In recent years, binder jetting 3D printing (BJ3DP) technology has attracted growing attention in construction materials. Although ordinary Portland cement (OPC) is one of the most widely used building materials, its slow hydration reaction limits its effectiveness in BJ3DP. In contrast, calcium aluminate cement (CAC) undergoes a rapid hydration reaction, achieving sufficient strength at an early stage, which is advantageous for BJ3DP applications. Nevertheless, there is still limited research on the suitability of cementitious materials for BJ3DP. Therefore, this work aimed to identify the optimal material mixing proportions and suitable post-processing solutions for BJ3DP by comprehensively evaluating the flowability of the dry materials, print bed characteristics, print quality, and mechanical strength of the printed specimens. The results showed that replacing 40% of ordinary Portland cement with calcium aluminate cement optimized the flowability of the dry mixture, allowing for the production of specimens with favorable print quality. Furthermore, at this optimal ratio, the printed specimen's compressive strength achieved 12.7 MPa after curing in Na2SiO3 solution for 28 d, representing a 42.6% increase in compressive strength compared with the specimens cured in water. Simultaneously, the compressive strength of the printed samples increased by 57.7% compared to our previous results and showed a further 10% increase under the same curing condition (water curing). Based on these findings, we proposed a novel composition ratio for high-alumina cementitious materials in BJ3DP, consisting of a 60% OPC and 40% CAC mix, along with post-treatment of the printed samples using a Na2SiO3 solution. This study broadens the range of cementitious materials suitable for BJ3DP and demonstrates the feasibility of Na2SiO3 as an effective post-treatment solution, providing a theoretical foundation for future research and practical applications of high-alumina cementitious materials.

The influence of thermal deformation in lattice structure and inter-atomic distance on the residual mechanical properties of calcium silicate hydrate (CSH)/polycarboxylate superplasticizer (PCE) composites at varying Ca/Si ratios remains ambiguous. Here, correlations between the multiscale structural transformation and mechanical properties of CSH/PCE composites with Ca/Si ratios ranging of 0.6-1.0 are investigated using ex situ small-angle X-ray scattering (q = 0.005-2.7 & Aring;- 1 ) and in situ loading wide-angle X-ray scattering (q = 1-18.5 & Aring;- 1 ). The temperature-driven CSH/PCE composite transformation stages were divided into dehydration (105-200 degrees C) and decomposition (300-500 degrees C) stages. Increased pore pressure, attributed to the well-packed particles in composites with low Ca/Si ratios and high PCE contents, induced significant thermal deterioration of residual elastic modulus. Swollen CSH building blocks with adsorbed PCE molecules at low Ca/Si ratios exhibited notable shrinkage after exposure to high temperatures, whereas the particle-to-particle distance decreased with constant radii of CSH blocks at high Ca/Si ratios.

The interfacial transition zone (ITZ) is the most vulnerable and porous phase in concrete, corresponding to the interface between aggregates and the cement matrix. To densify the ITZ microstructure, this study proposes the incorporation of graphene oxide (GO) into silica fume-modified mortars with high and low water-to-binder (W/B) ratios of 0.4 and 0.2, respectively. GO enhanced the degree of hydration by providing nucleation sites and promoting the pozzolanic reaction of silica fume and Ca(OH)2, leading to the formation of secondary amorphous phases. In the mortars containing GO, the calcium-to-silicon ratio of the phase around the aggregate surface increases because the GO nanosheets absorbed Ca2+ ions and migrated with the mixing water. The comparative effects of GO on the mechanical properties of the ITZ in the mortars with high and low W/B ratios were assessed using nanoindentation tests. The results indicate that the GO nanosheets interlocking with the Ca2+ ions resulted in the formation of denser hydration products with high elastic modulus in the vicinity of the aggregates at a W/B ratio of 0.4 while reducing the amount of unhydrated cement grains at a W/B ratio of 0.2. Overall, this study highlights the comparative effects of GO on enhancing the performance of normal- and high-strength cement composites and mortars, with a particular focus on the improvement of ITZ.

