Hydration Of Fly Ash Research Articles

The effect of graphene oxide-fly ash hybridisation on the hydration, microstructure and mechanical characteristics of cement concrete

Carbon-based nanomaterial graphene oxide (GO) can improve cement composites strength and toughness but reduce fluidity. Fly ash is the most common cement substitute for improved fluidity and durability, but early strength growth may be slower. The effects of GO and fly ash on concrete hydration, morphology and mechanical characteristics should be further investigated. GO addition to concrete at 0.025% and 0.05% was investigated by partially replacing cement with fly ash at 10% and 20% by weight. GO addition reduced the concrete fluidity, but the substitution of fly ash increased the fluidity. Moreover, GO compensated for the delayed strength growth of fly ash-based concrete at early-age. The strength properties of GO and fly ash-based concrete at 7 and 28 days are greater than control concrete. Hydration, morphology and elemental composition of concrete were characterised using TGDTA, SEM and EDX. The results demonstrated that GO accelerated cement hydration at early age. Furthermore, GO influenced the total hydration of concrete by expediting secondary hydration of fly ash at later age. Similarly, the morphology of concrete matrix was improved by interweaving hydrated products. This study suggests that using GO and fly ash to produce concrete is an effective process for achieving high performance.

Mechanical Properties and Neural Network Prediction of Cement Fly Ash-Enhanced Roadbed Soil in Seasonal Frozen Zones Under Short-Term Curing

The freezing and thawing of roadbed soils in seasonal frozen zones can cause uneven settlement and other road problems, which puts road operation at risk. This paper focuses on the rapid construction of expressways and analyzes the effects of using fly ash and cement as modifiers on the physical properties and chemical composition of subgrade soil. The study found that cement admixtures can improve soil mechanical attributes and frost resistance, while also increasing the degree of fly ash hydration. And the freeze-thaw cycle process can enhance the mechanical characteristics of soils with higher cement admixture content. This is due to the development of hydrates in cement fly ash-enhanced soils during freeze?thaw cycles, resulting in a denser interior structure. The use of neural network prediction analysis showed that using ANN4-10-3 to forecast soil mechanical property parameters can produce superior results. Therefore, it is recommended to use cement fly ash-enhanced soil for roadbed construction in seasonal freezing areas, and neural network can be used to predict soil mechanical parameters.

Inner superhydrophobic materials based on waste fly ash: Microstructural morphology of microetching effects

Superhydrophobic technology in construction can effectively improve the barrier ability of the substrate to harmful water-based erosion media, among which the coating scheme is the most practical choice at present, but it may easily fail due to mechanical damage. Therefore, the current research focus has shifted to the development of internal superhydrophobic materials (ISMs) that maintain stable hydrophobicity even under harsh conditions of use and do not rely on expensive fluor-based surface modifications. In this study, the topological structure of waste denitrification fly ash (FA) was constructed using an innovative method, by precisely controlling the etching medium concentration and the form of FA, a one-step process was developed to create ISM with excellent matrix strength and strong waterproof properties. In this study, the alkaline environment generated by FA hydration was used to induce the silica-hydrogen bond dehydrogenation of polymethylhydrosiloxane (PMHS) and the free ammonium ion deamination to toughen the film; PMHS and liquid phase are used to produce the oil-water phase, and the oil film adheres to the surface of the generated NASH and sodium silicate crystals; The porous inner surface of ISM is formed with hydrogen- and ammonia-induced rough structure; The synthesized ISM retains superhydrophobicity and less compressive strength reduction value, and its superhydrophobicity even at damaged or friction interfaces. The material preparation method provides a good foundation and innovative design concept for the seepage prevention and maintenance of concrete structures, the construction of aerated FA bricks, the preparation of foam insulation materials, and the batch utilization of waste FA.

