Mortar cracks when drying because it shrinks, and the material can’t stretch enough to absorb that shrinkage without pulling apart. The volume loss comes from two sources: water evaporating from the surface and chemical reactions happening inside the mix. Both processes reduce the total volume of the mortar, creating internal tension that eventually exceeds the material’s ability to hold together.
Two Types of Shrinkage at Work
When you mix cement with water, a chemical reaction called hydration begins. The products of that reaction actually take up less space than the original ingredients. The combined volume of dry cement powder plus water is greater than the volume of the hardened paste they become. This is called chemical shrinkage, and it starts the moment water hits the cement.
The second type is drying shrinkage, which is more intuitive. As water evaporates from the mortar, tiny pores inside the material lose moisture. The surface tension of the remaining water in those pores pulls the walls of the pores inward, like a sponge contracting as it dries. This creates compressive forces on a microscopic level that translate into the mortar physically getting smaller. When the surface dries faster than the interior (which it almost always does), the outer layer tries to shrink while the still-wet core resists. That mismatch creates tensile stress at the surface, and since mortar is strong in compression but weak in tension, it cracks.
Early Cracks vs. Long-Term Cracks
Not all mortar cracks appear at the same time, and the timing tells you a lot about what went wrong.
Plastic shrinkage cracks form in the first few hours after placement, before the mortar has hardened. They happen when evaporation from the surface outpaces the rate at which water bleeds up from deeper in the mix. Because the material is still soft, these cracks have distinctive features: they’re V-shaped, widest at the top, and the edges look rounded or deformed rather than clean. You might even see tiny bridges of paste stretching across the crack. They often appear in irregular, wandering patterns across the surface.
Drying shrinkage cracks develop after the mortar has set and hardened. These look different because they form in a brittle material. The crack lines are thinner and sharper, more like a line scored into glass. They tend to appear over days, weeks, or even months as the mortar continues to lose moisture to the surrounding air. Most of the chemical shrinkage happens within the first 24 hours, but drying shrinkage can continue for much longer depending on conditions.
How Water Content Affects Cracking
You might assume that wetter mixes shrink more, but the relationship is more nuanced than that. Research on mortar with varying water-to-cement ratios found that lower ratios actually increased drying shrinkage. When there’s less water in the mix, a greater proportion of the mortar’s volume is cement paste. That means more of those tiny capillary pores become unsaturated as hydration consumes the limited water, which intensifies the internal tension that drives shrinkage.
When the water-to-cement ratio exceeded 0.3, mortars had enough internal moisture to buffer against this effect, and drying shrinkage decreased. The practical takeaway: a mix that’s too dry can crack just as readily as one that’s too wet, though for different reasons. Too much water weakens the mortar and leaves more volume to be lost as it evaporates. Too little water starves the hydration process and amplifies capillary tension.
Sand Quality Makes a Bigger Difference Than You’d Think
The sand in your mortar isn’t just filler. It acts as a skeleton that resists shrinkage. Larger aggregate particles are particularly effective at reducing drying shrinkage because they create larger internal pores. Bigger pores hold water with less surface tension, which means less of that inward-pulling force that makes the mortar contract.
Research comparing different aggregate sizes confirmed this consistently: the larger the sand particles, the lower the drying shrinkage. Larger particles also pack together more tightly, increasing the bulk density of the mix and allowing the aggregate to interlock and resist the forces trying to pull the mortar apart. On the other end of the spectrum, sand with too many fine particles (silt, clay, or dust) increases shrinkage because fines demand more water to coat their greater surface area, and they fill the spaces between grains with material that shrinks rather than material that resists it. Industry standards note that sands with excessive fines produce weaker mortars and increase shrinkage.
Heat, Wind, and Low Humidity
Environmental conditions during and after application are one of the most controllable factors in mortar cracking. High evaporation rates create a steep moisture gradient between the surface and interior, concentrating tensile stress right where cracks start. Research on plastic shrinkage tested mortars at 30°C, 50% relative humidity, and wind speeds of 24 km/h (about 15 mph). Under those conditions, evaporation was aggressive enough to reliably produce surface cracks.
Any combination of heat, wind, and dry air accelerates surface drying. A hot day with a breeze is one of the worst scenarios for fresh mortar. Even moderate wind over a sun-exposed wall can dry the surface fast enough to crack it before the interior has a chance to supply moisture upward.
How to Reduce Cracking
Since cracking is driven by uneven moisture loss, the most effective prevention strategy is slowing that loss down. For the first seven days after application, keeping the mortar damp is critical. On larger jobs, this means misting the surface with water five to seven times per day. That sounds like a lot, but the goal is to never let the exposed surface dry out completely while the interior is still curing.
If frequent misting isn’t practical, covering the mortar with plastic sheeting after thoroughly wetting it traps moisture and dramatically slows evaporation. Weight the edges down with bricks or stones so wind doesn’t lift the cover. Even with sheeting in place, removing it once daily to rewet the surface for the first week gives the best results.
Timing your work helps too. Applying mortar during cooler parts of the day, avoiding direct sun exposure when possible, and shielding fresh work from wind all reduce the evaporation rate that triggers plastic shrinkage cracks. If you’re working in hot or windy conditions, dampening the masonry units before laying them prevents them from pulling water out of the mortar too quickly.
Getting the mix right matters just as much as curing. Use clean, well-graded sand with a range of particle sizes rather than uniformly fine sand. Adding lime to the mix (as specified in standard mortar types) improves workability and flexibility, giving the mortar more capacity to absorb minor shrinkage without cracking. A straight portland cement and sand mortar without lime tends to be more rigid, but industry specifications note it can actually offer greater resistance to cracking and rain penetration in wall applications because of how it bonds. The right mortar type depends on the specific job, but in general, a well-proportioned mix with adequate lime, properly graded sand, and a balanced water content gives you the best defense against the shrinkage that causes cracks in the first place.