Mineral Admixtures

Cement

Aggregates

Admixtures

Mixture Design

Fresh Concrete

Hardened Concrete

Dimensional Stability

Durability

 

 

Introduction

  • Also called ‘Supplementary Cementing Materials’
  • Used when special performance is needed: Increase in strength, reduction in water demand, impermeability, low heat of hydration, improved durability, correcting deficiencies in aggregate gradation (as fillers), etc.
  • Result in cost and energy savings: Replacement of cement leads to cost savings; energy required to process these materials is also much lower than cement
  • Environmental damage and pollution is minimized by the use of these by-products – about 6 – 7% of total CO2 emission occurs from the production of cement
  • Usage depends on supply and demand forces, as well as the market potential and attitudes

Typical compositions

% by mass PC GGBFS F-FA C-FA SF
SiO2 21 35 50 35 90
Al2O3 5 8 25 20 2
Fe2O3 2 3 10 5 2
CaO 65 40 1 20 -

PC: Portland cement, GGBFS: Ground granulated blast furnace slag, F-FA: Type F fly ash, C-FA: Type C fly ash, SF: Silica fume

Figure 1 shows the compositions of cement and supplementary cementing materials plotted on a C-S-A ternary diagram.

Figure 1: Composition of cement and mineral admixtures

Classification of mineral admixtures by RILEM

  • Cementitious
  • Highly pozzolanic: Silica fume, Rice husk ash (controlled burning)
  • Normally pozzolanic: Class F fly ash
  • Cementitious and pozzolanic: GGBFS, Class C fly ash

An additional category is also suggested by researchers – Weak pozzolans, such as slowly cooled and ground slag, bottom ash, and field-burnt rice husk ash

Pozzolans

Pozzolans are siliceous or aluminous materials, which possess by themselves little or no cementitious properties, but in finely divided form react with calcium hydroxide in the presence of moisture at ordinary temperatures to form compounds possessing cementitious properties (definition according to ASTM C595).
Portland-pozzolan cement reaction:

C3S/C2S + H2O —›C-S-H + CH
CH + Reactive SiO2 —›Pozzolanic C-S-H

The pozzolanic C-S-H is generally more porous than the normal C-S-H, and also has a lower C/S ratio. A pozzolan may also have reactive Al2O3, in which case the reaction with CH leads to the formation of C-A-H, which can give rise to problems in sulphate attack.

Pozzolanic activity is evaluated using the Pozzolanic Activity Index test, which defines the index as:

PAI (%) = Strength (PC/pozzolan mixture)*100 / Strength (PC mixture)
In this test, the mix design is done using a volumetric replacement of cement by the pozzolan (ASTM C311) as opposed to the Slag Activity Index test (ASTM C989) where a mass replacement is used.
Characteristics of pozzolanic reaction:

  • Lime consuming
  • Pore refiner (although there is not much decrease in the total pore volume)
  • Interface refiner (leads to higher strengths)
  • Slow rate of reaction; low heat of hydration. Time to attain same strength as PC may be termed as teff, or effective time. Figure 2 shows some typical values of teff for various mineral admixtures
  • Better durability, due to reduced permeability and reduced CH content
  • Alkalis and gypsum accelerate the pozzolanic reaction

Figure 2: Strength of pozzolanic mixtures

Fly Ash

Fly ash is a by-product obtained during the combustion of coal in thermal power plants. Nearly 73% of India’s electricity generation is through coal-burning thermal power stations. About 120 million tons of coal ash now being generated annually; could reach 200 million tons in 2010. Only about 10% of fly ash is being utilized in various industries; remaining amount gets dumped in landfills. According to World Bank estimates, by 2015, the disposal of coal ash would require 1000 square kilometers or one square meter of land per person. Thus, the utilization of fly ash is a matter of primary concern.

The quality and composition of fly ash depends on the type of coal being burnt. The rank (or purity) of coal increases in this order:

5. Lignite (brown coal)
4. Sub-bituminous coal (70 – 80% C)
3. Bituminous coal (80 – 90% C) – Soft coal, used for ordinary purposes
2. Semi-bituminous coal – Good heating value, has a smokeless flame
1. Anthracite (90 – 95% C) – hard coal; high temperature needed to burn it

Low rank coals contain impurities such as clay, shale, quartz, carbonates, and sulfides. It is these impurities which give fly ash its composition.

