Development and Promotion of Concrete Strength at Initial 24 Hours.

Knowing and promoting the strength development of concrete at an earlier age is essential for accelerating formwork circulation of the on-site construction and precast product manufacture. The strength development rate at earlier ages of less than the initial 24 h was investigated. The effect of measures of adding silica fume, calcium sulfoaluminate cement, and early strength agent on the strength development of earlier concrete at ambient temperatures of 10, 15, 20, 25, and 30 °C was studied. The microstructure and long-term properties were further tested. It is shown that the strength increases exponentially first and then logarithmically, different from what is commonly recognized. Increasing cement content exhibited a certain effect only above 25 °C. When the cement content increased from 420 to 460 kg/m3, the strength only increased from 6.2 to 6.7 MPa after 12 h at 25 °C. The early strength agent could increase the strength significantly, the strength could be increased from 6.4 to 10.8 MPa after 20 h at 10 °C and from 7.2 to 20.6 MPa after 14 h at 20 °C. All measures for promoting earlier strength did not have an evident negative effect. The results could be potentially referred for the formwork removal at a suitable moment.

Creep Deformation and Its Effect on Mechanical Properties and Microstructure of Magnesium Phosphate Cement Concrete.

Creep deformation is an important aspect of magnesium phosphate cement (MPC) used as a structural material. In this study, the shrinkage and creep deformation behaviors of three different MPC concretes were observed for 550 days. The mechanical properties, phase composition, pore structure, and microstructure of MPC concretes after shrinkage and creep tests were investigated. The results showed that the shrinkage and creep strains of MPC concretes stabilized in the ranges of -140 to -170 µÎµ and -200 to -240 µÎµ, respectively. The low water-to-binder ratio and the formation of crystalline struvite were responsible for such low deformation. The creep strain had almost no effect on the phase composition; however, it increased the crystal size of struvite and reduced the porosity, especially the volume of pores with diameters <20 nm and >200 nm. The modification of struvite and densification of microstructure led to an improvement in both compressive strength and splitting tensile strength.

Investigation and Optimisation of the Rheological Properties of Magnesium Potassium Phosphate Cement with Response Surface Methodology.

Magnesium phosphate cement (MPC) is a promising alternative cement. However, the rheological property of this new binder is still to be explored. In this study, Response Surface Methodology (RSM) was adopted with Central Composite Design (CCD) to establish mathematical models describing the rheological characteristics of MPC in terms of initial mini slump (Y1), mini-slump loss (Y2), yield stress (Y3) and plastic viscosity (Y4), as a function of three independent variables, namely, water-to-solid ratio (W/S ratio, X1), MgO to MKP ratio (M/P ratio, X2) and borax dosage (X3). The results show that the M/P ratio and borax dosage could significantly affect the yield stress and mini-slump loss of MPC, while the W/S ratio was the significant coefficient influencing plastic viscosity and initial mini slump. The numerical optimised values of X1, X2 and X3 were 0.280, 7.528 and 0.170, respectively, and an MPC paste with desirable rheological characteristics (Y1 161.858 mm, Y2 11.282, Y3 0.680 Pa, Y4 0.263 Pa·s) with the highest desirability of 0.867 can be obtained.

Preparation and Properties of Magnesium Phosphate Cement-Based Fire Retardant Coating for Steel.

Magnesium phosphate cement (MPC) is a potential inorganic binder for steel coating due to setting and hardening rapidly, and bonding tightly with steel. NH4H2PO4-based MPC as a fire-retardant coating for steel was investigated in this work. MPC coatings were prepared from MPC paste and MPC mortar with expanded vermiculite (EV). The physical-mechanical properties and fireproof performance of MPC coatings were investigated in detail. An infrared thermal imager was employed to collect the temperature distribution and temperature rise with time on the coating samples automatically. The X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses were carried out on the MPC coating after the fireproof test. Re-fire test and corrosion resistance were performed preliminarily on the MPC coating. The results showed that the fireproof performance of MPC coating met the fire protection requirement for steel as long as the thickness of the MPC paste coating was up to 10 mm, while the thickness of MPC mortar coating decreased to 4 mm when adding 40% EV (by mass). Dehydration and decomposition of reacted products in the hardened MPC coating were, to some extent, contributed to the excellent fireproof performance during the fire test. The slight ceramic formation and integration of MPC coating during the fire test would compensate for the decreasing of strength due to the dehydration and decomposition, so that the MPC coating would keep certain fireproof performance when undergoing fire again. MPC is suitable for a fire-retardant coating, while higher tensile bonding strength with steel and potential corrosion resistance on steel, as well as rapid surface drying and hardening can be achieved.

Preparation and Performance of Ternesite-Ye'elimite Cement.