Novel agricultural waste based hardening accelerator for early strength development of fly ash-based concrete

Rapid urbanization is leading to a rise in demand for precast elements to reduce the time and cost of construction. However, precast elements consume the time to remove formwork during production depends on the early strength development of fresh concrete. On the other hand, thermal power plants produce a significant amount of fly ash, which also cause environmental issues. One solution to this problem is to use fly ash in cement and concrete to enhance their pozzolanic properties and address environmental concerns. However, using fly ash can delay the early strength development of concrete resulting deacceleration in the production cycle of precast elements. To speed up the process, some may opt for chemical-based hardening accelerator and high temperature curing. However, this may negatively impact the hydration kinetics of the concrete, pose environmental risks, and compromise the reinforced concrete’s durability.To overcome such limitations, impurity free C-S-H seed based hardening accelerator was synthesized from agricultural waste product namely, bagasse ash using novel chemical synthesis process which could effectively accelerates the early strength development of concrete without altering the conventional hydration process. The proposed synthesis process provides a platform for the reaction between the target calcium and silicate ions simultaneously eliminating the harmful elements (like chloride etc.) responsible for degradation of health of reinforced concrete structure.In order to study the effectiveness of a synthesized C-S-H seed, a blended ordinary Portland cement (OPC) was prepared by replacing 10%, 15%, and 20% of the cement weight with fly ash. Various amounts of the C-S-H seed were then added to the blend. Presence of developed C-S-H seed effectively reduced the setting time and eliminated the induction period during hydration of fly ash blended cement. Presence of additional seeds effectively reduces the critical free energy of nucleation which in turn accelerates the precipitation of hydration product responsible for the early strength development of fly ash-based concrete. Based on the setting time and heat of hydration analysis, it has been observed that the presence of C-S-H seed has the potential to accelerate the early strength gain process of fly ash-based concrete.

Effect of new modified materials on the microscopic pore structure and hydration characteristics of sealing materials in coal seam boreholes

To improve the efficiency of coal mine gas extraction, it is necessary to optimize the performance of sealing materials. In this paper, fly ash, nano-silica, and triethanolamine was selected to optimize the pore structure of the sealing materials. The qualitative and quantitative analysis of the seal material was performed by low-temperature liquid nitrogen adsorption experiments, scanning electron microscopy, and combined with fractal theory. The results show that the pores of common sealing materials are mainly mesoporous, accounting for 52.19%, and the volume of micropores is the smallest, accounting for 9.88%. The single doping of fly ash made the pore volume of the material decrease by 9.5%, and the percentage of mesopores increased by 1.3%, but the percentage of micropores decreased by 0.9%. Co-doping with nano-silica and fly ash increased the microporous percentage of the material by 0.77% and the peak stress by 17.43%. The synergistic effect of fly ash, nano-silica, and triethanolamine was most effective in optimizing the pore structure of the sealing material. The pore volume was reduced to 0.08198 cm3/g, and the fractal dimensions D1 and D2 were increased by 2.88% and 2.48%, respectively. The microscopic morphology of the material was also flatter and denser, and the hydration of fly ash was also more complete.

Experimental and numerical investigation on the long-term performance of engineered cementitious composites (ECC) with high-volume fly ash and domestic polyvinyl alcohol (PVA) fibers

High-volume fly ash (FA) has been widely adopted to improve the mechanical properties and material greenness of engineered cementitious composites (ECC). However, the secondary hydration of fly ash leads to delayed strength gain over the long term, which may cause the composites to invalidate the long-term strain hardening performance. In this study, multiple-scale experiments conducted on ECC with high-volume FA (FA/cement = 4 by weight) and domestic polyvinyl alcohol (PVA) fibers at curing ages of 28, 90, 180, and 365 days showed that the fiber/matrix interfacial bonding, matrix compressive strength, and fracture toughness gradually increased with curing time and plateaued at a certain value after 180 days, while the tensile strain capacity continued to decrease with age. Based on the micromechanical parameters tested at different ages, a physical model was developed to simulate and reasonably predict the tensile performance of ECC with time. Design guidelines were proposed based on the model to enhance the robustness of the ECC's long-term performance.

