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A model for hydrated Portland cement paste as deduced from sorption-length change and mechanical properties

  • Colloque RILEM Munich. 1–3 Avril 1968
  • Causes Physiques et Chimiques du Fluage et du Retrait du Béton
  • Published:
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Résumé

Des résultats obtenus récemment à la Division des Recherches sur le Bâtiment et dans d'autres laboratoires ont rendu nécessaire une révision du modèle jusqu'ici accepté pour représenter la pâte de ciment Portland hydraté. Ce modèle admet comme valeurs les plus sûres et les aires superficielles et les porosités déterminées par adsorption d'azote, et fait dépendre un grand nombre des phénomènes observés du comportement de l'inter-couche d'eau d'hydratation. La nature des liaisons dans les «cristallites», et entre elles, sont clairement définies; on voit que les liaisons «inter-cristallites» ne sont pas affectées par adsorption d'eau.

Ce rapport expose quelques-unes des équations thermodynamiques relatives aux phénomènes de l'adsorption, variation de longueur et fluage, ainsi que quelques principes de base qui régissent l'adsorption et des types d'hystérésis.

Les résultats ont été fournis par deux séries de travaux: A. des expériences sur la chimie des surfaces; B. la mesure des propriétés mécaniques des systèmes compactés et coulés de ciment hydraté et de plâtre.

Les expériences (A) intéressaient les phénomènes d'adsorption, et les modifications de longueur des réseaux d'isothermes ou de l'eau et du méthanol, et les résultats de ces expériences aboutissaient aux conclusions suivantes:

  1. (i)

    L'isotherme de l'eau pour le ciment hydraté amené à siccité est irréversible en toute partie des courbes primaires, la cause étant la ré-introduction d'une intercouche d'eau d'hydratation.

  2. (ii)

    Les calculs de Brunauer, Emmett et Teller de l'eau isotherme ne sont pas valables. Les aires superficielles et les porosités obtenues par l'adsorption d'azote sont des valeurs plus sûres.

  3. (iii)

    A l'aide du réseau d'isothermes et du calcul, il est possible de séparer l'inter-couche et l'eau physique (adsorbée); les deux méthodes ont été utilisées pour décrire les propriétés du ciment hydraté.

Les mesures (B) comprenaient celles du module et de la résistance en fonction de l'humidité relative et de la porosité; elles ont conduit aux conclusions suivantes:

  1. (i)

    L'eau adsorbée ne se trouve pas aux limites «intercristallites».

  2. (ii)

    Les liaisons formées au cours de l'hydratation ne sont pas plus fortes que celles formées par compactage, contrairement à ce qui se passe avec le plâtre. Pourtant, les liaisons sont assez fortes et impliquent des proportions variables de forces agissant à long terme et à court terme. L'eau adsorbée normalement n'influence pas cette répartition.

  3. (iii)

    Le fluage réversible et une partie du fluage irréversible sont probablement associés à la lente décomposition de l'intercouche hydratée due à l'application d'une charge.

  4. (iv)

    Le fluage irréversible est probablement le résultat de la rupture et de la reconstitution des liaisons «inter-cristallites» dans des conditions de contrainte contrariée.

Summary

Recent findings in DBR and other laboratories has made it necessary here to revise the accepted model for hydrated portland cement paste. The model recognizes surface areas and porosities obtained by N2 adsorption as the most reliable values, and attributes many of the phenomena observed, to the behaviour of interlayer hydrate water. The bonds between and within individual “crystallites» are clearly defined; it is shown that the “inter-crystallite” bonds do not separate when exposed to physically adsorbed water.

The paper describes some of the thermodynamic equations pertinent to the phenomena of adsorption, length change and creep, and some basic principles governing physical adsorption and types of hystereses.

The results were obtained from two areas of work: A. Surface chemical experiments. B. Measurement of mechanical properties of compacted and cast systems of hydrated cement and gypsum.

The experiments in (A) included sorption and length change scanning isotherms of water and methanol and led to the following conclusions:

  1. (i)

    The water isotherm for “d-dried” hydrated cement is irreversible at all regions of the primary curves, due to the re-entry of inter-layer hydrate water.

  2. (ii)

    B.E.T. calculations on the water isotherm are invalid. Surface areas and porosities obtained from N2 adsorption are more reliable values.

  3. (iii)

    By use of the scanning isotherms and calculation it is possible to separate interlayer and physical water; each of these were used to describe properties of the hydrated cement.

The experiments in (B) included measurements of modulus and strength versus relative humidity, and porosity, and led to the following conclusions:

  1. (i)

    Physically adsorbed water is not present at “inter-crystallite” boundaries.

  2. (ii)

    The bonds formed during hydration are not stronger than those formed by compaction, in sharp contrast to the gypsum system. However, the bonds are fairly strong and involve a varying proportion of long-range and short-range forces. Physically adsorbed water does not normally influence these bands.

  3. (iii)

    The reversible and part of the irreversible creep is probably associated with the slow decomposition of the interlayer hydrate due to application of stress.

  4. (iv)

    The irreversible creep is probably the result of breaking and re-making of “inter-crystallite” bonds under confining stress conditions.

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Authors and Affiliations

  1. Division of Building Research, National Research Cuncil of Canada, Ottawa

    P. J. Sereda

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  1. R. F. Feldman
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  2. P. J. Sereda
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Feldman, R.F., Sereda, P.J. A model for hydrated Portland cement paste as deduced from sorption-length change and mechanical properties. Matériaux et Constructions 1, 509–520 (1968). https://doi.org/10.1007/BF02473639

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