Industry Insight
Why Water Retention Matters in Dry Mix Mortars
Water retention is a mortar's ability to hold on to its mixing water against suction and evaporation. Here is why it matters and how water retaining additives help.
5 min read UKCP Technical Team
Water retention is the ability of a fresh mortar to hold on to its mixing water rather than losing it to absorbent backgrounds or to evaporation. In dry mix mortars it is a key property: it keeps the mortar workable while it is being placed and makes sure enough water stays in the mix for the cement to hydrate, which is what gives the mortar its strength and bond.
Key takeaways
- Water retention keeps mortar workable and leaves enough water for the cement to hydrate.
- Poor water retention leads to rapid stiffening, weak bond, powdery surfaces and cracking.
- Absorbent backgrounds, thin layers, heat and wind all increase the need for water retention.
- Water retaining additives, most commonly cellulose ethers, are used in dry mix mortars to control it.
- Dosage must be balanced against workability and setting, ideally as part of a complete additive package.
What is water retention?
When a mortar is spread onto brick, block, concrete or plaster, the background starts to draw water out of it by capillary suction. At the same time, warmth and air movement remove water from the exposed surface. Water retention describes how well the mortar resists those losses.
It matters particularly in dry mix mortars because many of them are applied in thin layers, such as tile adhesives, skim coats and thin bed mortars, where there is very little water to spare. It also matters for factory made products, because the manufacturer cannot control the backgrounds and weather conditions the mortar will meet on site.
Why does water retention matter?
Cement hydration and strength
Cement needs water to hydrate and gain strength. If water is lost too quickly, hydration is incomplete, particularly at the interface with the background. The result can be a weak, powdery layer exactly where strength is needed most.
Bond to the background
Bond forms at the interface between mortar and background. A mortar that loses its water at the interface bonds poorly, which is a particular concern for renders and tile adhesives, where adhesion is critical to performance.
Workability and open time
Retained water keeps the mortar plastic for longer. Bricklayers can bed and tool units, renderers can rule and finish, and tilers can place tiles within the adhesive's open time.
Resistance to cracking
Rapid water loss from fresh mortar increases shrinkage and the risk of cracking, especially in thin layers and in warm, windy conditions.
What increases the need for water retention?
| Condition | Why it increases the demand |
|---|---|
| Highly absorbent backgrounds, such as some bricks, aircrete blocks and old masonry | Strong suction draws water out of the mortar quickly |
| Thin layers | There is less water held in reserve |
| Hot, dry or windy weather | Evaporation from the surface is faster |
| Long working times | Large tiles, render panels and slow detailed work need the mortar to stay workable for longer |
How do water retaining additives work?
Cellulose ethers are the most widely used water retaining additives in factory made dry mortars. Common types include methyl cellulose (MC), hydroxyethyl methyl cellulose (HEMC, also written MHEC) and hydroxypropyl methyl cellulose (HPMC). They dissolve in the mixing water, increase its viscosity and hold water within the mortar so it is released more slowly to the background and the air.
Cellulose ethers influence more than water retention. They also affect consistency, stickiness, sag resistance and, at higher dosages, setting time and air content. Starch ethers and other thickeners are sometimes used alongside them to fine tune consistency.
Where is water retention most important?
- Tile adhesives. Cementitious tile adhesives are covered by BS EN 12004-1, and open time, which depends heavily on water retention, is tested using the methods in BS EN 12004-2.
- Renders and plasters. Rendering and plastering mortars are covered by BS EN 998-1. Good water retention helps renders bond to absorbent backgrounds and reduces the risk of cracking.
- Masonry mortars. Masonry mortars are covered by BS EN 998-2. Water retention helps mortar bond to absorbent units and gives the bricklayer time to bed and tool.
- Skim coats and thin bed products, where very thin layers leave little margin for water loss.
- Bagged and silo mortars, which must perform consistently across a wide range of site conditions.
How does water retention interact with other additives?
Water retainers rarely work alone. A typical dry mix mortar may also contain air entrainers, accelerators, retarders and water repellents, and each of these can influence the others. Air entrainment, for example, also affects workability and consistency, so the two additives need to be balanced together. Our article on air entraining agents explains how they work.
This is one reason many manufacturers choose a single additive package developed for their own materials rather than combining separate products. UKCP's in-house development team creates bespoke "black box" additive packages using the raw materials from the customer's production site. Read more in bespoke additive packages for dry mortar manufacturers.
How do you get the balance right?
- Too little water retention leads to rapid stiffening, poor bond, powdery surfaces and cracking.
- Too much water retention can make a mortar sticky, slow its stiffening and affect setting.
- Materials matter. Sand grading, fillers and cement type all influence how a water retainer performs, so trial the additive with your own raw materials.
- Consider the end use. A thin bed tile adhesive and a bricklaying mortar need very different levels of water retention.
How UKCP can help
UKCP supplies Powder Water Retainer as part of its mortar admixtures range, alongside Powder AEA and Liquid AEA. With over 30 years' technical experience developing additives and admixtures for dry mortars, silo mortars and concrete applications, the UKCP team can also build water retention into a bespoke formulation. For datasheets and advice, visit the datasheets page, speak to our technical support team or request a sample.