This study explored the influence of alumina type and gypsum content on the hydration, mechanical properties, and chemical changes of Portland-limestone cement (PLC) using isothermal calorimetry, inductively coupled plasma-optical emission spectroscopy (ICP-OES), ionic chromatography (IC), compressive strength testing, X-ray diffraction (XRD), 27Al magic angle spinning nuclear magnetic resonance (27Al MAS NMR) spectroscopy, and mercury intrusion porosimetry (MIP) testing. The addition of alumina shortened the induction period, accelerated the acceleration period, and increased the mechanical strength of the PLC pastes with 3 and 7 wt% gypsum. The acceleration effect and strength enhancement were not distinct in the PLC pastes with less gypsum (0.1 wt %). The addition of gamma-alumina led to the greatest accelerating effect and strength improvement in the PLC pastes with 3 and 7 wt% gypsum, and the compressive strength achieved was comparable to those of pure Ordinary Portland cement (OPC) pastes without limestone. The addition of gamma-alumina induced the rapid dissolution of gypsum and consumption of sulfate ions. The chemical change affected by the addition of alumina varied greatly depending on the gypsum content, and trends in AFt and AFm formation in the PLC pastes with different gypsum contents were confirmed. Finally, pore size refinement was observed through the addition of alumina to PLC pastes with 3 and 7 wt% gypsum.

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In this study, we aim to evaluate the different dispersion methods that affect the dispersion behavior of graphene nanoribbons (GNRs) and to conduct a contrastive analysis of the different surfactants that influence the dispersion and stability of GNRs in deionized (DI) water and alkaline solution. Moreover, we studied uniformly dispersed GNRs with respect to the hydration, mechanical properties, and microstructure of the cement paste. Owing to the surface defects of the GNRs, electrostatic repulsion was sufficient to overcome the van der Waals force and provide a long-term period of high stability. The results show that by using SPs and 60 min of ultrasonication, the dispersibility of GNRs improved by 8.5% and 25.7% in DI water and alkaline solution, respectively. Uniformly dispersed GNRs accelerate the hydration and C-S-H polymerization, resulting in significant increases in compressive strength and splitting tensile strength by 17% and 33%, respectively, and reduced porosity by 14.6% after 28 days of curing.

This study investigated the effectiveness of graphene nanoribbons (GNRs) as nanoadditives for enhancing fire resistance of cementitious composites and compared their performance with conventional carbon nanotubes (CNTs). First, GNRs with striped structures exhibited superior dispersion stability compared to CNTs. Second, GNRs demonstrated excellent thermal stability, remaining structurally intact at 800 °C, while CNTs began decomposing above 450 °C. At room temperature, the GNRs significantly promoted the hydration reaction of cement clinkers, reduced the porosity, and refined the pore structure, thereby enhancing the mechanical properties of the cementitious composites. In high-temperature environments, GNRs act as bridges for cracks, effectively mitigating the deterioration of the mechanical strength caused by the dehydration and decomposition of hydration products. Moreover, the GNRs inhibited pore development and elongation, improving fire resistance. Overall, this study highlights the potential of GNRs as promising nanomaterials for enhancing the performance of cementitious composites under high-temperature conditions.

Although carbon nanotubes (CNT) and nanosilica (NS) have shown substantial potential for enhancing the thermal resistance and post-fire mechanical recovery of cementitious composites, their combined synergistic effects remain unclear. This study aimed to elucidate the influence of CNT and NS double-hybrids on the physicochemical properties of cementitious composites under different heating temperatures (200, 500, and 800 C-degrees) and re-curing conditions (25 degrees C/65 % RH and water immersion). We assessed the changes in the compressive and tensile strengths, bulk density, surface morphology, hydration products, and pore characteristics using focused ion beam scanning electron microscopy to visualize the evolving nanoscale pore structures. Our findings reveal a remarkable synergistic effect on the thermal resistance and strength recovery properties of the CNT/NS hybrid samples, owing to the stable matrix observed after heating to 800(degrees)C. Following exposure to 800 C-degrees, the tensile strength exhibited a remarkable 69.6 % increase compared to its pre-heating state, without any indication of crack formation. The CNT served as nucleation sites, expediting the pozzolanic reaction of NS during heating and resulting in a homogenized pore structure with interconnected hydrates. The CNT/NS hybrid samples exhibited uniform shrinkage of hydrates without creating nanoscale rod-like pores typical in ordinary cement paste, while the increase in pore volume was predominantly attributed to the expansion of existing pores and the formation of nearby new pores.

