Test and Calculation Model of Thermal Conductivity of Concrete

In order to study the influence of water content and hydration degree on the thermal conductivity of concrete, based on the steady-state plate method, the influence of water content, temperature, hydration degree and other factors on the thermal conductivity of early concrete was studied, and the calculation model of thermal conductivity of early concrete was established according to the test results and influence rules. The results show that there is a linear relationship between the thermal conductivity and water content at 28d, the higher the hydration degree is, the lower the thermal conductivity of concrete is; the difference between the measured value and the calculated model value is small, and the calculation formula can meet the requirements of engineering calculation.

Effect of Nitrate/Bromide on the Hydration Process of Cement Paste Mixed with Alkali Free Liquid Accelerator at Low Temperature

The effects of different inorganic salt accelerators (CaBr2, NaBr, Ca(NO3)2, NaNO3) and an alkali-free liquid accelerator were researched at a low temperature of 10 °C. The results showed the effects of 1.5% NaBr and 1.5% NaNO3 inorganic accelerator were pronounced. The 1-d compressive strengths of the mortar with these two inorganic salts were increased by 185.8% and 184.2%, respectively, and the final setting times were shortened from 7.74 to 6.08 min and 6.12 min, respectively. The hydration temperatures at 10 °C were measured, and the promotion effects of the inorganic accelerators were calculated: the relationship between the hydration degree was αAS + NN > αAS + NB > αAS + CB > αAS + CN > αAS. In addition, the reaction of C3A with NaBr and NaNO3 was used to analyze the products in an ettringite phase, i.e., Ca4Al2O6Br210·H2O, 3CaOAl2O3Ca(NO3)2X·H2O. The formation of these phases was detected in the hydration products of the cement paste hydration for 12 h, 24 h, and 28 d. Combined with the mass loss of the ettringite phase at 90–120 °C, determined using TG/DTG, the synergetic acceleration mechanism of the inorganic accelerators was comprehensively inferred.

A Comparative Study on the Mechanical Properties and Microstructure of Cement-Based Materials by Direct Electric Curing and Steam Curing

Direct electric curing (EC) is a new green curing method for cement-based materials that improves the early mechanical properties via the uniform high temperature produced by Joule heating. To understand the effects of EC and steam curing (SC) on the mechanical properties and microstructure of cement-based materials, the mortar was cured at different temperature-controlled curing regimes (40 °C, 60 °C, and 80 °C). Meanwhile, the mechanical properties, hydrates and pore structures of the specimens were investigated. The energy consumption of the curing methods was compared. The results showed that the EC specimens had higher and more stable growth of mechanical strength. The hydration degree and products of EC samples were similar to that of SC samples. However, the pore structure of EC specimens was finer than that of SC specimens at different curing ages. Moreover, the energy consumption of EC was much lower than that of SC. This study provides an important technical support for the EC in the production of energy-saving and high early-strength concrete precast components.

Additives Imparting Antimicrobial Properties to Acrylic Bone Cements

PMMA bone cements are mainly used to fix implanted prostheses and are introduced as a fluid mixture, which hardens over time. The problem of infected prosthesis could be solved due to the development of some new antibacterial bone cements. In this paper, we show the results obtained to develop four different modified PMMA bone cements by using antimicrobial additives, such as gentamicin, peppermint oil incorporated in hydroxyapatite, and silver nanoparticles incorporated in a ceramic glass matrix (2 and 4%). The structure and morphology of the modified bone cements were investigated by SEM and EDS. We perform experimental measurements on wettability, hydration degree, and degradation degree after immersion in simulated body fluid. The cytotoxicity was evaluated by MTT assay using the human MG-63 cell line. Antimicrobial properties were checked against standard strains Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. The addition of antimicrobial agents did not significantly affect the hydration and degradation degree. In terms of biocompatibility assessed by the MTT test, all experimental PMMA bone cements are biocompatible. The performance of bone cements with peppermint essential oil and silver nanoparticles against these two pathogens suggests that these antibacterial additives look promising to be used in clinical practice against bacterial infection.

