 56 days reached 0.95 MPa,2.28 MPa and 3.37 MPa, respectively, with a 10 wt% content of LFB. The strength satisfied the strength requirement of backfill for supporting the surrounding rock of stopes in the Yinshan lead-zinc mine (0.8 MPa, 2.0 MPa, 3.0 MPa). (iii) The pore-filling effect of the secondary hydration products, which was mainly produced by LFB, played a significant role in the early stage (<7 days), while the pozzolanic activity worked mostly in the mid-long period (>28 days). (iv) The LFB reduced the slump value of CFTB slurry by 2.6%–9.4% compared with OPC when the mass concentration increased from 58% to 64%, which was acceptable to satisfy the requirements of better fluidity and less transportation resistance in the Yinshan lead-zinc mine. Therefore, the LFB could be utilized as an alternative cementitious material for CFTB, which also provides a safe and economical approach to recycle LS and FA in an underground mine.

Insights into material design, extrusion rheology, and properties of 3D-printable alkali-activated fly ash-based binders

Material design of alkali activated fly ash-based binders for extrusion-based 3D printing, the rheological responses that are influential in ensuring printability, and the properties of such binders are discussed in this paper. Fly ash is supplemented with fine limestone, slag, or portland cement to provide adequate microstructural packing required for printability. The alkaline activators help reduce the yield stress and enhance the cohesiveness of the mixtures. Based on the measured shear yield stress at different times and concurrent printing of a filament, the printability window and yield stress bounds for printability, applicable for the chosen printing parameters, are established. This approach could be used for mixture qualification for extrusion-based printing. The Benbow-Bridgwater model is implemented on extrusion rheology results of pastes to determine the extrusion yield stress and wall slip shear stress, which are useful process-related parameters. It is shown that these parameters can also be related to shear and extensional rheological properties of alkali-activated pastes, thus ensuring a much-needed link between parameters related to material design and the process of extrusion. Mechanical properties and pore structure similar to those of conventionally cast mixtures are achieved.

Use of alkali activated high-calcium fly ash binder for kaolin clay soil stabilisation: Physicochemical evolution

This study addresses the use of alkali activated high-calcium fly ash-based binder to improve engineering characteristics of soft clay-rich soils as an alternative to common stabilisers. The physico-chemical reaction sequence has been investigated by considering the binder alone and the binder mixed with kaolin. An insight into the reactivity evidenced that calcium-containing phases derived from high-calcium fly ash represent the reactive phases and, hence, pozzolanic activity is the dominant process. New compounds are formed, thenardite Na2SO4 and an amorphous silicate consisting of chains combined with calcium probably incorporating three-dimensional four-fold aluminium environments.

Mechanical and microstructural characterization of alkali sulfate activated high volume fly ash binders

This paper presents a detailed characterization of cementitious blends containing high volumes of fly ash, activated using pH-neutral alkali sulfates. It is shown that this methodology, while resulting in a clinker factor reduction of 70%, provides requisite early-age strengths while compromising the 28-day strengths by only 30–40% as compared to plain OPC mixtures. The early age heat release for blends containing Class F fly ash is reduced by about 50% as compared to the straight OPC mixture. The overall pore volume increases with sulfate addition for the Class C fly ash based binder while it decreases when Class F fly ash is used, indicating the beneficial effect of the sulfate activation process in conjunction with a low calcium fly ash. The differences in reaction product constitution are brought out using thermal analysis and FTIR spectroscopy. 29Si NMR spectroscopy coupled with Gaussian spectral deconvolution on Class F fly ash-OPC blends provides valuable information on the changes in Qn(mAl) structures with addition of sodium sulfate, indicating the changes in the reaction products. From a durability perspective, Class F fly ash-based binders are found to be less susceptible to external or internal forms of sulfate attack as compared to plain OPC or the corresponding unactivated mixtures.

