High Curing Temperature Research Articles

Synthesis and application of a halogen-free epoxy resin flame retardant curing agent

In this study, the intermediate of SPDPC flame retardant curing agent was successfully synthesized through the reaction between phosphorus oxychloride and pentaerythritol under the catalysis of 4-dimethylaminopyridine. Using 1,2-propylenediamine and the prepared SPDPC, a novel phosphorus and nitrogen flame retardant curing agent named poly1,2-propylenediamine pentaerythritol diphosphate (PDS) was successfully synthesized. The target product was subjected to characterization using infrared spectroscopy, mass spectrometry, hydrogen NMR spectroscopy and thermogravimetric analysis. The molecular structure of the product was determined and its decomposition temperature curve obtained, leading to the conclusion that it could be cured at high temperatures. DDS was selected as the curing agent for epoxy E-51, the specific curing conditions were obtained by DSC test. Subsequently, E-51, DDS and PDS were mixed for high temperature curing resulting in obtaining test spline of PDS-E-51 flame retardant composite material. Then, the flame retardancy and mechanical properties of the spline were tested. It was observed that pure EP cured by DDS had an LOI of only 19.5%, indicating its flammability. However, upon addition of PDS as a flame retardant, the LOI significantly increased with 20 parts resulting in an LOI of 29.7%. The addition of 25 parts results in an increase in limiting oxygen index to 30.3%, while the tensile strength and impact strength are measured at 38.27 MPa and 5.087 kJ/m2 respectively. The CCT test shows that the addition of PDS can significantly reduce the HHR and THR of the system. CCT digital photos show that the addition of PDS can make the combustion residue of the system expand obviously, showing a good expansion and flame retardant effect. TG-FTIR gas phase infrared absorption indicates that the addition of PDS can reduce the concentration of combustible gas in the combustion process. Test results indicate that the mechanical properties of PDS-E-51 flame retardant composites experience a certain degree of decline, but with increasing amounts of PDS curing agent added, their flame retardancy is significantly enhanced.

Early warping deformation of high-speed railway CRTS III track slabs

Excessive flatness deviation issues in the track slabs of railway tracks make it difficult to achieve precise adjustments for both the track slabs and rails. This study aimed to analyze the deviation in flatness of the high-speed railway CRTS III track slabs. Experimental measurements were conducted on the warping deformation of the track slabs before water curing during the maintenance period. Six improved designs were implemented for the steaming process and structural construction. A numerical model was established to predict the warping deformation of the track slab before water curing, considering the temperature and humidity fields as driving forces. Using the sequential coupling method, the warping deformation of CRTS III track slab was calculated under the combined action of temperature field, shrinkage strain field, and prestressed tendons. Based on the results, it is concluded that: (1) High-temperature steam curing of the track slab increases the effective maintenance time and accelerates its strengthening rate. (2) Early shrinkage of the track slab is the primary factor causing warping deformation. (3) During the steam curing period, the warping deformation of the track slab can be controlled economically and effectively by employing secondary water spraying and presetting a reverse arch of 0.5 mm–0.8 mm.

Hydration and Fractal Analysis on Low-Heat Portland Cement Pastes Using Thermodynamics-Based Methods

Low-heat Portland (LHP) cement is a kind of high-belite cement, which has the characteristic of low hydration heat. Currently, it is extensively used in the temperature control of mass concrete. Based on the thermodynamic database of OPC-based materials, the thermodynamic software GEM-Selektor (noted as GEMS) is used for simulating the hydration products of the LHP cement paste. Then, according to the GEMS thermodynamic simulation results, MATLAB is used to visualize the initial and ultimate stages of LHP cement pastes; the effects of curing temperature and water to cement (w/c) ratio on hydration products are addressed; and the porosity, fractal dimension, and tortuosity of different pastes are calculated. It is found that an appropriately high curing temperature is important for reducing porosity, especially in the early hydration stage. Hydration time also has a significant impact on the hydration of LHP cement paste; long hydration time may reduce the impact of temperature on hydration products. The w/c ratio is another important consideration regarding the hydration degree and porosity of LHP paste, and under different curing temperatures, hydration times, and w/c ratios, the porosity varies from 5.91–32.91%. The fractal dimension of this work agrees with the previous findings. From tortuosity analysis, it can be concluded that the high curing temperature may cause significant tortuosity, further affecting the effective diffusivity of LHP cement paste. For cement pastes with low w/c ratio, this high curing temperature effect is mainly reflected in the early hydration stage, for ones with high w/c ratio, it is in turn evident under long-term curing.