Behaviour of structural engineered cementitious composites under dynamic tensile loading and elevated temperatures

This paper presents a systematic study on dynamic splitting tensile properties of polyvinyl alcohol (PVA) fibre reinforced engineered cementitious composites (ECC) after exposure to elevated temperatures up to 800 °C under strain rates of 4.34 to 10.95 s−1. A series of tests were conducted to characterise the weight loss, elastic modulus, uniaxial tensile properties, static and dynamic splitting tensile behaviour, and pore pressure, as well as the chemical and microstructural evolution of ECC. Results indicate that the dynamic splitting tensile properties of ECC went up with the increasing strain rate regardless of heated temperatures due to strain rate effect. The dynamic splitting strength, dynamic increase factor and energy dissipation of ECC reached the maximum values at 105 °C, which can be ascribed to the enhanced fibre-matrix bond and further hydration of fly ash. However, the dynamic tensile properties of ECC reduced at over 250 °C due to the melting of PVA fibres, thermally induced microcracks and dehydration of hydration products. At 800 °C, the dynamic splitting tensile strength and dissipated energy of ECC dropped by 81.0–83.2% and 88.4–89.1% over the measured strain rate range, relative to that at 20 °C. In addition, the main role for resisting dynamic loading was gradually transferred from fibre debonding-elongating action to crack propagation at over 400 °C. Accordingly, the failure patterns of ECC changed from tensile failure with the main central crack to coupled tensile-shear failure with multiple cracks.

Shrinkage of blended cement concrete with fly ash or limestone calcined clay

This study investigates the shrinkage of two sustainable aluminosilicate blends with fly ash or limestone-calcined clay (LC3). Paste and concrete were prepared using these SCMs for the highest possible replacement of binder without compromising the strength. The chemical and autogenous shrinkage were assessed for paste samples and further investigation were conducted on hydration by thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). Opting for an engineering approach, comparison among different segments of shrinkage i.e., autogenous, drying and total shrinkage of concrete having a specific compressive strength were considered. The initial investigation on paste samples highlighted the dissimilarities in shrinkage and hydration of fly ash and calcined clay. LC3 hydrated faster compared to fly ash leading to greater autogenous shrinkage. The high autogenous shrinkage in the LC3 blend was compensated by a low drying shrinkage for a specific compressive strength. Considering the replacement level of cement, shrinkage, and ecological impact, LC3 proved to be a more sustainable and eco-friendly concrete compared to fly ash.

Orthogonal analysis and mechanism of compressive strength and microstructure of the metakaolin-fly ash geopolymer

It is essential to give easier preparation and maintenance methods for geopolymers to facilitate practical application of metakaolin-fly ash based geopolymers (MK-FA geopolymers) in the engineering field. In this article, 16 sets of samples were prepared by orthogonal experiments design (OED) to quantify the order of contribution of three hypothetical influence factors in the growth of compressive strength. Test results revealed that the promotion of the calcium phase was significant. The calcium phase and fly ash acted synergistically during the early stage of strength development, with the former accelerating the hydration process of the precursor materials and the latter providing additional silicon and aluminum phases, which facilitated the formation of the polymeric skeleton and the densification of the matrix. In the later stages of the experiment, the promotion of gel particle formation by calcium effectively mitigated the disadvantage of reduced average reactivity due to the incomplete hydration of fly ash. Moreover, the highest compressive strength was obtained for the MK-FA geopolymer prepared according to the theoretical optimum ratio derived from the orthogonal analysis, which demonstrated the correctness and feasibility of the OED.

Investigations on the Performance of Shotcrete Using Artificial Lightweight Shale Ceramsite as Coarse Aggregate.