Fly ash use is not very common for the following reasons:

  1. Difficult quality assurance
  2. Poor marketing
  3. Conservative attitudes
  4. Storage problems
  5. Presence of toxic chemicals inside fly ash
  6. It is called a ‘waste’ instead of pozzolan or cement

Collection of fly ash

During combustion of coal, 75 – 80% of the ash flies out with the flue gas, and is thus called ‘fly ash’. The ash that doesn’t fly out is called ‘bottom ash’. This can be processed as aggregate, but is generally not used in concrete.
The collection of fly ash is done using the following two types of precipitators (see Figure 3):

  1. Bag-house precipitator
  2. Electrostatic precipitator

The bag-house precipitator is found to be more efficient, and removes out very fine material.

Sometimes, beneficiation of fly ash is done where coarse particles are ground to a fineness compatible with the intended use.

Figure 3: Electrostatic precipitator for fly ash collection

Uses of fly ash in concrete

  • As a mineral admixture
  • As a synthetic aggregate: Fly ash aggregate can be produced by sintering. The resultant aggregate can be used for lightweight concrete. However, it is very expensive. Aggregate can also be synthesized by agglomeration using lime or cement as binder, as in ‘cold bonding’.

Types of fly ash (ASTM classification)

  • Type C: This is also called High Calcium fly ash, and possesses both cementitious and pozzolanic properties. 10 – 15% of the material has a particle size greater than 45 µm, and the fineness (Blaine) is 300 – 400 m2/kg. The particles are primarily solid spheres with a smooth texture. The average particle size is less than 20 µm.
  • Type F: This is also called Low Calcium fly ash, and is a normally pozzolanic material. 15 – 20% of the material is larger than 45 µm, and the fineness is 200 – 300 m2/kg. Particles are solid spheres with a smooth texture, and the average particle size is 20 µm.

Apart from solid spherical particles, there also may exist hollow spheres. The small hollow spheres with entrapped gas are called cenospheres, while the large hollow spheres with solid spheres inside them are called plerospheres (see Figure 4).

Figure 4. Structure of fly ash

The loss on ignition of fly ash can represent the amount of unburnt carbon present. Too much of unburnt carbon can interfere with the air-entraining agent, leading to poor air void parameters. Restrictions are also placed on the sulfate (SO3) content, MgO content, alkali content, and moisture content of fly ash.

Effect on fresh concrete properties

  • The setting time is increased when fly ash is used.
  • Workability and flow of concrete are increased due to the spherical shape of the fly ash particles, which lends a ball-bearing type effect on the concrete mixture.
  • Bleeding and segregation are usually reduced for well-proportioned fly ash concrete.
  • The paste volume is increased when mass replacement of cement by fly ash is done.

Effect on hardened concrete properties

  • Strength gain of fly ash concrete is slower than normal concrete, as stated in the previous sections. The potential for thermal cracking is much reduced compared to ordinary PC concrete. Ultimate strengths are usually improved when fly ash is used.
  • Pozzolanic activity is proportional to the amount of particles under 10 µm in diameter.
  • Creep and shrinkage of fly ash concrete are typically higher than normal concrete, because of the increased amount of paste in the concrete (when mass replacement is done).
  • More air-entraining admixture is needed to entrain air in fly-ash concrete.
  • The results on the effects of fly ash on sulphate resistance are inconclusive.
  • Expansions during alkali aggregate reaction are reduced by the use of fly ash, because of the dilution of Portland cement (implying there are lesser alkalis available).
  • For properly cured fly ash concrete, the rate of chloride diffusion is reduced compared to ordinary PC concrete.

Use of fly ash in specialized applications

  • In high strength concrete, as an additional cementitious material.
  • In roller-compacted concrete. fly ash is good for bonding in-between the layers of this concrete.
  • In controlled low-strength materials (CLSM), which are flowable mortars used as backfill
  • As a synthetic aggregate
  • High volume fly ash concrete: Concrete with 50% of the Portland cement replaced by Class F fly ash. Has a low water content, generally less than 130 kg/m3. For slumps of 150-200 mm, the use of a superplasticizer is mandatory. The range of characteristic compressive strengths that can be achieved using HVFA concrete is 20-50 MPa. This concrete possesses excellent pumpability, and exhibits little bleeding (therefore, prone to plastic shrinkage cracking) and low drying shrinkage. It has applications in mass concrete blocks, building columns and foundations, caissons and piles, dams, highways, shotcrete and self-compacting concrete.

Silica Fume

According to ACI 116R, silica fume is a very fine amorphous (noncrystalline) silica produced in electric arc furnaces as a byproduct of the production of elemental silicon or alloys containing silicon; also known as condensed silica fume or microsilica. Despite previous knowledge about the benefits of silica fume, its use as mineral admixture for concrete really picked up only in the 80s.

