Multi-layered cement-hydrogel composite with high toughness, low thermal conductivity, and self-healing capability.

The inherent quasi-brittleness of cement-based materials, due to the disorder of their hydration products and pore structures, present significant challenges for directional matrix toughening. In this work, a rigid layered skeleton of cement slurry was prepared using a simplified ice-template method, and subsequently flexible polyvinyl alcohol hydrogel was introduced into the unidirectional pores between neighboring cement platelets, resulting in the formation of a multi-layered cement-based composite. A toughness improvement of over 175 times is achieved by the implantation of such hard-soft alternatively layered microstructure. The toughening mechanism is the stretching of hydrogels at the nano-scale and deflections of micro-cracks at the interfaces, which avoid stress concentration and dissipate huge energy. Furthermore, this cement-hydrogel composite also exhibits a low thermal conductivity (around 1/10 of normal cement) and density, high specific strength and self-healing properties, which can be used in thermal insulation, seismic high-rise buildings and long-span bridges.

Hierarchical Toughening of a Biomimetic Bulk Cement Composite.

Because of the inherent quasibrittleness and heterogeneity, matrix-directed toughening of concrete and cement composites remains to be a huge challenge. Herein, inspired by nacre materials, a novel biomimetic bulk cement composite is fabricated via a facile and efficient process based on compacting prefabricated multisized cement-polymer hybrid prills. This method combines with the three-dimensional "brick-bridge-mortar" structure design and synchronously the intrinsic and extrinsic toughening strategies. Such an approach shows the remarkable maximum toughness enhancement of 27-fold with 71% increase in flexural strength via cooperation with only 4 wt % organic matter. More attractively, it alters the traditional brittle fracture of cement composites to a distinct ductile fracture. In addition, such a biomimetic composite demonstrates the long-term ever-increasing strength and toughness, performing the excellent ductile-fracture retention ability. The hierarchical toughening mechanisms are further revealed with the synergy of microscopic characterizations and simulation methods. This strategy provides a new route for the development of high toughness biomimetic cement-based materials for potential applications in civil engineering domain.

Factors influencing the stability of AFm and AFt in the Ca-Al-S-O-H system at 25 °C.

The stabilities of Al2O3-Fe2O3-mono (AFm) and -tri (AFt) phases in the Ca-Al-S-O-H system at 25 °C are examined using Gibbs energy minimization as implemented by GEM-Selektor software coupled with the Nagra/PSI thermodynamic database. Equilibrium phase diagrams are constructed and compared to those reported in previous studies. The sensitivity of the calculations to the assumed solid solubility products, highlighted by the example of hydrogarnet, is likely the reason why some studies, including this one, predict a stable SO4-rich AFm phase while others do not. The majority of the effort is given to calculating the influences on AFm and AFt stability of alkali and carbonate components, both of which are typically present in cementitious binders. Higher alkali content shifts the equilibria of both AFt and AFm to lower Ca but higher Al and S concentrations in solution. More importantly, higher alkali content significantly expands the range of solution compositions in equilibrium with AFm relative to AFt phases. The introduction of carbonates alters not only the stable AFm solid solution compositions, as expected, but also influences the range of solution pH over which SO4-rich and OH-rich AFm phases are dominant. Some experimental tests are suggested that could provide validation of these calculations, which are all the more important because of the implications for resistance of portland cement binders to external sulfate attack.

Microstructural Origins of Cement Paste Degradation by External Sulfate Attack.

A microstructure model has been applied to simulate near-surface degradation of portland cement paste in contact with a sodium sulfate solution. This new model uses thermodynamic equilibrium calculations to guide both compositional and microstructure changes. It predicts localized deformation and the onset of damage by coupling the confined growth of new solids with linear thermoelastic finite element calculations of stress and strain fields. Constrained ettringite growth happens primarily at the expense of calcium monosulfoaluminate, carboaluminate and aluminum-rich hydrotalcite, if any, respectively. Expansion and damage can be mitigated chemically by increasing carbonate and magnesium concentrations or microstructurally by inducing a finer dispersion of monosulfate.

Effect of the length of the side chains of comb-like copolymer dispersants on dispersion and rheological properties of concentrated cement suspensions.

Two different groups of comb-like copolymer dispersants with side chain lengths ranging from 8 to 48 were synthesized and characterized: one group with the carboxylic content per mole molecule constant and the other group with the carboxylic contents per gram polymer constant. The effects of side chain length on comb-like copolymers on adsorption, dispersion, rheological behavior, and zeta potential for cement suspensions were investigated systematically to elucidate the governing dispersing mechanism. The adsorption of comb-like copolymer on the cement surfaces, controlled by COO(-) content in copolymer backbones, side chain length, and also polymer conformation in solution, governs the dispersion and rheological behavior of the cementitious system. The dispersion effect increases with the adsorbed amount; especially the adsorbed side chain density increases. But the dispersion power of comb-like copolymers varies depending on the side chain length in comb-like copolymer. The long side chain polymer has higher dispersion power than the shorter ones due to the stronger steric hindrance effect of the former; for the short side chain polymer with high ionic content, the electrostatic repulsion and steric hindrance together should be responsible for its dispersion effect. Such information also suggests that their exists the geometrical balance between the main chain and the side chains in comb-like copolymer dispersants which should be very useful in designing optimum molecular structures of high efficient dispersants.