In this study, a type of artificial lightweight shale ceramsite (ALSC) was used as the coarse lightweight aggregate for shotcrete (LAS), of which the mechanical properties, chloride penetration ion resistance, and rebound behavior were investigated. Based on the experimental results on compressive, tensile, and bond strength, LAS meet the strength requirements, and the replacement rate of fly ash (FA) and silica fume (SF) are suggested to be kept at ~15% and 10%, respectively, to result in the best mechanical properties of LAS. Adding FA and SF to the mixture significantly improved the chloride ion penetration resistance (CPR) of LAS because of morphology effects and secondary hydration of FA and SF that lead to a denser microstructure of the mixture. The electric flux and chloride ion migration coefficient (DRCM) of LAS decreased by 56% and 67%, respectively, with FA increasing from 0 to 10%. The electric flux and DRCM further decreased by 71% (153C) and 66% (3.24 m2/s), respectively, with FA increasing from 10 to 20%. As 5–10% SF was further added, the electric flux and DRCM of LAS decreased to extremely low levels; for instance, with FA = 10% and SF = 10%, DRCM = 1.61 m2/s, and the electric flux was too small and could be ignored. The contact stresses between aggregate and shotcrete mixtures were measured to investigate the rebound trend of ALSC in shotcrete. According to the analyses of the theoretical model of the rebound behavior of aggregate in shotcrete proposed by Armelin and Banthia, because of the reduced contact stresses between ALSC and mortar and the smaller density of LAS compared with normal-weight aggregate, the rebound rate of ALSC was about half of that of normal-weight aggregate in the shooting process of the shotcrete.

Research on engineering properties of cement mortar adding stainless steel reduction slag and pozzolanic materials

How to save energy and effectively use resources has become the goal of all countries. In Taiwan, except for blast furnace slag that can partially replace cement consumption and achieve the purpose of resource utilization, other steelmaking slags such as desulfurization slag, electric arc furnace (EAF) slag, etc., are in the research and promotion stage. This study added fixed proportions of fly ash, ground-granulated blast-furnace slag (GGBFS), and different proportions of stabilized stainless steel reduced slag (SSRS) to replace a portion of cement to make environmentally friendly cement mortar. We explore the engineering properties of this mortar with different water-binder ratios (W/B) and different ages and evaluate the feasibility of using Portland materials and SSRS to replace a portion of cement. Experiment results show that using these industrial byproducts to replace cement can improve workability. Under the appropriate amount of 20% GGBFS and 10% fly ash to replace cement, a limited amount of 5–10% SSRS to replace cement can ensure and strengthen the engineering and physical properties of the mortar. Before curing for 28 days, with the increase of SSRS replacement amount, the mortar exhibits more internal pores, which indirectly leads to poor engineering performance. After 28 days of age, due to the later effect of GGBFS and fly ash hydration, the later compactness becomes better and meets the engineering performance of the mortar application specification.

Microstructure and Chloride Diffusion Properties of Hardened Fly Ash Cement Paste with Three-dimensional Graphene

In this paper, the influence of three-dimensional graphene (3DG) on the microstructure and chloride diffusion properties of fly ash cement paste was investigated. 3DG accounting for 0.0–0.2% of cement mass was evenly dispersed in hardened cement paste containing fly ash by using ultrasonic and polycarboxylate superplasticizer. The microstructure of the cement paste was characterized by mercury intrusion test (MIP), X-ray diffraction (XRD), and scanning electron microscope (SEM). The results indicate that an appropriate amount of 3DG accelerates the secondary hydration of fly ash and provides a platform for the growth of cement hydration crystals. Moreover, the hydrated crystal fills the pores, reduces the harmful porosity, and refines the pore structure. The enhanced microstructure significantly improved mechanical properties and chloride diffusion resistance of hardened fly ash cement. With the addition of 0.1% 3DG, the compressive strength increased by 31.33%, and the chloride diffusion coefficient was reduced by 49.44%. The findings are beneficial to promote the generalized application of 3DG in cement in the marine environment.

Study on the Optimization of Filling Ratio and Strength Variation Characteristics of Cemented Backfills Containing Fly Ash