Carbon nanotubes (CNTs) enhance the thermal resistance and post-fire mechanical recovery of cementitious composites; however, the underlying mechanism has not been fully elucidated. This study aimed to reveal the effect of the incorporation of CNTs on the physicochemical properties of cementitious composites subjected to heating and rehydration. We assessed the variations in compressive and tensile strengths, volume, bulk density, surface morphology, and hydration products of Portland cement composites based on the amount of CNTs incorporated, heating temperature (200, 400, 600, and 800 degrees C), and re-curing conditions (under 25 degrees C/65 %RH and in water). Synchrotron three-dimensional X-ray nanoimaging was performed to visualize the internal nano- and micro-sized pore structure evolutions of the composites due to heating and rehydration. The CNT-incorporated samples exhibited improved thermal resistance and noticeable mechanical strength recovery by water re-curing, even after heating temperatures higher than the CNTs decomposition temperature (>600 degrees C). The CNTs improved the stability of the volume and density changes of the paste matrix and did not affect the formation of additional hydrates during the heating process. X-ray nanoimaging results revealed that cement hydrate formation on the dispersed CNTs resulted in a fastened and interconnected structure with dense pores (<2.5 mu m(3) and rod-like shape); these pores resist thermal deterioration and are advantageous for strength recovery by rehydration.

Graphene nanoribbons (GNRs) possess superior electrical properties due to their unique structures, making them increasingly valuable in composite materials. This study investigated the ability of GNRs to serve as nanoreinforcing agents that modify the electrical and self-sensing properties of cementitious composites compared to conventional nanomaterials such as carbon nanotubes (CNTs). Because they are highly dispersible in alkaline environments, GNRs can form effective conductive networks within a cement matrix. Incorporating 0.05 wt% GNRs into a cementitious composite significantly reduced the electrical resistivity of the sample after 28 d of curing by 64.61 % compared to the control sample. After drying, its electrical resistivity was still 42.82 % lower than that of the control sample. Furthermore, the GNRs-incorporated sample (dry state) exhibited a remarkable 63.65 % fractional change in resistivity when subjected to cyclic compressive stress. These results suggest that GNRs hold significant potential for enhancing the electrical and self-sensing properties of cementitious composites.

With the advent of rapid climate change and global warming, the cement industry has been actively striving to minimize its carbon footprint. An effective approach for reducing carbon emissions is to utilize supplementary cementitious materials (SCMs). Limestone calcined clay cement (LC3) has garnered significant attention because of its ability to reduce clinker use by approximately 45 wt%, compared to ordinary Portland cement. However, the raw materials involved in LC3 production, which are obtained through mining and heating of raw clay, also counts toward the carbon footprint. Therefore, the present study aims to explore the feasibility of using oyster shells as a replacement for limestone in LC3 systems. Various amorphousness of calcined clay were tested to investigate the hydration reaction and strength of the oyster shell calcined clay cement (OC3). Data collected using techniques such as X-ray diffraction, thermogravimetry, scanning electron microscopy, isothermal calorimetry, compressive strength testing, and 29Si nuclear magnetic resonance spectroscopy were analyzed to elucidate the hydration reaction mechanisms of OC3, considering the type of calcite (oyster shell or limestone) and the extent of the transformation of kaolinite clay to metakaolin. The findings of this study demonstrate that oyster shells can effectively replace limestone as a raw material for LC3. Furthermore, the irregular morphology of the shell particles enhanced the hydration reaction and development of the cement's microstructure.

