---
title: "Admixture vs additive; apparent vs absolute specific gravity"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:specific-gravity-differences"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/specific-gravity-differences
site: BCT Engineering
---

# Admixture vs additive; apparent vs absolute specific gravity

Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials

- Asked in: 2082 Baisakh, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/specific-gravity-differences
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

What is the basic difference between (i) admixtures and additives, and (ii) apparent specific gravity and absolute specific gravity?

## Short answer

Admixtures are added during mixing in small doses to modify fresh/hardened concrete, while additives are incorporated at cement production in larger amounts as binder. Absolute specific gravity considers only solid volume with zero pores, whereas apparent specific gravity includes impermeable pores but excludes permeable ones, giving a slightly lower value.

## Answer

#### i) Admixtures and additives
- **Admixture**: a material added to the concrete batch at the time of mixing (small dose, usually below 5% of the cement mass) to modify the properties of fresh or hardened concrete, e.g. plasticiser, retarder, air-entraining agent.
- **Additive**: a material added to the cement, usually at the manufacturing (grinding) stage, or in larger quantities as part of the cementitious material, e.g. gypsum, fly ash, slag, silica fume. It forms part of the binder.

#### ii) Apparent and absolute specific gravity
- **Absolute specific gravity**: the ratio of the mass of the solid material to the mass of an equal volume of water, where the volume includes only the solid matter and no pores at all (found with the aggregate crushed to powder).
- **Apparent specific gravity**: the ratio of the mass of the oven-dry aggregate to the mass of water of a volume equal to the solid volume plus the impermeable (sealed) pores; the water-permeable pores are excluded.

$$
G_{apparent} = \frac{A}{A - (C - B)}
$$

where $A$ = oven-dry mass of the sample, $B$ = mass of the pycnometer full of water, and $C$ = mass of the pycnometer with the sample, topped up with water.

Since the apparent volume includes the closed pores, the apparent specific gravity is slightly lower than the absolute specific gravity (they are equal only if there are no impermeable pores). For most aggregates the two are close (2.6-2.7).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is an additive rather than an admixture?
   A. Superplasticiser
   B. Retarder
   C. Fly ash or gypsum added to the binder
   D. Air-entraining agent
   Answer: C. Additives such as gypsum and fly ash become part of the binder. (Key point: Admixture vs additive)

2. Absolute specific gravity uses a volume that:
   A. Includes all pores
   B. Includes only solid matter, no pores
   C. Includes surface water
   D. Includes bulk voids between particles
   Answer: B. Absolute volume is that of solids only (aggregate crushed to powder). (Key point: Absolute specific gravity)

3. Apparent specific gravity compared with absolute is:
   A. Slightly lower, since its volume includes sealed pores
   B. Much higher
   C. Always equal
   D. Slightly higher always
   Answer: A. Closed pores add volume, lowering apparent value; equal only if none exist. (Key point: Apparent vs absolute)

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