J.M.J.M. Bijen

BENEFITS OF SLAG AND FLY ASH

Abstract The use of ground granulated blast furnace slag and powder coal fly ash as an addition to either cement or concrete is well-established. Concrete made with these secondary raw materials as a part of the binder does show distinctive advantages over concrete with Portland cement only. Especially, the performances with respect to chloride-initiated corrosion of rebars, alkali-silica reaction and sulphate attack are substantially improved. These improvements and their causes are discussed.

THE CHEMISTRY OF THE PORE FLUID OF SILICA FUME-BLENDED CEMENT SYSTEMS

The behaviour of silica fume in cement systems, particularly, the pozzolanic reaction was investigated by monitoring the chemistry of the pore solution of silica fume-blended cement pastes. This was supported by heat of hydration studies on similar mixes. Silica fume accelerated cement hydration during the early hours. The increased rate of hydration is believed to be due to enhanced precipitation of hydration products on the submicroscopic silica fume particles which possibly served as nucleation sites for crystallization during the early hours when it existed as chemically inert filler. The study also revealed evidence of pozzolanic reaction within three days after the formulations of the mixes, irrespective of the water-to-binder ratio used. This evidence is apparent in blends made with portland as well as blastfurnace slag cements. In both cases, OH−, Ca2+, K+, and Na+ were chemisorbed from the cement solution.

Reactivity of Fly Ash At High pH

The reactivity of fly ash in cementituous systems has been an object of many studies, and is only understood in broad terms. It is generally agreed that the particle size distribution of fly ash correlates with strength development. It has also been demonstrated that temperature and pH development of the blended cement paste have an effect on fly ash reactivity. It is however not clear whether ash chemistry and/or ash glass structure have an influence on its reactivity, as is the case with some blast furnace slag cements. In order to discriminate between the many variables controlling fly ash reactivity, the dissolution kinetics of class-F fly ash have been studied. The fly ash was first separated into equal size and density fractions. These were reacted with NaOH, with a pH ranging from 13.0 to 13.7 and temperatures between 20 and 40°C. If the results of the dissolution experiments are corrected for differences in glass content and particle size, the effects of temperature and pH seem to be of more importance than differences in ash chemistry. The dissolution proceeded virtually congruent. TEM micrographs of some of the ash fractions indicate that a wide variety of glasses may be encountered in fly ashes; most glasses show evidence of phase separation. The results are discussed with respect to glass structure and glass dissolution-theory.

CEMENT EQUIVALENCE FACTORS FOR FLY ASH

Abstract An investigation has been carried out into the cement equivalence factors for fly ash in concrete. The work was part of a pre-normative research aimed at taking the contribution of fly ash to strength development and the other properties of concrete into account on the minimum cement content and maximum water-cement ratio required to achieve these properties as by the Dutch Concrete Standards. Three cements were studied. These are ordinary Portland cement, rapid hardening Portland cement and Portland blast furnace slag cement. Four fly ashes, including two Low-NOx ashes, were used for the study. Concrete compositions with a range of water-cement ratios, two fly ash-cement ratios, and three curing regimes were studied as functions of concrete compressive strength development. The test results show that the contribution of fly ash to strength is strongly dependent on the water-cement ratio, the type of cement, the fly ash quality, and the concrete age. The equivalence factor increases when the water-cement ratio decreases. This dependance is highest for the rapid hardening Portland cement and lowest for the Portland blast furnace slag cement. For the latter cement the equivalence factor decreases with increasing age, while for the Portland cements investigated the reverse is the case. The period of curing has influence up to a period of 3 days.

THE INTERFACE BETWEEN POLYMER-MODIFIED CEMENT PASTE AND AGGREGATES

Abstract The effects of polymer dispersions on the structure of the interfacial zone between portland cement paste and aggregates (limestone and granite) have been investigated. EDAX analyses of the polymer modified composites showed that a relatively high polymer content is present at the paste-aggregate interface. Scanning electron microscope (SEM) observations of fractured surfaces of the plain cement paste-aggregate interface revealed large calcium hydroxide crystals which are orientated with their c-axes perpendicular to the interface. With increasing polymer content cement hydration products become indistinct, and microcracks appeared to be bridged-up by the polymer film.