Ternesite-ye'elimite (TCSA) cement is a new type of environmentally advantageous binder prepared by introducing ternesite, a reactive phase, into belite calcium sulfoaluminate cement clinker. This paper reports the laboratory production of TCSA cement by the addition of minor elements to achieve the coexistence of ternesite and ye'elimite. The influence of dopants on the mineralogical composition of clinkers and the clinkering conditions for the preparation of TCSA cement clinkers were investigated by X-ray powder diffraction and scanning electron microscopy. The mechanical properties and hydration products of the cement pastes were also studied. The results indicated that the addition of CaF2, P2O5 and Na2O can promote the coexistence of ternesite and ye'elimite, and that Na2O is the most effective candidate. TCSA cement clinkers could be successfully prepared at 1150 °C for 30 min by doping 0.3% Na2O. The TCSA cement clinkers exhibited shorter setting times than the BCSA cement clinkers. The later strength of TCSA cement showed a significant increase compared with BCSA cement. The effect of Na2O was different on the strength development for TCSA and BCSA cement. The dissolution of ternesite could promote the formation of ettringite. The reactivity of belite was higher in TCSA cement due to the formation of strätlingite.

Preparation and Application of Self-Curing Magnesium Phosphate Cement Concrete with High Early Strength in Severe Cold Environments.

The early strength of magnesium phosphate cement (MPC) decreases sharply in severe cold environments ≤-10 °C, with the 2 h compressive strength falling to 3.5 MPa at-20 °C. Therefore, it cannot be used as a repair material for emergency repair construction in such environments. In this study, MPC is adapted for use in such cold environments by replacing part of the dead-burned magnesia (M) in the mixture with a small amount of light-burned magnesia (LBM) and introducing dilute phosphoric acid (PA) solution as the mixing water. The heat released by the highly active acid-base reaction of PA and LBM stimulates an MPC reaction. Moreover, the early strength of the MPC significantly improves with the increase in the Mg2+ concentration and the initial reaction temperature of the MPC paste, which enables MPC hardening in severe cold environments. Although the morphology of the reaction products of the MPC is poor and the grain plumpness and size of the struvite crystals are remarkably reduced, the early strength of MPC prepared in the severe cold environment is close to that of MPC prepared under normal temperature. Furthermore, the increases in the early reaction temperature and early strength of magnesium phosphate cement concrete (MPCC) are significantly improved when the PA concentration in the mixing water and the LBM/M ratio are 10% and 4-6% at -10 °C and 20% and 6-8% at -20 °C, respectively. Moreover, self-curing of MPCC can be realized even at -20 °C, at which temperature the 2 h and 24 h compressive strength of MPCC reach 36 MPa and 45 MPa, respectively.

Low Temperature Synthesis of Large-Size Anatase TiO2 Nanosheets with Enhanced Photocatalytic Activities.

TiO2 nanosheets have continuously been intriguing due to their high surface activities as photocatalyst but still challenging to synthesis large-scale 2D nanostructures. A special microstructure evolution of TiO2 , ripening in aqueous solution at low temperature (≈4 °C), is found for the first time, i.e., from the initial aperiodic atom-networks gradually into low crystallized continuous spongy structure with small crystal facets and ultimately forming large-size anatase nanosheets with exposed (101) and (200) facets. Based on this finding, the synthesized anatase TiO2 nanosheets possess monodispersed large-scale 2D nanostructure so as to exhibit appreciable quantum size effects and remarkable enhanced optical absorption capacity. Using photocatalytic reduction of Cr (VI) to Cr (III) as the probe reaction to evaluate photocatalytic activities of the TiO2 nanosheets, the reductivity of Cr (VI) achieves 99.8% in 15 min under irradiation of 200-800 nm light. At the same time, an in situ Cr (III)-doping occurs spontaneously and triggers pronounced visible light driven photocatalysis, reducing 99% of Cr (VI) in 100 min under irradiation of 400-800 nm light.

Comparison of low crystallinity TiO2 film with nanocrystalline anatase film for dye-sensitized solar cells.

Dye adsorption and microstructure of TiO(2) film are important properties when it is used as photoelectrode of dye-sensitized solar cells (DSCs). This study investigated the application of a low crystallinity TiO(2) film in DSCs. The low crystallinity TiO(2) film is composed of interconnected spherical particles with an average size of 20 nm and has homogeneous mesoporous inner structure. A DSC based on the anatase nanocrystalline mesoporous film prepared by P25 was used for comparison purpose. It is shown that although loaded with much less dye, the DSC based on the low crystallinity TiO(2) film generated I(sc) (short circuit photocurrent) as much as the one based on the conventional anatase nanocrystalline film does and obtained higher V(oc) (open circuit photovoltage) as well as ff (fill factor). The overall light-to-electricity efficiency (eta) of the DSC based on the low crystallinity TiO(2) film reached 5.37%, while the eta of the DSC based on anatase nanocrystalline film was 4.69% in this work condition. It is suggested that a low crystallinity TiO(2) mesoporous film with a proper microstructure is as efficient as the anatase nanocrystalline mesoporous film when used in DSCs.

Application of tetrahydrofuran dispersant in microemulsion for fabricating titania mesoporous thin film.

A new strategy was used to fabricate titania mesoporous thin film by incorporating tetrahydrofuran (THF) into the CTAB/n-butyl alcohol/cyclohexane/water reverse microemulsion as a micelle disperser. Highly dispersed and congregated TiO(2) particles in the microemulsion with and without THF were observed by transmission electron microscopy (TEM), respectively. The photographs observed by field-emission scanning electron microscopy (FE-SEM) show that a uniform titania mesoporous thin film with monodisperse TiO(2) spherical nanoparticles of ca. 20 nm was obtained using the microemulsion with THF.