The Building Conservation Directory 2022

52 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 2 C AT H E D R A L C O MM U N I C AT I O N S MORTAR MIX RATIOS A practical guide to making repeatable and consistent mortars ROZ ARTIS O VER A PERIOD of nearly 30 years, the Scottish Lime Centre has observed that the most common occurrence of failed mortars is down to the incorrect proportioning of binders and sand. This is usually due to insufficient binder in a mortar mix and/or variations between batches causing problems of consistency in performance. It is therefore important to understand what is meant by the specification and how we can ensure the right ratio of binder to sand. THE MORTAR MIX RATIO NOTATION Common mortar specifications include 1:3, 1:2:9 or 1:1:6 mixes. The first one or two digits refer to the binder content (lime, cement or both) and the last digit always refers to the filler, which is usually sand. So a 1:3 mix could mean one part by volume of lime or cement to three parts by volume of sand. A 1:2:9 could mean one part by volume of cement (the gauging material) to two parts by volume lime, to nine parts by volume of sand. In all three mixes the ratio of binder (lime and/or cement) to sand is one to three. The aim is to coat all the sand grains with binder to make a complete mortar – but why the one to three ratio? THE VOID RATIO It may seem surprising but a 1:3 mix when prepared as a mortar will still have the same volume as the three volumes of sand you started with (the only science where one and three doesn’t make four)! This is because there are voids or spaces between the sand grains, which when combined with just the right amount of binder will all be filled. The mix ratios quoted above are based on commonly available building sands which presume they have void ratios or spaces equating to around a third of their volume, hence the one part by volume of binder to three parts by volume of sand, where a third of the sand volume is filled by binder. Unfortunately, not all building sands are so predictable, particularly if sand and other aggregates have been chosen to match historic work. It’s therefore important to know the void ratio of your sand so that you use sufficient binder to make a complete mortar. If you don’t you might have an expensive failure on your hands. It’s easy to measure the void ratio of sand on site. Just use a clear straight-sided jam jar of a known volume (at least 100ml). Using a measuring cylinder, pour into the jar exactly 100ml of bone-dry sand (dry it out in stages in a microwave oven) and tamp down lightly. Now fill the measuring cylinder to the 100ml mark with water and slowly pour the water into the sand until the water level is equal with the top of the sand. This needs to be done in stages to allow water to percolate into the voids or spaces. The volume of water required to reach the top of the sand is the amount missing from the measuring cylinder, and is equivalent to the void ratio in percentage terms. So, if 25ml of water is needed to reach the top surface of the sand, then the void ratio is 25 per cent. In some instances void ratios can exceed 50 per cent and binder content should be increased accordingly. MIXING BY VOLUME OR WEIGHT? When quoting mix proportions, the ratios are given nominally by volume. On a practical basis though, measuring hydraulic limes and other powder binders by volume is unlikely to be accurate as their volume changes substantially from one type of binder to another, the extent to which they have been tamped down, and according to how long they have settled in transit. Relying on volume alone can therefore lead to the production of inconsistent mortars of differing strengths and performance qualities, and sometimes failure. For this reason dry hydrate powders should be measured by weight in order to be accurate and consistent and rule out at least one of the many causes of mortar failure. Volume differences are particularly obvious when 25kg bags of different binders are lined up side by side, with the bag of ordinary Portland cement (OPC) being the smallest (with the least volume). This relates to the size of the particles, their surface area and the number of particles in a material. For example, on an area 1cm square, a single layer of particles could contain 3,600 particles of standard OPC or 13,000 particles of a particular NHL 2 natural hydraulic lime. In short, cement particles have a greater surface area and are heavier than those of Weighing out dry hydrate (powdered) binders is most accurately undertaken by weight not volume (All photos: ©Roz Artis, Scottish Lime Centre Trust