Lime in cement-lime mortar
ASTM C270-19a Table 2 is the proportion specification, in parts by volume. With 1 part cement, the hydrated lime or lime putty is:
- Type M: 1/4 part
- Type S: over 1/4 to 1/2 part
- Type N: over 1/2 to 1 1/4 parts
- Type O: over 1 1/4 to 2 1/2 parts
More lime means a weaker mortar. In C270-19a, Table 1 is property and Table 2 is proportion.
Sand is a multiple of the binder
Aggregate, measured damp and loose, is not less than 2 1/4 and not more than 3 times the sum of the separate cementitious volumes. Add the cement and the lime first, then multiply.
Type S at 1 cement to 1/2 lime: cementitious volume = 1 + 1/2 = 1 1/2 parts.
- Minimum = 2.25 × 1.5 = 3 3/8 parts
- Maximum = 3 × 1.5 = 4 1/2 parts
Type N at 1 cement to 1 lime: cementitious volume = 2 parts, so the most sand is 3 × 2 = 6 parts and the least is 2 1/4 × 2 = 4 1/2 parts.
Three traps:
- Forgetting to multiply, which gives a bare 2 1/4 to 3 parts.
- Multiplying the cement alone and ignoring the lime (3 × 1 = 3 parts).
- Using 3 1/2 as the upper multiplier. The limit is 3, not 3 1/2.
When the spec says neither
ASTM C270, as quoted in BIA TN 8 (2020): when the specification names neither proportion nor property requirements, the proportion requirements govern. Field-batched cement-lime mortar defaults to proportions. The mason does not choose, property is not automatic for being stricter, and work does not stop for a clarification.
The one exception
Preblended dry mortar under ASTM C1714 is the exception: for it, the property requirements are the default. Type M is a mortar type, not a specification method, and BIA TN 8's hot-weather guidance is a time limit, not a default rule.