Low Shrinkage Research Articles

Free radical-initiated direct ink writing of liquid polycarbosilane slurry and its conversion into SiC ceramic parts with low shrinkage

Additive manufacturing technology is being developed for manufacturing SiC ceramic parts with designed shape and microstructure. In this study, direct ink writing (DIW) was adopted to print SiC ceramic parts. To avoid incorporating volatile organic solvents or water, a SiC ceramic precursor liquid polycarbosilane (LPCS) was used as the binder of SiC powder and the rheological adjustment additive. The content of LPCS with the content and diameter of SiC powder were optimized to improve the fluidity of slurry, increase its ceramic yield and reduce its shrinkage during pyrolysis. According to the results of rheological analysis and shrinkage during pyrolysis, 78 wt% SiC powders composed of 40 nm SiC powder and 5 μm SiC powder in a ratio of 1:6 was appropriate. For achieving rapid solidification rate of the slurry at low temperature, a free‐radical initiator which can trigger the crosslinking of allyl group on LPCS was incorporated. According to the results of rheology, DSC and FT-IR, the preferred platform temperature for fast curing of DIW slurry was 150 °C. As the slurry accumulated layer by layer on heating platform, the cured green bodies were successfully prepared. After pyrolysis to 1200 °C, SiC ceramic parts were obtained with only 4.43% linear shrinkage associated with the precursor-to-ceramic conversion. Because the shrinkage was limited, no obvious deformation or crack was found.

A green inorganic binder for material extrusion of ultra-low shrinkage and relatively high strength metakaolin ceramics at low sintering temperature

Ultra-low shrinkage and relatively high strength metakaolin ceramics printed by material extrusion were innovatively fabricated using inorganic aluminum dihydrogen phosphate (Al(H2PO4)3) as a binder and low sintering temperature. The optical microscopy, scanning electron microscopy (SEM), electronic vernier caliper, three-point bending test, Archimedes method and X-ray diffraction (XRD) were used to measure and evaluate the surface morphology, microstructure, dimensional shrinkage, flexural strength, porosity and phase composition of the printed ceramics sintered at different temperatures. The results showed that when the mass ratio of metakaolin, Al(H2PO4)3 and deionized water was 17:6:2, the rheological characteristic of the purely green inorganic ceramic slurry was very suitable for material extrusion additive manufacturing, and the corresponding printed ceramic green bodies possessed high-quality formability. The ceramic samples sintered at 750 °C possessed the best whiteness, the lowest shrinkage (<2%), relatively high flexural strength (9.02 MPa). As the sintering temperature increased, Al(H2PO4)3 transformed to aluminum metaphosphate Al(PO3)3, and then decomposed into aluminum phosphate (AlPO4) and phosphorus pentoxide gas (P2O5), which caused pores and reduced strength, and alumina oxide (Al2O3) and silicon oxide (SiO2) in metakaolin gradually transformed to mullite and cristobalite with more stable structure and higher strength.

Dynamic response characteristics of coal/rock during water injection and freezing process under gas atmosphere and its control effect on gas outburst

The freezing method compensates for the defect of sacrificing coal integrity to reduce gas content, which is the case with traditional methods, achieving the improvement of coal body strength while reducing coal seam gas energy storage, improving the safety of coal and gas outburst accidents in deep coal seams during the process of rock cross-cut coal uncovering. This study conducted water injection and low-temperature freezing experiments on coal/rock samples under the gas atmosphere, analyzing the effects of water and temperature on sample temperature, deformation, and gas adsorption and desorption characteristics. The results indicate that water can displace adsorbed gas in coal/rock samples, and the relationship between the gas displacement and the water content of the sample satisfies an improved exponential function. The center temperature Tm of low water content coal/rock samples decreases with time and gradually tends to stabilize, while the Tm of high water content samples experiences a short-term deceleration or stagnation due to the phase transition heat release of water when it drops to around 0 °C. The cooling rate of samples with low water content and no gas is higher and that of rocks is higher than that of coal samples. Coal/rock samples with high water content experience frost heave during the freezing process, but the overall deformation is still dominated by cold shrinkage, and the amount of deformation is negatively correlated with temperature and water. The gas adsorption capacity of coal decreases linearly with the temperature. At the same time, an increase in water content and a decrease in freezing temperature will significantly reduce the gas desorption capacity of coal samples, effectively reducing the gas expansion energy of coal samples, especially the desorption gas expansion energy. In engineering implementation of this method, the ice phase network can fill the coal pores and cracks and improve the mechanical properties of the coal/rock mass, and the gas pressure in the coal seam and stress concentration near the coal rock interface can be reduced by low temperature and cold shrinkage, thereby achieving safe exposure of the coal seam and preventing accidents from occurring.

