Low-grade gypsum use in the building industry-code of practice
1989 Edition

Scope Overview of the Code - Dimensions and Tolerances

• Block Nominal Sizes (Clause 8.2.1):

• Rounding Procedures (Clause 8.2):
  Test or calculation results should be rounded following IS 2:1960, maintaining the same significant figures as the specified values.

• Physical Characteristics (Clause 8.4):
  Specifies mandatory physical properties for block quality assurance.

Summary:

• Blocks adhere to standardized length, breadth, and height.
• Rounding rules ensure consistency in data reporting.
• Physical properties guarantee durability and performance consistency.

flowchart TD
    A[Scope of Standard] --> B[Block Dimensions & Tolerances]
    B --> C[Length ≤ 700 mm, increments of 100]
    B --> D[Breadth: 75, 100, 125, 150 mm]
    B --> E[Height ≤ 300 mm, increments of 100]
    A --> F[Rounding as per IS 2:1960]
    A --> G[Physical Requirements]

Ensures uniformity and quality control for gypsum blocks.

Standards Referenced in This Code

Relevant Indian Standards Cited in IS 12654 (Clause 2.1):

Rounding Standards (IS 2:1960)

• Final results from tests or calculations must be rounded according to IS 2.
• Number of significant digits retained equals those in the original specified value.

Block Dimensions & Tolerances (Clause 8.2.1)

Summary

• Rounding should follow IS 2 guidelines.
• Referenced IS codes provide test methods and terminology.
• Blocks are dimensionally standardized with tolerances.
• Adherence to referenced standards assures quality and uniformity.

flowchart TD
    A[IS 12654] --> B[IS 1288 - Gypsum Testing]
    A --> C[IS 2250 - Masonry Mortar Code]
    A --> D[IS 2469 - Gypsum Terms]
    A --> E[IS 1/Sec 1-12 - Plaster Tests]
    A --> F[IS 2542 - Gypsum Product Tests]
    A --> G[IS 8272 - Fibrous Plaster Boards]

Definitions and Key Dimensions as per IS 12654

Block Nominal Sizes (Clause 8.2.1)

• Length and height must not exceed maximums and should be multiples of 100 mm.
• Breadth options are fixed.

Rounding Off (IS 2:1960)

• All final measured or calculated values must be rounded in accordance with IS 2.
• The rounded figure must preserve the significant figures from the specified value.

Summary

• Blocks must meet prescribed nominal sizes within allowed tolerances.
• Dimensional and physical compliance is confirmed after rounding.

flowchart TD
    A[Block Dimensions] --> B{Length}
    B -->|Max 700 mm| C[Multiples of 100]
    A --> D{Breadth}
    D -->|75, 100, 125, 150 mm| E[Standard Sizes]
    A --> F{Height}
    F -->|Max 300 mm| G[Multiples of 100]

This guarantees interchangeability and uniformity in block production.

Purity Criteria for Gypsum (IS 12654)

• Minimum Purity: Low-grade gypsum must contain at least 60% purity as calcium sulfate dihydrate (CaSO₄·2H₂O), as specified in Clause 4.1.
• Typical Impurities: Include silica, dolomite, and carbonate compounds that lower purity.
• Testing Method: Purity determination is conducted following IS 1288:1982.

Important Specifications for Low-Grade Gypsum Plaster (Clause 7.1.1 Table)

Additional Notes:

• Gypsum plaster must be free from harmful substances such as iron pyrites, alkalis, organic matter, mica, and clay.
• Purity directly influences the mortar’s strength and longevity.

flowchart LR
    A[Raw Low-Grade Gypsum] --> B[Purity Testing (IS 1288)]
    B -->|≥ 60% CaSO4·2H2O| C[Suitable for Use]
    B -->|< 60%| D[Reject or Further Processing]
    C --> E[Preparation of Calcined Gypsum]
    E --> F[Gypsum Plaster Production]

Ensures gypsum plaster quality in line with IS 12654.

Preparation Guidelines for Calcined Gypsum According to IS 12654

Key Requirements:

• Low-grade gypsum should have a minimum purity of 60% CaSO₄·2H₂O (per IS 1288:1982).
• Common contaminants include silica, dolomite, and carbonates.

Preparation Process (Clause 5):

• Calcined gypsum is produced by carefully heating low-grade gypsum to eliminate a portion of crystallization water.
• This calcined product is utilized in masonry mortar, interior plastering, and lightweight block production.

Gypsum Plaster Specifications (Clauses 7.1.1 & 8.1.1):

Typical parameters include:

Important Considerations:

• Calcination temperature is controlled between approximately 150°C and 180°C to retain reactivity.
• Excessive heating results in dead-burnt gypsum, which has poor setting properties.
• Moisture content must be minimized to prevent premature setting.

flowchart LR
    A[Low-Grade Gypsum] --> B[Calcination at 150-180°C]
    B --> C[Calcined Gypsum (Plaster)]
    C --> D{Applications}
    D --> E[Masonry Mortar]
    D --> F[Internal Plaster]
    D --> G[Lightweight Blocks]

Summarizes preparation and quality criteria as per IS 12654. Refer to the standard for detailed Table 1 values.

