Investigation of properties of fluorogypsum-slag composite binders – Hydration,strength and microstructure

A composite binder of high strength and low water absorption has been developed using industrial by-products fluorogypsum, granulated blast furnace slag and Portland cement. The development of strength in the binder at an early age is attributed to the conversion of anhydrite into gypsum and at later age is due to the formation of ettringite and tobermorite, as a reaction of slag with lime produced during the hydration of cement. These cementitious phases fill in pores and voids of the hydrating gypsum crystals to form a dense and compact structure of low porosity and low pore volume. The reaction products formed during the hydration period were confirmed by scanning electron microscopy and X-ray diffraction. The reduction in porosity and low pore volume of binders, as studied by mercury intrusion porosimetry, are responsible for attainment of high strength and better stability towards water in composite binders than the conventional gypsum plaster.     

Optimization of calcium chloride content on bioactivity and mechanical properties of white Portland cement

This research investigates the optimization of calcium chloride content on the bioactivity and mechanical properties of white Portland cement. Calcium chloride was used as an addition of White Portland cement at 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10% by weight. Calcium chloride was dissolved in sterile distilled water and blended with White Portland cement using a water to cement ratio of 0.5. Analysis of the bioactivity and pH of white Portland cement pastes with calcium chloride added at various amounts was carried out in simulated body fluid. Setting time, density, compressive strength and volume of permeable voids were also investigated. The characteristics of cement pastes were examined by X-ray diffractometer and scanning electron microscope linked to an energy-dispersive X-ray analyzer. The result indicated that the addition of calcium chloride could accelerate the hydration of white Portland cement, resulting in a decrease in setting time and an increase in early strength of the pastes. The compressive strength of all cement pastes with added calcium chloride was higher than that of the pure cement paste, and the addition of calcium chloride at 8 wt.% led to achieving the highest strength. Furthermore, white Portland cement pastes both with and without calcium chloride showed well-established bioactivity with respect to the formation of a hydroxyapatite layer on the material within 7 days following immersion in simulated body fluid; white Portland cement paste with added 3%CaCl₂ exhibited the best bioactivity.     

A hydration study of various calcium sulfoaluminate cements

The present work studies the hydration process and microstructural features of five calcium sulfoaluminate (CSA) cements and a ternary mixture including also ordinary Portland cement (OPC). The pastes were studied with simultaneous differential thermal-thermogravimetric (DTA-TG) analysis, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and expansion/shrinkage tests. The DTA-TG analysis confirmed the role of the hydration reactions involving the main CSA clinker constituent, tetracalcium trialuminate sulfate, which produced (i) ettringite when combined with lime and calcium sulfate, (ii) ettringite and aluminum hydroxide in the presence of calcium sulfate alone, and (iii) monosulfate and aluminum hydroxide in the absence of both lime and calcium sulfate. The MIP and SEM were able to discriminate between expansive (ternary mixture and CSA cement containing 50% gypsum) and non-expansive cements. Expansive cement pastes had (i) a nearly unimodal pore size distribution shifted toward higher radii and (ii) ettringite crystals smaller in size during the first day of curing. In a SEM image of a hardened paste of the CSA cement containing 50% gypsum, a stellate ettringite cluster was observed.     

Electron microscopy of heavy metal waste in cement matrices

Ordinary Portland cements mixed with various amounts of chromium metal in the form of nitrates (Cr(NO₃)₃), to simulate industrial waste, have been studied by electron microscopy techniques, i.e. scanning electron and scanning transmission electron microscopy. Trivalent chromium was found to be chemically incorporated in all hydrated cement phases, and appeared to substitute for silicon in calcium silicate hydrate (C-S-H), which is the major product of hydration. Chromium was also concentrated in polycrystalline Ca-Cr-rich phases.     

Fractographic studies of microstructural development in hydrated Portland cement

A comprehensive study of early hydration and morphological development in freeze-dried Portland cement paste has been carried out using scanning electron microscopy in conjunction with energy dispersive X-ray analysis. The microstructure develops from its early appearance as a porous C-S-H spherulite structure containing hexagonal CH crystals, with Hadley grains and large inherent pores being significant features, to one of large CH dispersions in massive C-S-H. Some aspects of microstructural influence on mechanical properties are discussed.     

