POZZOLANIC ACTIVITY IN SYNTHETIC FLY ASHES: I, EXPERIMENTAL PRODUCTION AND CHARACTERIZATION

THIS INVESTIGATION ATTEMPTED TO EVALUATE INDEPENDENTLY THE EXTENT OF INFLUENCE OF THREE PARAMETERS ON THE POZZOLANIC ACTIVITY OF FLY ASH; NAMELY, FINENESS, CRYSTALLINITY AND CHEMICAL COMPOSITION. TO THE EXTENT THAT THIS WAS POSSIBLE, THE FOLLOWING CONCLUSIONS WERE MADE: 1. COMPRESSIVE STRENGTH VALUES OF A LIME-MONOMINERALIC FLY ASH MORTAR ARE DIRECTLY PROPORTIONAL TO SPECIFIC SURFACE AREAS, AND THE RATE-OF- STRENGTH GAIN DECREASED PROPORTIONALLY WITH AN INCREASE IN PARTICLE SIZE. 2. WITHIN A CURING PERIOD OF EIGHT WEEKS, THE AMOUNT OF GLASSY PHASE IN A MONOMINERALIC FLY ASH INFLUENCES POZOLANIC STRENGTH TO A GREATER EXTENT THAN DOES PARTICLE SIZE. THERE IS SOME INDIRECT EVIDENCE THAT THE STRAIN NATURE OF THE GLASS MAY ALSO BE INVOLVED IN DETERMINING THE DEGREE OF REACTIVITY. 3. IRON IN AN OTHERWISE PURE ALUMINOSILICEOUS ASH REDUCES POZZOLANIC STRENGTH CONSIDERABLY. A SLIGHT BENEFICIATION MAY BE EFFECTED BY MAGNETIC SEPARATION, WHICH REMOVES THE COARSER MAGNETIC PARTICLES. 4. CALCIUM IN AN ALUMINOSILICEOUS FLY ASH RESULTS IN A VERY REACTIVE ASH WHICH, WHEN MIXED WITH LIME, YIELDS VERY HIGH COMPRESSIVE STRENGTHS. STRENGTHS OF THESE ASHES ARE NOT A FUNCTION OF THE AMOUNT OF CALCIUM HYDROXIDE, BUT MAY INSTEAD BE REALTED TO INCREASED SPECIFIC SURFACE AREA AND POSSIBLY THE PRESENCE OF HYDRAULIC CEMENT COMPOUNDS. 5. CALCIUM IN THE TRIMINERALIC ASHES OFFSETS THE DELETERIOUS INFLUENCE OF IRON. IT IS NOT CLEAR WHETHER THIS IS DUE TO A CHANGE IN THE NATURE OR COMPOSITION OF THE GLASSY PHASE OR TO FORMATION OF CEMENT COMPOUNDS. 6. TETRACALCIUM ALUMINATE HYDRATES ARE FOUND BY X-RAY ANALYSIS AFTER ONE WEEK OF CURING; CRYSTALLINE CALCIUM SILICATE HYDRATES, AFTER EIGHT WEEKS. MORTARS OF THE K+P+C ASHES CONTAIN ETTRINGITE, WHICH CONVERTS TO THE MONOSULFATE FORM WITH TIME. 7. REGRESSION ANALYSIS OF UNCONFINED COMPRESSIVE STRENGTH ON THE SQUARE ROOT OF CURING TIME CONFIRMS THE DELETERIOUS EFFECT OF IRON IN FLY ASHES. CALCIUM IN THE SYNTHETIC ASHES NOT ONLY COUNTERACTS THE NEGATIVE INFLUENCE OF IRON, BUT ALSO INCREASES ULTIMATE POZZOLANIC STRENGTH. THE FACT THAT THE K+C ASHES ATTAIN HIGHER COMPRESSIVE STRENGTHS THAN THE K-CYC ASH MAY BE RELATED TO THE HIGH SPECIFIC SURFACE AREAS OF THE K+C ASHES AND/OR MAY BE INDIRECT EVIDENCE FOR THE PRESENCE OF LATENT HYDRAULIC CEMENT COMPOUNDS IN THESE ASHES. 8. THE DATA OBTAINED FROM CHEMICAL METHODS FOR ASSESSING POZZOLANIC ACTIVITY IN THE SYNTHETIC ASHES INDICATE THAT THE TOTAL AMOUNT OF ACID-SOLUBLE SILICA AND ALUMINA IS A FUNCTION OF THE FLY ASH COMPOSITION. FURTHER RESEARCH SHOULD CONFIRM IF THIS IS A PURELY CHEMICAL OR A PHYSIOCHEMICAL FUNCTION. /HRIS/

  • Authors:
    • Joshi, R C
    • Rosauerea
  • Publication Date: 1973-5

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  • Accession Number: 00238164
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Jul 16 1974 12:00AM