Cement & concrete composites最新文献

The high water absorption and porosity of recycled aggregate often led to a compromised interface transition zone (ITZ), thereby adversely impacting the mechanical properties and durability of recycled aggregate concrete. This research presents a feasible, straightforward, and targeted strategy to reinforce the ITZ between recycled fine aggregate (RFA) and paste by utilizing RFA particles adsorbed with graphene oxide (GO), termed WGO@RFA. The experimental outcomes demonstrate that incorporating WGO@RFA can enhance the 28-day compressive and flexural strengths of recycled mortars by approximately 25 % and 20 %, respectively, compared to mortars containing only RFA. Furthermore, it can decrease the water sorptivity and chloride ion diffusion coefficients of recycled mortars (28 days) by about 20 % and 27 %, respectively. Notably, using WGO@RFA particles offers significant advantages, such as enhanced mechanical strengths, reduced transport properties, and a densified microstructure within the ITZ, compared to the conventional method of modifying the cement matrix with GO and then binding it with RFA. Highlighting the application of WGO@RFA shows a targeted strengthening of the ITZ, as the sub-nanometer thickness of GO adsorbed on the uneven RFA surface facilitates localized cement hydration at the ITZ. The findings of this research offer novel avenues for reusing aggregate and developing sustainable concrete.

Mixtures of powders of waste glass (WG), limestone (LS), Na₂CO₃ and CaO were used to formulate novel one-part in situ alkali-activated cement (WG-AAC). The in-situ interaction Na₂CO₃-CaO-H₂O promoted the formation of CaCO₃ and NaOH, which promoted the WG and LS dissolution and influenced the micro- and molecular features of the resulting cementitious products. Pastes and mortars developed 1-year strengths of up to 29 MPa and were stable underwater. Characterization by XRD, SEM, EDS, and ²⁹Si-NMR indicated that a Na₂CO₃:CaO ratio close to 1:1 resulted in polymerized C-S-H, CaCO₃, silica gel, and Ca-modified silica gel, which were intimately intermixed and possibly crosslinked through Q³ bonds. Such phases interacted synergistically improving the underwater stability of the WG-AAC, indicating that in-situ caustification is a suitable and practical alkaline activation for SiO₂-rich precursors.

This study explores the electrical and piezoresistive properties of ultra-high toughness cementitious composites (UHTCC) enhanced with multi-walled carbon nanotubes (MWCNTs) ranging from 0 to 1 wt% of cementitious binders. The observed polarization behavior is found to be analogous to the charging process of a capacitor. The polarization process and resistivity drift over time in the piezoresistive response are explained using an existing equivalent electrical circuit model incorporating a capacitor. The average results of electrical conductivity initially decrease and subsequently increase with higher MWCNTs concentrations, a phenomenon attributed to increased porosity and reduced matrix conductivity. The percolation threshold is identified at a volume fraction of 0.00387. Notably, even in the absence of MWCNTs, UHTCC materials exhibit piezoresistive properties due to the presence of metal impurities and ionic compounds. The insufficient polarization process results in an increasing trend in fractional change in resistance (FCR). The highest FCR sensitivity to external load occurs within the percolation threshold. Additionally, three equations are proposed to calculate electrical conductivity, incorporating the effects of interfaces, porosity, and matrix conductivity reduction, which align well with the experimental findings. These insights contribute to a deeper understanding of the electrical properties of UHTCC-MWCNTs composites, enabling more precise conductivity measurements and improved sensor sensitivity.