The thermal stability of geopolymer pastes with different Si/Al ratios (1.5 - 3.0) and sodium activators (Na 2 SiO 3 and/or NaOH) were evaluated in this study. The inclusion of Na 2 SiO 3 induced an active geopolymer reaction, enhancing pre-heating compressive strength, but gradually reduced from 300 degrees C due to vapor pressure and microstructural deterioration caused by entrapped physically bound water. Conversely, the strength of geopolymers containing only NaOH remained stable or increased up to 600 degrees C, owing to additional geopolymerization from unreacted raw materials and compact matrix induced by thermal shrinkage. The aluminosilicate networks of all geopolymers remained relatively stable until crystalline nepheline formed above 800 degrees C; larger crystalline particles were formed in geopolymers with Na 2 SiO 3 owing to its denser geopolymer matrix, whereas smaller crystalline particles were formed in geopolymers with smaller Si/Al and only with NaOH, resulting in fewer voids and stable mechanical strength. Swelling of silica was notable in Na 2 SiO 3 -acti- vated geopolymer, resulting in greater strength loss upon heating, indicating that Na 2 SiO 3 hinders the thermal stability of geopolymers at extreme temperatures.

This study explores the influence of the interatomic structure of sodium aluminosilicate hydrate (N-A-S-H) with varying silica contents on the mechanical properties of metakaolin-based geopolymer. Geopolymer pastes comprising Si/Al ratios between 2.0 and 3.0 were synthesized. A larger number of Si-O-Si linkages compared to Si-O-Al linkages and a higher atomic number density were found in the geopolymers with higher silica contents, which enhanced the compressive strength of the geopolymer pastes up to the optimal Si/Al ratio of 2.5. The paste with a Si/Al = 2.5 exhibited a greater portion of Q4(1Al and 2Al) and denser morphology compared to the other geopolymer pastes. Furthermore, in-situ high-energy synchrotron X-ray scattering experiments were conducted to assess the elastic modulus of the aluminosilicate structure at a local atomic scale. The modulus value in real space decreases with increasing silica contents up to Si/Al = 2.5 and increases with the presence of excessive unreacted silica fume. The modulus value in reciprocal space for the axial and lateral directions both presented a positive value at the geopolymer comprising a Si/Al ratio higher than 2.5, indicating that the load-bearing property of N-A-S-H changed at higher Si/Al ratios. Moreover, the smallest difference between the strains along the axial and lateral directions was detected for the geopolymer with Si/Al = 2.5 in both the real and reciprocal space, owing to the most interconnected and flexible nanostructure, which led to the highest mechanical strength.

The effects of various initial carbonation curing environments on the phase evolution and resulting mechanical characteristics of tricalcium silicate paste were studied. For the analyses of the reaction products and microstructure, synchrotron X-ray diffraction, thermogravimetry, Fourier transform-infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and high-resolution X-ray computed tomography were utilized. C3S cured under carbonation environment pressurized by 0.1 MPa showed excellent mechanical properties owing to the highest degree of reaction and homogeneous generation of CaCO3 with low-Ca/Si calcium silicate hydrates, resulting in a dense matrix with refined pore structure. C3S paste treated under other carbonation conditions underwent deteriorative microstructural phase transitions, including void evolution by decalcification of C-S-H and an inhomogeneous composition of crystalline phases, resulting in inferior properties.

Nanocrystalline calcium???silicate???hydrate (C-S-H) is a typical heterogeneous material with a multiscale structure spanning a wide length scale from angstrom to micrometer, and whose structure is determined by the Ca/Si ratio. In this study, we directly applied compressive loads on synthetic C-S-H pastes with Ca/Si ratios of 0.6???1.2 and investigated their mechanical properties using the elastic modulus calculated at three length scale levels (i.e., angstrom to nanometer, micrometer, and millimeter) via in-situ synchrotron X-ray scattering, nanoindentation tests, and strain gauges, respectively. Further, 29Si nuclear magnetic resonance spectroscopy was conducted on the C-S-H pastes to elucidate the alterations in the silicate polymerization. The experimental results confirmed the deformation behavior of the C-S-H paste with different Ca/Si ratios under external loading, which was demonstrated to be transferred from the surface of the pastes to particles owing to the presence of multiscale pores.

