The effect of latex on macrodefect-free cement

The effect of adding latex to macrodefect-free cement was studied. The porosity of the modified pastes was smaller than the normal macrodefect system but the pore size was not significantly changed. The traditional relationship between strength and flaw size in the cement system was found to be inadequate to explain the observed mechanical behaviour of the latex-modified systems. Scanning electron microscopy of the fracture surfaces of the normal and latex-modified pastes revealed that the use of different polymers resulted in different fracture paths through the cement composite. This was due to the difference in chemical nature of the polymer. The results demonstrated that the modulus (E) and fracture energy (R) terms in the traditional Griffith equation become limiting factors for strength determination in low-porosity cement systems.     

The microstructure of macrodefect-free cement with different polymer contents and the effect on water stability

The effect of different polymer contents on the microstructure of ordinary Portland cementbased macrodefect-free cement was studied by high voltage transmission electron microscopy. The microstructure, especially the morphology and composition of the gel matrix, of material with a low polymer content was significantly different from that of material with a high polymer content. The former consisted mainly of gelatinous material with some microcrystallites and with some porosity. The latter consisted of mainly pure non-porous polymer gel with little metallic content. Although the dry strength of the material with high polymer content was higher than that of the lower polymer content material, the former was extremely unstable after immersing in water. This was attributed to the difference in microstructure of the polymer gel as revealed by electron microscopy.     

Environmental degradation of macrodefect-free cements

The effect of different environments on the flexural strength of a range of macrodefect-free (MDFs) cements was studied. The results showed significant differences in behaviour of the MDFs. These differences were found to be clearly related to the cement system the MDF was made from.Initial measurements clearly established that high alumina cement (HAC) based MDFs were significantly stronger than ordinary Portland cement (OPC) based ones.Upon immersion in water the flexural strengths were observed to drop initially. In the OPC MDFs some recovery in strength was subsequently observed. No such behaviour was found in the HAC MDFs. One of the MDFs studied had had the polymer removed prior to testing. This showed no sign of loss of strength on immersion in water but this could be due to the processing rate used to prepare it.Gamma irradiation was more damaging to HAC MDFs than OPC MDFs. Gas analysis suggested that polymer degradation was occurring and it is proposed that the polymer matrix in HAC MDFs may be responsible to a degree for the high strength of these materials.     

Microstructure and mechanical properties of macrodefect-free cements

The mechanical properties of macro-defect-free cements were investigated with the help of fracture mechanics and rheology; transmission electronic microscopy (TEM) and scanning electronic microscopy (SEM) were used to characterize the microstructure. The microstructure was found to consist of nanosized hydration products embedded in a cross-linked polymer matrix. The nanoscale composite structure of the cement is thought to play an important part in the mechanical properties.     

Thermoluminescence of cement

Cement is found to be a thermoluminescent material. The glow curves of ordinary portland cement obtained from two different sources are found to be qualitatively similar. White cement (J K Brand) exhibits intense thermoluminescence (TL) compared to Birla white brand. The emission of J K white cement seems to be mainly due to Mn impurities which enters the product through limestone, the major ingredient in the preparation of cement. The possible use of TL as a spectroscopic technique to characterize cement is discussed.     

Energy and cement quality optimization of a cement grinding circuit

This study aimed at optimizing both the energy efficiency and the quality of the end product by modifying the existing flowsheet of the cement grinding circuit. As a general application, mill filter stream is sent to the air classifier owing to its coarser size distribution than the desired product. However, the study proved that some further evaluations i.e., quality tests and chemical assays, could make it possible to treat this stream as a final product. Consequently, directing this stream to the final product silo could be considered. Within the study, sampling survey was undertaken initially that was followed by the modelling and simulation works. The calculations implied that the production rate increased by 4.45% that corresponded to energy saving of 4.26%. As the plant decided to change the flow sheet, another sampling campaign was arranged to validate the outputs of the simulation studies. In that case, the real data showed that the increase in production rate was 3.68% and 28-Days strength of the cement improved by 2.9%. As a result, the simulation outputs were found to be in agreement with the real data hence the efficiency of the cement production, both quality and energy, for a given circuit was improved.     

