Identification of Chemical Bonds and Microstructure of Hydrated Tricalcium Silicate (C3S) by a Coupled Micro-Raman/BSE-EDS Evaluation

Identifying the phase evolution and revealing the chemical bonds of hydrated cements accurately is crucial to regulate the performance of cementitious materials. In this paper, a coupled Raman/BSE-EDS analysis was proposed to determine the chemical bonds of tricalcium silicate hydrates and the interface transition zone (ITZ) between inner C-S-H and anhydrates. The results show that the Raman/BSE-EDS method can accurately identify the chemical bonds of inner C-S-H and inner ITZ regions, which confirms the mixed structure of inner C-S-H and nano calcium hydroxide (CH). The inner ITZ shows a lattice change region with a thickness of 700–1000 nm, which can be attributed to the pre-disassembly process of C3S crystal. The successful application of coupled Raman/BSE-EDS provides new insight into the hydration process and multi-structure features of traditional cementitious materials.

A Comparative Study of the Properties of Recycled Concrete Prepared with Nano-SiO2 and CO2 Cured Recycled Coarse Aggregates Subjected to Aggressive Ions Environment

This research focused on the modification effects on recycled concrete (RC) prepared with nano-SiO2 and CO2 cured recycled coarse aggregates (RCA) subjected to an aggressive ions environment. For this purpose, RCA was first simply crushed and modified by nano-SiO2 and CO2, respectively, and the compressive strength, ions permeability as well as the macro properties and features of the interface transition zone (ITZ) of RC were investigated after soaking in 3.5% NaCl solution and 5% Na2SO4 solution for 30 days, respectively. The results show that nano-SiO2 modified RC displays higher compressive strength and ions penetration resistance than that treated by carbonation. Besides, we find that ions attack has a significant influence on the microcracks width and micro-hardness of the ITZ between old aggregate and old mortar. The surface topography, elemental distribution and micro-hardness demonstrate that nano-SiO2 curing can significantly decrease the microcracks width as well as Cl− and SO42− penetration in ITZ, thus increasing the micro-hardness, compared with CO2 treatment.

Study on the Properties and Mechanisms of a Cement-Stabilized Aggregate Mixture With Vibration Mixing

In order to understand the influence of the vibration mixing method on the performance of cement-stabilized aggregate mixture (CSAM), in this study, an unconfined compressive strength test, drying shrinkage test and the influence of gradation characteristics on compressive strength were used to systematically analyze the performance of CSAM based on vibration mixing. At the same time, the influence mechanism of vibration mixing on the interface transition zone of cement aggregate was analyzed by use of a scanning electron microscope (SEM), and the influence of the mixing method on cement dispersion uniformity was studied by the ethylene diamine tetraacetic acid (EDTA) titration method. The results show that 7 days of unconfined compressive strength and crack resistance of the CSAM with vibration mixing is better than those of the ordinary mixing, and the aggregate grade has a great influence on the compressive strength of the CSAM. Vibration mixing can improve the micro-water-cement ratio uniformity of CSAM and significantly improve the bonding condition of the interface transition zone between cement paste and aggregate, thus enhancing the structural compactness of CSAM. The dispersion uniformity of cement under the vibration mixing is better than that of ordinary mixing.

Examining The Mechanical and Durability Properties of Concrete Containing Recycled Fine Aggregates Possessing Different Contamination Levels

Three types of recycled fine aggregates (RFA) possessing different contamination levels (washed – unwashed) were used in the present study. RFAs were used instead of natural fine aggregate (NFA) at the concentrations of 10%, 20%, 40%, and 80%. No significant negative characteristics were observed in green and hardened characteristics of concrete when using RFA at up to 80% concentration. However, using less-washed or unwashed RFA at 10% or 20% concentration is generally appropriate. The use of less-washed or unwashed RFAs at the concentration of 40% or more negatively affects the slump values of concrete. Especially the use of unwashed RFA at 80% concentration yielded more significant damages. The non-hydrated cement particles in the RFA show insemination or hardening acceleration behavior and affected the 7-day pressure strength of concrete at a higher level. However, together with the use of RA having a weak interface transition zone, the splitting tensile strength values of concretes more significantly decreased. In conclusion, it was determined that RFAs can be used up to 80% concentration after an appropriate washing procedure. From the aspect of costs, in order to prevent durability problems, it should not be used at any concentration higher than 20% without a washing procedure. From the aspect of protecting the sustainable environment and natural sources, it is very important to increase the use of RFAs in concrete production.

Laboratory Investigation on the Performance of Cement Stabilized Recycled Aggregate with the Vibration Mixing Process

The objective of this article is to investigate the effect of the vibration mixing process (VMP) on the performance of cement stabilized recycled aggregate (CSRA). The ordinary mixing process (OMP) was also used for comparative analysis with the VMP. The performance of CSRA with different cement content was tested and comprehensively analyzed by the unconfined compressive strength test, dry shrinkage test, and temperature shrinkage test. The results showed that compared with OMP, VMP can greatly improve the compressive strength, dry shrinkage, and temperature shrinkage properties of CSRA. The optimum parameters of vibration time and vibration frequency in VMP were 30 s and 40 Hz, respectively. The reason why VMP can improve the performance of CSRA was that VMP strengthened the aggregate and interface transition zone and introduced microbubbles to optimize the pore structure.

Fiber cement boards modified with styrene-acrylic copolymer: An approach to address dimensional stability and cellulose fiber preservation

The performance of polymers such as acrylics, styrene-acrylics, styrene-butadiene, ethylene vinyl acetate, type added in cementitious composites are well reported in the literature to boost properties in the fresh mortar stage as workability, anti-bleeding, and hard stage as deformation, adhesion strength, crack bridging, cohesion, durability and reduced water uptake. Polymer treatment was performed in fiber cement boards by adding 5% w/w (dry basis) of styrene-acrylic copolymer aiming to investigate the impact on the mechanical and physical properties at initial curing period (28 days) and after the 200 soak and drying ageing cycles. Dimensional stability at 28 days and the cellulose fiber/cement interface transition zone were assessed by scanning electron microscopy combined with dispersive energy X-ray spectroscopy (EDS). These experiments confirmed that the water absorption value in polymer modified cement boards was reduced by 50% after the ageing cycles when compared to the unmodified boards. Additionally, improvements on board’s rigidity with reduction of modulus of elasticity (MOE) values up to 40% and 15% reduction of board shrinkage was noticed, enhancing boards dimensional stability and preventing fibers from the mineralization process by keeping the cellulose fiber adhered on the cementitious matrix, providing a dense and cohesive fiber-cement interface transition zone after the ageing cycles. This achievement can open important fields of application for the reinforcement of flat panels.