The Essential IS Code Guide to Mineral Admixtures: Fly Ash, GGBS, Silica Fume, and More

 

Mineral Admixtures – Detailed Specifications (as per IS Codes)

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1. Fly Ash (IS 3812 Part 1 & Part 2)

 

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IS 3812 (Part 1): For Use as Pozzolana in Cement, Cement Mortar, and Concrete

This part specifies the requirements for fly ash when used as a pozzolanic material for the manufacture of cement (e.g., Portland Pozzolana Cement) or as a partial replacement for cement in mortar and concrete.³

1. Classification

The standard classifies fly ash based on its chemical properties, particularly the content of Reactive Calcium Oxide (CaO):⁴

• Siliceous Pulverized Fuel Ash (Low-Calcium Fly Ash): Reactive CaO is less than 10% by mass.⁵ This is typically produced from burning anthracite or bituminous coal (similar to ASTM Class F).⁶

• Calcareous Pulverized Fuel Ash (High-Calcium Fly Ash): Reactive CaO is not less than 10% by mass. This is typically produced from burning lignite or sub-bituminous coal (similar to ASTM Class C).⁷

2. Key Chemical Requirements (Maximum Limits in % by Mass)

Characteristic	Siliceous Fly Ash	Calcareous Fly Ash
${\text{SiO}}_2+{\text{Al}}_2{\text{O}}_3+{\text{Fe}}_2{\text{O}}_3$ (Min.)	70	50
${\text{SiO}}_2$ (Min.)	35	-
$\text{MgO}$ (Max.)	5.0	5.0
Total ${\text{SO}}_3$ (Max.)	2.75	2.75
${\text{Na}}_2\text{O}$ (Available Alkalis) (Max.)	1.5	1.5
Total Chlorides (Max.)	0.10	0.10
Loss on Ignition (LOI) (Max.)	5.0 (May be 7.0 if strength/soundness met)	5.0 (May be 7.0 if strength/soundness met)

3. Key Physical Requirements

Characteristic	Requirement
Fineness (Specific Surface by Blaine's method, ${\text{cm}}^2/\text{g}$) (Min.)	3200
Particles Retained on $45\mu$m IS Sieve (Wet Sieving) (% by mass) (Max.)	34
Lime Reactivity (Avg. Compressive Strength, $\text{N}/{\text{mm}}^2$) (Min.)	4.5
Pozzolanic Activity Index (with Portland Cement at 28 days)	Not less than 80% of the strength of corresponding plain cement mortar cubes.
Soundness (Autoclave Expansion) (% by mass) (Max.)	0.8

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IS 3812 (Part 2): For Use as Admixture in Cement Mortar and Concrete

This part specifies the requirements for fly ash when it is added to concrete or mortar primarily to improve workability, pumpability, or other fresh/hardened concrete properties, rather than as a major cement replacement. The fly ash is typically used as collected, or with minor beneficiation.

1. Classification

Part 2 does not specify different grades like Part 1. It covers Pulverized Fuel Ash for use as an admixture.⁸

2. Key Chemical Requirements

The chemical requirements are generally less stringent than Part 1, particularly for ${\text{SiO}}_2+{\text{Al}}_2{\text{O}}_3+{\text{Fe}}_2{\text{O}}_3$ and ${\text{SiO}}_2$.

Characteristic	Requirement (Max. Limits in % by Mass)
${\text{SiO}}_2+{\text{Al}}_2{\text{O}}_3+{\text{Fe}}_2{\text{O}}_3$ (Min.)	50
$\text{MgO}$ (Max.)	5.0
Total ${\text{SO}}_3$ (Max.)	2.75
${\text{Na}}_2\text{O}$ (Available Alkalis) (Max.)	1.5
Total Chlorides (Max.)	0.10
Loss on Ignition (LOI) (Max.)	12.0 (Higher than Part 1)

3. Key Physical Requirements

The physical requirements are similar to or slightly more relaxed than Part 1, reflecting its use as an admixture where high reactivity might not be the primary goal.

