Studies on structural interaction and performance of cement composite using Molecular Dynamics

In order to promote the application of molybdenum (Mo) tailings in concrete industry, this paper studies the grinding characteristics of Mo tailings, the mechanical and hydration properties of mortars containing Mo tailings by X-ray diffraction (XRD), scanning electron microscope (SEM), laser particle size analysis, specific surface area analysis and strength test. The optimal grinding time is 80 mins to obtain the ground Mo tailings (-0.16 mm) with a specific surface area of 501 m2 kg-1 as mineral admixtures to the cementitious materials. With the coarse Mo tailings (+0.16 mm) as fine aggregate, composite mortars with a good workability were obtained with the compressive strength of 83.5 MPa and the flexural strength of 14.4 MPa after cured for 28 days. The comprehensive utilization rate of Mo tailings is 71 % in the composite mortars. The hydration of the cementitious composite containing Mo tailings at early age is primarily due to the hydration of the cement clinker (CC). While the large amount of quartz remains inert, the pozzolanic reaction of ground Mo tailings leads to the strength increases in long term. The main hydration phases in the cementitious composite are calcium silicate hydrates (C-S-H gels) and ettringite (AFt).

Full process of calcium silicate hydrate decalcification: Molecular structure, dynamics, and mechanical properties

Novel tricalcium silicate/magnesium phosphate composite bone cement having high compressive strength, in vitro bioactivity and cytocompatibility.

Calcium silicate hydrate (C-S-H) is a main product of Portland cement hydration. Although C-S-H is treated as an amorphous phase in macro scale, the atomic-scale structure of C-S-H is considered to be nanocrystalline and short-ordered. Various studies have been carried out and many models have been proposed in the past few decades to describe the structure of C-S-H at nano scale. Tobermorite and jennite are considered as two naturally existing crystalline solids sharing similar structures with C-S-H, especially tobermorite. As a result, tobermorite-based models are predominant in the literature. Besides, combination models with both tobermorite and jennite as well as exceptive models have also been developed in the same period. In this paper, the C-S-H models developed in the past decades are reviewed systematically. The reliability of the models is discussed based on the fundamental requirements of the C-S-H structure from consideration of experimental observations and structural chemistry, including charge balance, Ca/Si (C/S) ratio and Qndistribution. Finally, some considerations on the understanding of C-S-H are put forward for possible development of C-S-H structure in the future.

Cement based composites i.e. paste, mortar and concrete are the most utilized materials in the construction industry all over the world. Cement composites are quasi-brittle in nature and possess extremely low tensile strength as compared to their compressive strength. Due to their low tensile strength capacity, cracks develop in cementitious composites due to the drying shrinkage, plastic settlements and/or stress concentrations (due to external restrains and/or applied stresses) etc. These cracks developed at the nanoscale may grow rapidly due to the applied stresses and join together to form micro and macro cracks. The growth of cracks from nanoscale to micro and macro scale is very rapid and may lead to sudden failure of the cement composites. Therefore, it is necessary to develop such types of cement composites possessing higher resistance to crack growth, enhanced flexural strength and ductility. The development of new technologies and materials has revolutionized every field of science by opening new horizons in production and manufacturing. In construction materials, especially in cement and concrete composites, the use of nano/micro particles and fibers in the mix design of these composites has opened new ways from improved mechanical properties to enhanced functionalities. Generally, the production or manufacturing processes of the nano/micro sized particles and fibers are energy intensive and expensive. Therefore, it is very important to explore new methods and procedures to develop less energy intensive, low cost and eco-friendly inert nano/micro sized particles for utilization in the cement composites to obtain better performance in terms of strength and ductility. The main theme of the present research work was to develop a family of new type of cementitious composites possessing superior performance characteristics in terms of strength, ductility, fracture energy and crack growth pattern by incorporating micro sized inert carbonized particles in the mix design of cementitious composites. To achieve these objectives the micro sized inert carbonized particles were prepared from organic waste materials, namely: Bamboo, coconut shell and hemp hurds. For comparison purposes and performance optimization needs, another inorganic waste material named as carbon soot was also investigated in the present research. The experimental investigations for the present study was carried out in two phases; In the first phase of research work, a methodology was developed for the synthesis of the micro sized inert carbonized particles from the above mentioned organic raw materials. In the second phase of research, various mix proportions of the cementitious composites were prepared incorporating the synthesized micro sized inert carbonized particles. For micro sized inert carbonized particles obtained from bamboo and coconut shell three wt.% additions i.e. 0.05, 0.08, 0.20 were investigated and for particles synthesized from hemp hurds 0.08, 0.20, 1.00 and 3.00 wt.% additions were explored. The cement composites were characterized by third-point bending tests and their fracture parameters were evaluated. The mechanical characterization of specimens suggested that 0.08 wt.% addition of micro sized inert carbonized bamboo particles enhances the flexural strength and toughness of cement composites up to 66% and 103% respectively. The toughness indices I5, I10 and total toughness of the cement composites were also enhanced. The carbonized particles synthesized from coconut shell resulted in improved toughness and ductility without any increase in the modulus of rupture of the cement composite specimens. Maximum enhancements in I5 and I10 were observed for 0.08% addition of both carbonized and carbonized-annealed particles. For the carbonized hemp hurds cement composites the results indicate that the micro sized inert carbonized particles additions enhanced the flexural strength, compressive strength and the fracture energy of the cement composites. The microstructure of the cement composites was also studied with the help of field emission scanning electron microscope (FESEM) by observing small chunks of cement composite paste samples. The FESEM observations indicated that the micro sized inert carbonized particles utilized in the mix design of these mixes were well dispersed in the cement matrix. It was also observed that the fracture paths followed by the cracks were tortures and irregular due the presence of micro particles in the matrix. The cracks during their growth often contoured around the inert particle inclusions and resulted in enhanced energy absorption capacity of the cement composites. The study was further enhanced to the cement mortar composites and their performances were studied. The results indicated that the energy absorption behavior of the composites was enhanced for all the cement composites containing micro carbonized particles. Finally, it is concluded that the ductility and toughness properties of the cement composites can be enhanced by incorporating the micro sized inert carbonized particles in the cement matrix. The fracture energy, ductility and toughness properties enhancement of the cement composites greatly depends upon the source and synthesis procedure followed for the production of micro sized inert carbonized particles