Summary of Physical Requirements (IS 12654)

Block Dimensions (Clause 8.2.1, Table 8.2)

Permissible Tolerances (Clause 8.2)

• Blocks should meet specified dimensional tolerances to maintain uniformity.
• Typical tolerances: ±3 mm for length and breadth, ±5 mm for height (refer IS 12654 for exact figures).

Physical Quality Requirements (Clauses 7.4 & 8.4)

• Blocks must have consistent shape and size.
• Surfaces should be free from cracks, chips, or other defects.
• Density and water absorption values must comply with durability requirements.

Quick Reference:

Nominal Dimensions (mm):
- Length ≤ 700 (in multiples of 100)
- Breadth = 75, 100, 125, or 150
- Height ≤ 300 (in multiples of 100)

Tolerances:
- Length & Breadth: ±3 mm
- Height: ±5 mm

Ensures consistent block quality for masonry applications supporting structural integrity and aesthetics.

Essential Information on Gypsum Mortar (IS 12654)

1. Gypsum Plaster Standards (Clauses 6.1 & 7.1.1)

• Must satisfy the specifications outlined in Table 1, including purity, setting time, and fineness.
• Ensures reliable quality for mortar preparation.

2. Mortar Mix Proportions (Clause 7.5)

• Recommended ratio:
  • 1 part gypsum plaster
  • 2 parts sand
  • Water equal to 50% of the combined dry materials by mass
• Intended for interior plastering and masonry uses only.
• Not recommended for exterior exposure except in very dry climates.

3. Compressive Strength (Clause 7.4.2)

• Mortar should attain adequate compressive strength, as specified in IS 12654 tables or via testing.
• Strength is assessed post-setting to confirm durability.

4. Performance Expectations (Clause 7.5)

• After 48 hours on brickwork, mortar should exhibit:
  • No shrinkage cracks, chalking, or crazing
  • Strong adhesion without peeling

Summary Table: Mortar Proportions & Characteristics

flowchart TD
    A[Gypsum Plaster] --> C[Mortar Mix]
    B[Sand] --> C
    D[Water (50% mass)] --> C
    C --> E[Apply on Clean, Moist Brick Surface]
    E --> F[Set for 48 Hours]
    F --> G{Inspect for Defects}
    G -->|No defects| H[Accept Mortar]
    G -->|Defects| I[Reject or Remediate]

Guarantees gypsum mortar meets durability and application standards.

Lightweight Solid Gypsum Blocks for Non-Structural Walls (IS 12654)

Key Properties (Clause 8)

• Usage: Specifically designed for non-load bearing internal walls.
• Material Source: Calcined low-grade gypsum.
• Block Type: Lightweight solid gypsum blocks.

Physical Characteristics (Clause 6 & Table 1)

Mortar Mix for Blocks (Clause 7.5)

• Composition:
  • 1 part gypsum plaster
  • 2 parts sand
  • Water amounting to 50% by mass
• Usage: Internal plastering and masonry only; avoid external exposure except in low rainfall zones.
• Performance: No shrinkage cracks, chalking, or crazing after 48 hours; good bonding without peeling.

Mortar Mix Formula

[ \text{Gypsum Mortar} = 1 \times \text{Gypsum Plaster} + 2 \times \text{Sand} + 0.5 \times \text{Water (mass fraction)} ]

flowchart LR
    A[Calcined Low-Grade Gypsum] --> B[Lightweight Solid Blocks]
    B --> C[Non-Load Bearing Wall Application]
    A --> D[Gypsum Mortar Preparation]
    D --> E[1 part Gypsum + 2 parts Sand + 50% Water]
    E --> F[Use for Internal Plastering and Masonry]

Note: Verify block dimensions and strength certificates from suppliers as per IS 12654.

Method for Evaluating Fineness of Gypsum Blocks (IS 12654)

Specification:

• Must have at least 95% passing through a 150 µm IS sieve (Clause 7.1.1, Table 1).

Procedure (Annex A, Clause 1.1):

[ \text{Fineness (% passing)} = 100 - \left(\frac{\text{Mass retained (g)}}{100} \times 100\right) ]

Summary:

• Use 150 µm IS sieve.
• Sieve duration: 5 minutes.
• Required fineness: ≥ 95% passing through sieve.

Related Requirements (Table 1):

• Setting Time: 10 to 30 minutes (IS 2542).
• Compressive Strength: Minimum 7.0 N/mm² (IS 8272).

flowchart TD
    A[100 g dried Gypsum Sample] --> B[Sieve on 150 µm IS sieve for 5 min]
    B --> C[Break lumps gently]
    C --> D[Weigh material retained]
    D --> E[Calculate % passing = 100 - retained %]
    E --> F{Fineness ≥ 95%?}
    F -- Yes --> G[Passes fineness test]
    F -- No --> H[Fails fineness test]

This procedure ensures gypsum plaster quality in accordance with IS 12654.