Characteristic	Requirement
Fineness (Particles Retained on $45\mu$m IS Sieve) (% by mass) (Max.)	34
Uniformity (Variation in Specific Gravity)	The specific gravity of any individual sample shall not vary from the average of the previous 10 tests by more than $\pm 5\%$.
Increase in Water Requirement	Should not increase the water required to produce a slump of $100\pm 5\text{ mm}$ by more than 5% of the control concrete.
Drying Shrinkage	The drying shrinkage of concrete containing fly ash shall not exceed that of the control concrete by more than $0.03\%$.
Soundness (Autoclave Expansion) (% by mass) (Max.)	0.8

2. Ground Granulated Blast Furnace Slag (GGBS) – IS 12089, IS 16714

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IS 12089: Specification for Granulated Slag

IS 12089 was the earlier standard, specifically for granulated slag (the granular material) used for the manufacture of Portland Slag Cement (PSC).

Key points and typical requirements mentioned in the context of IS 12089 (often in its 1987 revision):

Characteristic	Requirement (as per context)
Physical State	Granules
Lump Size	Proportion of lumps $>50\text{ mm}$ size: $\text{Max }5\%$ of mass
Chemical Requirement	Hydraulic Index $[(\text{CaO}+\text{MgO}+{\text{Al}}_2{\text{O}}_3)/{\text{SiO}}_2]:\text{Min }1.0$ (or $1\%$ according to some sources, suggesting a minimum value rather than a percentage)
Magnesium Oxide ($\text{MgO}$) Content	$\text{Max }17.0\%$
Sulphide Sulphur Content	$\text{Max }2.0\%$
Glass Content	$\text{Min }85.0\%$

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IS 16714: Specification for Ground Granulated Blast Furnace Slag for Use in Cement, Mortar and Concrete

IS 16714:2018 is the more recent, active standard that specifically covers Ground Granulated Blast Furnace Slag (GGBS)—the finely ground powder.² It provides a comprehensive specification for its use in cement, mortar, and concrete.

The standard defines detailed requirements for chemical and physical properties, including:

Requirement Category	Typical Specified Limits (as per IS 16714:2018 context)
Physical Requirements	Fineness: Measured by the specific surface (e.g., Blaine fineness).
	Slag Activity Index: Must meet minimum strength requirements at $7\text{ days}$ and $28\text{ days}$ when tested with reference cement.
Chemical Requirements	Magnesium Oxide ($\text{MgO}$) Content
	Sulphide Sulphur Content
	Glass Content: $\text{Min }85\%$ (or other values)
	Insoluble Residue
	Loss on Ignition
	Moduli Value $[(\text{CaO}+\text{MgO}+{\text{Al}}_2{\text{O}}_3)/{\text{SiO}}_2]:\text{Min }1.0$
	Moisture content

Note: The exact numerical values for maximum and minimum contents are specified in the tables within the standard itself. For example, some technical data sheets referencing IS 16714:2018 suggest:

• Insoluble Residue: Max ³$3\%$

• Loss of Ignition: Max ⁴$3\%$

• Glass Content: Min ⁵$85\%$

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Relationship Between IS 12089 and IS 16714

The key distinction is in the scope and product form:

• IS 12089 covers the specifications for granulated slag (the material before fine grinding) primarily for the manufacture of Portland Slag Cement (PSC).⁶

• IS 16714 covers the specifications for Ground Granulated Blast Furnace Slag (GGBS) (the finely powdered cementitious material) for direct use in cement, mortar, and concrete.⁷

The test method for sampling in IS 16714 sometimes refers to IS 12089 for procedures concerning the source granulated slag.

The video below discusses mineral admixtures like GGBS and their effect on the durability of cement and concrete.

41 Mineral Admixtures | GGBS | Part 3 | Hydration of Slag & Durability of Slag Cements This video is relevant as it provides an in-depth lecture on Ground Granulated Blast Furnace Slag (GGBS), its hydration, and its impact on the durability of cement, which is the underlying purpose of the Indian Standard specifications.

