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The first hours' hydration of fast-setting binder is difficult to monitor due to the limited temporal resolution of common micro-structure-chemistry probes and the complicated product compositions. This study explored using T 2 relaxation time distribution data from in-situ 1H nuclear magnetic resonance (NMR) measurement to quantify the early-hydration kinetics of two OPC/CAC/calcium sulfate ternary binders. The calculated phased composition and porosity evolution were compared with evidence from in-situ XRD, SEM, TGA, temperature and tensile splitting strength evolution. We demonstrate that the deconvoluted T 2 signal of water can provide a reliable estimation of microstructural information, e.g., the ettringite product and the porosity. There exists a linear relationship between the averaged T 2 signal and the porosity of the paste. In addition, the averaged T 2 signal provides a good index of strength, which applies equally to all studied samples despite their different in the cement type, w/c and presence of fine aggregates. [ABSTRACT FROM AUTHOR]

Construction with cementitious materials via 3D printing requires a highly thixotropic behavior, which is a challenge for construction industry. In this work a facile and low-cost gelatin-modified thermal responsive smart cement paste formulation was developed for extrusion-based 3D printing in cold environments The paste's temperature-dependent rheological properties were characterised, and the underlying mechanism was explored. At room temperature, the modified paste presents similar rheological properties as the reference cement paste. After resting for 10 min at 5 °C, the modified cement paste has a yield stress of 1900 Pa which is 10 times that of the reference paste. Moreover, the mechanical properties of the modified paste are comparable to or higher than that of the reference paste at later age. Our approach may inspire a facile manipulation of paste rheology through temperature, which facilitates smartly controlled 3D concrete printings. [ABSTRACT FROM AUTHOR]

The ingress of water-soluble aggressive species is a major cause to the reduced service life of concrete structures. Bulk modification by adding hydrophobic agents in the cementitious materials is commonly applied to reduce water penetration into the concrete matrix. However, conventional hydrophobic modifications of concrete are associated with several unresolved major drawbacks such as high cost and toxicity. Here, we investigate the impact of safe and easy-to-culture bacteria on the hydrophobic properties of concrete. We demonstrate that hydrophobic concrete can be conveniently created by using the liquid culture of bacteria without the need of growing biofilms as suggested by previous research, even with a strain that does not present innate hydrophobicity. These key findings greatly improve the feasibility for the large-scale application of water resistance cementitious materials. Scanning electron microscopy and X-ray diffraction analyses indicated higher levels of micro-calcite formations with the increased addition of bacteria. These micro-calcite structures account for the dramatic increase in surface roughness which leads to elevated hydrophobic properties of the hybrid concrete. This phenomenon is reminiscent of the "lotus-leaf effect", with ultrahydrophobicity arising from the nano- and microscopic topologies of surfaces. Bacteria-modified cement presents distinct heat-flow profiles during hydration consistent with the increased calcite formation. Taken together, this research gives unprecedented mechanistic insight into the design of microbial-modified concrete with hydrophobic properties particularly useful in floating and offshore structures. Given the large repertoire of bacteria still to be used, our work sets the stage for the future design of microbial-based hydrophobic concrete that can be tailored with additional functions such as self-healing and odor removal. [ABSTRACT FROM AUTHOR]

The strength origin of C-S-H matrix is a key question in cement and concrete research, which remains largely unclear despite a century of study. Here we present a novel approach to endow strength integrity to C-S-H powders via mechanical compaction at room temperature. The unprecedented 2000 MPa compaction load results in a bulk C-S-H with tensile strength (~24 MPa) one order of magnitude higher than C-S-H matrix from hydration. This practice enables observations on how Ca/Si ratio (C/S) and initial moisture conditions affect their overall mechanical properties (hardness and strength), which are correlated with the micro-structures studied by 1H NMR and electronic microscope. We conclude that the bond between closely-contacted C-S-H particle, rather than the total porosity, is key to the bulk strength. The bond is clearly weakened when excessive water exists between C-S-H particles. This work provides novel insight to the strength origin of C-S-H. [ABSTRACT FROM AUTHOR]

Cemented paste backfill (CPB) is an efficient and sustainable technology for the disposal of tailings in mining industry. However, the flowability of CPB is not completely clear due to the lack of robust characterization methods and sufficient studies. In this study, the effects of pre-shear rate and time, resting time, and shear protocol on the rheological measurements of fresh CPB were examined. Additionally, a yielding model was built, and the 2-h structural evolution was investigated. The structural evolution of CPB was observed using the oscillatory shear rheology method. The underlying mechanisms were investigated using zeta potential, thermogravimetry, ionic concentration, focused beam reflectance measurement, and 1H nuclear magnetic resonance. The pre-shear rate of 100 s−1, pre-shear time of 100 s, pre-shear resting period of 150 s and the stress ramp-up protocol were all recommended by the results. The dual yielding properties of CPB are adequately represented by the proposed model. It is possible to categorize the structural evolution of CPB at rest into three stages, each of which can be attributed to flocculation brought on by particle interactions, packing density enhancement from the massive formation of rigid links, and an increase in the volume fraction of solid, respectively. [ABSTRACT FROM AUTHOR]

• A simple thixotropy model for superfine-tailings cemented paste backfill (SCPB) was proposed. • The H-B model provides the most stable dynamic yield stress estimation of SCPB. • The effect of pre-shear time on the transient flow and time-dependent rheological behavior of SCPB was studied. The fresh superfine-tailings cemented paste backfill (SCPB) exhibits strong thixotropy, but quantification of the resulting transient flow (non-steady state) and time-dependent yield stress is lacking. In this study, a simple qualitative model was developed to describe the transient flow and time-dependent yield stress of SCPB. The effect of pre-shear time on the rheological behavior of SCPB was investigated. In addition, the adaptability of conventional non-Newtonian rheological models to SCPB was also evaluated. The results showed the Herschel-Bulkley model provides the most stable dynamic yield stress estimation of SCPB compared to the Bingham and modified Bingham models. A longer pre-shear time (within 500 s) led to smaller initial static yield stress and delayed recovery kinetics of static yield stress, but hardly affected the steady state of SCPB and the time required to reach it. The proposed model provides a good quantification of the transient flow at a given shear rate and time-dependent yield stress of SCPB. [ABSTRACT FROM AUTHOR]