The validity of the formation factor concept from through-out diffusion tests on Portland cement mortars

The diffusion coefficients of ions and radionuclides in cementitious materials are the basic parameters to evaluate the state of the degradation of structures. In this article, three different tracers (two ions, and a radionuclide) were tested on the same formulations of mortars (sand volume fractions from 0 to 60%) in terms of the through-out diffusion, to determine the effective diffusion coefficients of each tracer and each formulation. The aim of this study is to prove the validity of the formation factor equation relating the effective diffusivity of a tracer in cementitious material to its diffusion coefficient in pure water. This result is extremely interesting because once the geometric formation factor of a material is known, it is possible to determine the values of the effective diffusion coefficients of any other diffusing species in this material.     

Effect of barium hydrosilicates on the early hydration rate of Portland cement

We have studied the effect of a barium hydrosilicate-based modifier on the phase composition of Portland cement hydration products. The results demonstrate that the addition of a modifier containing barium hydrosilicates, silicic acid, and calcium carbonate makes it possible to reduce the nucleation rate of crystals of new phases during the induction period of the hydration process, increase alite (3СaO · SiO₂) hydration, and reduce the rate of aluminate phase (3CaO · Al₂O₃) hydration. The use of such a modifier increases the degree of cement hydration and the amount of calcium hydrosilicates and reduces the amount of forming portlandite and ettringite, thereby improving the mechanical properties and durability of the set cement.     

Combined effect of chemical nature and fineness of mineral powders on Portland cement hydration

This paper focuses on the influence of the chemical nature and the fineness of the fillers on the hydration process and on the compressive strength development. Four different types of fillers are considered in combination with Portland cement: quartzite filler, alumina filler, limestone filler, and silica fume. The study deals with blended mortars having a 0.45 water to powder (cement and filler) ratio with a 10% substitution of cement by filler. Quartzite fillers do not seem to accelerate the hydration process in a significant way. No positive effect is noticed on the strength development either. The presence of a fine inert alumina powder increases the rate of early hydration of Portland cement. The greater the fineness, the faster the rate of hydration heat development. This reactivity leads to an increase in the compressive strength at early age for mortar containing the finest alumina powders. In case of coarse alumina powder, no acceleration effect is obtained. Finely ground limestone (calcite) fillers promote heterogeneous nucleation of hydrates which significantly accelerates hydration. At early age, this also results in an increased mortar compressive strength in comparison with the control mortar. From the obtained results, it is clear that both chemical natures as well as fineness are important with regard to the accelerating effect of the hydration process. With increasing fineness, the accelerating effect increases. For powders with comparable fineness, it is clear that limestone powder has a more significant accelerating effect than silica fume and alumina filler. Quartzite filler seems to have no significant effect.     

Effect of curing conditions and concrete mix design on the expansion generated by delayed ettringite formation

Many studies have shown the effect of different parameters on the expansion induced by delayed ettringite formation (DEF), but there is not yet a general agreement on their relative effects due to discrepancies on experimental results generally attributed to different experimental conditions. The aim of this study is to assess the coupling effects between some of the main parameters of this reaction using the experimental design method. The main interactions concern two factors: temperature and duration of heating during curing. Each interaction has been subjected to statistical tests in order to be validated. The resulting empirical models can reflect the experimental fields and therefore provide useful information about the risk of expansion related to␣DEF, knowing the temperature and the duration of curing, the alkali content of the concrete, the aggregate nature and the water to cement ratio.

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Résumé De nombreuses études ont montré l’influence de différents paramètres sur les expansions induites par la formation différée de l’ettringite (DEF). En raison des divergences de résultats attribuées à des conditions expérimentales différentes, il n’existe pas d’accord unanime sur l’effet de ces paramètres. L’objectif de cette étude consiste à étudier les couplages entre certains des principaux paramètres de cette réaction par la méthode des plans d’expériences. Les principales interactions concernent la température et la durée de l’échauffement. Chaque interaction a subi un test statistique afin d’être validée. Les modèles empiriques déterminés peuvent reproduire le champ expérimental étudié et ainsi fournir des informations pertinentes sur les risques d’expansion attribuée à la DEF, à partir de la connaissance de la température et de la durée de l’échauffement, la teneur en alcalins du béton, la nature des granulats et le rapport E/C.