The interfacial transition zone (ITZ) between aggregates and cement binders has long been known as the most vulnerable to crack resistance among the three phases in heterogeneous concretes [i.e., aggregate, ITZ, cement binders] owing to its porous microstructure. The characteristics of the ITZ are determined by the aggregate types, water-to-binder (W/B) ratios, and supplementary cementitious materials. Furthermore, the correlations between the physicochemical properties of the interface in the vicinity of saturated lightweight aggregates (LWAs) and the mechanical properties of cement concretes are still controversial because of the rough surface and ambiguous water desorption behavior of LWAs. To clarify such correlations, in this study we systemically investigated the effect of different water desorption behaviors of LWAs in cement mortars for various W/B ratios ranging from 0.2 to 0.5 on the mechanical properties of cement mortars via isothermal calorimetry tests, scanning electron microscopy, and nanoindentation grid tests. In addition, the experimental results were compared to those of mortars containing fine normal-weight aggregates (NWAs). These investigations clarified the effect of mechanical properties and porosity of the ITZ on the inherent strength of cement mortars with different aggregate types and W/B ratios. An improvement in the ITZ morphology around the LWAs was observed in relation to that around the NWAs for W/B > 0.3. Conversely, the porosity increased significantly on the LWA surface with respect to that on the NWA surface for W/B = 0.2. Nanoindentation tests revealed the deterioration of the elastic moduli and hardness of mortars containing LWAs at all investigated W/B ratios.

Several functional fillers have been used to modify the mechanical and electrical properties of cement-based materials. Graphene nanoribbons (GNRs) have attracted considerable attention as the next generation of carbon-based nanomaterials owing to their excellent properties. This paper aims to assess the influence of GNRs with varying incorporation amounts (0.05, 0.10, and 0.15 wt% of cement) on the mechanical properties, hydration behavior, and electrical properties of ordinary Portland cement paste via mechanical strength test, isothermal calorimetry, thermogravimetric analysis, X-ray diffraction, and alternating current electrochemical impedance spectroscopy. The enhanced performance of GNRs in cementitious composites was compared with that of carbon nanotubes (CNTs), which are one of the most popular and conventional nano-reinforcing agents for cement-based materials. The results of this study show that, compared to CNTs, GNRs significantly decrease the resistivity of cementitious composites owing to their excellent electrical properties. Furthermore, after the exfoliation of CNTs, GNRs possess a larger specific surface area and exhibit improved dispersibility. Thus, GNRs provide better nucleation and filling effects for cementitious composites to promote the hydration reaction of cement clinkers and modify the microstructure of the cement matrix, resulting in enhanced mechanical properties of the cementitious composites.

This study explored the effect of M-S-H formation on the local atomic arrangements and mechanical properties of C-S-H. The elastic moduli of the samples were calculated using shifted atomic distances (r) and d-spacings (d) acquired by applying an external load on the pastes during X-ray scattering experiments. The experimental results indicated that the crystal structure of C-S-H remained intact with MgCl2 addition. At the highest Mg/Si ratio (Ca/Si = 0.6, Mg/Si = 0.2), change in the dominant phase occurred from C-S-H to M-S-H because the low pH environment hindered the formation of C-S-H and facilitated the formation of M-S-H. The elastic modulus decreased with increasing Mg/Si ratio up to 0.1 owing to both C-S-H destabilization and low M-S-H content in the samples. Conversely, the elastic modulus increased in the paste synthesized with the highest Mg/Si ratio because considerable M-S-H had formed, which exhibited a higher elastic modulus than C-S-H.

This study investigates the synergistic strengthening mechanism of graphene oxide (GO), functionalized carbon nanotubes (f-CNT), and nano-silica (NS) triple hybrid-reinforced Portland cement composite. GO was selected as the variable owing to the synergistic effect of GO with both f-CNTs and NS upon dispersion by forming bonds with both nanomaterials. At a low GO dosage (similar to 0.03 wt% of cement), the bond between GO and NS deteriorated the dispersion in Ca2+-rich solution due to the overly attached NS on the GO surface. The highest GO fraction (0.05 wt%) also led to poor dispersion as the excess GO was agglomerated by Ca2+ ions. However, an optimal amount of GO (0.04 wt%) significantly improved the dispersion quality. The enhanced dispersion of the triple hybrid positively influenced the hydration degree and mechanical performance of the cement paste (133 % and 156 % for compressive and tensile splitting strength compared to OPC) and the pozzolanic reactivity of NS.