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    Mineral Admixtures & Blended Cements: Supplementary Cementing Materials in Concrete

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    Steven Kua
    Mineral admixtures can be added to concrete to replace cement, improve workability, reduce heat generation, and enhance durability. They are divided into pozzolanic materials like fly ash, cementitious materials like slag, and non-reactive materials. The addition of mineral admixtures improves concrete properties like strength, permeability, and resistance to sulfate attack and alkali-silica reaction.

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    Mineral Admixtures & Blended Cements: Supplementary Cementing Materials in Concrete

    Uploaded by

    Steven Kua
    67 views29 pages
    Mineral admixtures can be added to concrete to replace cement, improve workability, reduce heat generation, and enhance durability. They are divided into pozzolanic materials like fly ash, cementitious materials like slag, and non-reactive materials. The addition of mineral admixtures improves concrete properties like strength, permeability, and resistance to sulfate attack and alkali-silica reaction.

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    MINERA~1

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    Mineral admixtures can be added to concrete to replace cement, improve workability, reduce heat generation, and enhance durability. They are divided into pozzolanic materials like fly ash, cementitious materials like slag, and non-reactive materials. The addition of mineral admixtures improves concrete properties like strength, permeability, and resistance to sulfate attack and alkali-silica reaction.

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    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    67 views29 pages

    Mineral Admixtures & Blended Cements: Supplementary Cementing Materials in Concrete

    Uploaded by

    Steven Kua
    Mineral admixtures can be added to concrete to replace cement, improve workability, reduce heat generation, and enhance durability. They are divided into pozzolanic materials like fly ash, cementitious materials like slag, and non-reactive materials. The addition of mineral admixtures improves concrete properties like strength, permeability, and resistance to sulfate attack and alkali-silica reaction.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
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    of 29

    Mineral Admixtures & Blended Cements

    • Primary purposes of adding mineral admixtures or


    supplementary cementing materials in concrete
    – To replace cement (economical & technical reasons, and
    environmental consideration)
    – To improve the workability of fresh concrete
    – To reduce heat and early temperature rise
    – To enhance the durability of hardened concrete
    • Mineral admixtures are divided into 3 main categories
    1. Pozzolanic materials
    2. Cementitious materials
     supplementary cementing materials

    3. Non-reactive materials (ground lime stone, silica flour)


    • May react weakly with cement under certain conditions
    Pozzolanic Materials
    • History and origins
    – Greeks: Addition of
    naturally occurring
    materials from volcanic
    origin to hydraulic
    limes

    – Romans adopted &


    extended Greek
    technology (e.g.
    Parthenon)
    Pozzolanic Materials
    • Pozzolan
    – Pozzoli, a town in the Bay of Naples that was the source
    of a highly prized deposit of ash from Mt. Vesuvius
    – The name “pozzolan” is now applied to any alumino-
    silicate materials, of either natural or industrial origin
    – Powder: naturally occurring or to be ground to cement
    fineness
    (Mindess et al 2003)
    Pozzolanic Materials
    • Natural Pozzolans
    – Volcanic ash etc
    • By-product materials
    – Fly ash – inorganic, non combustible residue of
    powdered coal after burning in power plants
    – Silica fume – a by-product in the manufacture of Silicon
    metal and alloys
    – Rice husk ash – siliceous residue that remains after the
    rice husks are burnt under controlled conditions
    – Calcined clay under controlled temperatures (700-900 oC)
    to produce a highly reactive amorphous aluminosilicate
    (metakaolin)
    Composition
    (Mindess et al 2003)

    OPC 20 6 3 65
    – ASTM C 618 for fly ash & natural pozzolans
    • Class F fly ash
    – produced from bituminous and sub-bituminous coals
    – (SiO2+Al2O3+Fe2O3)  70%
    • Class C fly ash
    – produced from lignitic coals
    – (SiO2+Al2O3+Fe2O3)  50%
    Composition
    • Crystalline components (identified by X-ray diffraction
    method)
    – A good pozzolan should
    have a high fraction of
    reactive glassy or
    amorphous material
    – Most pozzolanic materials
    contain various
    quantities of crystalline
    phases

