Admixture vs additive; apparent vs absolute specific gravity

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).