Cleaner production of the precast concrete industry: comparative life cycle analysis of concrete using recycled aggregates from crushed precast rejects

The in-plant use of recycled aggregate concrete derived from precast rejects (termed PRAC herein) can promote a circular economy in the precast industry. However, the environmental implications associated with this practice remain poorly understood. A refined life cycle assessment (LCA) model was therefore developed to highlight the environmental benefits of using PRAC compared to natural aggregate concrete and conventional recycled aggregate concrete. Some key factors influencing PRAC’s environmental performance were also examined. The results indicate that PRAC exhibits around 15% lower energy consumption and carbon dioxide emissions compared to other recycled materials. This reduction is attributed to the favourable quality of PRAC and the elimination of long-distance transport. However, emissions allocation and raw material prices play significant roles in determining the overall environmental impact of PRAC. The equivalent mortar volume mixing method is best suited for PRAC production, as it saves energy, reduces emissions, and maintains similar mechanical properties. Nevertheless, the high-temperature curing, which is often necessary in a precast factory setting, can be energy-intensive and thus diminishes the eco-friendliness of PRAC. Overall, the findings support the use of precast rework as recycled aggregate for cleaner production in the precast industry.

The Effect Of Curing Types On Compressive Strength Of High Performance Concrete

The present investigation considers the effect of curing temperatures (30, 40, and 50˚C) and curing compound method on compressive strength development of high performance concrete, and compares the results with concrete cured at standard conditions and curing temperature (21˚C). The experimental results showed that at early ages, the rate of strength development at high curing temperature is greater than at lower curing temperature, the maximum increasing percentage in compressive strength is 10.83% at 50C˚ compared with 21C˚ in 7days curing age. However, at later ages, the strength achieved at higher curing temperature has been less, and the maximum percentage of reduction has been 5.70% at curing temperature 50C˚ compared with 21C˚curing temperature in 91 days curing age. Also, the results showed that the specimens which are cured under field condition (using curing compound) have a various strength development rate, and the results indicate 92.11% as minimum field-standard curing strength ratio.

Incorporation of Lignin in Bio-Based Resins for Potential Application in Fiber–Polymer Composites

Bio-based resins, obtained from renewable raw materials, are a more sustainable alternative to oil-based resins for fiber-reinforced polymer (FRP) composites. The incorporation of lignin in those resins has the potential to enhance their performance. This paper presents results of an experimental study about the effects of Lignoboost lignin incorporation on a partially bio-based vinyl ester (VE) resin. Two resins were prepared—without (reference) and with lignin addition (4% by weight) to its main chain—and their chemical, thermophysical, and mechanical properties were compared using Fourier transform infrared (FTIR) spectroscopy, gel permeation chromatography (GPC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and tensile and shear tests. Results suggest that the addition of lignin to the base resin resulted in a copolymer of increased heterogeneity and higher molecular weight, incorporating stiff and complex aromatic structures in the polymer chain. While requiring high-temperature curing, the VE–lignin copolymer presented improvements of 27% in tensile strength, 4% in shear strength, and increased glass transition temperature by about 8 °C, thus confirming the potential of this natural biopolymer for FRP composite applications.

Evaluation of Residual Stress in MEMS Micromirror Die Surface Mounting Process and Shock Destructive Reliability Test

Light Detection and Ranging (LiDAR) devices based on Micro-Electro-Mechanical Systems (MEMS) micromirrors have been promising sensors for automatic driving owing to the advantages of MEMS devices. The packaging plays a vital role in the reliability of MEMS micromirror but introduces the residual stress from the surface mounting process. The shock reliability of micromirrors is influenced by residual stress. In this work, the residual stress from the surface mounting process are investigated through resonant frequency tests. The frequency drift can be 718 Hz and -281 Hz under the residual stress of 107.0 MPa and -25.0 MPa respectively. As the difference of CTE rises, the residual stress proportionally increases but decreases with the thickness of the substrate. In addition, a higher curing temperature can introduce a more significant stress level. The dynamic response in shock environment under different residual stress are analyzed. Drop fall experiments verify the theoretical conclusions that the stiffness hardening effect can improve the shock level tolerance. The tensile stress levels of 41.7 MPa and 107.0 MPa are corresponding to 100 g and 200 g respectively. The influences of residual stress on frequency drift and reliability need to be considered seriously and simultaneously during the packaging design for MEMS micromirrors. This work provides a deep insight into commercial applications of MEMS-based LiDAR in autonomous driving.