The fly ash for underground filling can effectively utilize solid waste, improve the strength of the backfill, and reduce the cost, thus creating good social and economic benefits. Relying on the filling requirements of a gold mine in Jilin, this paper carried out the filling ratio experiments containing fly ash and analyzed the reasons for the variation of the backfill strength based on the hydration characteristics of cement and fly ash and scanning electron microscope. The results show that fly ash has an overall effect on the strength of the backfill, and the strength development is mainly concentrated in the period of 28–56 d; when the filling slurry contains tailings, the excessive amount of fly ash is likely to cause a large number of fine particles to obstruct the hydration of cementitious materials; when the concentration of the filling slurry is 74%, the cement content is 5%, the mass ratio of waste rock-tailings-fly ash is 6:2:3, and the CaO content is 6:3, the strength of the backfill is significantly higher than the current strength of the backfill of the mine, and the cost can be saved by RMB 0.56 per cubic meter; the strength characteristics of the backfill mainly depend on the pore structure; when the filling slurry is better matched, the cement and fly ash hydration generates a large number of C-S-H gel particles, which wraps the aggregate to form a dense structure with less pore structure, and the strength of the backfill increases; the strength variation process of backfill containing cement and fly ash is divided into cement hydration period, fly ash infiltration period, and slurry hardening period. To enhance the strength of the backfill, it is necessary to determine the appropriate cementitious material ratio to maximize the excitation of fly ash hydration during the fly ash infiltration period, and the hydration produces a gel structure with an excellent aggregate ratio. In addition, the slurry hardening reduces the porosity of the backfill. The results can provide basic data and theoretical guidance for further promotion and application of fly ash in mine filling.

Experimental Research on Compressive and Shrinkage Properties of ECC Containing Ceramic Wastes Under Different Curing Conditions

Engineered cementitious composites (ECCs) suffer from high shrinkage and low early strength due to large dosage of cementitious materials and slow hydration of fly ash. This study aims to improve compressive properties and reduce drying shrinkage of ECC using ceramic wastes and hydrothermal curing. Experimental results have indicated that ceramic polishing powder (CPP) and recycled ceramic sand (RCS) exert opposite effect on the compressive strength of ECC. Hydrothermal-cured ECC enhances elasticity modulus and compressive strength and reduces later drying shrinkage as compared with that under standard curing. A CPP dosage of 35% and a hydrothermal curing regime with a temperature of 70°C and age of 7 days are recommended for the engineering application of ECC.

Performance of Fly Ash blended Crushed Sand Concrete

Achieving low permeability in concrete property is an important requirement for safety-related nuclear reprocessing plant structures. Towards this need, in view of the scarcity of river sand, M50 grade crushed sand (CS) aggregate concrete with controlled fines content and fly ash has been designed. This paper discusses workability, mechanical strength, chloride permeability and temperature rise due to heat of hydration aspects of fly ash blended crushed sand concrete used in five major plant buildings of a nuclear fuel reprocessing facility. Fly ash blended CS concrete has performed well and exhibited rapid chloride permeability values less than 750 coulombs. Statistical evaluation of 200 Nos. of fly ash blended CS concrete samples tested for compressive strength showed lesser standard deviation than that of same grade RS concrete samples. Temperature rise due to heat of hydration of fly ash replaced CS concretes measured for mass concrete is comparable to that of RS concrete. This paper reports the experience of the authors on the performance of low permeable fly ash blended crushed stone concrete during the construction of safety-related nuclear reprocessing plant structures and highlights how it exceeded the properties set viz. workability, compressive strength, modulus of elasticity, tensile strength and chloride permeation resistance.

Polyacrylamide modified geopolymer cement slurry: A potential barrier for channel behind casing

The present work is based on development of novel cement for water shut off through the application of Super Absorbent Polymer (Polyacrylamide) and Geopolymer (Fly ash). The hydration of Fly ash and polyacrylamide leads to plugging of channels in the cement sheath. The plugging characteristics of PAA was studied in saline water and distilled water. Further, decrease in contact angle with time complements the hydration of cement. The SEM analysis suggest the homogeneous dispersion FA and PAA in the cement sheath imparting the cement integrity. Cement slurry with 30% FA and 1% PAA demonstrates excellent compressive strength along with slurry displacement characteristics.

