The influence of water removal techniques on the composition and microstructure of hardened cement pastes

The removal of water from hardened cement paste for analysis or to arrest ongoing hydration has been reported to affect the composition of hydrated phases and microstructure. The effect that arresting the hydration of hardened cement paste by replacing the pore water with acetone before drying, and by removing the water by freeze, vacuum and oven drying has on the hardened cement paste has been investigated. Two pastes were studied, a cemented iron hydroxide floc where a high proportion of ordinary Portland cement (OPC) had been replaced by pulverised fuel ash, and a pure hydrated OPC. The results showed that none of the water removal techniques caused any major deterioration in the composition and microstructure of the hardened cement pastes studied, but the pores appeared better preserved after arresting hydration using acetone quenching. Freeze drying appeared to cause more cracking of the microstructure than the other water removal techniques.     

The effect of desiccation on the porosity and pore structure of freeze dried hardened portland cement and slag-blended pastes

Mercury porosimetry studies of hardened cement pastes of Portland cement and blast furnace slag blended cements have been conducted after freeze drying cured samples. The samples have been aged after freeze drying in a desiccating atmosphere so as to evaluate the effect of aging on the pore structure of freeze dried hardened Portland cement paste prepared with and without admixtures. Results indicate that after 24 hours of freeze drying the hardened paste shows negligible change in porosity and pore structure with aging in a desiccating atmosphere.     

Variability of the nitrogen surface area of hydrated cement paste

The difference between the BET specific surface areas computed from water and nitrogen adsorption isotherms of bottle hydrated cement paste were found to vanish if special techniques for drying the samples were applied. Typical features of the methods were replacement of the pore water by methyl alcohol which in turn was substituted by pentane and final drying was achieved by distillation either in vacuo or above the critical temperature. The nitrogen and water surface areas of the specially dried paste are both approximately 200 m²/g. It is suggested that the commonly observed discrepancy in surface areas is due not to deficiencies of the measuring method but to real differences evoked by the particular drying, technique applied.     

Impact of drying on pore structures in ettringite-rich cements

Drying techniques affect the properties of cement pastes to varying extents. The effect of different drying techniques on calcium sulfoaluminate-based (C$A) cements and their constituent phases is reported for a range of simulated and commercial C$A pastes which are benchmarked against an OPC paste. The recommended methodologies used to dry samples were identified from the literature and include D-drying and solvent exchange. These methods were used in conjunction with mercury intrusion porosimetry (MIP) and X-ray powder diffraction (XRPD) measurements to assess the changes in pore structure and the damage to crystalline phases, respectively. D-drying and isopropanol exchange are the most satisfactory and least damaging methods for drying C$A based pastes.     

New insights into the effects of sugar on the hydration and microstructure of cement pastes

The effects of adding sugar to cement paste on hydration and microstructure were observed. While 1% sugar delayed hydration as expected, the delay period was shortened by increased curing temperature. When samples containing sugar began to react, hydration progressed very quickly and the degree of hydration soon surpassed that of control samples. Sugar addition increased the surface area and altered the pore size distribution, as measured by nitrogen, of cement pastes. Results indicate that sugar not only alters the rate of cement paste hydration, but the microstructure of calcium-silicate-hydrate (C-S-H) as well.     

碱-磷渣-粉煤灰胶凝材料的性能与硬化浆体结构

为充分利用磷渣和粉煤灰两种工业废渣生产高性能胶凝材料,研究了不同磷渣/粉煤灰配合比的碱-磷渣-粉煤灰胶凝材料性能,并用扫描电子显微镜和压汞仪分析了硬化浆体的细观结构和孔结构.结果表明:碱-磷渣-粉煤灰胶凝材料的凝结时间正常,在粉煤灰掺量为0～30 %(质量分数)范围内,随粉煤灰的掺量的增加,碱-磷渣-粉煤灰胶凝材料的凝结时间略有延长.与普通硅酸盐水泥相比,碱-磷渣胶凝材料的抗压强度较高,其3d和28d抗压强度分别可达到30.9MPa和98.8MPa,但其抗折强度相对较低.掺加粉煤灰后碱胶凝材料的抗压强度降低,而抗折强度提高.碱-磷渣-粉煤灰胶凝材料的耐蚀性和抗冻性能均显著优于硅酸盐水泥,其干缩比硅酸盐水泥的大.用部分粉煤灰取代磷渣粉可一定程度减小干缩.碱-磷渣-粉煤灰胶凝材料硬化浆体的结构非常致密,其孔隙率和平均孔径均小于普通硅酸盐水泥硬化浆体.     

