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