Low sintering shrinkage porous ceramics: Principles, progress, and perspectives

Low sintering shrinkage porous ceramics: Principles, progress, and perspectives

Incorporating Limestone Powder and Ground Granulated Blast Furnace Slag in Ultra-high Performance Concrete to Enhance Sustainability

While ultra-high performance concrete (UHPC) offers numerous advantages, it also presents specific challenges, primarily due to its high cost and excessive cement content, which can pose sustainability concerns. To address this challenge, this study aims to develop cost-effective and sustainable UHPC mixtures by incorporating ground granulated blast furnace slag (GGBFS) and limestone powder (LP) as partial replacements for portland cement. Eight fiber-reinforced UHPC mixtures were investigated, with a water-to-cementitious materials (w/cm) ratio of 0.15. In four of the UHPC mixtures, 25% of the cement was replaced with GGBFS, and further, LP was added as a mineral filler, partially substituting up to 20% of the cement. In the remaining four mixtures, cement was replaced with only LP up to 20% (without GGBFS). The 28-day compressive strength of the UHPC mixture with 25% GGBFS and 20% LP was 149 MPa, 3.50% lower than the mixture without GGBFS. Its 28-day flexural strength decreased by 30%. Increasing LP replacement reduced drying and autogenous shrinkage, with a 29% shrinkage reduction at 20% LP replacement. Moreover, UHPC mixtures with GGBFS exhibited lower shrinkage compared to those without GGBFS for all LP replacements up to 20%. For evaluating the sustainability of UHPC mixtures, the cement composition index (CCI) and clinker to cement ratio (CCR) were determined. For 20% LP replacement with 25% GGBFS, CCI was 3.6 and the CCR was 0.5, 38% decrease from the global clinker to cement ratio. Overall, 20% LP replacement UHPC mixtures with and without GGBFS can produce UHPC class performance and reduce the environmental impact.

Novel ultralight carbon foam reinforced carbon aerogel composites with low volume shrinkage and excellent thermal insulation performance

Lightweight carbon aerogels are attractive for thermal insulation due to their low thermal conductivity and excellent high-temperature resistance under extreme environments. However, the preparation of monolithic carbon aerogels from phenolic resin precursor always faces the problem of large volumetric shrinkage during the drying and carbonization processes, thus resulting in the increasing density and thermal conductivity of aerogels. Here, to solve such issues, ultralight and rigid carbon foam was designed and synthesized as the reinforcement to fabricate carbon aerogel composites (CACs), which could significantly enhance the shrinkage resistance of monolithic carbon aerogels. The high rigidity of the carbon foam reinforcements (CFRs) was achieved through a pre-carbonization process, which also endowed the CFRs with a matched shrinkage with the monolithic carbon aerogels. As a result, the obtained CACs reinforced by the rigid CFRs showed not only crack-free structures, but also quite low shrinkage, which was about 5.9 % after carbonization. The low shrinkage of CACs then endowed them with quite low density (21.5 mg cm−3) and excellent thermal insulation performance (25.9 mW m−1 K−1). Furthermore, due to a highly rough nanostructure, the CACs also possessed outstanding hydrophobicity. These merits make the CACs a promising thermal insulation material even in humid environments.