Identification of viscoelastic C-S-H behavior in mature cement paste by FFT-based homogenization method

A powerful and robust numerical homogenization method based on fast Fourier transform (FFT) is formulated to identify the viscoelastic behavior of calcium silicate hydrates (C-S-H) in hardened cement paste from its heterogeneous composition. The identification is contingent upon the linearity of the creep law. To characterize cement paste microstructure, the model developed by Bentz at the National Institute of Standards and Technology, which has the resolution of 1 μm, is adopted. Model B3 for concrete creep is adapted to characterize the creep of C-S-H in cement paste. It is found that the adaptation requires increasing the exponent of power law asymptote of creep compliance. This modification means that the rate of attenuation of creep with time is lower in C-S-H than in cement paste, and is explained by differences in stress redistribution. In cement paste, the stress is gradually transferred from the creeping C-S-H to the non-creeping components. The viscoelastic properties of C-S-H at the resolution of 1 μm were identified from creep experiments on cement pastes 2 and 30 years old, having the water-cement ratio of 0.5. The irreversible part of C-S-H creep, obtained from these old specimens at almost saturated state, is found to be negligible unless the specimens undergo drying and resaturation prior to the creep test.     

Properties of concrete with and without calcium chloride during drying and wetting

The effect of calcium chloride on the direct tensile strength of hardened cement paste and concrete cylindrical and prismatic specimens are investigated. Two different techniques for the determination of the direct tensile strength i.e. cylinders with embedded bars and prisms with glued end blocks are used. The results indicate that the addition of 2% calcium chloride significantly increases the direct tensile strength and the dynamic modulus of hardened cement paste. It is concluded that the addition of calcium chloride can partially inhibit the cracking caused by drying and sorption induced microcracking in the concrete system.     

Reactions between methanol and portland cement paste

The paper concerns the suitability of methanol replacement as a method for drying cement paste specimens prior to microstructural examination. Thermogravimetric tests on both cement paste and calcium hydroxide which were soaked in methanol for various periods show that methanol alters sample composition by reacting to form a carbonate-like product. Also, some of the methanol is found to remain with the solid at temperatures above 300°C. It is concluded that until more is known about the interaction between methanol and the cement paste constituents, the correct interpretation of test results of methanol treated specimens is likely to be very difficult.     

Acoustic Emission in the Drying of Hardened Cement Paste and Mortar

Acoustic emission (AE) during the drying of cement and mortar was studied extensively; AE in direct fracture of notched beams was also measured. A new method for observing the hydration of cement by measuring acoustic conductivity was developed and used to aid in the interpretation of the drying experiments. Acoustic emission due to drying increased rapidly with the time of hydration. It reached a maximum value at ∼60 days for hardened cement paste and 240 days for a standard mortar. It is shown to be caused by fractures in structures that develop, at least in part, during the drying process.     

锂渣和钢渣对水泥浆体力学性能与微观结构的影响

利用锂渣粉和钢渣粉替代部分P·O 42.5水泥制备了复合水泥净浆试样。通过SEM、XRD、FT-IR等测试方法分析了二者对试样的影响及作用机制。结果表明,锂渣替代部分水泥会降低浆体的流动性,钢渣替代部分水泥有利于提高浆体流动性。锂渣具有促凝效果,而钢渣在浆体中可发挥缓凝作用。锂渣、钢渣复合掺入时可调控浆体的流动性和凝结时间。锂渣对浆体力学性能的提高相比钢渣具有更明显的优势,当水胶比为0.4时,掺入20%(质量分数)锂渣的试样28 d抗压强度可达62.3 MPa,相比空白样可提高23%左右。SEM结果显示掺20%锂渣可使试样28 d微观结构更致密。XRD结果显示试样的水化产物主要为C-S-H凝胶和Ca(OH)₂。FT-IR结果显示Si—O键峰位发生了一定的红移,H—O—H键发生了蓝移。     

Effect of admixture on hydration of cement, adsorptive behavior of admixture and fluidity and setting of fresh cement paste

The adsorptive behavior of admixtures and the hydration of cement in the presence of admixtures were examined and the relationships of them with the physical properties of fresh cement paste including fluidity, variation of fluidity with time and setting time were discussed with the quantitative determination of organic admixture adsorbed on the cement, and with the observation and determination of the surface microstructure and composition of polished clinker dipped in aqueous solution containing a specified quantity of admixture by advanced method of surface analysis.