Study of Portland cement fracture surfaces by scanning electron microscopy techniques

An extensive scanning electron microscopy study was carried out with respect to the fracture surfaces of Portland cement hydrated for various times. It is shown that the two major products of hydration are calcium silicate hydrate spherulites, which consist of radiating fibres and calcium hydroxide platelets. These fibres bond with one another to hold the spherulites together. The volume between the spherulites consists of calcium hydroxide platelets. The fracture is frequently found to be across the weakly bonded basal planes of the calcium hydroxide, and is believed to limit the strength of the Portland cement.     

Mechanisms of hydration reactions in high volume fly ash pastes and mortars

This paper describes investigations of high-volume fly ash (HVFA)-Portland cement (PC) binders, the physical and chemical properties of which have been characterized up to 365 days of curing. Physical investigations were made of compressive strength development, pore structure by porosimetry, and morphology by scanning electron microscopy. Chemical examination was conducted for solid phase composition and degree of hydration by X-ray diffraction and thermal analysis, and for pore-fluid composition by high pressure extraction and analysis.

Up to 365d the cement in the HVFA pastes is not fully hydrated. However, the ash participates in both early (sulpho-pozzolanic) and late (alumino-silicate) hydration reactions. In addition to the usual products of cement hydration, ettringite (AFt) has been identified as a product of the early hydration of the fly ash. It has not been possible to identify long term hydration products of fly ash which appear to be non-crystalline. A two-step mechanism for pozzolanic reaction between fly ash and Portland cement has been proposed involving: (a) depolymerization/silanolation of the glassy constituents of the ash by the highly alkaline pore fluids, followed by (b) reaction between solubilized silicate and calcium ions in solution to form C---S---H.     

0～20℃硅酸盐水泥的水化性能

采用X射线衍射仪(XRD)及环境扫描电子显微镜(ESEM)研究0℃、5℃、10℃及20℃时硅酸盐水泥的水化过程,并进行凝结时间及力学强度测试。结果表明,温度越低,硅酸盐水泥的初凝和终凝时间越长,水泥早期的水化程度和力学强度也越低;但水化后期,水泥水化产物C-S-H凝胶明显细长,水化产物间距较小,大孔减少且孔隙分布更均匀,水化程度和后期强度较高。     

微波辐照活化煤矸石-硅酸盐水泥体系试验研究

通过宏观试验和扫描电镜(SEM)微观测试分析技术研究了微波辐照活化煤矸石及掺量对硅酸盐水泥体系的细度、凝结时间、体积安定性、火山灰活性和强度等性能的影响,结果表明,大掺量微波辐照活化煤矸石对硅酸盐水泥体系各项技术性能无不良影响;微波辐照活化煤矸石硅酸盐水泥在水化硬化过程中有微膨胀,但体积安定性满足国家相关规范要求;在煤矸石中添加少量CaO利用微波技术可获得质地优良的水泥辅助性胶凝组分;与常规热活化煤矸石方法相比微波辐照活化煤矸石的节能和生产成本降低效果十分显著。     

氯氧镁水泥胶凝材料的研究进展

氯氧镁水泥是由轻烧菱镁石粉和氯化镁水溶液按一定比例混合而成的一种新型镁质气硬性胶凝材料。它有很多优于普通硅酸盐水泥的性能,但其耐水性差等缺点也很突出。为了使氯氧镁水泥得到更好的应用,总结和分析了氯氧镁水泥的研究进展,包括水化行为、水化产物组成和结构、耐水性以及生产应用方面。指出了氯氧镁水泥在当前的发展过程中存在的不足和问题,以期为后续氯氧镁水泥的进一步研究提供参考。     

硫铝酸钙-贝利特水泥熟料的低温制备及其水化性能研究

以煤矸石、脱硫石膏、石灰为主要原料,采用水热合成-低温煅烧方法制备了硫铝酸钙-贝利特水泥熟料,通过X射线衍射(XRD)、扫描电镜(SEM)等分析了水热合成物、煅烧试样及水化产物的矿物相,利用等温量热仪测定了水泥早期水化放热随时间的变化,并测定了水泥的力学性能。结果表明:在120℃下水热合成前驱体后再经煅烧,可在1 050℃低温下制成硫铝酸钙-贝利特水泥熟料。与一步法相比,该低温水泥的早期水化放热速率较高。当水泥中的二水石膏掺量为13%(质量分数)时,水泥1d、28d的抗压强度分别为30.2 MPa和57.3 MPa,28d的水化产物主要为长纤维状的AFt。     