3D Printed Chondrogenic Functionalized PGS Bioactive Scaffold for Cartilage Regeneration.

Tissue engineering is emerging as a promising approach for cartilage regeneration and repair. Endowing scaffolds with cartilaginous bioactivity to obtain bionic microenvironment and regulating the matching of scaffold degradation and regeneration play a crucial role in cartilage regeneration. Poly(glycerol sebacate) (PGS) is a representative thermosetting bioelastomer known for its elasticity, biodegradability, and biocompatibility and is widely used in tissue engineering. However, the modification and drug loading of the PGS scaffold is still a key challenge due to its high temperature curing conditions and limited reactive groups, which seriously hinders its further functional application. Here, a simple versatile new strategy of super swelling-absorption and cross-linked networks locking is presented to successfully create the 3Dprinted PGS-CS/Gel scaffold for the first time based on FDA-approved PGS, gelatin (Gel) and chondroitin sulfate (CS). The PGS-CS/Gel scaffold exhibits the desirable synergistic properties of well-organized hierarchical structures, excellent elasticity, improved hydrophilicity, and cartilaginous bioactivity, which can promote the adhesion, proliferation, and migration of chondrocytes. Importantly, the rate of cartilage regeneration can be well-matched with degradation of PGS-CS/Gel scaffold, and achieve uniform and mature cartilage tissue without scaffold residual. The bioactive scaffold can successfully repair cartilage in a rabbit trochlear groove defect model indicating a promising prospect of clinical transformation.

Energy saving phase change energy storage thermochromic liquid crystal display

Phase change energy storage microcapsules (PCESM) improve energy utilization by controlling the temperature of the surrounding environment of the phase change material to store and release heat. In this paper, a phase change energy storage thermochromic liquid crystal display (PCES-TC-LCD) is designed and prepared for the first time. The as-prepared PCES-TC-LCD has a stable bilayer structure, where the lower layer is a thermochromic LC layer produced by centrifugal spin coating and high temperature curing, and the upper layer is a polymer-stabilized liquid crystal (PSLC) film with phase change energy storage properties obtained by polymerization-induced phase separation method. The prepared PCES-TC-LCD can realize the functions of triple-field driven and phase change energy storage, which has strong research value. In the performance experiments, the electro-optical properties of PCES-TC-LCD were analyzed and optimized by three sets of experiments on liquid crystal content, polymerization time, and PCESM content. In the film formation study, PCES-TC-LCD based on rigid/flexible substrates were prepared, respectively. The phase change energy storage function of PCES-TC-LCD was performed by differential scanning calorimetry (DSC) and infrared thermography. The devices formed on rigid or flexible substrates with stable phase change energy storage performance, which has great application value by reducing the external energy input, prolonging the thermochromic time and achieving energy saving effects.

Effect of curing temperatures on geopolymerization and heavy metal solidification in alkali-activated zeolite/MSWI fly ash specimens

