CON 123 - Session 5 - Chemical Properties

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  • Sulfate attack is often the most severe at the location of the most wetting and drying, which is usually near the soil line. Here concrete posts have been attacked by sulfates near the soil line. The concrete is in better condition deep within the soil where it is moist.
  • CON 123 - Session 5 - Chemical Properties

    1. 1. CON 123Cementitious MaterialsSession 5Chemical Properties
    2. 2. Chemical Compounds of Portland Cement Polished thin-section examination of Portland clinker shows alite (C3S) as light, angular crystals
    3. 3. Chemical Compounds of Portland Cement The darker, rounded crystals are belite (C2S)Scanning electron microscope (SEM) micrograph of alite (C3S) crystals in Portland clinker.
    4. 4. Chemical Compound Reactions Chemical Name Formula Hydrate Reaction Tricalcium SilicateTricalcium Silicate C3S C-S-H Hydrate Dicalcium SilicateDicalcium Silicate C2S C-S-H HydrateTricalcium C3A Ettringite C6AS3H32AluminateTricalcium Calcium Mono- C3A C4ASH12Aluminate sulfoaluminate Calcium SulfateGypsum CaSO4:2H2O CHS2 Dihydrate CalciumCalcium oxide CaO CH HydroxideMagnesium Magnesium MgO MHoxide Hydroxide
    5. 5. Portland Cements Influence of Compounds C3S Hydration:  Heat of Evolution  Setting characteristics  Hardening properties  Early Strength Development C2S Hydration:  Heat of Evolution  Setting characteristics  Hardening properties  Later Strength Development
    6. 6. Portland Cements Compound InfluenceFineness Finer the cement, the faster the hydration, higher initial strength 3 and 7 day strengths are increased by fineness Increased water demand of high fineness cement may decrease strength
    7. 7. Heat EvolutionHeat evolution as a function of time for cement paste. Stage 1 is heat of wetting or initial hydrolysis (C3A and C3S hydration). Stage 2 is a dormant period related to initial set. Stage 3 is an accelerated reaction of the hydration products that determines rate of hardening and final set. Stage 4 decelerates formation of hydration products and determines the rate of early strength gain. Stage 5 is a slow, steady formation of hydration products establishing the rate of later strength gain.
    8. 8. Heat Evolution
    9. 9. Heat of Hydration Heat of hydration Compound Name Bogue Compound Cal/gTricalcium Silicate C3S 120Dicalcium Silicate C2S 62Tricalcium C3A 207AluminateTetracalcium C4AF 100aluminoferrite
    10. 10. Relative VolumesRelative volumes of themajor compounds in themicrostructure ofhydrating Portlandcement pastes (left) as afunction of time and as afunction of the degree ofhydration.
    11. 11. Relative VolumesValues are given for anaverage Type I cementcomposition:C3S=55%, C2S=18%, C3A=10% and C4AF=8%. “AFt andAFm” includes ettringite(AFt) and calciummonosulfoaluminate (AFm)and other hydrated calciumaluminate compounds.
    12. 12. Portland CementsChemical Compound Contribution to Strength
    13. 13. Reactivity of Cement Compounds Relative reactivity of cement compounds. The curve labeled “overall” has a composition of 55% C3S, 18% C2S, 10% C3A, and 8% C4AF, an average Type I cement composition.
    14. 14. Compound Influence on Setting Time: Time of Final Set C3S is major influence in final set time Fineness of cement Temperature of mixture  (Nearly all exothermic reactions are accelerated by heat) Admixtures Mass of Cementitious Material Type of Cementitious Materials  Fly ash and slag are slower in winter  Large mass of fly ash may be slower
    15. 15. Setting Time of CementCoagulation-Crystalline layer of ettringite is thixotrophic network of high surface area off ofwhich C-S-H can grow. Coagulation-Crystalline C-S-H network of ettringite H2O time •C3A •C3S •C3A •C2S •Gypsum •C4AF •H2O C3S+H2O •Gypsum Thixotrophic Hardening Mixture Mixture
    16. 16. Time of Set for Portland Cements
    17. 17. Portland CementsAlkalies (K2O & Na2O) - usually expressed as Na2Oeq or total alkalies Present in all raw materials More soluble alkalies -  Shorter set times  Higher early strength  Lower 28 day strength  Lower AEA dose  Higher WR dose Alkali - reactive aggregates (opal, chert) “Low alkali cement” contains less that 0.60% total alkalies 18
    18. 18. Portland CementsAlkalies (K2O & Na2O) - usually expressed as Na2Oeq or totalalkalies Present in all raw materials More soluble alkalies -  Shorter set times  Higher early strength  Lower 28 day strength  Lower AEA dose  Higher WR dose Alkali - reactive aggregates (opal, chert) “Low alkali cement” contains less that 0.60% total alkalies
    19. 19. Portland Cements
    20. 20. Sulfate Attack Sulfate attack is often the most severe at the location of the most wetting and drying, which is usually near the soil line. Here concrete posts have been attacked by sulfates near the soil line. The concrete is in better condition deep within the soil where it is moist.
    21. 21. Effect of C3A on Sulfate ResistanceSulfate Attack 4 Month % Expansion Y = 0.116075 - 3.71E-02X + 5.80E-03X**2 R-Sq = 0.320 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 4 5 6 7 8 9 10 11 12 C3A
    22. 22. Alkali-Silica Reactivity ASR Expansion vs. Total Equivalent Alkali ASR 12 Week Percent Expansion 0.5 Y = -3.2E-02 + 7.88E-02X + 0.417862X**2 R-Sq = 84.1 % 0.4 0.3 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Total Equivalent Alkali Content (%)Alkali Silica correlation at 14 weeks is not as great as at greater ages.
    23. 23. Fly Ash and Natural Pozzolans Reduced Permeability & Diffusivity Resistance to ASR  Consumption of Ca(OH)2  Reduction in water mobility Resistance to Sulfate (Low CaO pozzolans)  Dilution of C3A  Consumption of Ca(OH)2  Iron stabilizing effect
    24. 24. Blast Furnace Slag Impact Reduced Permeability & Diffusivity Resistance to ASR (>35%)  Reduction in water mobility Resistance to Sulfate  Dilution of C3A  Iron stabilizing effect
    25. 25. Silica Fume Impact Higher Strength Reduced Permeability & Diffusivity Resistance to ASR  Consumption of Ca(OH)2  Reduction in water mobility Resistance to Sulfate (Low CaO pozzolans)  Consumption of Ca(OH)2
    26. 26. Chemical PropertiesPlease return to Blackboard and watch thefollowing video: Video 1: Cement Hydration

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