Multiscale prediction model for autogenous shrinkage of early-age concrete incorporating high volume fly ash

High volume fly ash concrete (HVFAC) manufacture is an effective strategy to mitigate the autogenous shrinkage, achieve green production and low cost in building engineering. Nevertheless, a reasonable and robust multiscale prediction model of autogenous shrinkage of HVFAC and relative input parameters remains lacking. Thus, the objective of present study is to make up the missing gap. The research offered the input parameters dataset of microscopic pore structure, internal relative humidity (IRH) and validation dataset of autogenous shrinkage of cement pates and concretes with a water to binder ratio of 0.30/0.40, with fly ash in cement mass replacing-levels of 0, 30 %, 50 % and 70 %. The multiscale autogenous shrinkage prediction model was established by three parts of bulk modulus homogenization, capillary pressure calculation and composite models. The needed volumetric ratios of multi-phases were obtained by VCCTL software; bulk modulus was homogenized by Mori-Tanaka method, the proposed prediction models of IRH and LF-NMR-based microscopic pores characterization were used for capillary pressure calculation at cement paste level; composite models for autogenous shrinkage transformation from paste level to concrete level. The results show that HVFAC is capable of improved shrinkage-related pore structure, IRH and low autogenous shrinkage, which backs up the application potential of HVFAC in building engineering, and the input parameters dataset is confirmed to be reasonable. The proposed multiscale model was validated to be robust via comparing with test results and references dataset. The achievement of this study bear significance for HVFAC design and application in early-age cracking prevention in building engineering.

Development of novel mortar using fine river sand and pretreated waste flue gas desulfurization gypsum powder and pond fly ash

The objective of this study is to investigate the potential of combined pond fly ash (PFA) and flue gas desulfurization gypsum (FGDG) as a partial replacement for cement by mixing them with fine river sand for green mortar development. We studied the physical, mechanical, and microstructural properties of mortars by using techniques such as scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDXS) and X-ray diffraction (XRD) for chemical component identification of the hydrated phase. These experimental results show that mortar containing pond fly ash, FGDG, and cement concentrations of 5 wt%, 5 wt%, and 90 wt% yields the maximum compressive strength (8.9 MPa) and flexural strength (3.4 MPa) after 28 days. This mortar has 12.6% higher compressive strength and 48% lower shrinkage in comparison with control specimens. A reduction in shrinkage is attributed to the denser structure of the mortar, as the calcium silicate hydrate (C–S–H) gel is found to exist in the SEM image and also identified through XRD studies. It is also concluded that excessive ettringite formation causes expansion in the volume of mortar, voids formation, and results in the reduction of strength. The application of this mortar can be in the internal plaster, bricks, and masonry.

Low sintering shrinkage and high strength of porous mullite–zirconia ceramics: Effect of AlF3 and ZrSi2 contents

AbstractLow‐shrinkage porous mullite–zirconia ceramics were prepared using Al and ZrSi2 as raw materials and AlF3 as the whisker catalyst through foam‐gelcasting and freeze‐drying methods. The oxidation of Al and ZrSi2 combined with the Kirkendall effect and volume expansion was employed to reduce the sintering shrinkage, while the mullitization was promoted by the reaction between AlF3 and extra ZrSi2, and ZrO2 were generated to enhance the compressive strength as the reinforcing phase. The effect of AlF3 and ZrSi2 content on the microstructure and properties of porous mullite‐zirconia were explored. These ceramics exhibited high porosity (73.75%–85.02%) and compressive strength (2.98–17.71 MPa), low shrinkage (0.47%–10.89%) and thermal conductivity (0.228–0.462 W/(m·K)). In particular, the porous mullite–zirconia ceramics with great compressive strength (4.01 MPa) companied with near‐zero shrinkage (0.47%) at the AlF3 content of 15 wt% and extra ZrSi2 content of 20 wt% by introducing the reinforcing phase (ZrO2). Additionally, the temperature difference around 950°C of the porous ceramics during the heating test proved the exceptional application on thermal insulation.

