Optimization of strength in cement pastes

It has been demonstrated that porosity is by far the dominant controlling factor limiting strength of hydrated cement paste. Mechanical means have been employed in the present study to minimize this porosity, “hot pressing” under rather modest temperatures and pressures, producing materials having very low porosity and unusually high strength. A new relationship to describe the interrelation of strength and porosity is given, and the effect of maturity of specimens, composition and microstructure are illustrated. Though theoretical density has not yet been achieved, the cement pastes have compressive strengths (as well as tensile and shear strengths) an order of magnitude higher than in normally hydrated cements, are stable and durable, and have dense interpenetrating microstructures.     

Characteristics of pastes from a Portland cement containing different amounts of natural pozzolan

This work is concerned with assessing the influence of natural pozzolan on the physical, mechanical and durability properties of blended Portland cement pastes. The results indicate that final setting times of natural pozzolan blended Portland cement pastes range from 4 to about 5 h. Naphthalene-type superplasticizer tends to retard the hydration process of plain and natural pozzolan blended Portland cement pastes. These blends show slightly higher setting times than those without superplasticizer. The use of superplasticizer is found to have a significant influence on the workability. At a lower level of Portland cement replacement by natural pozzolan, the addition of 1% superplasticizer by weight of blended Portland cement leads to a significant decrease in the water to Portland cement plus natural pozzolan ratio for a given workability. However, for the blended Portland cement with a high proportion of natural pozzolan, the increase in water content causes the porosity to increase with an accompanying decrease in compressive strengths. The variations in composition and cure time are found to provide significant changes in compressive strength. Depending on these parameters, the variation in compressive strength can be estimated by using the equation, σ=σ₀/[1+exp(a+bp+cp²)]ⁿ, where σ is the compressive strength of natural pozzolan blended Portland cement paste at a given cure time and natural pozzolan replacement level (MPa); σ₀ is the compressive strength of plain Portland cement pastes with or without superplasticizer at a given cure time (MPa); p is the natural pozzolan replacement level (%); a, b, c, n are the empirical constants to be determined. The blend with a composition of 80% Portland cement and 20% natural pozzolan and 1% superplasticizer provides superior strength and durability characteristics in comparison to the counterparts without superplasticizer and to the blends with a high proportion of natural pozzolan. At high contents of natural pozzolan, the resistance to freezing and thawing is found to be impaired. Moreover, these blended cements do not provide high durability performance against sulfate attack.     

High strength generation in cement pastes

Unusually high strengths have been generated in materials produced by employing “hot-pressing” techniques, and intermediate ranges of strengths have been achieved by applying high pressures at room temperature, to portland cement pastes. By pressing at ca. 250°C and 50, 000 psi strengths are as high as 95, 000 psi (compressive), and 9250 psi (indirect tensile). The hot-pressed materials are volume stable when immersed in water and subsequently evacuated. The microstructures of such materials are very compact, consisting of an intergrowth of dense hydrated cement “gel” surrounding residual unhydrated cement grain cores. The lowest porosity of the materials measured was approximately 1.8%, by far the closest approach to zero porosity or theoretical density yet achieved in cement pastes. The effect of microstructure and porosity are discussed, and high pressure techniques are compared with other methods of strength generation.     

Physical properties of high strength cement pastes

Recent developments in methods of processing ordinary Portland cement have shown that above average strengths (in flexure) are easily obtained without expending energy on high pressure compaction techniques. This paper reports strengths and other physical properties of both ordinary pastes and modified (macro defect free) pastes and attempts to compare and contrast the two types. The apparently greater notch sensitivity of the modified pastes is explained in terms of a reduced inherent flaw size. Optical microscopy shows that large pores are absent in the modified paste but transmission electron microscopy (TEM) shows that the fine scale microstructures of ordinary pastes and high strength pastes are very similar.     