Characterization of fly ash-cement paste and molecular structure in the presence of seawater by 27Al and 29Si MAS NMR spectroscopy

Cement-based materials commonly suffer attacking from detrimental ions in marine environment and the degradation mechanism for material is complicated. To study the effect of curing stage on degradation process, the microstructure and molecular structure of fly ash and Portland cement paste curing at 1 day, 7 day, 14 day and 28 day with w/c ratio of 0.35 in seawater solutions for 90 days are investigated by 27Al and 29Si magic-angle spinning (MAS) NMR spectroscopy, XRD and SEM tests. It reveals that more Friedal salt and Aft hydration phase are formed when the samples curing at early stage (1 day) are immersed in the sea water due to unhydrated and porous microstructure subject to sulfate and chloride ions. Based on the 29Si spectrum analysis, the increasing of mean silicate chain length of C-S-H and the degrading for AlO4/SiO4 ratio demonstrate that detrimental ions in seawater lead to the de-aluminization and polymerization for the cement hydrate. Furthermore, 27Al NMR test observes phase transformation from Al[4] in C-S-H gel to Al[6] Aft phase for samples in marine environment. Prolonging the curing age and incorporating fly ash in cement paste can significantly help resist the degradation for aluminate-silicate chains in C-S-H gel and aluminate phase transformation. More importantly, the hydration degrees for cement and fly ash computed separately illustrate that the aggressive ions in seawater act as activating role on fly ash, contributing to the secondary hydration of fly ash–cement paste. The activation effect is more pronounced for samples suffer to marine environment especially in the very early curing age, which significantly help resist aggressive ions as the cement paste in low hydration degree. Hopefully, this study can provide valuable insights on design of durable and sustainable marine concrete.

Synergistic effects of nano-silica and fly ash on properties of cement-based composites

The combined utilization of mineral admixtures and nano-materials in cement-based composites is an inevitable trend based on the purpose of improving the durability. However, synergistic effect of mineral admixtures with nano-materials and their formation mechanism remains poorly understand. Therein, synergistic effect of nano-silica and fly ash on mechanical property, hydration process, non-evaporative water content, pozzolanic activity were investigated, the pore size distribution of hardened paste were tested and synergetic effect formation mechanism was analyzed. The results indicate that nano-silica improved compressive strength in the early stage and high content of fly ash with moderate fineness contributed to the strength growth in the later stage. In addition, nano-silica promotes cement hydration in early stage and provides favorable hydration conditions for later hydration of fly ash. Synergistic effect of nano-silica and fly ash increases content of non-evaporative water and improves pozzolanic activity. Moreover, synergistic effect of nano-silica and fly ash significantly optimizes pore structure in hardened paste. The suggested synergistic effect mechanism is attributed to the optimization of hydration process, mixing ratio matching and fineness matching among cement, nano-silica and fly ash.

Effects of colloidal nanosilica/polycarboxylate ether superplasticizer nanocomposite and graphene oxide on properties of fly ash blended cement

The ternary effects of nanosilica/polycarboxylate ether superplasticizer nanocomposite (NS@PCE), graphene oxide (GO) and fly ash (FA) with different dosages on workability, fly ash hydration, water sorptivity, compressive strength and microstructure development of cement paste were investigated. The results revealed that the NS@PCE addition improves the workability of cement paste significantly. The single addition of NS@PCE or GO have favorable effect on hydration of FA particle, whereas the hybrid of both in cement cause a bit loss of the FA hydration degree (FA HD). 1% of NS@PCE addition and 0.01% of GO dosages in FA-blended cement are sufficient for compensate its early strength loss caused by the FA incorporation. Microstructure investigation verified the formation of new polyhedron-like crystals, with an average length of 1.25 µm and Ca/Si ratio of 12.99, in cement of 28 days.

Evolution of aluminate hydrate phases in fly ash-cement system under the sulfate conditions

Abstract The aluminate hydrate phases play an important role in the sulfate attack of concrete. In this paper, the aluminum hydrate phases evolution of fly ash blended cement paste powders immersed in different sodium sulfate solution is studied. In order to accelerate the reaction, the fly ash blended cement paste powders are immersed in different sulfate solution at ambient temperature up to 28 d. The results show that the hydration of fly ash and the distribution of aluminum hydrates phases in the hydration products of fly ash-cement blends are significantly influenced by the sulfate. The addition of fly ash into the cements introduces the reactive Al3+ into the systems, leading to an increase of Al(IV)/Si and mean chain length of C-A-S-H. Furthermore, although the pozzolanic reaction of fly ash consumes the calcium hydroxide, where the calcium hydroxide is required for the formation of ettringite, the addition of fly ash is favorable for the formation of ettringite.