Synthesis of structure controllable carbon aerogel with low drying shrinkage and Scalable size as High-Temperature thermal insulator

Carbon aerogels, owing to the high-temperature durability and low thermal conductivity, are promising candidates for high-temperature thermal insulation used in inert atmospheres. However, significant drying shrinkage poses challenges in fabricating large-sized carbon aerogels and optimizing their thermal insulation properties. Herein, we report the synthesis of monolithic carbon aerogels with low drying shrinkage and controllable nanostructures by adding self-crosslinked chitosan to provide reaction sites and three-dimensional network support based on the traditional resorcinol–formaldehyde method in an acetic acid environment. Owing to the increased cross-linking degree of resorcinol–formaldehyde gel from the polymerization and hydrogen bonding between chitosan and resorcinol–formaldehyde oligomers, the skeleton of gel network is strengthened and the shrinkage during supercritical drying is as low as 1.51 %, resulting in a good monolithic shape of carbon aerogel (130 × 130 × 15 mm). Besides, they exhibit typical nanoporous characteristics with uniform and small particle size (9–130 nm) and low density (0.070–0.221 g·cm−3), achieving a low thermal conductivity of 0.047 W·m−1·K−1 at 1100 °C and high compressive strength of 1.80 MPa (density of 0.138 g·cm−3). This method facilitates the production of carbon aerogels with low shrinkage, minimal defects, low density, and small pore size, rendering them suitable for applications requiring large dimensions and low defect levels, such as fiber-reinforced composites and high-performance thermal insulators.

Study on the polymerization shrinkage and mechanical-physical properties of dental resin composite modified by polyurethane dimechacrylate and glass flake fillers

ObjectivesWe intend to explore the polymerization shrinkage and mechanical-physical properties of the dental resin composite modified by polyurethane oligomer PU-MA and glass flakes fillers, and provide some experimental information for developing new dental materials. MethodsBased on Bis-GMA/TEGDMA, the new dental resin composite was hand fabricated by combining PU-MA with different content (65 wt% ∼ 75 wt%) of glass flakes/Si-Al-borosilicate glass mixed fillers. The kinetics of polymerization shrinkage was investigated by using an experimental set-up based on laser triangulation. The mechanical-physical properties of specimens after 60 days water immersion and 5000 times thermo-cycles were measured by three-point bending test and micro vickers hardness tester. Data were submitted to one-way ANOVA/LSD-test (α = 0.05). Independent sample T test was used to analyze the data before and after aging. Two-way ANOVA was adopted to analyze the effect of composition and aging condition on the mechanical-physical properties (α = 0.05). ResultsPU-PG-75 % performs lower volume shrinkage (1.35%). As the increase of filler content, the water sorption and solubility increased and the depth of cure decreased significantly (P < 0.05). After aging, the flexural strength, elasticity modulus and vickers hardness of commercial products dropped obviously and PU-PG-75wt% was relatively stable. Conclusions20 wt% PU-MA could decrease the volume shrinkage of dental resin composite effectively. 60 days water immersion and 5000 times thermal cycles simultaneously resulted in a significant reduction in the flexural properties for experimental materials. PU-PG-75 % may have the potential to be taken as a low shrinkage dental resin composite with good mechanical properties. Clinical SignificanceThis new synthesized composite could perform low polymerization shrinkage and better mechanical resistance, which may provide a new idea for improving the lifespan of direct restoration.

