Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C-S-H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C-S-H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C-S-H in hydrated Portland cement pastes. It is found that the relation of Na⁺ between the moles bound in C-S-H and its concentration in the pore solution is linear, while the binding of K⁺ in C-S-H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C-S-H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.     

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.     

Characterization of white Portland cement hydration and the C-S-H structure in the presence of sodium aluminate by Al and Si MAS NMR spectroscopy

The effects of hydrating a white Portland cement (wPc) in 0.30 and 0.50 M solutions of sodium aluminate (NaAlO₂) at 5 and 20 °C are investigated by ²⁷Al and ²⁹Si magic-angle spinning (MAS) NMR spectroscopy. It is demonstrated that NaAlO₂ accelerates the hydration of alite and belite and results in calcium-silicate-hydrate (C-S-H) phases with longer average chain lengths of SiO₄/AlO₄ tetrahedra. The C-S-H phases are investigated in detail and it is shown that the Al/Si ratio for the chains of tetrahedra is quite constant during the time studied for the hydration (6 h to 2 years) but increases for higher concentration of the NaAlO₂ solution. The average chain lengths of “pure” silicate and SiO₄/AlO₄ tetrahedra demonstrate that Al acts as a linker for the silicate chains, thereby producing aluminosilicate chains with longer average chain lengths. Finally, it is shown that NaAlO₂ reduces the quantity of ettringite and results in larger quantities of monosulfate and a calcium aluminate hydrate phase.     

Effect of various superplasticizers on the rheological properties of Portland cement pastes

In the present work, the influence of the addition of some superplasticizers employed for maximising the solid loading of Portland cement pastes has been investigated. Cement pastes were prepared from deionized water and a commercial manufactured ordinary Portland cement 32.5 R (produced by Buzzi Unicem). Cement and water were mixed with a vane stirrer according to ASTM Standard C305. The water/cement ratio was kept fixed at 0.32. Three commercial superplasticizing agents produced by Ruredil were used: they are based on a melamine resin (Fluiment 33 M), on a modified lignosulphonate (Concretan 200 L), and on a modified polyacrylate (Ergomix 1000). Rheological tests were carried out at 25 °C by using the rate controlled coaxial cylinder viscometer Rotovisko-Haake 20, system M5-Osc., measuring device MV2P with serrated surfaces. The tests were performed under both continuous and oscillatory flow conditions. Ergomix 1000 presents a different behaviour as that of the other two superplasticizers studied, because it shows a marked shear-thickening behaviour above a critical deflocculant concentration and slight elastic effects particularly at high dosages as well.     

The BET-specific surface area of hydrated Portland cement and related materials

Factors that may affect the results in the determination of the BET-specific surface area of hydrated Portland cement and related materials are analyzed on the basis of available experimental data. In addition to factors brought about by starting mix composition and curing conditions, the obtained result is also influenced by sample preparation, especially by the way free water is removed from the pore system. The consistently lower BET_N2 values, as compared to BET_H2O results, are brought about by the extremely restricted ability of nitrogen molecules to penetrate small pore entrances under conditions used in the measurement and to enter a part of the overall pore space.     

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。     

Early hydration of white Portland cement in the presence of bismuth oxide

Abstract

Abstract

Mineral trioxide aggregate (MTA) is a clinical product comprising a mixture of 80 wt-% Portland cement and 20 wt-% bismuth oxide, which is used as a root-filling material in dentistry. The influence of bismuth oxide on the hydration reactions of Portland cements is not well understood. In this study, the impact of 20 wt-% replacement of bismuth oxide on the hydration of white Portland cement was monitored by powder X-ray diffraction (XRD), ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and transmission electron microscopy (TEM). The findings of this research have confirmed that bismuth oxide is an inert additive in white Portland cement, which does not participate in the hydration reactions.     

Effect of Cr2O3 and NiO additions on the phase transformations at high temperature in Portland cement

Although there exists much work to characterize the effect of minor elements on the properties of Portland cement, the same is not valid for the knowledge about their impact on the clinkering process. The goal of this work is to study the effects of chromium and nickel oxide additions on the transformations during the production of Portland cement clinker. The experiments were carried out using a differential thermal analysis (DTA) piece of equipment to obtain the heating and cooling transitions. Samples were prepared using high-purity reactants. The achieved results showed that the Cr₂O₃ additions to cement raw meal act as an inhibitor of C₂S formation and increase the amount of liquid phase. On the other hand, NiO has very little effect on the clinkering reactions, but NiO does cause a small increase in the temperature of C₂S formation and an opposite effect on the temperatures of liquid and the polymorphic transformation of C₂S.     

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.     

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。     

Composition, morphology and nanostructure of C-S-H in white Portland cement pastes hydrated at 55 °C

The C-S-H present in water- and alkali-activated hardened pastes of white Portland cement hydrated at 55 °C has been characterized. The mean length of the aluminosilicate anions in the C-S-H was similar in both systems and increased with age. Inner product C-S-H generally had a fine scale, homogeneous morphology. Outer product C-S-H was generally fibrillar with water, and foil- or lath-like with alkali. There were some regions of C-S-H with coarse morphology. It was not possible to determine the chemical composition of C-S-H using the SEM; TEM-EDX was necessary. The C-S-H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water, which was offset by a larger quantity of calcium hydroxide. The potassium in the KOH-activated paste was present either within the C-S-H structure charge balancing the substitution of Al³⁺ for Si⁴⁺, or adsorbed on the C-S-H charge balancing sulfate ions.     