Durability of kraft pulp fiber–cement composites to wet/dry cycling

If pulp fiber–cement composites are to be used for exterior applications, the effect of cyclical wet/dry exposure must be known. In this research program the effects of three fiber treatments—beating, bleaching, and drying—were investigated to identify those that may minimize effects of environmental aging and degradation during wet/dry cycling. After 25 wet/dry cycles, all composites showed significant losses in first crack strength, peak strength, and post-cracking toughness. The majority of losses in mechanical properties occurred within the first 5 wet/dry cycles, though ductile fiber failure was still observed by scanning electron microscopy (SEM). A three-part progressive degradation mechanism during wet/dry cycling is proposed: (1) initial fiber–cement debonding, (2) reprecipitation of hydration products within the void space at the former fiber–cement interface, and (3) fiber embrittlement due to fiber cell wall mineralization. Unbeaten fiber–cement composites exhibited greater peak strength and post-cracking toughness, prior to cycling, while no significant differences were seen after 25 cycles. The effects of fiber beating varied prior to and after cycling. Unbleached fiber–cement composites exhibited the slowest progression of degradation during cycling. The initial drying state appeared to have no effect on composite performance after 25 wet/dry cycles.     

The chemical composition and microstructure of hydration products in blended cements

Pastes of neat and blended Portland cement (incorporating either 60% ground granulated blast furnace slag, or 30% pulverised fuel ash, or 22% volcanic ash) were cured for one year at temperatures ranging from 10 to 60 °C. The hydration products were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The apparent porosity of the pastes increased with increasing curing temperature. Chemical analysis data for the hydration products are presented in ternary composition diagrams, where it is noted that in the presence of the replacement materials the composition of the C–S–H shifted towards higher Si and Al contents, whereas that of Ca was lower.     

Pre-induction and induction hydration of tricalcium silicate: an environmental scanning electron microscopy study

Environmental scanning electron microscopy (ESEM) has been used to study the very early pre-induction, and induction physical processes that occur in the hydration of tricalcium silicate. An in situ experimental technique is described which allows direct, real-time observation of the sub-micrometre morphological changes that take place during this reaction. The results of this investigation are correlated with kinetic data obtained by differential scanning calorimetry (DSC). In this way, microstructural evolution has been identified with the stages of very early hydration. Upon first contact with water, a gelatinous coating was seen to form at grain surfaces and a crystalline secondary product was observed at the end of an extensive dormant period. These findings are viewed in the light of previous wet and dry microscopy studies, and are discussed within the framework of ordinary Portland cement as a possible explanation of induction. Comment is made as to the suitability of environmental SEM for analysis of such materials.     

Micro-mechanical properties of calcium sulfoaluminate cement and the correlation with microstructures

To investigate the micro-mechanical properties of calcium sulfoaluminate cement and the correlation with the microstructures, we apply a variety of advanced techniques of microstructural and micro-mechanical characterization, including scanning electron microscopy with backscattered electron and energy-dispersive X-ray spectroscopy detectors, X-ray fluorescence, X-ray diffraction and nanoindentation. For the first time, the micro-mechanical properties of material microstructures present in a calcium sulfoaluminate cement are estimated. In the calcium sulfoaluminate cement used in this research, two type of hydration product microstructures with the differentiable microstructural morphologies, compositions and micro-mechanical properties are identified and investigated. The correlation of the micro-mechanical properties with the microstructures shows that the hydration product microstructure containing more ettringite has lower indentation modulus and hardness than that containing more aluminum hydroxide.     