Performance evaluation of fly ash–copper slag-based geopolymer bricks

This study investigates the production and evaluation of geopolymer bricks made from a blend of fly ash, copper slag, soda ash activator, and sand as fillers. Locally abundant industrial and mining waste materials were selected as the primary components. The bricks were synthesized using two binders: 60% fly ash with 40% copper slag, or 70% fly ash with 30% copper slag. Both were milled with the activator at a 0.2 soda ash-to-precursor ratio. Fine sand was added to the mixes at 1:2 and 1:3 binders-to-sand ratios. The bricks’ physical, mechanical, and durability properties were examined through compressive strength, modulus of rupture, density, water absorption, drying shrinkage, and efflorescence test, and their performance was compared to established industry standards. The experimental findings indicate that bricks made with 60% fly ash, 40% copper slag, and a 1:2 binder-to-sand ratio exhibited optimal compressive strength (9.64 MPa) and water absorption (7.5%) at 28 days of curing age. Conversely, there was only a marginal increase of up to 4.7% in the strength of the formulation with 70% fly ash and 30% copper slag, attaining a compressive strength of 4.9 MPa between the curing ages. Furthermore, the results indicated a positive correlation between the density and compressive strength of the geopolymer bricks at similar curing ages. The bricks’ density showed minimal variation with curing age and the highest modulus of rupture value observed was 2.5 MPa. The optimal bricks also exhibited relatively low linear shrinkage, good resistance to efflorescence, and met the relevant industry standards.

Mechanical, physical and durability performance of engineered cementitious composites prepared with calcium aluminate cement

This research aims to fill a gap in existing studies by exploring the use of calcium aluminate cement (CAC) as an eco-friendly substitute for Ordinary Portland Cement (OPC) in Engineered Cementitious Composites (ECC). The study investigates the impact of CAC, both with and without the presence of fly ash (FA), on the mechanical, physical, durability, and microstructural properties of ECC-CAC mixtures. Various proportions of FA-to-CAC up to 1.5 were considered, while assessing different engineering parameters of compressive and flexural strengths, ductility, ultrasonic measurements, chloride penetrability, and drying shrinkage. Moreover, scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX) and X-Ray diffraction (XRD) were utilized to analyze the reaction products related to CAC and/or FA in selected mixtures. Higher strengths of CAC-ECC were achieved at earlier curing ages, though the transformation of CAC’s metastable phases led to reduced strengths in ECC-CAC mixtures compared to the control ECC during extended curing periods. Furthermore, the combination of FA with CAC played a crucial role in avoiding the hydrates conversion and minimizing the shrinkage in CAC-ECC, achieving even lower shrinkage levels than those observed in control OPC-ECC.

Solvent-free Acrylate/BCB drop-on-demand (DOD) inkjet dielectric ink for 3D printing

Currently, drop-on-demand (DOD) inkjet-printed dielectric inks used to produce next-generation high-frequency electronic devices fail to meet the requirements of circuit board substrates in terms of thermal, mechanical, and dielectric properties. To address this gap, we synthesized a low-viscosity acrylate monomer featuring an intrinsically low dielectric structure (hexafluorobisphenol A) and multiple cross-linking sites, and a benzocyclobutene monomer with low curing shrinkage and excellent thermomechanical properties. By mixing these monomers with other reactive diluents and verifying them through theoretical calculations and printing, we have developed a range of dielectric inks suitable for inkjet 3D printing. To enhance the overall performance of the cured inks, a sequential UV/thermal curing strategy was employed to form interpenetrating networks (IPNs). The optimal ink formulations met the requirements of circuit board substrates, demonstrating excellent heat resistance (Td5% = 334 °C, Tg = 181 °C), mechanical properties (tensile strength = 85 MPa, elongation at break = 13.5 %), and dielectric properties (Dk = 2.526 and Df = 0.113 at 15 GHz). These formulations meet the performance requirements for circuit board substrates and lead the industry in mechanical and dielectric properties. Furthermore, the feasibility of inkjet printing heterogeneous integrated electronic devices was verified, showing excellent print resolution and continuity of the conductive network. This work provides new insights into the future development of dielectric materials for inkjet 3D printing.

