Chungkong Chau

Potentiometric Study of the Formation of Magnesium Potassium Phosphate Hexahydrate

AbstractThe formation process of magnesium potassium phosphate hexahydrate was studied by monitoring the pH development of the MgO-KH2PO4-H2O system. It is found that a typical potentiometric curve goes through several rises and drops, forming two local peaks and two local valleys, before reaching a plateau at pH of approximately 10.5. The characteristic points of the curve are identified and then utilized to distinguish the different reaction stages during the formation of the hexahydrate. By comparing the potentiometric curves of the ternary systems with different molar ratios, and from X-ray diffractometry analysis of the precipitates extracted at different stages, the formation of the hexahydrate most likely proceeds stepwise through two crystalline intermediates, namely MgHPO4·7H2O and Mg2KH(PO4)2·15H2O. The thermal behavior of the formation process of magnesium potassium phosphate hexahydrate, including the formation and transformation among the intermediates and final product, is further verified b...

New Water Permeability Test Scheme for Concrete

This paper proposes an efficient test scheme for concrete permeability based on the principle of the test of water penetration depth. By utilizing an autoclave, at least 18 concrete samples can be evaluated simultaneously under the same operating condition. Experimental results show that the proposed test scheme yields permeability measurements with high reproducibility. Testing conditions equivalent to those of a conventional flexible wall permeameter are simulated by fixing a water reservoir on the top and an air buffer at the bottom of the test sample. In addition, the whole is put under uniformly applied pressure inside the autoclave. Boundary leakage in the traditional concrete permeability test is eliminated by a new vacuum coating method that offers an excellent bonding between the concrete surface and the resin coating compound. Thus, the proposed test scheme not only greatly increases the testing efficiency, but also improves the consistency of the results of the water permeability test on concrete.

Property Assessment of Magnesium Phosphate Cement

Magnesium phosphate cement (MPC) paste is prepared by mixing dead burned magnesia powder and potassium di-hydrogen phosphate with a proper proportion of water. The superior properties such as fast setting, high early strength, low shrinkage and good bonding makes MPC suitable for applications in rapid repair of infrastructures. However, the setting time of MPC is sometimes too fast, only taking a few minutes, which causes difficulties in manipulation. How to retard and control the setting time of MPC becomes a challenging topic in both scientific and engineering fields. It was found that the setting of MPC was greatly influenced by the reactivity of magnesia, which could be determined by a quick, accurate and convenient test method. Two kinds of magnesia, namely, high-reactivity magnesia and low-reactivity magnesia were blended in order to control the setting time of MPC. Effects of magnesia to phosphate molar ratio, blending ratio and borax addition on setting time, workability and mechanical properties of MPC cement paste were investigated. Finally, the suitable ranges of the different constitutes of MPC based repair materials with satisfied workability and strength development were proposed for the practical engineering applications.

Recent Development of Magnesium-Based Cements - Magnesium Phosphate Cement and Magnesium Oxychloride Cement

The recent development of two types of environmental friendly cementitious materials, magnesium oxychloride cement and magnesium phosphate cement, at HKUST are presented. Both of them can develop high strength without heat treatment under elevated temperature, i.e. the bonding of these cementitious materials can be achieved at low temperature through chemical reaction, as opposed to fusion or sintering at high temperature. The preparation process of the two cements can not only save a lot of energy but also emit no carbon dioxide. For magnesium oxychloride cement, our research includes parametric study of the formulation, strength development, water resistance, and also identification of phase composition in the cement paste. Magnesium phosphate cement is mainly applied as rapid repair material in civil engineering. In this paper, the formulation, mechanical properties and performance in patch repair of mortar specimen including strength, bond ability to old concrete substrate, volume stability are studied.

Viscoplastic Analysis of Cementitious Extrudates under Squeeze Flow

A successful extrusion offiber-reinforced cementitious composites requires a thorough understanding and proper design of the rheological properties of extrudates. The squeeze flow test is regarded as an effective method to evaluate the rheological properties of semi-solid-state fresh extrudates. Based on the strain rates derived from a generalized velocity field, the viscoplastic behavior of cementitious extrudates can be analyzed using the von Mises yield criterion combined with the nonlinear Herschel-Bulkley constitutive relationship. As far as the squeezing force is concerned, the theoretical predictions show good agreement with the experimental measurements. It is found that the bulging phenomenon of a specimen during the squeeze flow process can be depicted by the radial velocity component. Moreover, the velocity field of fresh cementitious extrudates under large plastic deformation is obtained. The present viscoplastic analysis of short fiber-reinforced cementitious extrudates under plastic flow is believed to be a promising mathematical tool toward optimization of formulation for the booming extrusion application.

Extrusion of MOC-based panel

Magnesium Oxychloride Cement (MOC) is an air-hardened material with chemical reaction between magnesium oxide and magnesium chloride solution. It has many properties superior to Portland cement. It does not need wet curing, has very good fire resistance, low thermal conductivity. In this study, wood composites were developed by using MOC as binder and wood powders as filler. It was demonstrated that MOC was suitable for binding wood particles in large quantities. In order to ensure smooth extrusion process, additives and rheological modifiers were used to modify the rheology behaviors and properties of the mixture and the rheology behaviors of fresh materials were also investigated with upsetting and relaxation tests simultaneously. Then MOC-based panels with different cross-sections were produced by using extrusion technology and mechanical properties of them were investigated.