Abstract
Magnesium phosphate cement (MPC) is an excellent repair material for civil and road engineering, but its volume stability under various environmental conditions significantly influences these applications. In this study, the volume stability of MPC under different curing conditions (e.g., air, standard, and water curing) is investigated. Moreover, the phases, weight loss, microstructure, and pore structure of the samples have been determined by X-ray diffraction, thermogravimetry, scanning electron microscopy, and Brunnauer–Emmet–Teller method. The results show that MPC will shrink by 8 × 10−4 under air-curing conditions. At the same time, MPC will expand by 9 × 10−4 under water-curing and standard curing conditions, which means that curing conditions influence the volume stability of MPC. Not only that, compared with air-curing conditions, the compressive strength of MPC under standard curing and water-curing conditions will decrease by 30% and 60%, respectively, which implies that greater humidity will reduce the mechanical properties of the repair material. Therefore, air curing is the best curing condition for MPC. To get a better repair effect, the environment should be avoided as much as possible in a humid state. The microscopic analysis results show that the volume expansion of MPC is related to hydration products, and the volume shrinkage occurs owing to drying shrinkage caused by internal moisture evaporation.
Highlights
Nowadays, concrete is widely used in civil and road engineering
E samples’ phase compositions for 28 d were analyzed by the XRD (Dmax1400, Rigaku, Japan) using Cu Kα radiation at 70 mA and 40 kV. e scanning step was 0.02, and the scanning rate was 8°/min from 3° to 80°. e samples’ microstructure was investigated using a MAIA3 scanning electron microscope (TESCAN, Czech Republic). e thermal property of the magnesium phosphate cement (MPC) was analyzed by the TG (STA8000, Perkin Elmer, USA). e samples were heated from 30°C to 600°C in a nitrogen atmosphere at a heating rate of 20°C/min. e samples’ pore structure was investigated using a specific surface area and porosity analyzer (Autosorb iQ, Quantachrome, USA). e gas adsorption type is nitrogen, and the samples were degassed at 40°C for 12 h
Under the standard curing and watercuring conditions, the volume deformation of MPC is shown as a slight expansion
Summary
Concrete is widely used in civil and road engineering. Due to heavy traffic loads and various environmental conditions, the concrete structure is damaged by increasing service life [1, 2]. Due to its fast setting time, high early strength, and bond strength with the old concrete, magnesium phosphate cement (MPC) is often used for the fast repairing of the concrete structure [5]. MPC is a cementitious material formed through a chemical reaction between dead-burned MgO and phosphate (KDP) [12, 13]. In the early MPC period as a repair material in civil engineering applications, ammonium dihydrogen phosphate (APD) was typically used as the phosphate. Ammonia (NH3) is a toxic gas obtained as a by-product of the hydration reaction. E hydration reaction product between MgO and KDP is hexahydrate magnesium potassium phosphate (KMgPO4·6H2O) [10, 14, 16, 17]. Because of KMgPO4·6H2O having similar properties as struvite, it is often called K-struvite. e formation of K-struvite is depicted as follows [6, 18]: MgO + KH2PO4 + 5H2O KMgPO4 · 6H2O. (1)
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