A new mortar that cracks in the days following its application raises a specific question: which parameter slipped between the mixing, the substrate, and the drying conditions? Several mechanisms often combine, but their respective weight in the appearance of early cracks is not the same. This article measures the actual impact of each cause to allow for a quick diagnosis.
Drying shrinkage of the mortar: the mechanism that mixing alone cannot explain
Drying shrinkage remains the primary cause of cracking in new mortar. When water evaporates, the volume of the mortar decreases. If this contraction is hindered by the substrate or by an irregular thickness, internal stresses arise and cause cracks, sometimes within the first few hours.
This phenomenon is amplified by an excess of mixing water. A mortar that is too liquid dries faster on the surface than in depth, creating a drying gradient between the outer crust and the still moist core. The crust contracts, the core resists, and the crack opens.
The same problem is found in overly thick screeds. The revision of NF DTU 26.2, announced by AFNOR for publication no later than the third quarter of 2026, sets maximum thicknesses of 10 cm for constant thickness screeds and 15 cm at the highest point for sloped forms, precisely to limit these internal gradients. An article details why mortar crumbles according to La Maison Durable by crossing several of these factors.
Water-cement-sand mixing: table of common errors and their effects on cracking
The mixing is the most direct lever on early cracking. Three variables interact: the volume of water, the proportion of cement, and the condition of the sand.
| Mixing Error | Cracking Mechanism | Time of Appearance |
|---|---|---|
| Excess mixing water | Accelerated drying shrinkage, surface/core gradient | First 24 to 48 hours |
| Overdosage of cement | Excessive chemical shrinkage during hydration | First days |
| Too wet sand not corrected | Actual water volume exceeds calculated volume | First 24 hours |
| Too fine or clayey sand | Increased water demand, irregular porosity | First week |
The most frequent trap concerns the moisture of the sand not taken into account. Sand stored outdoors after rain contains a significant amount of water. If the mason adds the same amount of water as for dry sand, the actual water/cement ratio far exceeds what was planned.
Overdosage of cement produces a mortar that appears harder, but also more rigid and more prone to shrinkage. Mechanical strength increases, flexibility decreases, and microcracks appear even before the mortar has finished setting.
Poorly prepared substrate and climatic conditions: two underestimated causes of early cracks
A perfectly mixed mortar can still crack if the substrate has not been treated properly. Two situations consistently arise.
- A substrate that is too dry absorbs water from the fresh mortar by capillarity. The base of the mortar dehydrates faster than the surface, reversing the usual drying gradient. Cracks then appear at the substrate/mortar interface, compromising adhesion.
- A dusty or dirty substrate prevents mechanical bonding. The mortar dries on the surface without bonding to the wall, then detaches and cracks under its own weight.
- A substrate with significant thickness variations (hollow joints, irregularities) creates areas where the mortar is thicker. These areas dry more slowly, and the difference in shrinkage between thin and thick areas generates tensile cracks.
Generously wetting the substrate before application is not always sufficient. The humidification must be uniform and done well in advance so that the water penetrates deeply without leaving a film on the surface.
Heat, wind, and direct sunlight on fresh mortar
Climatic conditions act as an accelerator. Mortar applied in full sun on dry and windy days loses its surface water long before the hydration of the cement is complete. Surface shrinkage becomes abrupt.
The revision of NF DTU 26.2 also imposes a maximum covering delay of 8 weeks to prevent screeds from being exposed to the open air for too long, which deteriorates the surface condition and amplifies shrinkage cracks.
On the other hand, sharp cold slows down the setting of the cement without preventing evaporation. The mortar remains fragile longer, and daily temperature variations are enough to cause microcracks before the mechanical strength is reached.
Structural cracking or shrinkage cracking: distinguishing the cause to adapt the repair
Not all cracks in new mortar fall under the same diagnosis. A shrinkage crack is generally fine, superficial, and distributed in a network (crazing). It indicates a problem with implementation, not a defect in the structure.
A crack that follows the masonry joints in a stair-step pattern, which widens over time or crosses the wall from side to side indicates a structural problem: foundation movement, differential settlement, poorly distributed load. In this case, repairing the mortar without addressing the structural cause is pointless.
The most reliable distinguishing criterion remains the evolution over time. A shrinkage crack stabilizes within a few weeks. A structural crack continues to open, sometimes seasonally, depending on ground movements.
Before any repair, measuring the width of the crack and monitoring it over several weeks with a witness (a simple pencil line on either side) allows for a decision between a simple aesthetic defect and a disorder that requires the intervention of a building expert. A new mortar that cracks is not always poorly made mortar: it is sometimes the first visible symptom of a problem located much deeper.