Comportement pseudo-pouzzolanique d'une cendre volante de centrale thermique: Fid-Bau Portal

Comportement pseudo-pouzzolanique d'une cendre volante de centrale thermique

Gibergues, A. Carles / Vaquier, A.

Résumé Les auteurs étudient le comportement d'une cendre voltante riche en sulfates et ne libérant pas de silice, en vue de préciser le rôle des sulfates dans le caractère pouzzolanique. A cette fin, ils comparent les résistances mécaniques de mortiers riches en cendre volante à celles de mortiers témoins, suivent leur hydratation par examen aux rayons X et analysent l'activité de la cendre placée au contact d'eau pure et d'eau de chaux saturée. La cohésion initiale des mortiers de cendre s'accroît par formation de sulfoaluminates jusqu'à environ un mois. Par la suite, l'absence de réaction pouzzolanique empêche la progression des propriétés mécaniques des mortiers et enlève tout intérêt pratique à ce type de cendre.

Summary In earlier work we have shown that pozzolanic properties of some fly ashes used in Portland cement concretes result of two distinct reactions: first, fly ashes release sulphates which combine with calcium hydroxyde and calcium aluminate to form ettringite; then, silica passes into solution and reacts with lime to produce C.S.H. To define the effect of sulphates, we have undertaken the present study with another ash having a low soluble silica content. Cristalline components are the major constituent, some 60 %, and consist chiefly of quartz (35 %), magnetic (15 %), hematite (5 %) anhydrite (3 %). The specific surface value is 5200 cm2/g (Blaine). We have made four type of 1:3 sand mortars with Portland cement (CPA 400) and quartz aggregate having a maximum size of3.15 mm. In the first, cement only was used (w/c ratio of0.50). Second and third mortars contained equal volumes of cement and fly ash and w/(cement+fly ash)=0.50; fly ash added to the third mortar had been previously washed in water to eliminate soluble sulphates. A fourth mortar was made with equal volumes of cement and finely ground quartz. Initially, sulphates brought by fly ash make easy ettringite formation in mortars: X-ray examination show that the content of ettringite in raw fly ash mortars, is, at 2 days, twice more important that in washed fly ash ones. The main effect of this reaction is to increase strength of mortars of about40 to45%. Then, during the first month of hardening, calcium sulphoaluminate goes on forming in mortars; but the high-sulphate form is converted to low-sulphate. This last reaction starts at about seven days and is probably due to sodium and magnesium sulphates released by fly ash. During this first month strength of mortars continues to increase. At longer age, calcium sulphoadluminate formation is stopped and ettringite goes on converting to low sulphate form and tetracalcium aluminate. Fly ash does not release silica able to combine with calcium hydroxyde: therefore strength of mortars does not increase with age.