3. Silica Fume (IS 15388)

1. Forms of Silica Fume Covered by IS 15388

The standard recognizes silica fume in different physical forms:

Form	Description	Typical Bulk Density
Natural State	Silica fume taken directly from the collection filter.	150 to 350 kg/m3
Densified Silica Fume	Silica fume that has been treated to increase the bulk density by particle agglomeration.	Above 500 kg/m3
Silica Fume Slurry	A homogeneous, liquid suspension of silica fume particles in water.	Approx. 700 kg/m3 (dry content 50% by mass)

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2. Key Specifications (Chemical & Physical Requirements)

The standard specifies limits for several properties to ensure the material's quality and performance as a high-efficiency pozzolan. While the exact tables are within the official document, the critical requirements are:

A. Chemical Requirements (IS 15388, Table 1)

The primary chemical component is Silicon Dioxide (${\text{SiO}}_2$).

• Silicon Dioxide (${\text{SiO}}_2$) Content: The most crucial requirement; the minimum value must be ³85% by mass.⁴

• Loss on Ignition (LOI): Specifies the maximum permissible mass loss when the material is heated to a high temperature, ensuring low organic content. The limit is typically Max 4%.

• Moisture Content: The material must be relatively dry to ensure proper handling and mixing. The limit is typically Max 3%.

• Alkalies (as ${\text{Na}}_2\text{O}$ equivalent): The total content of alkalies should be low to mitigate the risk of Alkali-Silica Reaction (ASR). The limit is typically Max 1.5%.

B. Physical Requirements (IS 15388, Table 2)

• Pozzolanic Activity Index: This is a performance test to ensure the silica fume reacts effectively with calcium hydroxide from cement hydration.

  • $7\text{ Days}$ Index: Minimum percentage of compressive strength of a mortar cube made with silica fume replacement compared to the control cement cube. A common industry value is Min 105%.

• Oversize Material (Retained on ⁵$45\mu$ sieve):⁶ Due to the extremely fine nature of silica fume (particles are typically ⁷100× smaller than cement), the amount of coarse material is severely restricted.⁸ The limit is typically Max 10%.

• Specific Surface Area (Fineness): Though not always expressed by a traditional Blaine method, the standard specifies a high surface area (e.g., Min 15 m2/g), reflecting the ultra-fineness of the material.

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3. General Information

• Grades: IS 15388 does not classify silica fume into performance grades (like Grade 80/100/120 in ASTM C1240) based on pozzolanic activity. It provides a single set of minimum performance requirements that all silica fume must meet for use in concrete.

• Nature of Silica Fume: It is an ultrafine, non-crystalline (amorphous) silicon dioxide (⁹SiO2​) produced as a by-product of silicon and ferrosilicon alloy manufacturing in electric arc furnaces.¹⁰

• Purpose: It is used primarily to produce High-Performance Concrete (HPC), significantly improving strength, durability, and resistance to chloride ion penetration and sulfate attack.

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4. Metakaolin (IS 1344, IS 8112)

1. Specifications for Metakaolin under IS 1344

The relevant standard for Metakaolin when used as a cement additive is IS 1344: Specification for calcined clay pozzolana (most recent revision). Metakaolin is a highly refined and processed version of calcined kaolinite clay, and thus must meet the chemical and physical requirements specified for calcined clay pozzolana.

IS 1344 classifies the material into two grades based on the required performance:

Requirement	Grade 1	Grade 2
I. Chemical Requirements (Minimum/Maximum)
Sum of ${\text{SiO}}_2+{\text{Al}}_2{\text{O}}_3+{\text{Fe}}_2{\text{O}}_3$	$\text{Min }70\%$	$\text{Min }70\%$
${\text{SiO}}_2$ (Silicon Dioxide)	$\text{Min }40\%$	$\text{Min }40\%$
$\text{CaO}$ (Calcium Oxide)	$\text{Max }10\%$	$\text{Max }10\%$
Loss on Ignition (LOI)	$\text{Max }10\%$	$\text{Max }10\%$
II. Physical Requirements (Minimum/Maximum)
Fineness (Specific Surface, ${\text{m}}^2/\text{kg}$)	$\text{Min }320$	$\text{Min }320$
Lime Reactivity (Compressive Strength)	$\text{Min }4.0{\text{ N/mm}}^2$	$\text{Min }3.0{\text{ N/mm}}^2$
Drying Shrinkage	$\text{Max }0.12\%$	$\text{Max }0.15\%$
Soundness (Le Chatelier)	$\text{Max }10\text{ mm}$	$\text{Max }10\text{ mm}$
Reduction in Strength ($\text{Cement-Pozzolana}$)	$\text{Max }20\%$	$\text{Max }20\%$

Note on Metakaolin Purity:

Metakaolin often has much higher purity than the minimum requirements of IS 1344, with a typical sum of (SiO2​+Al2​O3​+Fe2​O3​) content well above 90%. Due to its controlled manufacturing process, it typically meets or exceeds the Grade 1 requirements by a large margin.