高细石灰石粉用作水泥混合材料的试验研究

实验室试验了在P·Ⅰ42.5R硅酸盐水泥中掺入高细石灰石粉对水泥物理性能的影响。试验结果表明,高细石灰石粉是一种优良的水泥混合材料;高细石灰石粉与矿渣粉按适当比例配合,比单独掺入矿渣粉各试验龄期强度均明显提高,特别是大幅度提高了3d强度。在P·Ⅰ42.5R硅酸盐水泥中掺入4%高细石灰石粉和26%矿渣粉,能够在保持水泥3d强度基本不变的前提下,大幅度提高水泥的28d强度。     

Influence of polymer on cement hydration in SBR-modified cement pastes

The influence of styrene-butadiene rubber (SBR) latex on cement hydrates Ca(OH)₂, ettringite, C₄AH₁₃ and C-S-H gel and the degree of cement hydration is studied by means of several measure methods. The results of DSC and XRD show that the Ca(OH)₂ content in wet-cured SBR-modified cement pastes increases with polymer-cement ratio (P/C) and reaches a maximum when P/C is 5%, 10% and 10% for the pastes hydrated for 3 d, 7 d and 28 d, respectively. With wet cure, appropriate addition of SBR promotes the hydration of cement, while the effect of SBR on the content of Ca(OH)₂ and the degree of cement hydration is not remarkable in mixed-cured SBR-modified cement pastes. XRD results illustrate that SBR accelerates the reaction of calcium aluminate with gypsum, and thus enhances the formation and stability of the ettringite and inhibits the formation of C₄AH₁₃. The structure of aluminum-oxide and silicon-oxide polyhedron is characterized by ²⁷Al and ²⁹Si solid state NMR spectrum method, which shows that tetrahedron and octahedron are the main forms of aluminum-oxide polyhedrons in SBR-modified cement pastes. There are only [SiO₄]⁴⁻ tetrahedron monomer and dimer in the modified pastes hydrated for 3 d, but there appears three-tetrahedron polymer in the modified pastes hydrated for 28 d. The effect of low SBR dosage on the structure of aluminum-oxide and silicon-oxide polyhedron is slight. However, the combination of Al³⁺ with [SiO₄]⁴⁻ is restrained when P/C is above 15%, and the structure of Al³⁺ is changed obviously. Meantime, the polymerization of the [SiO₄]⁴⁻ tetrahedron in C-S-H gel is controlled.     

熟料细度对粉煤灰水泥浆体强度和自收缩的影响

通过实验室球磨机制备出比表面积分别为280m2/kg、370m2/kg和670m2/kg的3种水泥熟料,与不同掺量的粉煤灰配制成不同颗粒级配的粉煤灰水泥,并测试了粉煤灰水泥浆体的抗压强度、自收缩、孔隙率和显微结构。结果表明：提高熟料细度能在很大程度上降低粉煤灰水泥浆体的孔隙率并提高复合水泥浆体早期抗压强度;粉煤灰的掺入降低了水泥体系的自收缩,提高了粉煤灰水泥浆体的体积稳定性;粉煤灰水泥浆体背散射图像表明,提高熟料细度可显著减少粉煤灰水泥浆体中未水化的水泥颗粒含量,并在一定程度上减少未水化粉煤灰颗粒含量。     

Influence of clays on the rheology of cement pastes

The fresh state of concrete is becoming increasingly important in furthering the types of applications of today's construction world. Processing techniques have resulted in technologies such as self-consolidating concrete and depend on the microstructural changes that take place during and immediately after mixing and placing. These changes to the microstructure reflect the flocculation behavior between the particles in suspension. The ability to modify this behavior allows control over the balance among flowability and shape-stability of concrete. This study investigates how clay admixtures affect the microstructure of cement pastes from a rheological stand point. Shear and compressive rheology techniques are used to measure how the solids volume fraction of suspensions with different admixtures evolves with stress. Based on these relationships, the effectiveness of clays on the balance between flowability and shape-stability is measured. Results are consistent with green strength tests performed on concrete mixes derived from the cement paste mixes.     