    – Examples
    • Class F ash: quartz, mullite, hematite, magnetite
    • Class C ash: free CaO, anhydrite (CaSO4), C3A, C2S, etc
    Composition (cont’d)
    • Minor components
    – Alkali oxides
    • in situations where pozzolanic materials are used to control
    AAR, alkali content of the materials should be determined

    – Unburnt carbon (determined by Loss on Ignition)


    • In SF; 1-2%
    • In FA: 0.5-3% (sometimes as high as 25%)
    • In RHA: 3-8%
    • Concern for air-entrainment
    • Color of concrete

    – SO3 and periclase (crystalline MgO)


    • unsoundness
    Physical Characteristics

    (Mindess et al 2003)
    »  silica fume
    » small size  packing effect
     reduce bleeding, reduce
    the size of capillary pores

    (Mindess et al 2003)

    fly ash 
    Pozzolanic Reactions
    • Primary reaction: amorphous silica reacts with CH from
    cement hydration
    S + CH + H  C-S-H
    – Composition of C-S-H from pozzolanic reactions
    • In cases of fly ashes or natural pozzolans: not very different
    from that formed in regular cement hydration, C/S is slightly
    lower generally
    • In cases on silica fume or rice husk ash, C/S is significantly
    different from that of cement hydration, ~1.0
    – Kinetics of the reaction of fly ashes is similar to the slow
    rate of hydration of C2S. Thus, the addition of pozzolan
    has a similar effect of  C2S content
    • Reduce early heat evolution and early strength, but not long-
    term strength so long as water is available
    • Increase the overall solid volume, reduce the porosity,
    increase strength and durability
    Pozzolanic Reactions
    • When a pozzolan has appreciable quantities of
    reactive alumina (natural pozzolans or calcined clay)
    A + CH + H  C-A-H
    – The composition of calcium aluminate hydrates
    depends on particular pozzolan used.
    Pozzolanic Reactions
    • The extent of a pozzolanic reaction can be followed by
    monitoring the  in CH over time

    • Slow rate of pozzolanic reaction requires prolonged period


    of moist curing
    • Pozzolanic reaction is more temperature sensitive than
    regular cement hydration
    Ground granulated blast-furnace slag (GGBFS)
    • Obtained from the production of iron
    • Typical composition
    – CaO = 35-45%, SiO2 = 32-38%, Al2O3 = 8-16%,
    MgO = 5-15%, Fe2O3 <2%, sulfur = 1-2%
    • Reactivity of slags can vary widely depends on
    – Glass content, composition, and fineness
    • ASTM C 989 assess slag reactivity by strength activity index
    (mortar with 50% OPC and 50% slag/mortar with 100% OPC)
    Slag Hydration
    • GGBFS reacts slowly with water, strength development
    too slow
    • Activation of slag by
    – Ca(OH)2
    – Soluble sodium salts, NaOH, Na2CO3, NaSiO3
    • Slags are commonly activated by Portland cement
    • In slag-cement blends, slag also shows pozzolanic
    behavior
    • Products of slag hydration
    – A mixture of C-S-H and AFm (monosulphoaluminate)
    • The rate of hydration of activated slag is similar to that
    of C2S, as is the heat of hydration
    Blended Cements

    - Setting time
    - Strength
    - Heat
    - Sulfate
    resistance
    Effects of Mineral Admixtures
    on Properties of Fresh & Hardening Concrete
    • Heat of hydration (fly ash and slag)
    – Reduce the overall heat of hydration
    – Reduce the rate of heat liberation
    – Reduce temperature rise in concrete
    • Workability
    – Improve cohesiveness
    – Fly ash and silica fume are particularly beneficial due to their
    spherical shape
    – Addition of fly ash allow w/cm (water/(cement + mineral
    admixtures)) to be reduced while maintaining slump
    – Silica fume is more beneficial provided that a water reducing
    admixture is used (spherical particles provide lubrication
    among cement particles; eliminate bleeding and segregation,
    but make concrete more susceptible to plastic shrinkage)
    Effects on Microstructure
    • Increase C-S-H & reduce CH leads to more homogenous
    microstructure
    • Improve pore structure, reduce overall porosity & pore
    size