Admixtures of pozzolanic materials or solid wastes with municipal solid waste incineration fly ash (MSWIFA) can improve heavy metal stabilization/solidification (S/S) properties in alkali-activated MSWIFA specimens. In this work, the compressive strength of MSWIFA admixes with zeolite activated by Na2O·1.7SiO2·nH2O (FZNS) specimens with different curing times (3, 7, 14, and 28 d) and temperatures (20, 50, and 80 ℃) is investigated. The results reveal that a 40 wt% zeolite replacement of MSWIFA in FZNS specimens (FZNS40) not only increases the early compressive strength by 129% but also reduces strength loss during continuous high-temperature (50 or 80 ℃) curing. Moreover, zeolite addition promotes the creation of geopolymers at 28.5°–30.5°in X-ray diffraction (XRD) curves. During 50 or 80 ℃ curing, the NaCl peak disappears and a marginal Friedel's salt peak can be detected at 11.4°. The gel contents are more concentrated on C-S-H in FZNS0 and C-(A)-S-H/N-A-S-H in FZNS40, as demonstrated by differential thermogravimetry (DTG) and magic angle spinning nuclear magnetic resonance (MAS NMR) analysis. With zeolite addition, long-time and high-temperature curing can improve the heavy metal solidification ratio in FZNS specimens. After high-temperature curing, the honeycomb or cluster morphologies of the gel products, visible by scanning electron microscopy (SEM) in the FZNS specimens, reveal high heavy metal stabilization properties. It is easier to form the fibrous C-(A)-S-H/N-A-S-H gel near zeolite owing to the high content of SiO2 and Al2O3. Zeolite modifies the compressive strength, high-temperature stability, and heavy-metal S/S efficiency in FZNS specimens, which promotes the utilization of MSWIFA as an ecologicalresource in the future.

Comparative study on mechanical properties and microstructure development of ultra-high performance concrete incorporating phosphorous slag under different curing regimes

In this study, the mechanical property and microstructure development of UHPC (ultra-high performance concrete) incorporating different PS (phosphorous slag) content is studied under five curing regimes. The microstructure of UHPC is also discussed by TG, XRD, MIP, the SEM-EDS and 29Si NMR. The result indicated that the high strength of UHPC containing PS is obtained by heat curing in a short time (e.g. The compressive strength of PS60 is 135.5 MPa and 134.4 MPa under curing conditions of 90 °C (48 h) and 1 MPa (6 h), respectively). The cement is completely hydrated, and the hydrated product of calcium hydroxide is completely consumed in UHPC that contains a lot of PS under the heating curing (90 °C, 1 MPa-180 °C and 2 MPa-250 °C) by the result of TG combined with XRD. In addition, the hydration products are tobermorite and xonotlite under high temperature and high pressure curing conditions.

UV–thermally dual-curable 1K clearcoat via urethane and radical reactions

Conventional polyurethane-based automotive clearcoat systems have mainly consisted of a polyol and an isocyanate crosslinker, requiring a metal catalyst and high-temperature curing conditions. Herein, we report a UV–thermally dual-curable 1K clearcoat that contains isocyanate crosslinkers blocked by methacrylate-functionalized pyrazole (BIPyA-3) and a photoactive benzophenone containing a polyol (BP–polyol). The blocked isocyanate can form a urethane bond with the polyol after deblocking, and the self-initiation of methacrylate in the blocking agent can participate in the radical reaction. In addition, under UV irradiation, the benzophenone group can generate radicals that can propagate the methacrylate functional group of BIPyA-3 as well as form CC bonds with the polyol, thereby increasing the overall crosslinking density in the clearcoat. Rheological behaviors of the novel clearcoat were investigated by oscillatory rheometry under combined thermal and UV curing conditions. The thermal and surface mechanical properties of the films cured under different conditions—(i) thermal, (ii) thermal–UV, (iii) UV–thermal, and (iv) UV—were compared, and their relationship with the overall crosslinking density was investigated. This novel dual-curable, urethane–methacrylate clearcoat cured via a UV–thermal process can not only contribute to enhancing the surface hardness of clearcoats without modifying their chemistry but also effectively consume unreacted methacrylate functional groups in the cured films.

Effects of Steel Slag Powder Content and Curing Condition on the Performance of Alkali-Activated Materials Based UHPC Matrix.

The accumulation of steel slag and other industrial solid wastes has caused serious environmental pollution and resource waste, and the resource utilization of steel slag is imminent. In this paper, alkali-activated ultra-high-performance concrete (AAM-UHPC) was prepared by replacing ground granulated blast furnace slag (GGBFS) powder with different proportions of steel slag powder, and its workability, mechanical properties, curing condition, microstructure, and pore structure were investigated. The results illustrate that the incorporation of steel slag powder can significantly delay the setting time and improve the flowability of AAM-UHPC, making it possible for engineering applications. The mechanical properties of AAM-UHPC showed a tendency to increase and then decrease with the increase in steel slag dosing and reached their best performance at a 30% dosage of steel slag. The maximum compressive strength and flexural strength are 157.1 MPa and 16.32 Mpa, respectively. High-temperature steam or hot water curing at an early age was beneficial to the strength development of AAM-UHPC, but continuous high-temperature, hot, and humid curing would lead to strength inversion. When the dosage of steel slag is 30%, the average pore diameter of the matrix is only 8.43 nm, and the appropriate steel slag dosage can reduce the heat of hydration and refine the pore size distribution, making the matrix denser.

