Investigation of the Interfacial Transition Zone between Aggregate-Cement Paste by AC Impedance Spectroscopy

Three different types and sizes of coarse aggregate were chosen,and the alternating current（AC） impedance of cement paste samples with and without aggregate was measured at different curing ages.Based on Song’s equivalent circuit model,the electrical properties from the AC impedance results were obtained,and the resistance of connected pores RCCP was used to characterize the microstructure of the interfacial transition zone（ITZ）.The results show that the RCCP of concrete sample with aggregate is lower than that of cement paste sample,which indicates that the introduction of aggregate in cement paste makes the ITZ porous.Furthermore,for the same type of aggregate,an increase in particle size leads to a more porous ITZ,which accounts for the “water effect” and a larger aggregate would accumulate a thicker water film around it.In addition,for the same size of aggregate,the physical interaction between aggregate and cement paste is dominant in early age,and the microstructure of the ITZ around limestone aggregate is denser,which mainly depends on its rough surface and high water absorption.However,the microstructures of the ITZ around granite and basalt aggregates are denser in later age,which may be due to their higher chemical activity,and the chemical interaction between them and cement paste resulting in the generation of more hydrates.AC impedance spectroscopy thus proves to be powerful for evaluation of the microstructure of the ITZ.

Effects of LiAl-Layered Double Hydroxides on Early Hydration of Calcium Sulphoaluminate Cement Paste

As a 3D micro-nano material, layered double hydroxides have been widely used in many fields, especially for reinforced composite materials. In this paper, Li Al-LDHs was obtained by a hydrothermal method. In order to investigate the effects of Li Al-LDHs on the early hydration of calcium sulphoaluminate（CSA） cement paste, compressive strength, setting time and hydration heat were tested while X-ray diffraction（XRD）, Fourier transform infrared spectroscopy（FT-IR）, scaning electron microscopy（SEM） and differential scanning calorimetry（DSC） analysis were employed. The results indicated that Li Al-LDHs could significantly improve the early compressive strength and shorten the setting time of calcium sulphoaluminate cement paste with 3 wt% concentration. Besides, the hydration exothermic rate within 5h was accelerated with increasing Li Al-LDHs content. Moreover, the addition of Li Al-LDHs did not result in the formation of a new phase, but increased the quantity of hydration products providing higher compressive strength, shorter setting time and denser microstructure.

Improvement of Cement Strength by Induction Method

The induction method of improving the strength of Portland cement by adding fine slag powder,high aluminate component and hydrated paste was investigated through determining the physical properties,hydration heat and pore size distribution,and its mechanism was discussed.The experimental results reveal that a certain content of high aluminate component,fine slag powder and hydrated paste can improve remarkably the strength of Portland cement.

Multiscale mechanical modeling of hydrated cement paste under tensile load using the combined DEM-MD method

In this paper, a combined DEM-MD method is proposed to simulate the crack failure process of Hydrated Cement Paste （HCP） under a tensile force. A three-dimensional （3D） multiscale mechanical model is established using the combined Discrete Element Method （DEM）-Molecular Dynamics （MD） method in LAMMPS （Large-scale Atomic/ Molecular Massively Parallel Simulator）. In the 3D model, HCP consists of discrete particles and atoms. Simulation results show that the combined DEM-MD model is computationally efficient with good accuracy in predicting tensile failures of HCP.