Cement and Concrete Research最新文献

The study of mixed Al³⁺/Fe³⁺ nitrate hydrates is gaining increasing interest with the emergence of new hydraulic binders and Supplementary Cementitious Materials (SCMs) containing significant amounts of iron and aluminum oxides that can be activated by nitrate addition. Layered AFm-NO₃ hydrates containing varying proportions of trivalent Al³⁺ and Fe³⁺ cations were synthesized and characterized. The AFm-NO₃ phase constitutes a continuous solid solution between the two end-members Ca₂Al(OH)₆·NO₃·2H₂O and Ca₂Fe(OH)₆·NO₃·2H₂O. The obtained Ca₂Al_1-xFe_x(OH)₆·NO₃·yH₂O solid solution proves stable for four hydration states, with a significant impact on the interlayer distance: y = 5 (at high relative humidity with an interlayer distance of 10.4 Å), y = 2 (at ambient conditions with an interlayer distance of 8.6 Å), y = 1 (above 60 °C with an interlayer distance of 7.4 Å) and y = 0 (anhydrous above 175 °C with an interlayer distance of 8.0 Å). In fact, thermal variations reveal some differences between the two Al-rich and Fe-rich sides. Finally, AFm-NO₃ stability was examined in the presence of other anions, demonstrating the low stability of NO₃⁻ with respect to Cl⁻ and CO₃²⁻ anions.

This study introduces a novel approach for in-situ CO₂ sequestration using recycled concrete fines (RCFs). The method employs heterogeneous dual‑calcium carbonate (Cc) precipitation from wet carbonation and calcium bicarbonate (Ca(HCO₃)₂) solution to form a Cc binder between RCFs. The results demonstrate that metastable aragonite is significantly promoted and stabilized in wet carbonation by leveraging the seeding effects from semi-dry carbonated RCFs. A carbonated cement paste layer attached to the surface of RCFs facilitates the combination of precipitated aragonite crystals, while K (Na)-feldspar and quartz-based aggregates exhibit a relatively lower affinity for combining with Cc. The entanglement of aragonite crystals provides most of the strength in the carbonated RCF system. In the non‑carbonated recycled cement paste powders system, wet carbonation primarily produces calcite, while precipitation from Ca(HCO₃)₂ yields aragonite, forming a Cc solid skeleton that contributes to the strength.

Structural build-up in fresh cement paste at rest is characterized by time evolutions of storage modulus and yield stress, which both increase linearly in time during the induction period of hydration, followed by an exponential evolution after entering the acceleration period. Here, we investigate structural build-up by coupling calorimetry and oscillatory shear measurements of Portland Cement at different w/c ratios and in the absence of admixtures, capturing how the storage modulus evolves with changes in cumulative heat. This allows the decoupling of hydration kinetics from the mechanisms dictating build-up at rest. We obtain an exponential relation between stiffness and heat, with the same exponent in both the induction and acceleration periods. This suggests that, at least in the absence of admixtures, the same mechanism dictates build-up at rest in both periods. Since it is understood that C-S-H dictates build-up at rest in the acceleration period, we deduce that the same mechanism holds in the induction period.

Mobile robots for 3D printing applications are ready to transition from factory floors to building sites. Their remarkable flexibility and adaptability support a variety of deposition-based 3D printing technologies that utilise materials ranging from concrete and earth for extrusion, spraying, or shotcreting to metals for processes like Wire Arc Additive Manufacturing. Not confined to new constructions alone, their mobility enables utilisation in corrective building maintenance, restoration, revitalisation, and repair. Their ability to cooperate with one another allows for deployment in multi-robot settings, offering scalability in speed by their number. Despite their promising potential, mobile 3D printing robots also encounter numerous technological challenges. These include ensuring the mechanical properties of printed structures meet required building codes, designing robust mechanical systems for large-scale construction projects, and integrating these systems seamlessly with existing architectural planning tools. Moreover, enhancing the precision and robustness of these robots through advanced sensing and control technologies is critical for their effective application in building manufacturing. With this paper, we detail selected current research trajectories and give insights into current challenges, open questions, and key prospects associated with mobile 3D printing robots for on-site construction within existing environments. To enrich the discussion, insights into potential architectural application scenarios for revitalising, repairing, and strengthening building structures are provided. The complex, interdisciplinary nature of these challenges underscores the need for a collaborative approach in advancing the field of mobile 3D printing technology.

This study describes advances in high-performance construction material development using a minimum of primary resources while enabling simultaneous CO₂ sequestration capacities. Two so far unutilized Austrian steel slags were combined with metakaolin and vegetable oil to produce alkali-activated materials exhibiting high compressive and flexural strength of up to 94 MPa and 13 MPa, respectively. This approach enabled a reduction in primary mineral resources of up to 82 wt%, with an average reduction in global warming potential (GWP) of 52 % compared to a traditional high-performance Portland cement material. Oil addition led to the formation of mainly water unsolvable metal soap phases precipitating within the pore spaces without significantly altering the phase assemblage and chemistry of the binder matrix, but further reducing the GWP by 74 %. The (heavy metal) leaching behavior coincides with that of traditional concrete materials and was even further reduced by the addition of oil.

Calcined clays, combined with limestone, exhibit significant potential as SCMs for achieving high clinker substitution levels. This is partly related to their high aluminum and silicon contents. Binary Portland cement – calcined clay (PC²) blends and ternary blends with limestone (LC³) have been examined using metakaolin (MK) as calcined clay for blends with low and high MK/clinker ratios. The hydration reactions (up to 420 days) and aluminum distribution within the C-(A)-S-H and calcium aluminate hydrate phases have been quantitatively assessed by ²⁷Al and ²⁹Si NMR spectroscopy. The presence of limestone induces higher amounts of ettringite in the LC³ blends, resulting in lower amounts of Al(4) in the C-(A)-S-H. For the high MK/clinker ratio blends, the increase of aluminum incorporated in the C-(A)-S-H phase after prolonged hydration coincides with a leveling off for the amount of AFm phases for the binary and ternary blends at 60 % and 40 % of MK reaction, respectively.

Moisture sorption and macroscopic deformation of hardened cement pastes (hcps) were studied in the first three drying–resaturation (D–R) cycles using low–field ¹H nuclear magnetic resonance relaxometry. Four types of water (interlayer, gel, interhydrate and capillary) were detected in the hcps. In principle, they were sequentially removed from larger to smaller pores during drying with decreasing relative humidity (RH), resulting in shrinkage of the hcps. Unexpected increases in gel and interlayer water contents as well as greater shrinkage during drying from 100 % to 69 % RH were observed, which is believed to originate from the re–arrangement of C–S–H structure. Because of the C–S–H re–arrangement, irreversible shrinkage was detected during D–R cycles. The drying and irreversible shrinkage in each cycle diminished with the D–R number. A lower water–to–cement ratio and higher curing temperature for hcps are beneficial for reducing drying and irreversible shrinkage.