Behaviour of concrete affected by sea-water under high pressure

Systematic research has been performed with respect to the behaviour of concrete in sea-water under high pressure. If was found that high hydrostatic sea-water pressure had no detrimental effect on strength, but increased the depth of sea-water penetration and the rate of swelling of the concrete, especially for ordinary Portland cement. A correlation between the amount of chloride penetration, the degree of swelling and the increase of mass of the concrete was observed. The permeability of concrete in sea-water with increase in time decreased to very low values.ResumeUne recherche expérimentale systématique portant sur le comportement du béton dans de l’eau de mer sous haute pression a été menée. La recherche prenait en compte les critères suivants: (i) résistance et solidité de la pâte de ciment après prise, (ii) pénétration et dissolution des ions, (iii) perméabilité et (iv) gonflement.Pour tester le premier critère, des échantillons de mortier ont été plongés dans un récipient haute pression d’eau de mer à 5°C et 10,0 MPa, pendant des périodes respectives d’une semaine, d’un mois et de six mois. L’étude des propriétés de résistance et de l’augmentation de l’absorption massique et de l’absorption du chlorure sur des échantillons d’essai plongés dans le récipient ainsi que sur des échantillons de référence a montré qu’il est très peu probable que l’eau de mer sous haute pression porte atteinte à la solidité du mortier de du béton. Rien ne permet de dire que le comportement de la pâte de ciment après prise plongée dans de l’eau de mer sous une pression élevée pouvant atteindre 10,0 MPa soit considérablement différent du comportement sous une pression (atmosphérique) normale.La pénétration du chlorure est plus rapide dans des conditions d’eau de mer sous haute pression que dans des conditions de pression atmosphérique. Cette pénétration rapide pourrait néanmoins être également influencée par la quantité de pores remplis d’air que comporte le béton avant l’essai d’exposition. La pénétration d’ions de Cl− et de Na+ ainsi que la dissolution d’ions de K+ est plus rapide dans le cas de béton ciment Portland ordinaire que dans le cas de béton de ciment Portland de haut fourneau. En diminuant le rapport eau/béton, on obtient une baisse considérable du taux de pénétration des ions de Cl−. A la longue, la perméabilité du béton en eau de mer diminue. En raison de la formation de sels tels que brucite/calcite/ aragonite, la taille des pores diminue.Le béton plongé dans de l’eau de mer gonfle de jusqu’à 10% en un an. Dans de l’eau de chaux saturée, le gonflement est plus faible que dans de l’eau de mer. Le gonflement devient plus important lorsque le rapport eau/ciment diminue et qu’on utilise du béton de ciment Portland ordinaire au lieu du béton ciment Portland de haut fourneau, lorsque la teneur en granulat diminue et que les températures décroissent (de 40 à 20°C).

THE INFLUENCE OF POLYMER MODIFICATION ON THE ADHESION OF CEMENT PASTES TO AGGREGATES

Abstract The adhesion strength development between polymer-modified cement paste and aggregates has been studied. Portland cement ASTM Type I, and three kinds of polymer dispersions namely: a styrene acrylate (SA), a copolymer of vinylpropionate and vinyl chloride (VVC) and an acrylate with a coupling agent (ACA) were used. The aggregates used included a limestone and a granite. The strength results were found to be strongly affected by the polymer nature, their contents, aggregate type, and the curing conditions of the composites.

The Interaction of Polymer Dispersions with Portland Cement Paste

Research has been done on the interaction of two types of polymer dispersions, a styrene acrylate (SA) and a polyvinylidene chloride (PVDLC), with portland cement by means of a COULTER particle size analyzer and a filtration technique. It was found that over 50% of the polymer SA particles were adsorbed onto the cement surface, the others did not adhere to cement grains and remained dispersed in the mixing water. However, the polymer dispersion PVDLC caused a quick flocculation and coagulation of cement grains and stiffening of the paste. Hardly any free polymer particles of this polymer were found in the liquid phase.

Active thin sections to study syneresis

Abstract Thin sections are made from hardened products. To achieve information on the very early hydration process of cementitious composites, one would need an “active thin section,” i.e., a thin section of a cement paste that is hardening while under investigation. In our research, it has been possible to construct such active thin sections. Using ordinary light microscopy it is possible to observe water movement and volume changes on hydrating specimens of about 100-μm thickness. This research is part of a larger project to investigate the formation of the interfacial transition zone. We believe that a process called syneresis might play an important role. Syneresis is the contraction of a gel under the expulsion of a liquid. This is addressed in the second part of this paper. This phenomenon is studied using active thin sections.

Fly Ash and Slag Reactivity in Cements: Tem Evidence and Application of Thermodynamic Modelling

Abstract An overview is provided of the microstructural development of Portland cements, blended with fly ash (PFA) or granulated blast furnace slag (BFS). The results are linked to recent advances on the thermodynamic modelling of such cements and are used to explain experimentally obtained information on reaction products of these blended cements with transmission electron microscopy. The techniques elaborated may provide a materials framework to explain leaching characteristics, and to predict very long term stability (and durability) of waste products immobilized by cements.