Impact of curing on the volumetric and mechanical properties of cold bitumen emulsion mix

Cold bitumen emulsion mix (CBEM) is an emerging sustainable material for constructing the base or surface layer of flexible pavements, reducing energy consumption and global warming effects compared to hot mix asphalt. The main challenges associated with CBEM are low initial strength, moisture susceptibility, etc. This is primarily due to water encapsulation. The curing process allows the water to evaporate gradually over time, improving mechanical properties. This study investigates CBEM’s volumetric and mechanical properties under different curing conditions. CBEMs were prepared using low (50 blows) and high (75 blows) compaction efforts and cured at 25, 40, and 55 °C for 1 to 28 days. In volumetric characteristics, the air voids and voids in mineral aggregates decreased slightly with curing, while total binder volume decreased. Effective binder volume and voids filled with binder increased slightly, then decreased. The mechanical properties, i.e., Marshal Stability, Marshal Quotient, Indirect Tensile Strength (ITS), and Moisture Susceptibility parameters like the Indirect Tensile Strength Ratio (ITSR) and Retained Marshal Stability (RMS), were found to improve over time at each curing temperature. Using a high curing temperature further improved the mechanical characteristics of CBEM. The curing process was decelerated in case of CBEM prepared by high compactive efforts compared to CBEM prepared by low compactive efforts. The Michaelis-Menten model can be used to analyze moisture loss growth and the development of mechanical properties over time. The strength-maturity model can be developed by using ITS as strength parameters. There was a strong correlation between all mechanical characteristics and moisture loss.

High-performance acetosolv lignin-incorporated DGEBA cured with aprotic imidazolium-based ionic liquid: Polymerization, chemical, thermal and combustion aspects of the thermosetting materials

The lignin valorization constitutes a chemical platform for several segments of chemical industry. The aim of this work was to evaluate the potential of acetosolv coconut fiber lignin (ACFL) as an additive to DGEBA, curing it using an aprotic IL ([BMIM][PF6]) and analyze the properties of the obtained thermosetting materials. ACFL was obtained by mixing coconut fiber with 90 % acetic acid and 2 % HCl at 110 °C during 1 h. ACFL was characterized by FTIR, TGA and 1H NMR. The formulations were fabricated by mixing DGEBA and ACFL at different concentrations (0–50 % wt.). The curing parameters and [BMIM][PF6] concentrations were optimized by DSC analyses. The cured ACFL-incorporated epoxy resins were characterized by gel content (GC), TGA, MCC and chemical resistance in different media. ACFL undergone a selective partial acetylation that favored its miscibility with DGEBA. High GC values were obtained at high curing temperatures and ACFL concentration. The crescent ACFL concentration did not affect the Tonset of the thermosetting materials significantly. ACFL has increased the resistance of DGEBA to combustion and different chemical media. ACFL has shown a great potential to be used as a bio-additive for enhancing the chemical, thermal and combustion properties of high-performance materials.

AFt/AFm distribution and microstructural properties of thermally cured concretes containing rice husk ash

The behaviour of concretes with and without rice husk ash was experimentally investigated with respect to the susceptibility to delayed ettringite formation. Concrete specimens have previously undergone a specific initial thermal curing up to 85°C, which was designed to trigger delayed ettringite formation. Specimens were subsequently exposed to a specific exposure environment by water immersion at 38°C, over 1 year. Expansion measurements and microstructural analyses were performed to evaluate the level of attack, as well as the integrity of concretes and chemical characteristics of compounds by means of SEM-EDS. Mechanical properties were also assessed over one year to detect the level of damage from delayed ettringite formation. The data has indicated that the incorporation of 8% of rice husk ash could partially reduce the level of DEF expansion and damage. The microstructure of cement matrices from both the reference and RHA concrete indicated different features, sulfate phase compositions and degradation stages. The reference concrete contained radial cracking from filled pores and nearby interfacial transition zone, whereas RHA concrete presented denser features, with microcracking scattered at random across cement matrix and minor neoformations.