Heat deformations of cement paste phases and the microstructure of cement paste

The paper presents the results of an experimental investigation of the influence of thermal deformations of phases present in hardened cement paste on its microstructure. For the observation of the thermal deformation course in the range of temperature20–800°C, a method of complex thermal analysis (DTA, DTD, TD) was employed. The microstructure of heated cement pastes was observed by both optic and electron microscopes. From an analysis of the results of the investigation it has been found that chemical processes stimulated by temperature in particular phases of cement paste have a significant influence on the thermal deformation course. It is shown that micracracks caused by different thermal deformations, appear first in the aregas of Ca (OH)₂ concentration (ca.300°C), and next (ca.400°C) as well as in the areas of occurrence of unhydrated large clinker grains.

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Résumé On rend compte d'une étude expérimentale sur les causes de la destruction de la microstructure de la pate de ciment portland exposée à la chaleur dans une gamme de températures allant de20 à800°C. On a prêté une attention particulière aux déformations thermiques de chacune des phases reconnues de la pate de ciment durci. On a utilisé une méthode d'analyse thermique complexe (DTA, TD, DTD) pour l'étude de l'évolution de déformations thermiques. On a observé les microstructures de la pate de ciment chauffée au microscope optique et électronique. On a étudié les phases suivantes de la pate de ciment: clinkers (C₃S, β-C₂S, C₃A, C₄AF), minéraux hydratés de ciment (hydrates C₃S, β-C₂S, C₃A, C₄AF), et produits d'hydratation [Ca(OH)₂, ettringite]. Les résultats de l'étude ont permis à l'auteur de déterminer l'allure de la déformation thermique des phases que présente la pate de ciment durcie sous températures croissantes. On a constaté que les processus chimiques que l'élévation de température induit dans chacune des phases de la pate de ciment exercent une influence notable sur l'évolution des déformations thermiques. Les microfissures causées par différentes déformations thermiques des phases apparaissent dans les zones de concentralisation de Ca(OH)₂ à environ300 et400°C aussi bien que dans les zones où se rencontrent des grains de clinker non hydraté.

     

砒砂岩对硅酸盐和硫铝酸盐水泥性能的影响

为了研究砒砂岩对硅酸盐水泥和硫铝酸盐水泥物理性能的影响,采用单因素多水平梯度实验,通过不同砒砂岩掺量的对比,测定硅酸盐水泥和硫铝酸盐水泥的凝结时间、标准稠度用水量和胶砂强度等性能。结果表明:砒砂岩对硅酸盐水泥和硫铝酸盐水泥皆有促凝作用,当砒砂岩掺量质量分数为5%时硅酸盐水泥的初凝时间会缩短30%,硫铝酸盐水泥初凝时间缩短47%,硅酸盐水泥和硫铝酸盐水泥的标准稠度用水量分别增加6.6%和21.7%;砒砂岩掺量质量分数为10%时,硅酸盐水泥的3 d和28 d的强度分别增加7.2%和6%,对其力学性能有较大影响;掺入砒砂岩后,硫铝酸盐水泥强度降低,且随掺量增加,抗压强度降幅增大。     

Strengthening of cement foams

Lightweight cellular concretes are attractive building materials for a number of reasons: they offer a unique combination of moderate thermal insulation and stiffness, low cost and incombustibility. They have relatively low strength, however, and are brittle. In this paper we describe the behaviour of composite cement/polystyrene foams with improved strength and ductility.     

Susceptibility of Portland cement and blended cement concretes to plastic shrinkage cracking

The market share of different types of blended cements is increasing year by year. Generally, blended cements are ground to higher fineness and exhibit a slower development of mechanical properties compared to Ordinary Portland Cement (OPC), which might affect the concrete performance in terms of shrinkage cracking at early ages.In this paper, the performance of concretes made with different cement types is compared according to the ASTM C1579-13 standard for plastic shrinkage cracking. The cracking behavior was further correlated to the deformations of both unrestrained and restrained specimens measured by a 3D image correlation system. The main factors influencing the cracking behavior were discussed based on poromechanics. It is concluded that the bulk modulus evolution has a dominant effect on controlling the plastic shrinkage cracking. Concretes made of more reactive cements, in particular with higher clinker content, are less susceptible to plastic shrinkage cracking. For cements with the same clinker content, increasing the cement fineness reduces the risk of plastic shrinkage cracking.     