Strength and hydration properties of reactive MgO-activated ground granulated blastfurnace slag paste

Ground granulated blastfurnace slag (GGBS) is widely used as a partial replacement for Portland cement or as the major component in the alkali-activated cement to give a clinker-free binder. In this study, reactive MgO is investigated as a potentially more practical and greener alternative as a GGBS activator. This paper focuses on of the hydration of GGBS, activated by two commercial reactive MgOs, with contents ranging from 2.5% to 20% up to 90 days. The hydration kinetics and products of MgO–GGBS blends were investigated by selective dissolution, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy techniques. It was found that reactive MgO was more effective than hydrated lime in activating the GGBS based on unconfined compressive strength and the efficiency increased with the reactivity and the content of the MgO. It is hence proposed that reactive MgO has the potential to serve as an effective and economical activator for GGBS.     

Early hydration of quick cements in the system C11A7 · CaF2C2SC2(A,F)

To explain the initial hydration of quick cements of the system C₁₁A₇ · CaF₂C₂SC₂(A,F), their SO₃/Al₂O₃ ratio was varied, Portland cement was added to increase the alcalinity of the pore solution in the cement stone, and setting retarded was used during the tests. The process of hydration was investigated with the help of X-ray diffraction and scanning electron microscopy as well as via the determination of the chemically bound water. The quantity of rapidly growing ettringite crystals increases with a rising SO₃/ Al₂O₃ value. The optimal SO₃/Al₂O₃ value of the system C₁₁A₇ · CaF₂C₂SC₂(A,F) is found between 0.26 and 0.38. The sole addition of setting retarder or Portland cement can distinctly reduce the formation of monocarbonate or even stop it completely. However, the simultaneous addition of both additives (setting retarded and a Portland cement) supports the formation of ettringite and monocarbonate crystals and through this also supports the increase of initial strength of quick cement.     

Revisiting the microstructure of hydrated tricalcium silicate––a comparison to Portland cement

The microstructures of 0.40 w/s-ratio pastes of tricalcium silicate (C₃S), alite and Portland cement have been studied at the hydration times 24 h and 1 month. A field-emission SEM (FE-SEM) was used to obtain high-resolution backscattered electron images. Comparison revealed no microstructural differences between C₃S and alite, but there were considerable differences in microstructure between C₃S and Portland cement pastes. The microstructure of the C₃S paste was simpler than that of the Portland cement, and could be described by a few characteristic features. Distribution of the reaction products differed substantially in the two systems. While hollow shells (Hadley grains) were a prominent feature of the Portland cement paste, their occurrence was more limited in the C₃S and alite pastes. Hollow shells were restricted to grains smaller than about 5 μm in the C₃S and alite pastes, and gapped hollow shells were not a common feature.     

Measuring adhesion between steel and early-hydrated Portland cement using particle probe scanning force microscopy

Particle probe scanning force microscopy was used to measure adhesion between steel and early-hydrated cement in the study. Particle probes, created by attaching steel microspheres to microcantilevers, were successfully used to collect adhesive forces between steel and early-hydrated Portland cement in air and in saturated lime water. Mixed Gaussian models were applied to predict phase distributions in the cement paste, i.e., low density calcium silicate hydrate, high density calcium silicate hydrate, calcium hydroxide, other hydrated products and the unreacted components. Consistent correlations were achieved for volume fractions between areas with different adhesion measurements and predictions from the hydration model. Results showed that low density calcium silicate hydrate, high density calcium silicate hydrate and other hydrated products exhibit intermediate adhesion to steel microspheres. Calcium hydroxide exhibits the smallest adhesion, while the unreacted components exhibits the largest adhesion among all groups.     

Aspects of Portland cement hydration studied using atomic force microscopy

Aspects of the mechanisms of hydration and microstructural evolution in Portland cement are still not fully understood. Atomic force microscopy (AFM) is in many ways a powerful tool for investigating changes in surface structure that accompany the hydration of Portland cement, especially because surfaces can be imaged under aqueous solutions at normal temperature, pressure and high magnification. We have investigated changes in the surface characteristics of sections of Portland cement clinker immersed initially in saturated calcium hydroxide solution which was then replaced by water, and in sucrose solution. In the case of the former, the observations are consistent with the early formation of a protective membrane and the subsequent growth of calcium silicate hydrate (CSH) structures by an osmotic process. The dissolution of the clinker in sucrose solution has also been directly observed. It is concluded that the use of AFM will help to resolve many questions relating to cement hydration.Sadly deceased.Editors note: I am very saddened by the sudden loss of Derek Birchall and acknowledge his outstanding contribution to the subject and this journal over many years.