Papers by Author: Zhu Ding

Abstract: Phosphoaluminate cement (PAC) is a kind of new cementitious material which has many special properties compared to Portland cement (PC). PAC sets quickly and develops early-high strength. In order to investigate the hydration mechanism, the hydration products and microstructure of PAC were studied with x-ray diffraction (XRD), electron scanning microscope (SEM) and electrochemical impedance spectroscopy (EIS). Heat evolution of PAC was also measured. The results show that the hydration mechanism of PAC is different from Portland cement, which is caused by the special minerals including CxP, CA(P), phase L, and so on. The main hydration products of PAC are calcium phosphorus aluminates hydrate (C-A-P-H), calcium phosphate hydrate (C-P-H), aluminates hydrate (C-A-H), the corresponding hydration microcrystal as well as gels. Also, there is no calcium hydroxide produced during hydration. The hydration procedure of PAC is divided into four stages which are dissolution and induction, acceleration, deceleration, stabilization.

Abstract: Phosphoaluminate cement (PAC) sets quickly and develops high early strength. As a novel cementitious material, its hydration mechanism and microstructure are more complex than ordinary cement. Based on nitrogen adsorption isotherm method, investigated characteristic of adsorption isotherm of PAC paste, adsorption loop and pore distribution at different hydration period with various W/C ratio, results show that adsorption and desorption isotherm are tight relate to pore structure and pore distribution. In addition, the study analyzed the scope of application of nitrogen adsorption isotherm method on investigating pore structure of cement paste.

Abstract: Electrodeless resistivity and impedance spectroscopy measurement are reliable equipments of test online for monitoring the hydration process of cement-based materials continuously and accurately. Phosphoaluminate cement (PAC) sets quickly and develops high early strength. In order to understand the mechanism, the hydration products and microstructure formation of PAC in early age need to be studied. In the study, early hydration process and impedance spectroscopy characteristics of PAC with different dosage of retarder were investigated. According to the test, resistivity of freshly mixed PAC paste decreases sharply and then rises slowly, some characteristic peaks appear at different hydration stages of PAC, which indicates the hydration process of PAC includes four stages which are dissolution, induction, acceleration and deceleration. Impedance spectroscopy can reflect the change of micro-structure and ion concentribution of PAC matrix by resistance and capacitance.

Abstract: The high brittleness of cement-based materials is the dominant reason that causes cracking, corrosion, and even losing the load bearing capacity of the structure. PVA fiber reinforced Strain-hardening Cement-based Materials (SHCC) which is characterized by strain-hardening and multi-micro cracking significantly improves the mechanics performance of cement-based materials. In this paper, 3-point-bending and direct tensile tests were conducted to contrast influence of the material mechanics on SHCC by aggregate gradation. Tests results demonstrate that appropriate particle gradation may improve the strain hardening section in the stress-strain curve. Therefore, the maximum diameter of aggregate can be enlarged from 110μm to 1.25mm. Comparatively large aggregate will be convenient for such materials in the practical application and popularization.

Abstract: Penetration and diffusion of chloride ions in concrete can lead to the corrosion of steel bar and shorten the service life of concrete structures. Phosphoaluminate cement (PAC) is a new cementitious material which has many special properties compared to Portland cement (PC). In the study, chloride ion diffusion in PAC concrete was tested with RCM method. The phase composition and morphology of hydration products, pore volume of hardened paste cured for 28d were analyzed with X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP). The results show that chloride ion diffusion coefficient of PAC concrete is much lower than that of Portland cement concrete under the same test conditions. The hydration products of PAC are main micro-crystalline phase and gel of phosphate and/or phophoaluminate, which formed a dense microstructure. There is no calcium hydroxide produced in the PAC hydration system. In hardened PAC paste, chloride ions might replace the atom group [OH] - and [PO4]3- of hydrates and become stable compounds. The resistance to chloride ion diffusion of PAC concrete will increase with the hydration age, because its microstructure becomes denser with the hydration age increasing.

Abstract: An electrodeless resistivity measurement system developed recently can provide a reliable method for monitoring the hydration process of cement-based materials continuously and accurately. Phosphoaluminate cement (PAC) sets quickly and develops high early strength. In order to understand the mechanism, the hydration products and microstructure formation of PAC in early age need to be studied. In the study, early hydration process of PAC with different dosage of retarder was investigated by the electrodeless resistivity equipment. According to resistivity-time curve, resistivity of freshly mixed PAC paste decreases sharply and then rises slowly, some characteristic peaks appear at different hydration stages of PAC. Heat evolution of PAC was also measured. The hydration mechanism and structure formation were studied according to these results. Depending on the dosage of retarder, the hydration process of PAC includes four stages which are dissolution, induction, acceleration and deceleration.

Abstract: Electrochemical chloride extraction (ECE) is used for the rehabilitation of chloride-contaminated concrete. High current densities of steel surface are applied between the steel and a temporary external anode which is placed on the concrete surface. Anions are pushed away from the cathode (steel reinforcement), and cations are accumulated at the cathode. In this study chloride ions were found in the electrolyte during ECE. The extraction efficiency of chlorides was obvious. After treatment, the amount of chlorides around the reinforcement was greatly reduced. ECE had insignificant effect on the compressive strength. Steel potentials in treated specimens showed a strong shift towards more positive values, while potentials in control specimens remained strongly negative.

Abstract: A novel phosphate bonded magnesia cement was developed recently, which sets quickly and develops high strength in the early ages. In the present research, the physical and mechanical properties, hydration products, and microstructure of the new cement were investigated. The new cement was mainly consists of dead burned magnesia, phosphate and fly ash. The results showed that it not only has high early strength, but also has high long-term strength. Interestingly, the strength of the cement increase with the content of fly ash, from 10 % - 40 % by weight. The hardened cement paste has a dense microstructure, low open porosity and water sorptivity due to the low water to binder ratio. The reaction products after hydration of the cement includes struvite mineral and amorphous species.

Abstract: The magnesium phosphosilicate cement (MPSC) is a novel inorganic binder, it sets quickly and has very high strength. Also, it is a promising material for the sustainable development. In the present study, the durability of MPSC were investigated, including deicer scaling resistance under freezing-thawing cycles, chemical corrosion resistance in sodium sulfate and magnesium sulfate solutions, and wet-dry resistance in fresh and natural sea water. For comparison, Portland cement samples were also prepared for parallel tests. Test results showed that the chemical durability of MPSC is superior that of Portland cement. The causes of the high performance may be attributed to the low water demand and a reasonable microstructure of hardened paste matrix.