Molecular simulation of calcium-silicate-hydrate and its applications: A comprehensive review

H2 diffusion in cement nanopores and its implication for underground hydrogen storage

Determination of fracture toughness of nano-scale cement composites using simulated nanoindentation technique

Microstructural damage from freeze-thaw cycles severely undermines the durability of cementitious materials. Although graphene oxide (GO) is known to refine cement microstructures, its molecular mechanism for enhancing frost resistance remains unclear. This study employed molecular dynamics (MD) to investigate the effect of GO on the properties of calcium silicate hydrate (CSH) under freeze-thaw cycles. Simulation results demonstrated that GO forms strong bonds with the CSH matrix, leading to a densified interfacial structure. Atomic-scale analysis revealed that GO stabilizes the hydrogen-bonding network within CSH. Consequently, GO mitigates the degradation of peak compressive stress and elasticity modulus relative to neat CSH under the same freeze-thaw protocol. As a result, the overall damage tolerance of the composite is enhanced. This study elucidates the mechanism behind the enhanced frost resistance of GO-modified cement mortar at the molecular level and provides theoretical support for research on the durability of nano-modified cementitious materials. All MD simulations were conducted using LAMMPS. The models employed ReaxFF force field. To simulate the freeze-thaw damage mechanism in cementitious materials, a cyclic strain with a maximum amplitude of 0.15 was applied along the Z-axis of the CSH model.

Hydration kinetics, pore structure, 3D network calcium silicate hydrate, and mechanical behavior of graphene oxide reinforced cement composites

The mechanical and rheological properties of ethylene α-olefin copolymers are governed by a molecular and supramolecular structure that depends on its molecular weight characteristics, comonomer concentration, and the uniformity of branching distributions for polymer chains with different molecular weights (compositional heterogeneity). Using state-of-the-art methods, we study the parameters of molecular weight distribution, molecular structure, and compositional heterogeneity of ethylene hexene-1 copolymers with different compositions, produced over highly active supported Ti-Mg and V-Mg Ziegler-Natta catalysts, and show the possibility of controlling these parameters to improve the quality of the resulting polymers.

Experiment and molecular dynamics simulation of functionalized cellulose nanocrystals as reinforcement in cement composites

Geometry conformations of six azotetrazolate nonmetallic salts such as guanidinium azotetra- zolate (GZT) etc. were optimized by density functional theory (DFT) at the level of B3LYP/6-31G. Molecu- lar structure parameters such as the energy of lowest unoccupied molecular orbital (ELUMO), the energy of highest occupied molecular orbital (EHOMO) and the atom charge of six azotetrazolate nonmetallic salts were calculated. Relationships between the impact sensitivity and those molecular structure parameters or ther- modecomposition parameters were investigated. Results showed that the impact sensitivity had good rela- tionships with parameters such as oxygen balance, thermodecomposition temperature, thermodecomposition activation energy or net atom charge on substituted groups in cation. More precisely, the impact sensitivities decrease with the decrease in oxygen balance or increase with the decrease in the thermodecomposition

Reactive molecular dynamics simulation of the mechanical behavior of sodium aluminosilicate geopolymer and calcium silicate hydrate composites

Chloride ions transport and adsorption in the nano-pores of silicate calcium hydrate: Experimental and molecular dynamics studies