阿利特—硫铝酸盐水泥与硅酸盐水泥复合性能的研究

研究了阿利特－硫铝酸盐水泥与硅酸盐水泥复合所制备的水泥的性能。结果表明,复合后水泥的强度性能优于单一品种水泥的性能;凝结时间则由复合体中占比例较多的一种水泥所控制。     

改性硅酸盐复合水泥早期力学性能的初探及机理分析

通过设计正交试验探索掺磷铝酸盐水泥的改性硅酸盐复合水泥的力学性能,从石膏、磷铝酸盐水泥熟料、外加剂等因素调控,以改性后的硅酸盐复合水泥水化浆体的早期抗压强度为主要指标确定最佳方案,并与未改性的硅酸盐水泥强度作对比,并通过数据分析及XRD、SEM、孔结构分析等现代测试手段,解释改性硅酸盐复合水泥早期力学性能得以改善的机理。     

A study on mechanical and pressure-sensitive properties of cement mortar with nanophase materials

The mechanical properties and self-monitoring capability of cement mortar containing nano-SiO₂ or nano-Fe₂O₃ were experimentally studied and compared with that of plain cement paste. The results showed that the compressive and flexural strengths measured at the 28th day of cement mortar containing nano-SiO₂ or nano-Fe₂O₃ were both higher than that of plain cement mortar with the same water-binder ratio (w/b). Furthermore, the self-monitoring capability of cement mortar with nano-Fe₂O₃ is also presented in this paper.     

铝酸盐水泥熟料对粉煤灰硅酸盐水泥性能的影响

以铝酸盐水泥熟料、硅酸盐水泥熟料和粉煤灰为原料,探讨了掺加少量铝酸盐水泥熟料对硅酸盐水泥及粉煤灰硅酸盐水泥复合体系水化、凝结和硬化性能的影响。结果表明,在硅酸盐水泥及粉煤灰硅酸盐水泥中掺加铝酸盐水泥熟料,可以明显缩短水泥的初、终凝时间,但复合体系的需水量增加;掺加少量铝酸盐水泥熟料(≤3%)可明显提高硅酸盐水泥的早期强度,但后期强度(28d)有所降低;当铝酸盐水泥熟料的掺量达5%时,水泥的各龄期强度均明显降低。少量铝酸盐水泥熟料掺加到粉煤灰硅酸盐水泥中,复合体系的各龄期强度都明显提高,且早期强度的提高幅度较大。     

The origin of nonlinear current-voltage behavior in fiber-reinforced cement composites

Two distinct slopes (resistances) are obtained in current-voltage (I-V) plots of discontinuous, conductive fiber-reinforced cement composites. The low-field resistance correlates with the DC resistance (R_DC) of each composite. The high-field resistance correlates with the intermediate frequency cusp resistance (R_cusp) in Nyquist (−Z_imag vs. Z_real) plots obtained using impedance spectroscopy (IS). A model is developed that is based on passive oxide film formation on copper or steel fiber surfaces at low fields (I-V) or low frequencies (IS) due to the high pH pore solution of cement paste. With increase of field, leading to film breakdown (active or transpassive corrosion behavior), or increase of frequency, leading to short-circuiting of the passive layer, the fibers act as short-circuiting elements in the composite microstructure, resulting in a decrease in overall resistance.     

Thermal analysis of borogypsum and its effects on the physical properties of Portland cement

Borogypsum, which consists mainly of gypsum crystals, B₂O₃ and some impurities, is formed during the production of boric acid from colemanite, which is an important borate ore. In this study, the effect of borogypsum and calcined borogypsum on the physical properties of ordinary Portland cement (OPC) has been investigated. The calcination temperature and transformations in the structures of borogypsum and natural gypsum were determined by differential thermal analysis (DTA), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. Thermal experiments were carried out between ambient temperature and 500 °C in an air atmosphere at a heating rate of 10 °C min⁻¹. After calculation of enthalpy and determination of conversion temperatures, borogypsum (5% and 7%), hemihydrate borogypsum (5%) and natural gypsum (5%) were added separately to Portland cement clinker and cements were ground in the laboratory. The final products were tested for chemical analysis, compressive strength, setting time, Le Chatelier expansion and fineness properties according to the European Standard (EN 196). The results show that increasing the borogypsum level in Portland cement from 5% to 7% caused an increase in setting time and a decrease in soundness expansion and compressive strength. The cement prepared with borogypsum (5%) was found to have similar strength properties to those obtained with natural gypsum, whereas a mixture containing 5% of hemihydrate borogypsum was found to develop 25% higher compressive strength than the OPC control mixtures at 28 days. For this reason, utilization of calcined borogypsum in cement applications is expected to give better results than untreated borogypsum. It is concluded that hemihydrate borogypsum could be used as a retarder for Portland cement as an industrial side. This would play an important role in reducing environmental pollution.     

