Mortar ! Behind this generic term, hide a large number of product variants, intended for building and public works applications. However, in absolute terms, nothing is simpler than making mortar. You need to knead a binder (usually cement or lime), sableof the’eau. The possible addition of one or more adjuvants gives the mortar special qualities. You thus obtain an adhesive paste, flexible and malleable which hardens in a few hours.
Choice of binder according to the application
Limes
Limes, are traditional natural binders. Widely used, still today, for the renovation of old buildings, for which they often constitute the original binder. They remain flexible after setting and show little shrinkage on drying, which accompanies (to a certain extent) the natural movements of the masonry structures. These qualities minimize the risk of cracking of the coatings. Lime mortars are impermeable to runoff, but permeable to water vapour, a characteristic allowing gas exchange between the ambient air of the building and the atmosphere. These walls are described as “breathable”, a particularity taken advantage of to fight against the proliferation of mold and fungi and which guarantees the longevity of the structures, especially in soft stones. Limes have, moreover, excellent capacities to regulate humidity and reinforce the healthiness of buildings by their antiseptic and antiparasitic power.
Three categories of lime share the market:
1 – Aerial lime or fat lime, has long been made in an artisanal way from pure natural limestone. It hardens by carbonation (absorption of carbon dioxide present in the ambient air). Associated today with hydraulic binders and/or plant aggregates, it contributes to the formulation of innovative thermal and sound insulation.
- Main qualities : white in colour, the aerial lime takes on a velvety appearance when it dries and is tinted, in the mass, with the color of the local sands or dyes. It is a healthy material for decoration (paints, whitewashes, etc.), interior coatings or the grouting of exposed stones. Its appearance acquires a patina over time.
- Main flaws : limited mechanical resistance, long setting time (several days) and drying time (several months).
2 – Hydraulic limes are made from a mixture of limestone and marl or clay (10 to 20%), with added additives. Hardening takes place in 2 stages: hydraulic jack rapid, by chemical reaction with the mixing water, then carbonation, spread over several weeks. White or grey, hydraulic limes are suitable for wet environments. There are 4 categories:
- Lime NHL 2 (takes between 10 and 20 days/resistance of 20 to 40 kg/cm²), is used for its flexibility, on walls made of soft materials (stone, raw earth, cob, etc.).
- Lime NHL 3.5 (setting time between 4 and 8 days/strength of 35 to 100 kg/cm²), is used for bonding most traditional or modern construction materials and for interior and exterior coatings.
- NHL5 lime (setting time between 5 hours and 3 days/resistance from 50 to 150 kg/cm²), is a pure lime whose resistance and drying time are close to those of cement, while letting the support breathe. It is used in all cement applications, including screeds.
- NHL-Z Lime is a bastard lime (mixed with cement), with high resistance, usable on all new baked supports and on hard stones. Its breathable properties are anecdotal.
Most of the commercial coatings are formulated with hydraulic lime, more or less added air lime and strongly admixed.
3 – Natural lime is a hydraulic lime without the addition of toxic additives. healthy material, par excellence, it is used pure, for the roughcasting of stones and interior or exterior coatings. It can be mixed with cement.
Cement
Cement is a hydraulic mineral binder that stands out clearly from lime, by its very high compressive strength and its quick grip. Very little permeable to air and water, cement mortars are particularly suitable:
- to secure or coat hard and inert materials (bricks, tiles, concrete blocks, etc.);
- to make hard screeds, especially in humid environments;
- to seal containers (tanks, reservoirs, swimming pools, etc.).
Cement mortars are strictly contraindicated for roughing, grouting or coating soft materials (limestone, raw earth, cob, aerated concrete, etc.). Brittle and impermeable to water vapour, they tend to crack and detach from their supports. Infiltration water or water coming from capillary rise remains blocked between the body of the wall and the plaster, due to insufficient gas exchange, the wall takes on water, becomes sensitive to frost, saltpetre and parasites and mushrooms are growing: the slow process of destruction has begun! .
There are 4 main categories of cement:
- Portland CEM I, fast-setting, can be used in cold weather;
- the Portland compound CEM II, is intended for general masonry work;
- CEM III blast furnace cements, suitable for corrosive environments;
- white cements, for decorative works;
- masonry cements, with high plasticity, for less resistant structures and the assembly of terracotta and concrete bricks.
All of these cements can be used to make mortars, but the most universal are Portland cements of the CEM I or CEM II categories of class 32.5 or 42.5. The labeling of these products includes a letter specifying the rate of hardening : N = normal ; R = fast ; L = lent.
Mortar aggregates
For common mortars, we mainly use construction sand (river or quarry), clean, without vegetable or organic soiling. The ideal grain size is between 0 and 2 millimeters in diameter (0/2), for roughing and rendering, and 0/4 to 0/6, for standard masonry work.
Dosages, of ingredients, for the most common applications
The following dosages are given as an indication, for dry sand, according to the recommendations of the DTU. Whatever the binder load, it takes ±1,600 kg of sand to make 1 m3 of fresh mortar, and 180 liters of water (to be adjusted according to the humidity of the sand and the desired viscosity). Mortar water should be clean and soft (non-saline).
Dosage value usual binders, for 1m3 mortar costs :
1 – Gobetis or primer layer, thickness ±4 mm (consistency of viscous soup):
- 450 kg of cement or lime/m3. Suitable for new and stable surfaces;
- 500 kg hydraulic lime/m3for renovation or on soft materials (limestone, cellular concrete, etc.);
2 – Coating bodythickness ±12 to 15 mm:
- 450 kg of cement or lime/m3, (in accordance with the nature of the gobetis);
- 300 kg of lime + 150 kg of cement/m3 (bastard mortar on cement render);
3 – Finishing plasterthickness ±8 mm:
- 300 kg of cement or lime/m3 (in accordance with the binder of the gobetis);
- Specialized industrial coatings ready to use.
Use : it is possible to lay lime on a cement-rendered surface, but not the other way around.
4 – 300 kg of cement/m3for assembling concrete blocks or terracotta bricks;
5 – 350 kg of cement/m3for floating screeds and all other common masonry work;
6 – 400 kg lime/m3for routine renovation work or the assembly and jointing of soft stones;
7 – 200 kg cement + 200 kg lime/m3for bastard mortars intended for the lining of all materials and the sealing of terracotta roof tiles and accessories;
8 – 600 kg of “R” class quick cement/m3adjuvanted with a non-chlorinated expansive product, for sealing and consolidating unstable elements.
The grouting of terracotta and tiles can be carried out without adding sand (1 volume of cement, for 0.5 volume of water).
Preparation of mortars
Whatever the binder, mixing the mortar is carried out according to the same ritual:
Manual method:
- pour in a container, trough or flat, hard and clean surface, the sablethen the cement ;
- stir with a trowel or shovel until a mixture of uniform color ;
- dig one crater sufficient to receive the volume of water;
- pour 2/3 the amount of water expected;
- PUSH gradually, in the water edges from the crater to moisten the mixture;
- knead adding water until desired consistency.
Mechanical methods:
In the cement mixer, tank rotating, at 45°:
- pour, in order, 1/2 the amount of water planned, the dose of sablethen the dose of binder ;
- let it spin 5 minutes ;
- add water until you get a paste desired viscosity.
- Empty into the wheelbarrow;
- Clean the tank immediately.
Remarks: any adjuvants must be diluted previously, in the mixing water. The fibers, metallic, organic or mineral, must be roughly distributed in the sand. The industrial adhesive mortars are prepared for kneaderat low speed.