     

Reduction of the cement content in mortars made with fine concrete aggregates

This study’s main objective is to show the viability of reducing the cement content of mortars by incorporating fine crushed concrete aggregates whilst simultaneously maintaining a good performance in terms of functional requisites. The advantages of this, if the results are positive, are both environmental and economic: less energy is consumed in cement manufacture and the mortars’ direct costs are lower. To evaluate the hypothetical binding characteristics of concrete fines incorporated in mortars, and thus allow a cement consumption reduction, various standard tests were performed to quantify their most important properties (e.g. mechanical strength, water-related performance, cracking susceptibility, shrinkage) and compare them with those of a reference mortar containing no recycled fines and not reducing the cement content.     

硬化砂浆中水泥含量试验制样方法探讨

为探索处理硬化砂浆试样的方法,使水泥含量测定结果更接近真实值,首先成型不同粉煤灰掺量的砂浆试样,在砂浆试样标准养护到一定龄期后,基于硬化胶凝材料中结合水的损失过程,利用Ca O测定法研究不同的试样处理方法对硬化砂浆中水泥含量测定结果的影响,得到处理硬化砂浆试样的方法:即无水乙醇终止水化后,60℃鼓风烘干,然后以10℃/min的速度由室温升到(520±10)℃,保持1 h,并且通过实验证明结合水含量是影响硬化砂浆中水泥含量测定结果的关键因素之一。采用该方法处理后,不同龄期相同胶凝材料含量和同一龄期不同粉煤灰掺量硬化砂浆中水泥含量测定结果的相对误差都在±5%以内。     

Effect of latex and superplasticiser on Portland cement mortar in the fresh state

This report deals with the individual and combined effect of latex and superplasticiser on Portland cement mortar in the fresh state. The compatibility of five commercial superplasticisers with four latexes of the vinyl polymer group and SBR latex in varying dosages was studied with respect to setting time, consistency of fresh cement pastes, subjective workability (surface texture, segregation), bleeding, air content, water reduction capacity and the flow-time relationship of fresh mortar. It has been observed that superplasticisers of melamine formaldehyde and a blend of melamine and naphthalene formaldehyde eliminated shortcomings like delayed setting, high air entrainment in the fresh latex-modified system whereas lignosulphonate and a blend of lignosulphonate and naphthalene formaldehyde aggravated it. In a consequent report, performances in the hardened state will be discussed.     

Hollow-shell formation – an important mode in the hydration of Portland cement

The characteristics and quantities of hollow shells (Hadley grains) in the microstructure of Portland cement paste have been studied. It is shown that at relatively early hydration stages, the hydration largely occurs in a manner resulting in hollow shells. At later ages, the “hollow-shell hydration” mode becomes much less prominent as “inner products” are increasingly formed. The intrinsic porosity associated with hollow shells can be significant at later ages, between about 1% and 9% depending on the mix composition and curing regime. The presence of silica fume increases the amount of hollow-shell porosity considerably, while the hollow-shell porosity appears to decrease with decreasing water/binder ratio. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of limestone content,fineness, and composition on the properties and microstructure of alkali-activated slag cement

The influence of the fineness, concentration, and chemico-mineralogical composition of limestone on the workability, reaction kinetics, compressive strength, microstructure, and binder gel characteristics of sodium carbonate–based waste-activated waste slag cement pastes was investigated in this work. Alkali-activated slag cements incorporated with limestone, containing 33–100% of calcite, at a content of up to 60% with a 28-day compressive strength of 26.2–48.8 MPa were proposed. The main reaction products of hardened alkali-activated cement pastes and those incorporated with limestone are CSH, CaCO₃, Na₂Ca(CO₃)₂·5H₂O, and Na₂CaSiO₄. “Physically active” limestone does not chemically react with the binder gel but it can improve the physical structure. The higher packing density of mixed cement, without an increase in the water demand, the satisfactory binding strength of limestone with the binder gel lead to the improvement in the physical structure and compressive strength of alkali-activated slag paste.     

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.     

Effect of curing time on granulated blast-furnace slag cement mortars carbonation

Currently, ground granulated blast-furnace slag cements use in cement-based materials is being increasing because perform well in marine and other aggressive environments. However, mortars and concretes made of this type of cement exhibit high carbonation rates, particularly in badly cured cement-based materials and when high blast-furnace slag contents are used. Concrete reinforcement remains passive but can be corroded if the pore solution pH drops as a result of the carbonation process promoting the reinforced concrete structure failure during its service life. Results show the very sensitive response to wet-curing time of slag mortars with regard to the natural carbonation resistance. Then, a minimum period of 3–7 days of wet curing is required in order to guarantee the usual projected service life in reinforced concrete structures. In this work, estimation models of carbonation depth and carbon dioxide diffusion coefficient in ground granulated blast-furnace slag mortars as a function of the curing period and the amount of ground granulated blast-furnace slag are proposed. This information will be useful to material and civil engineers in designing cement-based materials and planning the required curing time depending on their ground granulated blast-furnace slag content.     