Hydration reactions and physicochemical properties in a novel tricalcium-dicalcium silicate-based cement containing hydroxyapatite nanoparticles and calcite: A comparative study

The mineral trioxide aggregate (MTA) is Portland type cement whose main application in dentistry is retrograde filling. The purpose of this study was to analyze hydration reactions and physicochemical properties of a new tricalcium-dicalcium silicate-based cement containing nanocrystalline hydroxyapatite (nHAp) and calcite. The new formulation was compared with Biodentine™ and MTA-Angelus™ as control samples.Hydration reactions were monitored by Raman spectroscopy, X-ray diffraction, radiopacity, pH, setting time, and compressive strength. The compressive strength reaches its higher value at 7 days following the sequence: Biodentine™ (104.8 MPa) > Cement + 5% nHAp (59 MPa) > MTAAngelus™ (27.3 MPa), in agreement with the pH values measured at 24 h: Biodentine™, Cements + nHAp or + calcite (10.6–11.6) > MTA-Angelus™ (9.7). Mean setting times was around 30 min and no significative differences (p = 0.0001) were observed. In the Biodentine™ control samples, Ca₃SiO₅ diminishes until disappear at 28 days of hydration. On their turn, calcium silicate hydrate (CSH) increases continuously in the range of time analyzed. The present results suggest that the physicochemical properties were improved for the new cement with nanosized hydroxyapatite nanoparticles and relevant information on chemical properties is of valuable importance for testing predictive models for Biodentine™ and MTA-Angelus™.     

Statistical analysis of impact-fracture characteristics and microstructure of industrial Portland cement clinkers

Impact is the dominant breakage mode in most industrial grinding mills used in cement manufacture. The physical amenability of two industrial Portland cement clinkers to size reduction was determined through measurement of their fracture strengths under impact loading. It was found that the fracture strength of clinker is strongly dependent on size, which is consistent with the increasing expenditure of energy in fine grinding. Also, it was observed that the measured fracture strengths could be well described by either single or multiple Weibull distributions. The appearance of these distributions was consistent with the variability in the composition and microstructure of the clinker nodules, observed in a detailed examination under the microscope. Possible reasons for the appearance of these populations are given. It is concluded that the fracture strength of clinker is generally determined by porosity at coarser nodule sizes and by mineralogy and texture at finer sizes.     

New applications of calcium sulfoaluminate cement

This paper presents four innovative utilizations of calcium sulfoaluminate (CSA) cement:

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development of concrete with high early strength: 40 MPa, 6 h after its preparation, and higher than 55 MPa after 24 h,

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design of self-leveling screed with limited curling, when unbonded to its support,

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design of self-leveling topping mortar presenting the following properties: time of workability higher than 30 min, set within 75 min, and low drying shrinkage (<250μm/m).

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glass-fiber-reinforced cement (GFRC) composites that can be demolded 4 h after casting and present high ductility and durability after aging in different weathering conditions.

     

Effect of curing temperature and type of cement on early-age shrinkage of high-performance concrete

This paper presents the results of an experimental study on the influence of curing temperature and type of cement [Portland cement and blast-furnace slag (BFS) cement] on the autogenous deformations and self-induced stresses in early-age concrete. It was found that higher temperatures do not lead to higher deformations in the observed period, but generally cause a faster shrinkage and a faster development of self-induced stresses. Another experimental finding is that, at the temperatures tested, concrete made with BFS cement shows higher shrinkage in the first days than concrete made with Portland cement.     