There are numerous variants of this highly pozzolanic material available:
Condensed silica fume, microsilica, silica flour, fume silica (a white fluffy material produced from vapour phase hydrolysis of chlorosilanes such as SiCl4 in the flame of hydrogen and oxygen, used in the paint industry as filler), silica gel, and precipitated silica.

Silica fume is a by-product of the ferrosilicon industry. The purity of silica fume depends on the ferrosilicon alloy from which Si metal is being extracted (see Table 1).

Table 1. Silica purity in various ferrosilicon alloys

Ferrosilicon alloys SiO2 content
FeCrSi 18 – 48%
FeMgSi 44 – 48%
50% ferrosilicon 72 – 77%
70% ferrosilicon 84 – 88%
Silicon metal (98%) 93 – 98%

 

Figure 5 depicts the process of collection of silica fume. After being collected over the furnace, the silica fume is transferred, cooled, and physically trapped. Cyclones are used to remove oversize and other unwanted materials. Similar to fly ash, silica fume is also collected using baghouse precipitator.

Figure 5: Process of silicon metal extraction

Silica fume is available in the following states (see Figure 6):

  • As is bulk powder: Due to the low specific gravity of silica fume (~2.2), the bulk powder becomes very bulky and difficult to handle and transport. It is difficult to handle pneumatically; it is sticky and self agglomerating with a tendency to create small weak lumps. Furthermore, its low density yields small loads in bulk tankers. It is primarily used in bagged products (e.g., grouts, pre-mixed mortars)
  • Dry-densified silica fume: Densification is performed by aerating and tumbling the silica fume powder with compressed air, until the desired bulk density is reached due to the agglomeration caused by electrostatic charges that develop. An efficient superplasticizer is required to deflocculate and cause a good dispersion of the silica fume in concrete. Unlike the as is powder, the densified version is easy to handle and transport, and flows well pneumatically.
  • Slurry: 50% water + 47% silica fume + 3% chemical agent, that keeps the particles in suspension and prevents gelling. The slurry form is susceptible to gelling in cold climates. However, it is a very efficient way of dispensing silica fume. Also, storage space can also be reduced.

Figure 6: Different forms of silica fume

Properties

  • Specific gravity: 2.2
  • Typical fineness: 20000 m2/kg (average particle size ~ 0.1 – 0.5 µm)
  • Bulk density: As-produced - 130 to 430 kg/m3, slurry - 1320 to 1440 kg/m3, densified - 480 to 720 kg/m3
  • Colour: light grey to dark grey (lighter implies purer)
  • Cost: almost 10 times as much as PC
  • Typically used at 5 – 15% replacement level
  • Benefits from silica fume are due to the pozzolanic reaction that produces additional C-S-H, as well as due to the particle packing (filler effect) of the fine silica fume particles

 

Effects on fresh concrete properties

  • Because of its high fineness, the use of silica fume causes an increase in the water demand of concrete. Typically it is always used in conjunction with a superplasticizer.
  • Silica fume causes the mix to be sticky and cohesive. Also, concrete mixes with silica fume are prone to slump loss problems. Because of its cohesiveness, a higher slump is needed to place silica fume concrete.
  • Bleeding is reduced drastically. In fact, most silica fume mixes do not show any bleeding. In dry areas, if the evaporation rate exceeds the rate at which concrete sets, plastic shrinkage may occur. Figure 7 shows the increase in capillary pressure with a decrease in the pore diameter. Silica fume concrete is especially susceptible to this problem in case curing is not done properly. A simple calculation of the capillary tension due to plastic shrinkage can be shown as follows:
    Pc (capillary tension) = 0.001YS/(w/c), where Y = surface tension of water = 0.0073 N/m, and S = surface area of particles (20000 m2/kg for SF, 350 m2/kg for cement). Assuming a w/c of 0.35,
    For PC concrete, Pc = 0.07 MPa
    For SF concrete, Pc = 4.20 MPa
    (for more information on plastic shrinkage problems with silica fume, see the paper: M. D. Cohen, J. Olek, and W. L. Dolch, “Plastic Shrinkage Cracking in Portland Cement and Portland Cement-Silica Fume Paste and Mortar,” Cement and Concrete Research Vol. 20, 1990, pp. 103 – 119.