Effect of incorporating fibers in reactive powder concrete – A review

Modified form of High-Performance Concrete is a Reactive- Powder Concrete(RPC) which was first found by PierreRichard and Marcel Cheyrezy, they successfully built France laboratory using RPC. High compressive strength of over 200 MPa has been achieved, with flexural strength up to 40 MPa, and in certain cases crushing strengthup to 800 MPa with the introduction of hybrid fibre reinforcement. Cement OPC grade, silica fume, quartz sand, crushed quartz flour, water, a superplasticizer, and fibre reinforcement all contribute to making RPC.These materials have high durability characteristics as a result of their extremely low porosity, low permeability, limited shrinkage, and higher corrosion resistance. To increase package density, the powder combination of RPC should have a variety of granular particle classes. By dense packaging theory and grading optimization technique, the absence of coarse aggregate increased compactness. Because of its compactness, it reaches high strength. The rapid hydration of cement and silica fume at a high curing temperature of 100 °C as compete to 27 °C is something that causes the higher early strength. This paper discusses about the hybrid fibers incorporation in RPC under various curing conditions and its enhancement in strength than single fibers. Using micro and macro fibers dramatically improves the properties of RPC and thermal properties with various types of fiber. The mechanical characteristics of the RPC beam without the hybrid fiber, on the other end, are the lowest of all the mixtures. Apart from these characteristics, RPC can be utilized for industrial and nuclear waste storage facilities due to its ultra-dense microstructure, which provides ultra-high strength even at elevated temperature and water resistance. This paper infers that the addition of hybrid fiber in RPC considerably reduces the spalling of concrete at elevated temperatures and it exhibits superior strength and durability.

Effects of teff straw ash on the mechanical and microstructural properties of ambient cured fly ash-based geopolymer mortar for onsite applications

Although geopolymer cement (GPC) is a substitute for Portland cement, its application is restricted due to the need for high-temperature curing (40–90 °C), which makes it challenging to utilise for onsite applications. To address this issue, the current study examined the potential of substituting fly ash (FA) with teff straw ash (TSA) in geopolymer mortars cured at ambient temperature. The findings revealed that substituting FA with TSA can eliminate the need for high-temperature curing, and the compressive strengths of FA-TSA-based geopolymer mortar mixtures cured for 28 days ranged from 45 to 53 MPa. Further, increasing the TSA content enhanced the mortar's flexural and direct tensile strengths. A teff straw ash level of 10% increased compressive, flexural, and direct tensile strengths by 40%, 59%, and 30% at 28 days, respectively. Furthermore, the mineralogical phases of the mortar after 28 days confirmed the presence of gismondine coexisting with other phases, and microstructural analysis indicates that the inclusion of TSA resulted in a denser structure. These findings suggest that TSA could be a potential substitute for FA in GPC applications to lower energy usage and environmental impact.

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.

Optimizing the dielectric and mechanical properties of melamine based‐benzoxazine resin by copolymerizing with epoxy resin

AbstractBenzoxazine resins are suffering from high curing temperature, low crosslinking density, and poor toughness, which hampers their broad applications. The introduction of melamine during synthesis has been shown useful in improving these properties, but the dielectric property was deteriorated heavily. Epoxy resins show higher crosslinking density and better toughness but the thermal properties are relatively poor. Herein, the melamine based‐benzoxazine MBF was copolymerized with epoxy resin E51 in this work in order to improve the dielectric properties. The curing reactivity, polymerization mechanism and the comprehensive properties such as thermal, dielectric and mechanical properties were systematically verified. It is found that the abundant active functional groups such as amino and phenolic groups in MBF could catalyze and copolymerize with E51 and form uniform copolymers poly(MBF‐E) at various proportions. With the increasing of epoxy resins, more polar amino groups was consumed, thus the polarity and the water absorption of the copolymers decreased. Additionally, the dielectric constant decreased from 3.67 for poly(MBF) to 2.94 for poly(MBF‐60E). Meanwhile, the highest increasement of tensile strength and elongation at break value reached 130% and 70%, respectively, which may be caused by the higher crosslinking density and more flexible ether bond formed.