Hemp cellulose-based aerogels and cryogels: From waste biomass to sustainable absorbent pads for food preservation

This study presents a circular economy approach utilizing hemp stems and rice straw, typically perceived as low-value agricultural waste, to develop a sustainable alternative to traditional plastic absorbent pads for food packaging. The development of an active material was achieved through the utilization of hemp cellulose and a bioactive extract isolated from rice straw. In addition to reducing plastic pollution, this material demonstrates the potential to enhance food preservation. This research provides evidence of the benefits of repurposing agricultural by-products to create valuable and environmentally-friendly products. Hemp cellulose was extracted, characterized, and processed to develop stable aerogels and cryogels through supercritical CO2 drying and freeze-drying. The water stability and internal structure of the materials were guided via TEMPO-mediated oxidation and high-pressure homogenization. Both materials showed versatile physicochemical and mechanical properties. Nevertheless, with higher water sorption (2.20 mL/g), minimal dimensional changes, and lower shrinkage, cryogels were suitable for meat absorbent pad application. To enhance the cryogels functionality, they were impregnated with a rice straw bioactive extract in two different concentrations. The incorporation of the extract did not affect the structure of the cryogels, improved their mechanical properties and the antioxidant activity remained stable after drying (63.89–78.96 %). Finally, the performance of the developed materials was compared to commercial plastic pads and pristine meat preservation challenge test during 9 days at refrigeration conditions. The incorporation of rice straw extract improved meat color preservation. While moderate extract concentrations (75 mg/g) showed a protective effect against lipid oxidation, higher levels (187.5 mg/g) induced pro-oxidant reactions. This research highlights the potential of hemp cellulose-based cryogels as sustainable and functional packaging materials for meat products.

Molecular Confinement Engineering Induced Super Thermostable and RT‐Adjustable Gel for Tri‐Heat‐Channeled Smart Window

AbstractThermochromic hydrogel‐based smart windows hold great potential for building energy‐saving. However, most of thermochromic hydrogels possess high response temperatures (RT) and poor size stability under heat, resulting in limited energy‐saving performance for smart windows. Here, a printable thermochromic gel (PNC) is reported with low thermal shrinkage and tunable RT for smart windows applications via molecular confinement engineering. The RT of PNC gel is well within the comfort temperature zone of the human body and can be modulated (26.5–31.5 °C) by the introduced support networks. Importantly, the resultant gel exhibits a super transmittance (95%) in visible light and an outstanding modulation (87%) for sunlight. Based on PNC gel, a tri‐heat‐channeled smart window is fabricated using a multilevel thermal regulation strategy that couples conduction, radiation, and evaporative cooling. The tri‐heat‐channeled smart window demonstrates outstanding daytime cooling (10 °C lower than conventional windows) and can cut off 30% of annual heating, ventilation, and air‐conditioning (HAVC) energy consumption. Furthermore, a novel smart window is designed with invisible shutters that appear only when needed. This work offers novel clues for constructing versatile gels and innovative smart windows for building energy‐saving.

Promoting the circularity of ceramic materials through cold sintering of aggregates from construction and demolition waste

A ceramic composition containing 95 wt% of construction and demolition waste-like material was consolidated by cold sintering process at 200 °C using KOH water solutions as the liquid medium. The relative density of the samples reaches ∼90 % of the theoretical one for process conditions of 600 MPa and 60 min. A post-annealing process at 1100 °C of the as-cold sintered samples causes a slight increase in the relative density and of their mechanical strength compared with conventionally sintered samples at 1100 °C and increases the shape factor of the Weibull distribution, thus increasing the reliability of the component. It is shown that cold sintering of the material avoids its high pyroplasticity, providing low shrinkage and reducing internal defects in the ceramic. This work represents the first exploration of the viability of manufacturing ceramic tiles with high recycled content contributing to the transition to a greener world.

Octaphenyl-POSS nanoparticles dispersed thick polyvinyl alcohol/acrylamide photopolymer with low volumetric shrinkage and high diffraction efficiency.