Effect of fly ash fineness on compressive strength and pore size of blended cement paste

This paper presents an experimental investigation on the effect of fly ash fineness on compressive strength, porosity, and pore size distribution of hardened cement pastes. Class F fly ash with two fineness, an original fly ash and a classified fly ash, with median particle size of 19.1 and 6.4 μm respectively were used to partially replace portland cement at 0%, 20%, and 40% by weight. The water to binder ratio (w/b) of 0.35 was used for all the blended cement paste mixes.Test results indicated that the blended cement paste with classified fly ash produced paste with higher compressive strength than that with original fly ash. The porosity and pore size of blended cement paste was significantly affected by the replacement of fly ash and its fineness. The replacement of portland cement by original fly ash increased the porosity but decreased the average pore size of the paste. The measured gel porosity (5.7–10 nm) increased with an increase in the fly ash content. The incorporation of classified fly ash decreased the porosity and average pore size of the paste as compared to that with ordinary fly ash. The total porosity and capillary pores decreased while the gel pore increased as a result of the addition of finer fly ash at all replacement levels.     

Effect of the addition of chitosan ethers on the fresh state properties of cement mortars

The effect of two non-ionic chitosan derivatives (hydroxypropyl (HPCH) and hydroxyethyl (HECH) chitosans) and one ionic derivative (carboxymethylchitosan, CMCH) on the fresh-state properties of cement mortars was studied. Zeta potential measurements and particle size distribution were carried out in order to elucidate the action mechanism of the admixtures. Results were seen to be strongly dependent on substituents of the chitosan. Non-ionic derivatives had a weak dosage-related influence on the fresh-state properties. The ionic CMCH showed the more marked effect: it was found to act as a powerful thickener and to reduce the workable life of the fresh mixtures, whereas it caused a delay in the hydration of the cement particles. CMCH reduced the slump by 50% while commercial viscosity enhancers exhibiting larger molecular weights (hydroxypropylmethylcellulose, HPMC, and hydroxypropyl guaran, HPG) only reduced it by ca. 25%. The negative values of zeta-potential and the strong flocculating effect point to an adsorption of CMCH onto the positively charged cement particles. Optical microscopy and TEM observations showed the polymer giving rise to interlinking between cement particles.     

Effect of different geometric polypropylene fibers on plastic shrinkage cracking of cement mortars

Three kinds of different geometric (cross-section) polypropylene fibers were used to investigate their effects on plastic shrinkage cracking of cement mortars. The test results showed that polypropylene fiber made in drawingwire technique with circular cross-section (abbreviated as DW-PP fiber) had a more pronounced effect on the resistance to plasticshrinkage cracking than the polypropylene fiber made in a fibrillated film technique with rectangular cross-section (abbreviated as FF-PP fiber). Polypropylene fiber with Y-shape cross-section had a little more effective than that of DW-PP fiber. For the three kinds of polypropylene fibers, the plastic shrinkage cracking resistance for cement mortar increased with fiber addition changing from 0.05% to 0.15%. It was observed that there was almost no plastic shrinkage cracking even when the volume fraction of DW-PP fiber was as low as 0.10%. The mechanism of the resistance effect of different geometric polypropylene fibers on the plastic shrinkage cracking of cement mortars is discussed.

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Résumé Cet article présente l'influence des fibres de polypropylène aux différentes formes géometriques sur la fissuration sous retrait plastique du mortier de ciment. Les résultats expérimentaux indiquent que la largeur de fissure et la valeur pondérée de fissuration globale des mortiers de ciment diminuent clairement lorsque l'on incorpore un certain volume de fibres de polypropylène. Dans la résistance à la fissuration sous retrait plastique du mortier, la fibre fabriquée par le tirage de fil est plus forte que celle des fibres fibrillées, mais elle est un peu moins forte que la fibre en forme d'Y. En fonction du volume de fibres, la résistance à la fissuration sous retrait plastique des mortiers fibrés en volume de 0,05% à 0,15% augmente. La fissuration du mortier incorporé en volume de 0,10% de fibres, fabriquées par le tirage de fil, a disparu en substance. L'article présente enfin la recherche sur le mécanisme permettant d'atténuer la fissuration des mortiers par la fibre de polypropylène en différentes formes.

     

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。