    (Mindess et al 2003)
    Effects on Strength Development
    • Develop very good strength over time
    • May reduce early-age strength, can be offset by reducing
    w/cm
    • More reactive pozzolanic materials such as silica fume
    and calcined clay will reduce setting time and contribute
    to early strength

    (Mindess et al 2003)
    Effects on Drying Shrinkage and Creep
    • As an approximation, addition of mineral admixtures
    does not significantly affect the drying shrinkage or
    creep of concrete
    • However, if volume changes are critical, test should be
    made to determine the exact characteristics under the
    anticipated service conditions.
    Effect on Durability
    • Used extensively for improving the durability of
    concrete
    • Improvements in durability result from the reduction in
    CH, changes in pore structure, and reduction in w/c
    – Increase sulfate resistance
    – Control alkali-aggregate reaction
    – Reduce chloride diffusion
    – Reduce leaching and efflorescence
    Linear expansion of concrete
    exposed to standard ASTM tests
    for sulphate attack or alkali-
    aggregate reaction

    (Mindess et al 2003)

    Expansion limit
    for AAR

    Solid lines: w/c=0.5


    Comparison of ASTM and BS-EN Standards
    ASTM BS-EN
    Portland C 150 – Spec for Portland 197-1: 2000 (SS EN 197-1: 2008)
    cement cements CEM I – Portland cement
    Blended C 595 – Spec for blended 197-1: 2000 (SS EN 197-1: 2008)
    cements hydraulic cements CEM II – Portland comp. cem
    C 1157 – Performance Spec CEM III – blastfurnace cem
    for blended hydraulic CEM IV – pozzolanic cem
    cements CEM V – composite cem
    Mineral C 618 – Spec for coal fly ash 450-1: 2005 Fly ash for concrete
    admixtures and raw or calcined natural – Definition, spec, and
    pozzolans for use in concrete conformity criteria
    C 989 – Spec for GGBFS for 15167-1: 2006 (SS EN 15167: 2008)
    use in concrete and mortars Ground granulated blast furnace
    slag for use in concrete, mortars,
    and grouts
    C 1240 – Spec for silica fume 13263-1: 2005 Silica fume for
    used in cementitious concrete
    mixtures
    BS-EN Cement Types
    Mechanical and Physical Requirements (BS-EN 197-1)

    N – ordinary early strength


    R – high early strength
    Chemical Requirements (BS-EN 197-1)
    Reading Assignment
    • Chapter 6, Water, pp. 115 – 120

    • Chapter 7, Aggregates
    – Section 7.1, properties required for concrete mix design,
    pp. 122-140

    • Electronic teaching materials


    – Aggregates
    Recycled Concrete Aggregates
    • Produced by
    – Breaking up and removing old concrete from structures
    – Crushing the materials
    – Removing reinforcement & embedded
    items
    – Washing and screen (other methods to
    remove mortars)
    • Aggregate properties
    – Similar to crushed rock in particle shape
    – Much higher water absorption than virgin aggregates
    – Specific gravity ~ 5-10% lower than virgin aggregates
    – Good quality original concrete generally produces good
    quality recycled concrete aggregates
    Recycled Concrete Aggregates
    • Fresh concrete properties
    – Higher water demand
    – Higher cement content required
    – Lower workability and greater slump loss
    • Hardened concrete properties

    Compressive strength
    Tensile strength
    Modulus of elasticity
    Drying shrinkage
    Creep
    Permeability
    Depth of carbonation
    Comparison of ASTM and BS-EN Standards
    ASTM BS-EN
    Aggregate C 33 – Spec for concrete BS-EN 12620: 2002 (SS EN 12620: 2009)
    aggregates (not including (including natural, manufactured, and
    lightweight, heavyweight, and recycled aggregates with density >
    recycled aggregates) 2000kg/m3)

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