Relatively high volumetric shrinkage induced by polymerization of photopolymer severely limits its applications in high-density holographic storage. In this paper, what we believe to be a novel organic-inorganic hybrid nanoparticles (NPs) of octaphenyl-polyhedral Oligomeric silsesquioxane (POSS) are employed into polyvinyl alcohol (PVA)/acrylamide photopolymer with 300µm thickness. By optimizing the concentration of octaphenyl-POSS, the volumetric shrinkage is successfully suppressed from 1.11% to 0.41%, and satisfies the requirements of commercial holographic data storage (HDS) material. Moreover, the introduction of POSS NPs does not sacrifice other holographic recording capacities. Contrarily, the diffraction efficiency of photopolymer after dispersing 2‰(w/v) POSS increases from 86% to 90%. The physical mechanism of enhanced capacities is discussed by using photopolymerization and NPs diffusion process. To our knowledge, this is the best results of PVA/acrylamide photopolymer with low volumetric shrinkage, high diffraction efficiency and angular selectivity. The POSS-dispersed photopolymer system is expected to have good application prospects in angular-multiplexing high-density 3-D holographic storage.

Achieving high-strength, high-porosity and relatively low-shrinkage in extrusion-based 3D printed ZrO2 ceramics by adjusting CaCO3 addition and sintering temperature

High-strength, high-porosity and relatively low-shrinkage ZrO2 ceramics were successfully fabricated by material extrusion 3D printing technology via optimizing CaCO3 addition and sintering temperature. The effects of different CaCO3 additions on the performance, phase formation, and microstructure of porous ZrO2 ceramics sintered at different temperatures were systematically investigated and discussed. The results showed that flexural strength, porosity and shrinkage of the ZrO2 ceramics were significantly affected by CaCO3 addition and sintering temperature. The appropriate addition of CaCO3 could promote sintering and phase transformation of m-ZrO2, increase the number and size of pores in the matrix. The corresponding flexural strength and porosity of the sintered samples increased, while the shrinkage rate decreased. The raised sintering temperature appropriately facilitated the formation of CaZrO3. Microstructural analysis indicated that CaZrO3 exhibited a larger polyhedral bulk morphology, mainly distributed at the edges of pores, which could stabilize the pore structure and thicken the sintering necks, and thereby exhibiting well-balanced comprehensive performance. The ZrO2 ceramics with high flexural strength, high porosity and relatively low shrinkage rate were achieved by adding 15 wt% CaCO3 and sintering at 1450 ℃. The resulting flexural strength and porosity were 27.33 MPa and 67.62 %, respectively, presenting an improvement of 563.35 % and 13.69 % compared to ZrO2 ceramics without CaCO3. Moreover, the shrinkage rates in the X-direction (9.98 %), Y-direction (9.62 %), and Z-direction (9.94 %) significantly reduced, decreasing by 38.28 %, 38.10 %, and 41.46 %, respectively.

Shrinkage reduction mechanism of low Ca/Si ratio C-A-S-H in cement pastes containing fly ash

Understanding the drying shrinkage of low Ca/Si ratio cement pastes is crucial for promoting the use of low-clinker ratio cementitious materials and reducing the environmental impact of cement production. We prepared well-hydrated cement paste samples with various fly ash replacement and water-to-cement ratios. The long-term drying shrinkage was measured by 1 mm-thick samples. Results showed that fly ash containing samples exhibited lower shrinkage and the irreversible part of drying shrinkage was less compared to those without fly ash. Chemical composition analysis of the calcium aluminate-silicates hydrate (C-A-S-H) was conducted using X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS). Additionally, water vapor sorption isotherms and proton nuclear magnetic resonance (1H NMR) relaxometry were used to determine specific surface area and pore structure. By analyzing these results in conjunction with the C-A-S-H model, we attributed the reduced and more reversible drying shrinkage in fly ash cement to lower Ca ion amounts in the interlayer space and fewer trapped larger pores.