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2. IS 8112: 43 Grade Ordinary Portland Cement

IS 8112: Specification for 43 Grade Ordinary Portland Cement is a product standard for cement and sets the requirements for the OPC itself, not for mineral admixtures like metakaolin.

The key specifications in IS 8112 relate to:

• Fineness (Blaine's Apparatus): $\text{Min }225{\text{ m}}^2/\text{kg}$

• Compressive Strength:

  • $3\text{ days: Min }23{\text{ N/mm}}^2$

  • $7\text{ days: Min }33{\text{ N/mm}}^2$

  • $28\text{ days: Min }43{\text{ N/mm}}^2$

• Setting Time: Initial $\text{Min }30\text{ minutes}$, Final $\text{Max }600\text{ minutes}$

Metakaolin is used as a partial replacement for cement (or as an admixture) to improve the performance of the concrete produced using IS 8112 cement, but the metakaolin material itself must conform to IS 1344.

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5. Rice Husk Ash (RHA) – IS 8112 (Performance Improver) 

Relevance of IS 8112 to RHA

While IS 8112 does not specify RHA properties, it sets the benchmark for the cement that RHA is often used to partially replace or blend with. When RHA is used in concrete, the resulting blend must still ensure the concrete meets the relevant strength and durability requirements, which are indirectly linked to the quality standards of the base cement (like 43 Grade OPC defined in IS 8112).

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Specifications for Rice Husk Ash (RHA) in Cement

For RHA used as a pozzolana to be blended with cement, the specifications are typically drawn from standards like IS 16415:2016 (Portland Pozzolana Cement with Rice Husk Ash) or the general pozzolana standard IS 3812 (which covers Fly Ash and calcined clay, but RHA often aligns with its principles).

Key specifications for good quality RHA as a pozzolana generally include:

1. Chemical Requirements

• Silica Content (¹$Si{O}_2$): RHA is primarily a source of silica.² To be highly reactive (pozzolanic), the silica should be largely in an amorphous (non-crystalline) form, which is achieved by controlled burning (typically below ³700∘C).⁴ The total content of $Si{O}_2+A{l}_2{O}_3+F{e}_2{O}_3$ should ideally be greater than 70% (similar to requirements in standards like ASTM C618).

• Loss on Ignition (LOI): This indicates the unburnt carbon content. High LOI can negatively affect cement strength and color. A common limit for good quality pozzolana is $<6.0\%$ (though some standards allow up to 12% for certain types of ash).

• Sulphur Trioxide ($S{O}_3$): Should be low, often specified at $<5.0\%$ maximum.

2. Physical Requirements

• Fineness: RHA must be very fine to achieve high pozzolanic reactivity. Fineness is often measured by the surface area (e.g., Blaine air permeability test) or by the amount retained on a 45μm sieve (e.g., $<34\%$ retained, similar to ASTM requirements).

• Pozzolanic Activity Index: This is a crucial test, measuring the compressive strength of a cement-pozzolana blend mortar against a control OPC mortar. The index at 28 days is often required to be $\ge 75\%$ of the control.

• Specific Gravity: Typically low compared to cement, often around $2.05$ to $2.3$.

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IS 8112: 43 Grade OPC (Original Specifications)

The main specifications covered by IS 8112 for the base 43 Grade OPC (which RHA would replace a portion of) are:

Property	Requirement (IS 8112)
Fineness (Blaine)	$\ge 225m^2/kg$
Initial Setting Time	$\ge 30$ minutes
Final Setting Time	$\le 600$ minutes (10 hours)
Soundness (Le Chatelier/Autoclave)	$ \leq 10~mm$ (Le Chatelier)
Compressive Strength at 28 days	$\ge 43N/m{m}^2$

This video covers the uses of rice husk and rice husk ash in construction materials.

