Mortar does not cure at low temperatures, and frost in fresh mortar damages the masonry. Here are the temperature limits, winter mortar and additives, and how to protect fresh masonry from frost, rain and drying out.
Introduction
Mortar only achieves its strength if allowed to cure under the right conditions. Cold slows curing, and if the free water in fresh mortar freezes, the structure ruptures from within — the result is a weak, crumbling joint and loose masonry. Therefore, winter and weather protection measures are a concrete, season-critical subject that is directly linked to quality assurance and complaints.
Why cold is a problem
KC and cement mortar harden when cement reacts with water (hydration). This reaction is strongly inhibited when temperature falls, and practically stops near freezing point. If the temperature drops below 0 °C before the mortar has released enough of its free water to the stone, the water freezes and expands by approximately 9 %. This breaks the mortar's pore structure so the joint never reaches its strength — damage that cannot be repaired later without taking the masonry apart.
Temperature limit
In practice, winter bricklaying is reckoned when the temperature falls towards and below +5 °C. Then precautions should be taken: heated mortar, protection of the fresh masonry, and possibly a heated working area. The industry guide recommends that the mortar has a temperature of at least about 10 °C after mixing, and that the mixing water is heated — but no more than about 80 °C, so that the cement does not take damage ('burn').
Winter mortar and admixture
There are special winter mortars, and for lime and KC mortar, a freezing point depressant additive can be used. According to Danish experience, ethanol (denatured spirits) or an ethanol/isopropanol mixture (with up to approx. 40% isopropanol) can be used as a freezing point depressant in lime and KC mortar with at most approx. 55% cement in the binder, in the order of 1–4 litres per 100 litres of mortar. Always follow the supplier's and industry guidance's specific instructions — dosage and suitability depend on the mortar type.
Members only
The rest of the article is locked
Create a free user account to unlock all articles, calculators, templates and range — and get access to quizzes and classes. It takes less than a minute: name, email and a password. No payment card.
Want to try something first? See what you can use without an account — the junior zone, the encyclopedia, the videos and more.