Drying Kinetics of Biofilms Obtained from Chestnut Starch and Carrageenan with and without Glycerol

Drying kinetics of biofilms from chestnut starch and hybrid carrageenan were experimentally determined and modelled. The biofilms were obtained by mixing the respective solutions (4.0% w/w) of both substances in the proportion 80/20 of chestnut starch/hybrid carrageenan. Glycerol (at 5.0% w/w) was added to some biofilms. Biofilms were prepared using a casting method. Drying kinetics were performed at 30, 40, and 50°C for forced air convection (1.8 ± 0.1 m/s) and 30 and 50°C for natural convection. Constant drying rate period, above critical moisture content (5.4 and 4.2 kg water/kg d.b. without and with glycerol, respectively) allowed the evaluation of the heat transfer coefficients. The water diffusion coefficients were determined in the falling rate period assuming a semi-infinite slab with variable thickness. Water sorption isotherms of biofilms at 25°C were determined and fitted with the GAB model. Mechanical properties of different biofilms showed no significant differences for Young's modulus values (44.0 ± 4.6 MPa). Nevertheless, films prepared at 30°C under natural drying showed the lowest values of tensile strength and elongation (32.2 ± 4.9 MPa and 0.85 ± 0.11%).     

Examination of Portland cement paste hydrated in the presence of malic acid

When malic acid (MA) solutions are added to ordinary Portland cement (OPC), rapid heat evolution takes place, but the hydration is retarded considerably at all the MA concentrations. To understand the mechanism of retardation, UV-visible and IR spectral studies were made and the results have revealed that some interaction occurs between MA and certain constituents of OPC. X-ray diffraction (XRD) and thermogravimetric (TG)/differential thermal analysis (DTA) studies have proved the formation of a new reaction product due to interaction between MA and some of the mineral phases of Portland cement. The retardation of the hydration of Portland cement may be attributed to the formation of this new compound.     

Phase equilibria of hydrated Portland cement

A model is described for predicting the equilibrium phase assemblage in hydrated Portland cement and for calculating the relative contents and composition of phases present in the assemblage, from the chemical composition of the cement and the water/cement ratio. The method is also used to calculate the content of capillary pores using the best available data for the densities for each of the phases. These calculations were carried out on three different Portland cements (two white Portland cements and one grey) at water/cement ratios of 0.70. Energy dispersive spectroscopy (EDS) analysis on the SEM was used to verify the presence of all phases predicted by the model. A density for the C-S-H phase of approximately 2.10 g/cm³ and an evaporable water content in the C-S-H of approximately 19% provided the best agreement between the predicted values of chemical shrinkage, loss of ignition and content of evaporable water, and experimental data. Other results included compositional data on the C-S-H phase corresponding to molar ratios of Al/Ca=0.04 and S/Ca=0.03 in all cements investigated, and Fe/Ca=0.02 in the grey Portland cement.     

石膏对改性硅酸盐水泥性能的影响

将磷铝酸盐水泥熟料掺入硅酸盐水泥中改性后,运用XRD和SEM等测试技术,研究了石膏对改性硅酸盐水泥性能的影响.结果表明,石膏的掺入可以改善改性硅酸盐水泥的力学性能和抗冻性;在石膏掺量为3.5%时,改性硅酸盐水泥水化速度最快,硬化浆体的结构最致密,强度最高,抗冻性最好.     

Impedance spectroscopy study of hardened Portland cement paste

In this paper, the differential impedance analysis (DIA) has been applied to the study of the dielectric properties of hardened Portland cement paste. Two time constants are found in the impedance spectra obtained in the frequency region form 100 kHz to 15 MHz. One time constant has been attributed to the solid matrix and the other one to the liquid phase filling the pores. The effect of the cement paste-electrodes interface has been quantified using two different experimental set-ups. Measurements using direct contact between electrodes and cement paste have been compared with measurements using an air gap technique in which the specimen “floats” between the electrodes. The two referred time constants have been found in both types of measurements. The influence of drying on the dielectric parameters is also studied.     

Polymerised high internal phase emulsion cement hybrids: Macroporous polymer scaffolds for setting cements

We polymerised the continuous styrene/divinylbenzene monomer phase of high internal phase emulsions (HIPEs) containing 70 vol.% cement slurry as internal phase to synthesise polymer cement hybrid materials. These novel cement containing poly(merised)HIPEs have an interconnected bi-phasic structure consisting of an interpenetrating network of set cement and polymer. Incorporating 14 wt.% of polymer into the cement resulted in an increased compressive strain to failure as compared to pure set cement but both elastic modulus and crush strength decreased. These novel polymer cement hybrid materials have a better chemical resistance against acetic acid then pure cement and showed also no shrinkage when exposed to xylene and dodecane.     