A eco-friendly carbonation coating (HBCC) with a piezo-photocatalysis was developed using gamma-dicalcium silicate and hydrophobic BiOI/BaTiO₃ (HB), aiming at purifying pollutants by multi-dimensional energy (mechanical energy and visible light) and self-sequestrating CO₂ produced by degrading pollutants. Based on the self-floating effect induced by the hydrophobicity of HB, the increase of catalyst content on the surface of HBCC was studied to promote the formation of a hydrophilic-hydrophobic interface. The selective adsorption of CO₂ and H₂O molecules by the hydrophilic-hydrophobic interface of HBCC was confirmed by simulations and experiments, which accelerates carbonation. Also, carbonation degree (37.1 %), bonding strength (40.1 %), and anti-corrosion performance (15.4 %) enhanced induced by accelerating carbonation was further confirmed. Additionally, HBBC exhibits the prominent degradation effect of Rhodamine b (90.8 %), methylene blue (86.6 %), and sulfamethoxazole (74.7 %) under ultrasound and visible light within 60 min. Meanwhile, CO₂ emitted by piezo-photocatalytic degradation pollutants can be efficient sequestration by HBCC itself, and the carbonation can be enhanced to further improve its bonding strength. Finally, the enhancement mechanism of carbonation, water purification, and CO₂ self-sequestration of HBBC was explored and ascertained.

The chemically induced degradation of alkali-activated materials exposed to the surrounding environment is a critical concern for durability. In this study, the leaching of alkali activated slag mortars (AASs) subjected to a 6M NH₄NO₃ solution was investigated by integrating techniques including ICP-OES, XRD/QXRD, TGA/DSC, ATR-FTIR, and ²⁹Si MAS-NMR. The results revealed that the main leachable elements from the AASs and their leaching rates decreased in the following order: Na, K, Ca, and Mg. In contrast, Si and Al, the key elements in the C-A-S-H gel, displayed a remarkable resistance to leaching. Upon NH₄NO₃ attack, the primary phase (C-A-S-H) becomes more siliceous and has a greater mean chain length through decalcification and dealumination. The second phase, Mg, Al-layered double hydroxide (Mg, Al-LDH, or hydrotalcite), incorporated nitrate from the surrounding solution, sulfate from precursor dissolution, and Ca from gel decalcification to form nitrate/sulfate-bearing Ca, Al-LDH phases. Remarkably, the water-to-binder ratio exerted a nuanced influence, dictating the pace of element leaching, while exhibiting a relatively modest impact on the stability of the solid phases after 28 days of exposure. This work proposes a leaching mechanism for understanding the leaching process occurring in AASs based on an in-depth experimental exploration of mineralogical alterations.

Novel Ultra-High-Performance-Concrete (UHPC) structures reinforced with Fiber-Reinforced Polymer (FRP) rebars are promising candidates for applications in important infrastructures under exposed environments where normal concrete and steel rebars may falter. This paper aims to assess the bond-slip behavior between lapped sand-coated deformed Glass FRP (GFRP) rebars and UHPC using double-row splice tests, with parameters including bar diameter, splice length and lap clearance. Failure modes including the pullout of GFRP rebars and the splitting of UHPC were identified. For cases of pullout failure, the average bond strengths in samples with splice lengths of 5d_b were reduced by 17.6–22.1 % compared to those of 2.5d_b. Increasing the lap clearance from 0 to 1d_b and 2d_b led to 11.1 % and 30.2 % increases in average bond strengths. Furthermore, average bond-slip models for lapped sand-coated deformed GFRP rebars in UHPC were developed. The predicted curves matched the experimental ones, showing errors within 20 % for both average bond stresses and slips. When the cover is not less than 2d_b, the splice length is recommended to be at least 15d_b for sand-coated deformed GFRP rebars with diameters of 10 mm–16 mm in UHPC, approximately 1.25 times the corresponding development length proposed by the existing research.

In this study, we investigated the impact of introducing an in-situ activator, produced by carbonating cement particles in an aqueous solution, on the properties of cement mortars through secondary mixing. Two organic additives, ethylenediaminetetraacetic acid (EDTA) and glutamic acid (GLTA), were employed to enhance the leaching of calcium ions during carbonation, thereby improving the carbonation efficiency. A suite of characterization techniques revealed that the presence of organic additives could refine the carbonated particles and influence the morphology. The carbonated activators generated by this process were rich in silica gel and various polymorphic forms of calcium carbonate. These components, serving as fillers and nucleation for cement hydration, significantly accelerated the hydration process of cement mortar and promoted the formation of carboaluminate in the secondary mixing process. This approach effectively decreased the porosity of the cement mortar, refined the pore structure, and enhanced the mechanical strength.