Effect of components fineness on strength of blast furnace slag cement

The strength development of 1:1 mixes of clinker and blast furnace slag with varying fineness of components from 3000 to 6000 cm²/g has been studied. Overall results indicate that in manufacturing blast furnace slag cement (BFSC), it is not only the fineness of the clinker-slag mix but also of the individual components which govern the choice of the mix composition for a desired strength.     

浅谈试验小磨细度控制与大磨水泥强度的对应关系

惠州光大水泥公司在确定大磨生产水泥时各物料配比的情况下,以同样配比、同样物料进行了小磨试验,42.5R水泥粉磨至不同的比表面积,32.5R水泥粉磨至不同的45μm筛余,以各组试验强度与大磨生产水泥强度进行比较分析,确定大小磨强度,比表面积和45μm筛余控制差值。较好地实现了水泥质量的预前控制。     

The relationship between strength and the composition and fineness of cement

Data from new experiments and from work published during the past 40 years are subjected to regression analysis to determine the relationship between strength and the composition and fineness of cement. The original authors differed in their opinions on the relative importance of C₃S and C₃A. The controversy is examined against a background of experience with a group of materials that are representative of Portland cement in general, and in terms of an analysis based on a model in which, during the first weeks of hardening, the strength developed by C₃S depends on the proportion of C₃A in the cement.     

Influence of fly ash fineness on strength, drying shrinkage and sulfate resistance of blended cement mortar

In this paper, the influence of fineness of fly ash on water demand and some of the properties of hardened mortar are examined. In addition to the original fly ash (OFA), five different fineness values of fly ash were obtained by sieving and by using an air separator. Two sieves, Nos. 200 and 325, were used to obtain two lots of graded fine fly ash. For the classification using air separator, the OFA was separated into fine, medium and coarse portions. The fly ash dosage of 40% by weight of binder was used throughout the experiment. From the tests, it was found that the compressive strength of mortar depended on the fineness of fly ash. The strength of mortar containing fine fly ash was better than that of OFA mortar at all ages with the very fine fly ash giving the highest strength. The use of all fly ashes resulted in significant improvement in drying shrinkage with the coarse fly ash showing the least improvement owing primarily to the high water to binder ratio (W/B) of the mix. Significant improvement of resistance to sulfate expansion was obtained for all fineness values except for the coarse fly ash where greater expansion was observed. The resistance to sulfuric acid attack was also improved with the incorporation of all fly ashes. In this case the coarse fly ash gave the best performance with the lowest rate of the weight loss owing probably to the better bonding of the coarse fly ash particles to the cement matrix and less hydration products. It is suggested that the fine fly ash is more reactive and its use resulted in a denser cement matrix and better mechanical properties of mortar.     

A new approach to the study of the influence of cement fineness on the strength of cement mortars

A study is reported on the possibility to determine the influence of particle sizes of cement on the strength of cement mortars by taking into consideration all the particles in a set, under the presumption that apart of the independent effect of every particle in the course of hydration there is a simultaneous and intermediate effect of all the particles on the same physical property. The method has been applied to cement of identical chemical and phase composition but different particle size distributions.

Abstract

Es wird über Untersuchungen berichtet, die sich mit der Möglichkeit einer Einflussbestimmung der Teilchengrössen auf die Festigkeit von Zementmörteln befassen, wobei alle Teilchen einer Gruppe imbegriffen sind, unter der Annahme, dass abgesehen von der unabhängigen Einwirkung jedes Teilchens im Laufe des Hydratationsprozesses, auch eine simultane gegenseitige Einwirkung aller Teilchen auf dieselbe physikalische Eigenschaft besteht.

Dieses Verfahren wurde an einem Zement mit identischer chemischer und Phasenzusammensetzung, jedoch mit verschiedenen Korngrösseverteilungen durchgeführt.     

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.     