Figure 7: Relation between capillary pressure and diameter

Effects on hardened concrete properties

  • Pore size refinement and reduction in permeability occurs when silica fume is used. Due to a combined effect of silica fume as a highly reactive pozzolan and filler, the transition zone between aggregate and paste is strengthened.
  • Compressive and flexural strengths are increased, while the chloride permeability and diffusion are reduced significantly compared to ordinary PC concrete.
  • Elastic modulus is increased (ESFC ~ 1.15 EPCC), or, in other words, concrete becomes stiffer with the use of silica fume.
  • Creep and shrinkage are increased at high replacement levels (10 – 15%) because of an increase in the volume of the paste. However, due to the higher stiffness, the resistance to creep and shrinkage deformation is higher.
  • Amount of air-entraining agent required for a particular volume of air is increased in silica fume concrete. Freeze-thaw resistance is reduced slightly compared to normal concrete, but damage is usually limited owing to the extremely low permeability of SFC.
  • In most cases, silica fume concrete shows better resistance to chemical attack (exceptions being ammonium sulphate and magnesium sulphate attack), owing to the decreased permeability, as well as due to reduced CH in the paste.
  • Expansions due to ASR are reduced in silica fume concrete.
  • Corrosion rate is reduced with the use of silica fume. This is because of two reasons: the low permeability of SFC causes a lower availability of moisture and oxygen at the cathodic sites, and the high resistivity of SFC makes the flow of electrons difficult.
  • Carbonation depth is generally lowered.
  • SFC has very good abrasion and erosion resistance. This makes it an ideal choice for industrial flooring.
  • Fire performance of SFC is not very good. This is a consequence of the low permeability of silica fume concrete. When a fire occurs, the free water inside concrete transforms to steam and escapes through the interconnected voids. When this escape is prevented to the dense microstructure, significant pressures get built up inside, which ultimately cause the concrete to explode and spall. This type of failure occurred in the late 1990s in the English Channel tunnel.

Ground granulated blast furnace slag

Blast furnace slag is a by-product of the extraction of iron from iron ore. Coke and limestone are added as fluxes inside the blast furnace. The impurities in iron ore combine with the lime and rise up to the surface of the blast furnace (shown in Figure 8), while the molten iron, which is heavier, stays at the bottom.

The use of blast-furnace slag as a cementitious material is very old. 1892 was the first time that Portland-blast furnace slag cement was manufactured. In the present day scenario, slag is used almost in every country to varying degrees.

The reactivity of slag depends on the rate of cooling. In increasing order of reactivity, the cooling processes may be ranked as: Slow cooling (in air), Rapid cooling (by water spray), and Quenching (dipping in water).

Amongst mineral admixtures, slag possesses the highest specific gravity (~ 2.8 – 3.0). Because it is a processed material, the fineness can be controlled to any desired degree. However, for most typical applications, slag fineness is only slightly higher than cement fineness.

Figure 8: Blast furnace used in iron extraction

Types of slag

  • Air cooled slag: Low reactivity slag that finds use as aggregate. The strength and toughness of this aggregate makes it a very suitable material for railroad ballast.
  • Expanded or foamed slag: Low reactivity slag that is foamed with air. Makes a very good lightweight aggregate, and is used for thermal insulation.
  • Granulated: This is a high reactivity slag, and is usually quenched. The hardened matter is then ground to a fineness similar to cement. Thus the name: Ground Granulated Blast Furnace Slag (GGBFS).
  • Pelletized slag: The reactivity is similar to GGBFS, but the process of pelletization is a complex one. The schematic diagram in Figure 9 shows the process of pelletization. Typically, this type of slag is not used as much as GGBFS

Figure 9: Process of pelletization of slag

Factors determining cementitious properties of slag cements

  • Chemical composition of GGBFS – The amount of CaO in slag determines its cementitious properties. Since slag is almost entirely amorphous, the reactivity of the CaO will determine the overall slag reaction.
  • Alkali concentration of reacting system – When the alkali content is higher, the system will be more reactive.
  • Glass (reactive SiO2) content of GGBFS – Glassy SiO2 causes the pozzolanic reaction to take place with the hydrated lime.
  • Fineness of GGBFS and PC – Higher fineness implies a faster reaction.
  • Temperature during early phase of hydration – Slag hydration is greatly enhanced at high temperatures.