Water desorption and shrinkage in mortars and cement pastes: Experimental study and poromechanical model

The aim of the present research was to study moisture changes and strains induced by smooth water desorption of several cement based materials. The main advantage of this small-steps drying is to dramatically limit the structural effect within tested samples, by lowering moisture gradients and therefore cracking due to differential shrinkage. Resulting data are of importance as they allow water retention curves, porosity distribution and desiccation shrinkage to be determined versus a large range of relative humidity. Experiments were conducted on ordinary mortars and cement pastes with water-to-cement ratio of 0.5 and 0.8. The role of the cementitious matrix and of aggregates over water-related behaviour of these materials can also be studied. Finally, a simple numerical model, based on experimental poromechanical results, was proposed to predict the shrinkage when the material is submitted to drying.     

Physico-chemical and biological properties of novel Eu-doped carbonization modified tricalcium silicate composite bone cement

This paper introduces a novel composite bone cement (Eu:HAp/V-C₃S), which is composed of carbonated tricalcium silicate (V-C₃S) and europium-doped hydroxyapatite (Eu:HAp), and Eu:HAp was generated by hydrothermal synthesis. The physical and chemical properties, mineralization in vitro, biocompatibility and bacteriostasis of the composite bone cement were evaluated. The results show that the Eu:HAp/V-C₃S composite bone cement has good setting properties and a relatively low pH value. The composite bone cement with 0.1 wt% Eu has a higher compressive strength than pure V-C₃S (141.06% higher than pure V-C₃S), which can greatly improve the mechanical properties of the materials. The in vitro immersion test shows that composite bone cement has good mineralization ability. The cell test proves that it has good cell proliferation ability and low cytotoxicity. In addition, the bacteriostatic experiment also verifies that composite bone cement has bacteriostatic effect to some degree. These results indicate that Eu:HAp/V-C₃S composite bone cement is a promising biomedical material.     

Liquid-phase self-diffusion in hydrating cement pastes — results from NMR studies and perspectives for further research

The changes in the pore structure of hydrating cement are accompanied by changes in the dynamics of liquid phases contained in the pore system of the hydrating matrix. Dynamic NMR methods (relaxometry, diffusometry) allow the non-destructive observation of these changes. Relaxometry can be performed using quite simple equipment and has been widely used in studies of the kinetics of cement hydration. Diffusion studies, by contrast, require much more sophisticated equipment. On the other hand, the diffusion coefficient has a direct relevance for the transport of moisture or contaminants in the cement matrix while relaxation time measurements provide more indirect information. The purpose of the present paper is to review the possibilities of field gradient NMR in diffusion studies on hydrating cement and to provide an outlook on how this information can be used for improving our understanding of the properties and microstructure of hydrating cement. As an example, new results on the relationship between the diffusive exchange length in the sample and non-exponential relaxation in cement are discussed at the end of the contribution.     

Polymer derived silicon oxycarbide ceramic monoliths: Microstructure development and associated materials properties

Polymer derived SiOC and SiCN ceramics (PDCs) are interesting candidates for additive manufacturing techniques to develop micro sized ceramics with the highest precision. PDCs are obtained by the pyrolysis of crosslinked polymer precursors at elevated temperatures. Within this work, we are investigating PDC SiOC ceramic monoliths synthesized from liquid polysiloxane precursor crosslinked with divinylbenzene for fabrication of conductive electromechanical devices. Microstructure of the final ceramics was found to be greatly influenced by the pyrolysis temperature. Crystallization in SiOC ceramics starts above 1200?°C due to the onset of carbothermal reduction leading to the formation of SiC and SiO₂ rich phases. Microstructural characterisation using ex-situ X-ray diffraction, FTIR, Raman spectra and microscopy imaging confirms the formation of nano crystalline SiC ceramics at 1400?°C. The electrical and mechanical properties of the ceramics are found to be significantly influenced by the phase separation with samples becoming more electrically conducting but with reduced strength at 1400?°C. A maximum electrical conductivity of 10¹ S?cm^?1 is observed for the 1400?°C samples due to enhancement in the ordering of the free carbon network. Mechanical testing using the ball on 3 balls (B3B) method revealed a characteristic flexural strength of 922?MPa for 1000?°C amorphous samples and at a higher pyrolysis temperature, materials become weaker with reduced strength.