Influence of organic admixtures on the rheological behaviour of cement pastes

The rheological properties of cementitious pastes used to proportion Self-Consolidating Concretes (SCC) have been examined, in particular, the influence of High range water reducing admixture (HRWRA) with that of Viscosity Modifying Admixtures (VMAs) have been compared. HRWRAs are known to have dispersing effects on the cement particles through steric and/or electrostatic repulsion, while the effects of VMAs are expected to stabilise the paste by increasing the viscosity of the aqueous solution. Both transient and steady state rheological behaviour of the cementitious pastes proportioned with different dosages of HWRA and VMA were considered. Experimental results show that the influence of VMAs on rheological properties is actually minor compared to that of HRWRAs. These results are discussed in the framework of rheology of concentrated suspensions.     

Influence of spraying on the early hydration of accelerated cement pastes

In practice, most of the studies about the interaction between cement and accelerators is performed with hand-mixed pastes. However, in many applications mixing occurs through spraying, which may affect accelerators reactivity and the microstructure of the hardened paste. The objective of this study is to analyze how the mixing process influences the early hydration of accelerated cement pastes. Isothermal calorimetry, X-ray diffraction, thermogravimetry and SEM imaging were performed on cement pastes produced by hand-mixing and by spraying, using equivalent doses of an alkali-free and an alkaline accelerator and two types of cement. Results showed a great influence of the spraying process on the reactivity of accelerators and on the morphology of the precipitated hydrates. Variations in hydration kinetics caused by the mixing method are explained and the results obtained might have a significant repercussion on how future research on the behavior of accelerated mixes will be performed.     

A theoretical argument for the existence of high strength cement pastes

The purpose of this paper is to determine whether the volume of porosity or whether the size of individual pores is responsible for controlling strength in cement paste. The suggestion given here, and recently by Birchall, Howard and Kendall, is that the conventional strength — total porosity relation is largely fortuitous. Emphasis is placed upon the maximum pore size as being responsible for the strength characteristics of cement paste. Attention is drawn to the non-linear nature of the strength — critical flaw size curve and finally the influence of pore linkage is discussed.     

Mineral phases formation on the pozzolan/lime/water system

In Spain, the paper industry recycles large amounts of waste paper in the new paper production process. Paper sludge thermal activation (calcination at 700 °C for 2 h) is an environment-friendly alternative source for metakaolinite (MK) to be used for the manufacture of blended cements or cement based materials.This paper could contribute to standardization of the use of new pozzolanic products in cement based materials and its use mixed with saturated lime dissolution. In this process are obtained zeolite, CSH gel, hydrotalcite-like compounds, hydrated aluminate tetracalcium and stratlingite.These materials are formed in different times of pozzolanic reaction.     

Effects of slag content,fineness and additive to cement pastes on the corrosion behaviour of embedded reinforcing steel

Previous studies proved that the slag content of the blast furnace slag-Portland cement mix could be increased, while retaining the engineering properties of the produced slag cement pastes within the normal range and increasing the fineness of the mix. The corrosion behaviour of reinforcing steel embedded in the cement pastes giving the optimum mechanical properties was studied using the galvanostatic polarisation technique. The most corrosion-resistant mix has been determined. The effect of adding CaCl₂ to a paste of this mix on the corrosion behaviour of embedded reinforcing steel has been investigated. For the purpose of comparison, anodic polarisation tests were carried out using pastes having the composition of the most corrosion-resistant mix but at the Blaine area of the ordinary slag cement. Portland and normal slag cements were also tested. The threshold concentration of CaCl₂, below which breakdown of steel passivity did not occur, has been determined. The results of this study might have practical implications in the field.     

Compressive strength versus residual porosity in polymer impregnated cement pastes

Impregnated cement pastes were prepared from fine cement of Blaine area 600 m²/kg using various initial W/C ratios in the range 0.25–0.70, cured for various durations, and by using styrene or methyl methacrylate, which were polymerized $\text{in situ}$ by thermal treatment. The polymer effect on compressive strength was found to be variable and depends on pore size. Upon impregnation, low porosity samples show measurable reduction in zero strength porosity, whereas high porosity samples show measurable reduction in the strength at zero porosity. Enhancement of compressive strength takes place at an optimum range of porosity and gel/space ratio.