Critical evaluation of strength prediction methods for alkali-activated fly ash

The literature contains many proposed methods for proportioning alkali-activated binders for maximum compressive strength. Many of these methods have been developed using metakaolin, which is a relatively pure aluminosilicate powder. In recent years, fly ash has become a more common aluminosilicate source for alkali activation. However, fly ash is a more complex material than metakaolin, and activated fly ash may not follow the same trends as activated metakaolin. In this study, literature-recommended strength prediction methods for alkali-activated binders, using metakaolin and fly ash, are reviewed and compared with the compressive strengths measured for eight Class F fly ash-based binders made by activation with sodium hydroxide solution. Of the eight fly ash binders in the study, six had correct performance predictions considering SiO2/Al2O3 and Na2O/Al2O3 optimal ratios developed for metakaolin. A published empirical equation developed to predict alkali-activated fly ash concrete strength correctly predicted relative strengths for six of the eight fly ash binders. Modifier element content is another possible indicator of reactivity, and the fly ashes in this study generally showed that fly ashes with high contents of Ca2+, Mg2+, Na+, and K+ were likely to produce strong binders, although the correlation shown here was not as strong as that shown in prior studies. This work demonstrates that, while the proposed prediction methods are generally adequate, they do not cover all fly ashes and more work is needed improve prediction methods and account for the behavior of outliers.

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.

Gel nanostructure in alkali-activated binders based on slag and fly ash, and effects of accelerated carbonation

Binders formed through alkali-activation of slags and fly ashes, including ‘fly ash geopolymers’, provide appealing properties as binders for low-emissions concrete production. However, the changes in pH and pore solution chemistry induced during accelerated carbonation testing provide unrealistically low predictions of in-service carbonation resistance. The aluminosilicate gel remaining in an alkali-activated slag system after accelerated carbonation is highly polymerised, consistent with a decalcification mechanism, while fly ash-based binders mainly carbonate through precipitation of alkali salts (bicarbonates at elevated CO2 concentrations, or carbonates under natural exposure) from the pore solution, with little change in the binder gel identifiable by nuclear magnetic resonance spectroscopy. In activated fly ash/slag blends, two distinct gels (C–A–S–H and N–A–S–H) are formed; under accelerated carbonation, the N–A–S–H gel behaves comparably to fly ash-based systems, while the C–A–S–H gel is decalcified similarly to alkali-activated slag. This provides new scope for durability optimisation, and for developing appropriate testing methodologies.

Pelletizing steel mill desulfurization slag

Hot metal desulfurization slag is a high-metallic iron content slag produced at a typical steelmaking facility and is currently considered waste. Each year, 50,000 tons of this slag is produced at one particular steelmaking plant. This material can be beneficiated so that it can be utilized as blast furnace feed, but it is necessary to first develop a process for agglomerating it into pellets. A beneficiated slag concentrate was both conventional bentonite binder and a new fly ash-based binder (FBB) that was formulated by the investigators. The FBB used a high-carbon fly ash that is currently landfilled as waste. In addition to fly ash, FBB contained calcium hydroxide as an activator that also acts as flux known to improve the pellet's reducibility characteristics. The desulfurization slag concentrate pellets bonded with the FBB had superior dry compressive strengths compared to the bentonite-bonded pellets.

Can fly-ash extend bentonite binder for iron ore agglomeration?

There is great incentive to reduce bentonite use in iron ore pelletization by improving its effectiveness. In order to make bentonite more effective, it is necessary to understand the actual binding mechanisms so that they can be properly taken advantage of. Bentonite use could also be reduced by replacing bentonite with even lower-cost binders, such as high-carbon fly-ash based binder (FBB). While FBBs can be used alone as binders, it was considered possible that mixtures of FBB and bentonite could exhibit superior binding properties. In this study, it was found that bentonite bonds by a physical mechanism, while FBB bonds by a chemical mechanism. These mechanisms were determined to be incompatible. Mixtures of the two binders resulted in reduced dry magnetite concentrate pellet compressive strengths below the industrially acceptable value of 22 N (5 lbf). Activators and accelerators, which were necessary components of the FBB, deactivated the bentonite. The compatibilities and mechanisms of the two binders are explained in this paper. The classical theory of the binding mechanism of bentonite binder is challenged by the bentonite fiber mechanism that was recently identified by the authors.