Which type of hydrated lime is best to use with Portland cement for mortar?

Which type of hydrated lime is best to use with Portland cement for mortar?

• A. Type S
• B. Type N
• C. Type I
• D. Type III

Answer: (A)

The main concept is how lime sets: some lime mortars harden mainly by carbonation (air exposure), while others have hydraulic properties that give them strength even in damp conditions. When you mix lime with Portland cement, you want the mortar to gain strength early and resist moisture without losing breathability. The hydraulic lime type provides that hydraulic set, so it works well with Portland cement to deliver a mortar that has good early strength, durability in wet conditions, and still remains breathable. The lime that relies solely on carbonation would cure much more slowly and be less durable in damp exterior conditions, making it a poorer match for Portland-cement–based mortars. The hydraulic lime option is therefore the best choice for this combination.

The main concept is how lime sets: some lime mortars harden mainly by carbonation (air exposure), while others have hydraulic properties that give them strength even in damp conditions. When you mix lime with Portland cement, you want the mortar to gain strength early and resist moisture without losing breathability. The hydraulic lime type provides that hydraulic set, so it works well with Portland cement to deliver a mortar that has good early strength, durability in wet conditions, and still remains breathable. The lime that relies solely on carbonation would cure much more slowly and be less durable in damp exterior conditions, making it a poorer match for Portland-cement–based mortars. The hydraulic lime option is therefore the best choice for this combination.