In order to prepare the same fluidity of fresh cement paste, mortar and concrete, the required amount of an easily adsorbed admixture is larger than that of a hard adsorbed one. An admixture having a functional group producing a complex salt with Ca²⁺ decreases the concentration of Ca²⁺ in liquid phase at early age and delays the saturation of Ca²⁺, which influences the morphology of hydrate produced, causes fluidity loss with time and delays the setting time of cement. The microstructural and compositional estimations of the adsorption layer of admixture on the surface of clinker minerals by in-lens FESEM, ESCA-imaging, AES and AFM indicates that the admixture is partially adsorbed to the interstitial phase in a thick layer, forming characteristic three dimensional surface structure.     

A colloidal interpretation of chemical aging of the C-S-H gel and its effects on the properties of cement paste

The properties, structure, and behavior of cement paste, including surface area, drying shrinkage, creep, and permeability are discussed with the assumption that the C-S-H gel is an aggregation of precipitated, colloidal-sized particles that undergoes chemical aging. A basic thesis of this paper is that C-S-H particles bond together over time, increasing the average degree of polymerization of the silicate chains and causing the C-S-H to become stiffer, stronger, and denser. This process occurs slowly at ambient temperatures, but can be greatly accelerated by elevated temperature curing and is also encouraged by drying, which introduces large local strains that may provide a microstructural basis for creep sites. This chemical aging process of C-S-H can thus affect many of the physical properties of cement paste, and there is particular relevance for the complex shrinkage and creep behavior of this material. The effects of a short heat treatment, which causes rapid aging, depend strongly on the moisture of the paste when it is heated. Many of the observations and insights presented here are not new. The primary objective of this paper is to demonstrate, by reporting a variety of published findings in one place, the significant amount of evidence that has been generated over the past 50 years favoring this interpretation. Another objective is to show that the properties and behavior of the C-S-H gel, and of cement paste, do not require a layered microstructure. Separating chemical aging effects from other changes, such as continued hydration, may well lead to a better understanding of the microstructural causes of creep and shrinkage.     

不同养护条件下掺循环流化床固硫灰硬化水泥浆的膨胀性能

为探讨掺循环流化床固硫灰(简称固硫灰)硬化水泥浆的体积变化规律,将固硫灰以不同质量取代水泥制备水泥浆试件,结合扫描电子显微镜、压汞仪、X射线衍射仪及TG-DSC同步热分析仪分析了固硫灰及试件的微观形貌、矿物组成和热效应,对三种养护条件下试件的体积膨胀性能进行了试验研究。结果表明：固硫灰的掺入能显著提高浆体标稠用水量,固硫灰掺量80%(质量分数,下同)的浆体标稠用水量是基准组的1.6倍;封闭养护时,硬化浆体体积随固硫灰掺量的增加由收缩(掺量低于40%)变为膨胀(掺量高于60%),且收缩率高于基准组;相较于封闭养护,标准养护使硬化浆体内部水化更充分,大毛细孔含量减少,微观结构更密实,硬化浆体体积膨胀,固硫灰掺量80%的试件28 d膨胀率最高达0.42%;空气养护的试件体积均呈收缩趋势,80%固硫灰掺量的试件水灰比较大,但干燥引起的收缩变形高于膨胀产物产生的膨胀变形。     

硫酸腐蚀后油井水泥石的性能及微观结构

研究了硫酸对油井水泥石强度及微观结构的影响。结果表明：水泥石被硫酸腐蚀后,强度明显下降,硬化浆体中100 nm以上有害孔的数量显著增多,水化产物变得疏松多孔,硬化水泥浆体的物相组成发生变化,有新的腐蚀产物CaSO4·2H2O生成;水泥石抵抗酸性介质腐蚀的能力不仅与其致密程度有关,还与其硬化浆体的矿物组成密切相关;不同水化产物抵抗腐蚀的能力不同,Ca（OH）2比C-S-H凝胶更容易受到酸性介质的腐蚀;C-S-H凝胶被腐蚀后产生的孔隙主要是细小孔隙,而Ca（OH）2被腐蚀后产生的孔隙主要是100 nm以上有害孔,降低硬化浆体中Ca（OH）2的含量是提高水泥石抗腐蚀性能的关键。     

Microstructure and composition of the hydrated cement paste of an 84 year old concrete bridge construction

The hydrated cement paste of an 84 year old reinforced concrete bridge construction has been examined for its composition and microstructure by means of chemical analysis, SEM, X-ray diffraction analysis and thermal analysis. The main hydration products are: calcium hydroxide, fibrous calcium-silicate-hydrate incorporated frequently with the calcium hydroxide into hexagonal plates, and calcium-aluminate-carbonate-hydrate. The properties of the almost completely hydrated cement paste have not been adversely affected even after more than 80 years.     