Hydration of slag

GGBFS is a mineral admixture with both cementitious and pozzolanic properties. In fact, it is classified as a hydraulic cement in most codes. However, an activator is necessary to hydrate the slag. The activation of slag hydration can be done in the following ways:

  • Alkali activation: e.g. by caustic soda (NaOH), Na2CO3, sodium silicate, etc. The products formed are C-S-H, C4AH13 and C2ASH8 (Gehlenite).
  • Sulphate activation: e.g. by gypsum, hemihydrate, anhydrite, phosphogypsum, etc. The products formed are C-S-H, ettringite, and aluminium hydroxide (AH3).
  • Mixed activation: When both alkali and sulphate sources are present, such as in a cement system.

Effects on fresh and hardened concrete

  • Apart from delaying the initial set, slag does not significantly alter the fresh concrete properties. The workability of slag concrete is similar to an equivalent PC concrete, primarily because slag possesses the same level of fineness as PC.
  • The rate of strength gain is slowed down considerably when cement is replaced by slag. The delay increases with increasing replacement. 100% slag concrete is also possible, although the curing duration to produce the required strengths would need to be substantially increased
  • The ultimate strengths with slag are generally improved; the durability is also improved with the replacement of cement by slag.
  • Slag is the ideal admixture for marine concrete, as slag concrete shows excellent resistance to chemical attack and corrosion
  • Slag concrete is reported to have higher carbonation rates compared to ordinary PC concrete

Other mineral admixtures

Rice husk ash (RHA)

This is a high reactivity pozzolan obtained by controlled calcination of rice husk. Field-burnt rice husk is almost crystalline in nature, and makes a weak pozzolan. Thus, to obtain a high degree of pozzolanicity, a good control is needed while burning. RHA usually contains a large amount of unburnt carbon which might adversely affect air entrainment.
RHA is a fine material, with particle sizes less than 45 µm, and a surface area of 60000 m2/kg. The particles are typically cellular. A high amount of reactive silica is present in the system (> 90%).

Metakaolin

This is obtained from calcination of kaolinite clay in the range of 740 – 840 oC. The crystalline clay loses its structure at this temperature by the loss of bound water. Burning should strictly be done in this range, since beyond 1000 oC, recrystallization of the clay occurs.
A general formula of metakaolin can be written as AS2. This aluminosilicate compound reacts with CH produced during cement hydration in the following form (suggested by Murat – in Cement and Concrete Research, Vol. 13, 1983):
AS2 + 6CH + 9H —
C4AH13 + 2C-S-H
C-S-H formed in this reaction is aluminous, with a C/S ranging from 0.83 (for crystalline forms of C-S-H) to > 1.5 (for amorphous and semi-crystalline forms of C-S-H).
The content of C-S-H and its formation rate depends on the mineralogical characteristics of the kaolin precursor. Metakaolin has a performance comparable to silica fume as a mineral admixture in concrete. Since MK is not a by-product, its processing is an expensive affair. Thus the marketability of MK is not as good as silica fume, which is a proven by-product.

Proportioning methods for mineral admixtures

  • Simple replacement method: This is the traditional method of proportioning. Replacement of cement can be done either on a volume basis or a mass basis. Volume replacement does not change the overall volume of the paste. However, when mass replacement is done, volume of the paste increases, and this increase is usually compensated by a decrease in the volume if sand. Mixture characteristics can be adversely affect by the removal of sand if the volume removed is substantial (which can happen for large amounts of low specific gravity admixtures such as silica fume). For slow reacting pozzolans like Type F fly ash, this method results in low early age strengths. Another disadvantage of this method is that it does not account for the variations in characteristics of the mineral admixtures. The advantages of this method are its simplicity and positive effects on workability (when replacing material is fly ash).
  • Addition method: This method involves a direct addition of the mineral admixture to the concrete without replacing any part of the cement. In high performance concrete, this is the method of choice, since it increases the cementitious content. This increase is compensated by a decrease in the fine aggregate content. Addition method usually results in higher strengths.when fine materials such as silica fume are used, this method can cause a substantial increase in the water demand.
  • Modified replacement method: In this case, part of the admixture is added, and part of it is used as a replacement. The quantity of mineral admixture put into the mix is greater than the quantity of cement removed. This method is typically used to obtain sufficiently high early age strengths with fly ash. However, workability and water demand can be difficult to control in this method.
  • Rational method: This is an efficient method of proportioning admixtures. It quantifies the influence of the admixture using a factor K, which is the ‘cementing efficiency factor’. This factor qualifies the mineral admixture as a lower grade or higher grade cement. In other words, K represents the amount of the mineral admixture that can replace 1 unit of Portland cement in the mixture to achieve similar properties. This method is able to overcome the slow early age strength development for fly ash concrete.


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