Carbon fibre-reinforced cement

This review describes fabrication processes for aligned fibre and random fibre carbonreinforced cement and links important process parameters with composite theory. The way in which the material fits into the general framework of crack constraint and matrix cracking theories is discussed. A broad survey is made of the mechanical properties, durability and dimensional stability of a variety of carbon-reinforced cement composites, and economic constraints on potential applications are considered.List of symbols b breadth of three-point bend specimen - d depth of three-point bend specimen - E _c composite Young's modulus - E _f fibre Young's modulus - E _m matrix Young's modulus - l fibre length - l _c fibre critical transfer length - l _s specimen span in three-point bend test - m Weibull modulus - r fibre radius - P applied load - V _f fibre volume fraction - V _m matrix volume fraction - x length of fibre needed to transfer load _mu V _m - x _d crack spacing in a composite with short, aligned fibres - _fu fibre ultimate strain - _mu matrix ultimate strain - _fu fibre ultimate strength - _mu matrix ultimate strength - _cu composite ultimate strength - _MOR modulus of rupture - _T tensile strength - interlaminar shear strength - _i interfacial shear strength - _m matrix work of fracture - _F work of fracture     

Free and combined chloride in hydrating cement and cement components

Normal portland cement, low C₃A cement, tricalcium silicate, and C₃A—gypsum mixtures are hydrated in the presence of 0, 0.8, 1.5 and 3.0% CaCl₂ for 1, 3, 7, 14 and 28 days and the amounts of extractable chloride determined by applying a pressure of 80 000 lb (449 MPa) or by treating the pastes with excess water. At early periods of hydration both methods yielded similar values for immobilized chloride. The amount of free chloride in the pore solution increased as the dosage of initially added chloride was increased. The C₃A—gypsum mixture immobilized much higher amounts of chloride than did the C₃S phase. Normal portland cement had lower amounts of free chloride than low C₃A cement.

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Résumé Du ciment Portland normal, du ciment CAP, du silicate tri-calcique et des mélanges de C₃A-platre sont hydratés en présence de 0, 0,8, 1,5 et 3% de CaCl₂ en 1, 3, 7, 14 et 28 jours; les quantités de chlorure que l'on peut extraire sont déterminées par application d'une pression de 449 MPa ou par traitement des pates avec un excès d'eau. Dans les premiers temps de l'hydratation les deux méthodes donnent des quantités voisines de chlorure retenu. La quantité de chlorure libre dans la solution interstitielle augmente avec le dosage de chlorure initialement ajouté. Le mélange de C₃A-platre retient des quantités beaucoup plus grandes de chlorure que ne le fait la phase C₃S. Le ciment Portland normal montre des quantités plus faibles de chlorure libre que le ciment CAP.

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This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.     

Rubber-modified bone cement

A precursor rubber-toughened polymethyl-methacrylate (PMMA) powder developed by ball-milling was incorporated into a series of test bone cements, with different combinations of PMMA powder, rubber-toughened PMMA powder, MMA monomer and benzoyl peroxide (BPO) initiator. The resulting microstructures were characterized by electron microscopy and measurements made of the tensile properties, fracture mechanics parameters and curing features. It is demonstrated that rubber-toughened PMMA powder additions give a significant increase in elongation and fracture toughness, with a reduction in setting time.     

Superfine cement for improving packing density, rheology and strength of cement paste

Superfine cement is a cement ground to a much higher fineness than ordinary cement. The addition of a small quantity of superfine cement to fill into the voids of ordinary cement can improve the packing density of the cement and thereby reduce the amount of mixing water needed to fill the voids. In this study, the effects of superfine cement on the packing density of cement (directly measured by a wet packing test), the water film thickness of cement paste (taken as the excess water to solid surface area ratio), and the flowability, rheology and strength of cement paste were investigated. The results showed that the addition of 10% to 20% superfine cement can significantly increase the packing density of the cement and the water film thickness of the cement paste. Such increases in packing density and water film thickness would then improve the flowability, rheology and strength of the cement paste. Hence, superfine cement is an effective cementitious filler for improving cement performance.