Which of the following is NOT an advantage of using composite cement lime mortar over cement mortar?
🔬 Understanding Composite (Cement-Lime) Mortar
A composite mortar, also known as a gauged mortar, is a mix that uses both Portland cement and hydrated lime as binding materials. This combination is designed to leverage the best qualities of each binder: the high strength of cement and the excellent workability of lime.
The Trade-Off: Adding lime to cement mortar is a balancing act. It improves many of the mortar's plastic (wet) properties but generally reduces its final hardened strength compared to a pure cement mortar of the same richness.
⚖️ Detailed Analysis of the Options
The question asks us to identify which statement is NOT an advantage of adding lime to cement mortar.
(a) High Workability
This IS an advantage. Lime particles are extremely fine and have a lubricating effect on the mix. This makes the mortar "buttery," smoother, and much easier to spread on the trowel and apply to the wall. This is one of the primary reasons for using a composite mortar.
(c) High water retention
This IS an advantage. Lime has a high capacity to hold water. This prevents the mortar from drying out too quickly when it comes into contact with absorbent bricks. Good water retention ensures that there is enough water available for the cement to hydrate properly, leading to a better bond and higher strength development.
(d) Low drying shrinkage
This IS an advantage. Because of its high water retention and fine particle size, lime helps to reduce the amount the mortar shrinks as it dries. Lower shrinkage means a lower risk of developing cracks in the plaster or masonry joints.
(b) High drying shrinkage
This is NOT an advantage. High drying shrinkage is a significant disadvantage in any mortar, as it leads to cracking. As explained above, adding lime actually *reduces* shrinkage. Therefore, stating that composite mortar has "high drying shrinkage" is false, and it certainly is not an advantage.
💡 Study Tips for Mortar Properties
- Lime = Better Living for Masons: Think of lime as the ingredient that makes the mason's life easier. It improves Workability (smoothness), Water Retention (less drying out), and reduces cracks (Low Shrinkage).
- The Price of Lime is Strength: The main trade-off for all these benefits is a reduction in the mortar's final compressive strength compared to pure cement mortar.
- High Shrinkage is ALWAYS Bad: In construction, high shrinkage is a defect that causes cracks. It can never be considered an advantage.