Why Does Mortar Fail to Bond Well? Causes and Effective Solutions

 In masonry construction, poor mortar bonding is a common pain point that leads to reduced wall strength, hollow joints, cracks, and even water leakage. So, why does mortar often fail to bond with bricks, and how can it be prevented?

1. Common Causes

1. Low Mortar Strength Grade
   Using low-grade or pure cement mortar results in poor workability, making it hard to compact. This often causes voids in mortar joints, leading to insufficient bonding.

2. Early Mortar Dehydration
   Bricks absorb water too quickly, causing the mortar to lose moisture and bond strength. Dust on the brick surface further reduces adhesion.

3. Improper Construction Methods
   With the bedding method, if mortar is spread too long in advance, its water is absorbed prematurely, and bonding is lost.

4. Excessive Joint Recession
   In fair-faced brickwork, workers sometimes leave large recessed joints (over 20mm) to skip joint finishing, reducing mortar fullness and increasing later jointing work.

2. Preventive Measures

1. Improve Mortar Workability
   Use high-quality raw materials within shelf life. Add cellulose ether (HPMC) and redispersible polymer powder (RDP) to enhance adhesion and workability.

2. Optimize Masonry Methods
   Avoid excessive bedding or brick placing methods. Instead, promote the “One Brick, One Trowel of Mortar, One Press” method.

3. Control Bedding Length
   Under normal conditions, mortar bedding should not exceed 750mm. At temperatures above 30℃, limit it to 500mm.

4. Proper Brick Moistening
   Moisten bricks 1–2 days before construction: 10–15% water content for fired clay and perforated bricks, 8–12% for sand-lime and fly ash bricks.

5. Winter Construction Precautions
   Even under positive temperatures, moisten brick surfaces before laying. In freezing conditions, increase mortar consistency. In seismic fortification areas (Intensity 9), masonry work should be avoided in severe winter.

By following these measures, mortar bonding and joint fullness can be significantly improved, reducing risks of hollowing, cracking, and poor durability.