PROLITE - Insulating material made of rice husk ash - YouTube

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6. Lime (IS 712 – Building Lime)

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Classification of Lime as per IS 712:1984

Class	Name of Lime	Primary Use	Supply Form (Note 1)
Class A	Eminently Hydraulic Lime	Structural purposes (e.g., mortar, concrete, foundation works)	Hydrated form only
Class B	Semi-Hydraulic Lime	Masonry mortars, lime concrete, and plaster undercoat	Hydrated or Quicklime
Class C	Fat Lime (Non-Hydraulic)	Finishing coat in plastering, whitewashing, composite mortars	Hydrated or Quicklime
Class D	Magnesium/Dolomitic Lime	Finishing coat in plastering, whitewashing	Hydrated or Quicklime
Class E	Kankar Lime	Masonry mortars	Hydrated form only
Class F	Siliceous Dolomitic Lime	Undercoat and finishing coat of plaster	Hydrated or Quicklime

Note 1: The standard specifies that only Class A and Class E limes must be supplied in the hydrated (slaked) form. Other classes may be supplied as quicklime or hydrated lime.

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Key Specifications (Chemical & Physical)

IS 712 specifies detailed chemical and physical requirements for each class of lime (presented in Tables 1 and 2 of the standard). The specific values vary significantly by class, but the parameters tested are consistent.

1. Chemical Requirements

The chemical composition is determined on an ignited basis and includes limits for:

• Calcium and Magnesium Oxides (CaO + MgO): The total content is a major factor in classification. For example, Class C (Fat Lime) requires a very high content.

• Magnesium Oxide ($MgO$): Limited to ensure proper slaking and strength development.

• Silica, Alumina, and Ferric Oxide ($Si{O}_2+A{l}_2{O}_3+F{e}_2{O}_3$): These form the essential compounds that give hydraulic limes (Class A and B) their cementitious (setting) properties.

• Insoluble Residue in Dilute Acid and Alkali: Indicates the amount of inert impurities.

• Carbon Dioxide ($C{O}_2$): Limited, as it indicates unburnt limestone or carbonation of slaked lime.

• Unhydrated Magnesium Oxide ($uMgO$): Critical for hydrated limes, as high unhydrated content can cause delayed expansion (unsoundness) in the hardened mortar.

• Available Lime as $CaO$: A measure of the effective calcium content.

2. Physical Requirements

The physical properties specified ensure the lime is workable, sound, and develops adequate strength:

Test Property	Class A (Eminently Hydraulic) Key Requirement	Class C (Fat Lime) Key Requirement
Fineness	Amount retained on a $300\mu m$ IS sieve is limited.	Amount retained on a $300\mu m$ IS sieve is limited.
Setting Time	Initial: $\ge 120$ minutes (2 hours) Final: $\le 2880$ minutes (48 hours)	Not specified, as it sets by carbonation (very slow).
Compressive Strength	Mortar cube strength after 14 days and 28 days is specified (e.g., $1.75N/m{m}^2$ to $2.8N/m{m}^2$ at 28 days for Class A).	Strength is specified for a lime-pozzolana composite mortar.
Soundness (Le-Chatelier expansion)	Limited maximum expansion (e.g., $\le 10mm$ or $\le 5mm$ depending on the form).	Limited maximum expansion.
Workability/Volume Yield	Volume yield test is specified for quicklime.	High volume yield is characteristic of fat lime.

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Step-by-Step QC Checklist (All Admixtures)

1. Collect manufacturer test certificate with batch number.

2. Sample material as per IS method.

3. Test for:

   • Chemical composition (oxides, LOI, chloride, alkali, reactive silica).

   • Physical properties (fineness, specific surface, moisture, sieve residue).

   • Strength/activity (lime reactivity, SAI, pozzolanic index).

4. Compare results with IS limits.

5. Approve/Reject batch.

6. Store in dry, labeled, sealed containers.

7. Perform trial mixes for strength, durability, and workability validation before site use.

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