Difference between Internal and External Hydration of Hardened Cement Paste under Microwave Curing

In order to investigate the difference between internal and external hydration of hardened cement paste under microwave curing, a comparative study on the hydration products, hydration degree, fracture morphology, and pore structure between the inner part and outer part of hardened cement paste (Φ120 mm × 120 mm) under microwave curing was carried out by XRD-Rietveld refinement, TG-DSC, SEM, and MIP methods. The results show that the total hydration degree of the inner part is lower at early ages, but with the hydration, there is little difference in the hydration degree between inner and outer parts at later ages. Apart from granular AFt crystal formed in the inner part of hardened cement paste, there is little difference in the fracture morphology between internal and external hydration. The total porosity of the outer part is lower than that of the inner part.

Coupling Influence between Recycled Ceramics and Grazed Hollow Beads on Mechanical Properties and Thermal Conductivity of Recycled Thermal Insulation Concrete

This paper investigated the influence of recycled ceramics and grazed hollow beads on the mechanical, thermal conductivity and material properties of concrete. The results showed that the concentration of recycled ceramics and grazed hollow beads has significant optimization on the workability and thermal properties of the concrete. However, the superabundant concentration can reduce the hydration degree of the concrete, which results in the suppressed production of C-S-H gel and the increase of material defects. In summary, considering the coordinated development of key factors such as thermal insulation properties, mechanical properties and microstructure, 10% RCE and 60% GHB are the optimal material system design methods.

Effect of C-S-H Nucleating Agent on Cement Hydration

This work aims to study the effect of a nucleating agent on cement hydration. Firstly, the C-S-H crystal nucleation early strength agent (CNA) is prepared. Then, the effects of CNA on cement hydration mechanism, early strength enhancement effect, C-S-H content, 28-days hydration degree and 28-days fractal dimension of hydration products are studied by hydration kinetics calculation, resistivity test, BET specific surface area test and quantitative analysis of backscattered electron (BSE) images, respectively. The results show that CNA significantly improves the hydration degree of cement mixture, which is better than triethanolamine (TEA). CNA shortens the beginning time of the induction period by 49.3 min and the end time of the cement hydration acceleration period by 105.1 min than the blank sample. CNA increases the fractal dimension of hydration products, while TEA decreases the fractal dimension. CNA significantly improves the early strength of cement mortars; the 1-day and 3-days strength of cement mortars with CNA are more than the 3-days and 7-days strength of the blank sample. These results will provide a reference for the practical application of the C-S-H nucleating agent.

Early Cracking Risk Prediction Model of Concrete under the Action of Multifield Coupling

Through the adiabatic temperature rise experiment, the adiabatic temperature rise of concrete with hydration time was recorded. Based on the maturity degree theory, the relationship between the hydration degree of the concrete and the equivalent age was determined. Then, the hydration degree prediction model of the concrete's early elastic modulus and tensile strength was established. The local temperature and humidity of the concrete were measured by the shrinkage experiment, and based on the capillary water tension theory, a temperature-humidity prediction model for the early shrinkage of the concrete was designed. According to the ratio of the creep deformation and elastic deformation of concrete which were obtained through the restraint ring experiment, a model for predicting the early creep coefficient of concrete was proposed. Based on the coupling effect of “hydration-temperature-humidity,” a prediction model of early cracking risk coefficient of concrete under multifield coupling was proposed. Finally, several groups of slab cracking frame experiments were carried out, and the cracking risk prediction results of concrete were consistent with the actual situation, which indicated the correctness of the early cracking risk prediction model of concrete.

Simulation of chemo-thermo-mechanical problems in cement-based materials with Peridynamics

A chemo-thermo-mechanical problem is solved using a peridynamic approach to investigate crack propagation in non-reinforced concrete at early-age. In the present study, the temperature evolution and the variation of the hydration degree in conjunction with the mechanical behaviour of cement-based materials are examined. Firstly, a new peridynamic model is introduced to solve fully coupled chemo-thermal problems by satisfying thermal equilibrium condition and hydration law simultaneously and then the effects of the chemo-thermal analysis are imposed in the mechanical framework to investigate all the interactions. The proposed approach is used to solve 2D chemo-thermo-elastic problems and then it is applied to investigate the fracture of concrete structures. Additionally, we examine the accuracy of the method by comparing the crack paths, temperature and hydration degree with those achieved by applying other numerical methods and the experimental data available in the literature. A good agreement is obtained between all sets of results.