Why Excess Water Weakens Mortar and Why You Should Never Rehydrate Mortar

Why Excess Water Weakens Mortar and Why You Should Never Rehydrate Mortar

Why excess water weakens mortar is an important principle for anyone involved in paving and hard landscaping. Although adding extra water may seem like a simple way to improve workability, it can reduce strength, increase porosity and shorten the long term durability of the mortar.

With hotter summers becoming more common, mortar can lose workability much faster than many installers expect. Rehydrating, or “knocking up”, mortar by adding extra water after it has begun to stiffen is a practice many of us have used at some point, but it can significantly reduce the long-term performance of the mortar.

Why Water is Important in Mortar

Mortar is made from cement, aggregate and water. Water performs two essential functions.

Firstly, it makes the mortar workable so it can be mixed, spread and used for laying paving.

Secondly, it reacts chemically with the cement in a process known as hydration. Hydration is what gives mortar its strength. As the cement reacts with water, millions of microscopic crystals form and interlock, binding the aggregate particles into a strong, solid mass.

Not all of the mixing water is needed for hydration. The excess gradually evaporates as the mortar cures, leaving behind tiny microscopic voids within the hardened cement paste. These are known as capillary pores.

Why Excess Water Weakens Mortar

Capillary pores are a normal part of hardened mortar and concrete. However, the more excess water that is added, the greater the number and size of these pores.

An increase in capillary pores generally results in:

• Reduced compressive strength.
• Higher water absorption.
• Increased permeability.
• Greater susceptibility to freeze-thaw damage.
• A higher risk of efflorescence and moisture-related staining.

For this reason, only enough water should be added to achieve a workable consistency. Although extra water may make mortar easier to use initially, it produces a weaker, more porous material once hardened.

Why Rehydrating Mortar Weakens It

Once hydration has begun, the cement starts forming the microscopic crystal structure that gives mortar its strength. This process cannot simply be restarted by adding more water after the mortar has begun to stiffen or set.

Adding water and remixing at this stage breaks up the developing cement structure while also increasing the water-to-cement ratio. As the excess water later evaporates, it leaves behind even more capillary pores.

The result is mortar with reduced strength, greater porosity and lower durability.

A simple way to think about it is baking a cake. Once the cake has started baking, you cannot pour in more mixture and expect it to bake as though it were freshly mixed. Mortar behaves in much the same way. Once hydration is underway, the internal structure is developing and cannot be restored by simply adding water.

It is important to distinguish between mortar that has merely stiffened through slight evaporation and mortar that has started to set. Freshly mixed mortar that has only lost a little moisture may, in some cases, be re-tempered in accordance with the manufacturer’s guidance. However, once initial set has begun, additional water should never be added. Site-mixed cement mortars that have exceeded their recommended working life should be discarded and replaced with a fresh batch.

Top Tips

• Mix only the amount of mortar you can comfortably use within its working time.
• In hot, dry or windy conditions, reduce batch sizes to minimise waste.
• Temperatures above 30°C, especially when combined with low humidity and strong winds, create challenging conditions for laying paving and curing mortar.
• Always follow the mortar manufacturer’s guidance regarding working times and hot weather installation.

Why Mortar Should Not Dry Out Too Quickly

Although mortar eventually dries, it does not gain its strength simply by drying out. Strength comes from hydration, the chemical reaction between cement and water.

If moisture is lost too quickly, there may not be enough water available for the cement to complete hydration. When this happens, the mortar cannot develop its intended strength or durability.

What Causes Rapid Drying?

Rapid moisture loss is commonly caused by:

• Hot weather.
• Strong winds.
• Direct sunlight.
• Warm paving units.
• Dry or highly absorbent paving and substrates that draw moisture from the mortar.

These conditions can remove water faster than the cement is able to hydrate.

What Happens if Mortar Dries too Quickly?

If moisture is lost before hydration is sufficiently complete, the mortar may suffer from:

• Reduced compressive strength.
• Poor adhesion between the mortar and the paving.
• Increased shrinkage.
• Surface cracking.
• Greater porosity and water absorption.
• Reduced resistance to freeze-thaw cycles.
• A dusty or crumbly surface in severe cases.

These defects can shorten the service life of the paving and increase the likelihood of future maintenance or premature failure.

How Can Rapid Drying be Prevented?

Good curing practices help retain moisture while hydration continues. Depending on site conditions, this may include:

• Damping down dry paving units or substrates where appropriate.
• Avoiding installation during the hottest part of the day.
• Protecting freshly laid mortar from direct sunlight and strong winds.
• Covering new work with damp hessian or other suitable curing materials where necessary.
• Following the mortar manufacturer’s recommendations for curing and protection.

In Summary

Mortar gains its strength through hydration, not simply by drying. Drying is the evaporation of excess water, whereas hydration is the chemical process that creates the hardened cement matrix.

Using too much water increases the number of capillary pores within the hardened mortar, reducing its strength and durability. Likewise, adding water to mortar after hydration has begun does not restart the chemical reaction. Instead, it disrupts the developing cement structure and increases the water-to-cement ratio, producing a weaker, more porous mortar.

The simplest way to avoid future problems is to mix manageable batches, use only the amount of water required for workability, protect fresh mortar during curing, and always discard mortar that has exceeded its recommended working life rather than attempting to rehydrate it.