Natural cement as the precursor of Portland cement: Methodology for its identification

When cements appeared in the 19th century, they took the place of traditional binding materials (lime, gypsum, and hydraulic lime) which had been used until that time. Early cements can be divided into two groups, natural and artificial (Portland) cements. Natural cements were introduced first, but their widespread usage was short-lived as they were quickly replaced by artificial cements (Portland), still the most important and predominant today. The main differences between natural and artificial cements arise during the manufacturing process. The final properties of the cements are greatly influenced by differences in the raw materials and burning temperatures employed.The aim of this paper is to assess the efficiency of traditional analytical techniques (petrographic microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR)) used to differentiate natural and artificial cements.     

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.     

The hydration products of Portland cement in the presence of tin(II) chloride

The hydration products of Portland cement pastes cured using water containing tin(II) chloride have been compared with those using distilled water. In the latter case, the expected products—portlandite, ettringite and calcite—were observed. The X-ray diffraction patterns of the cement pastes cured in the presence of tin(II) chloride showed several additional peaks that have been attributed to the formation of calcium hydroxo-stannate, CaSn(OH)₆, and Friedel's salt (tetracalcium aluminate dichloride-10-hydrate), Ca₃Al₂O₆·CaCl₂·10H₂O. The amount of portlandite formed was reduced in the presence of tin(II) chloride. Calcium hydroxo-stannate contains tin in the +IV oxidation state and equations are presented to account for the oxidation of Sn(II) to Sn(IV) preceding the formation of CaSn(OH)₆ and Friedel's salt.     

硅酸盐与硫铝酸盐复合水泥孔隙液相pH值变化研究

为了解硅酸盐水泥与硫铝酸盐水泥复合之后性能变化的原因,通过对复合水泥孔隙溶液的pH值进行研究,探讨了pH值变化与性能之间的内在联系。研究表明,孔隙溶液pH值变化与强度有良好相关性,水化3h后复合水泥pH值在SAC掺量为20%-80%之间出现波谷,水化剧烈并开始大量放热,后期强度也出现明显波谷区,其中以其掺量为60%时pH值和后期强度最低;当SAC或PC掺量为0%-20%时,复合水泥孔隙溶液pH值和强度相对于纯的SAC或PC而言基本没有变化,甚至还会稍微提高,这对降低水泥生产成本和提升水泥性能具有积极意义。     

Hospital waste ashes in Portland cement mortars

Nowadays, most concretes incorporate mineral additions such as pozzolans, fly ash, silica fume, blast furnace slag, and calcareous filler among others. Although the technological and economical benefits were the main reasons for the use of mineral additions, the prevention of environmental contamination by means of proper waste disposal becomes a priority. The chance of incorporating hospital waste ashes in Portland cement-based materials is presented here. Ash characterization was performed by chemical analysis, X-ray diffraction, radioactive material detection, and fineness and density tests. Conduction calorimetry and setting time tests were developed on pastes including ash contents from 0% to 100%. Mortars were prepared including ash contents up to 50% of cement. The results of setting time, temperature development, flexural and compressive strengths, water absorption, density, and leachability are analyzed. Results indicate that Portland cement systems could become an alternative for the disposal of this type of ashes.     

Microwave-promoted burning of Portland cement clinker

A new method of burning Portland cement clinker is studied. The microwave sintering is adopted after the raw meal is heated to certain temperature in an electric furnace. The experimental results show that after the raw meal is heated at a low electric heating temperature (1000-1200 °C) and then further sintered with microwave for 1 to 2 min, Portland cement clinkers can be formed. The f-CaO contents of the clinkers are 1-2%. It has also been found that the higher the temperature of the samples put into the microwave cavity, the shorter the time needed for microwave burning. When the temperature is up to 1300 °C, the sample needs to be heated by microwave for only 40 s, and the f-CaO content decreases to 0.65%. It has been proved by the experiments that (1) the new burning technique can greatly increase the forming speed of Portland cement clinkers, (2) Fe₂O₃ can enhance the microwave clinkering. XRD patterns of the clinkers show that their mineral compositions and the characteristic XRD peaks are similar to those of clinkers by conventional burning method.     

A preliminary investigation of the in vitro bioactivity of white Portland cement

Freshly-mixed and partially-cured ordinary Portland cement (OPC) pastes have been shown to exhibit good biological compatibility with a range of cells and tissue-types; particularly those associated with bone formation. Formulations based on OPC have been used as dental restoratives and are now being investigated for their potential use in orthopaedic repair. Despite the current clinical interest in OPCs, very little is known about their chemistry in the physiological environment. In this respect, research to investigate aspects of the interactions between a white Portland cement (WPC) paste and simulated body fluid (SBF) has been carried out in vitro. Exposure to SBF has been found to promote the precipitation of a layer of ‘bone-like’ hydroxyapatite on the surface of WPC paste which underpins its ability to integrate with living tissue. The dissolution of portlandite and formation of calcite were also observed on contact with SBF.     

低热硅酸盐水泥性能评价

通过对比试验，证明低热硅酸盐水泥(HBC)的工作性、力学性能和耐久性等均优于纯硅酸盐水泥(PC)，完全能够满足高性能砼的三大技术要求，是制备高性能砼、大体积砼、蒸养砼制品、高温施工砼及有特殊抗化学侵蚀要求砼的理想胶凝材料。     

硅酸盐熟料挤压裂纹的观察试验

介绍了硅酸盐熟料挤压裂纹的观察试验过程,试图通过观察硅酸盐熟料受到挤压后,微观晶体裂纹的形成、裂纹产生的起始点及熟料受压后其它的受破坏情况,进而对挤压粉磨效果进行验证。