New methods to measure liquid permeability in porous materials

Several novel methods have recently been proposed for rapid measurement of the liquid permeability of saturated cement paste, mortar and concrete. The relative merits of the techniques are discussed, and some recent results obtained on pastes and mortars are presented. The low permeabilities seen in cement paste are inconsistent with the pore size distributions measured following drying, indicating that the pore structure is significantly changed by drying.     

Influence of hydration on the fluidity of normal Portland cement pastes

The rheological properties of cement paste strongly influence the workability of concrete. It is known that early hydration processes alter phase composition and microstructure of cement pastes. These processes affect fluidity and setting behaviour of cement paste. While many studies tried to measure and model rheological properties of cement pastes, only a few studies assessed the influence of the hydrate morphology on the fluidity of cement pastes.Results of the present study compare the influence of long prismatic hydrates (i.e. syngenite, secondary gypsum) on the fluidity of cement pastes with the effect of other hydrates (AFm).To induce the formation of certain hydration products the cement composition was modified by addition of set regulators and alkali sulphates. Furthermore a combination of various analytical methods such as fluidity (viscometric) testing and microstructural analysis (phase quantification by XRD-Rietveld analysis, investigation by Environmental SEM, BET analysis etc.) was performed. Results are implemented into a fundamental discussion on the influence of various hydration products on the fluidity of the paste.     

Changes in physicochemical properties and digestive characteristics of yam (Dioscorea opposita Thunb.) powder under different processing conditions using principal component analysis

Abstract

This study aimed to understand and establish changes in the physicochemical properties of yam powder with different processing methods, and select a suitable method for use in the development of hypoglycemic functional foods. Samples were dried using three methods, namely hot air drying, vacuum freeze drying, and microwave vacuum drying, and turned to powder using two smashing methods, namely ordinary smashing and superfine smashing (SS). Color changes, polarized light microscopy, differential scanning calorimetry, X-ray diffraction, polysaccharide dissolution rates, α-glucosidase inhibitory activities, glucose dialysis retardation indexes, and digestive characteristics were measured. The results showed that the drying method and the smashing method had significantly different effects on the physicochemical properties and digestion characteristics of the yam powder. Freeze-dried yam retained more nutrients, while SS increased the rapidly digestible starch content, reduced the digestible starch and resistant starch contents, and increased the polysaccharide dissolution rate in the yam powder samples. Six samples were compared using principal component analysis, with the highest composite score observed for the samples treated by freeze drying coupled with SS, followed by hot-air drying coupled with superfine smashing (HAO).     

高温条件下G级油井水泥原浆及加砂水泥的水化和硬化

利用X射线衍射仪和扫描电子显微镜研究了80～240℃温度范围内温度、硅砂对G级油井水泥水化硬化的影响,检测和分析了硬化体的水化产物、微观结构和强度,揭示了水化产物组成、微观结构及硬化体抗压强度的变化特点.结果表明:当养护温度超过110℃时,不添加硅砂的水泥原浆的主要水化产物由CSH(Ⅱ),C2SH2,C3S2H3转变为C2SH,硬化体微观结构由三维网络状结构转变为板快状或团块状结构,原浆水泥石抗压强度随温度升高而降低;在相对较高的温度条件下,添加硅砂的水泥主要水化产物则分别转变为C5S6H5,C6S6H(>150℃),C5S5A0.5H5.5,C3.2S2H0.8及其他类型的水化硅酸钙晶体,硬化体的微观结构相应地变为纤维网状、粗框架、短平行针状及团块状,在温度为100～150℃范围时,添加硅砂的水泥硬化体抗压强度随温度升高而增加,而在温度为150～240℃范围时.抗压强度随温度升高而降低.对于温度超过120℃的深井,合理的硅砂加量为30%～40%.