Construction & Building Materials

Factory mortar, render, autoclaved aerated concrete and calcium silicate brick look like four separate products from the outside. In process terms they are four lines asking the same three questions at different temperatures and pressures: in what form does calcium enter the system, when and in which phase does it react with silica, and how much of the mix water is chemically bound versus lost to the substrate and the air. Change the answers and you change the product.

The role of calcium compounds on these lines does not fit under one heading. Natural calcium carbonate looks like an inert filler in a dry mix, yet it packs into the voids between cement grains, lowers water demand and supplies nucleation surface for C-S-H. Calcined, the same raw material becomes the heat source that lifts an AAC cake to 70 °C; hydrated, it becomes the plate-like portlandite crystals that decide how a render holds its water.

On site, things rarely go wrong because of compressive strength. The cube cast from the mould reaches its class at 28 days without difficulty, while on the wall the render cracks, the mortar fails to bond and the AAC cake collapses before wire cutting. This page is not about the strength table; it is about the balance of water, particle size and timing behind it.

What decides how a mortar behaves on the wall is, first of all, the suction of the substrate. Aerated blocks, perforated brick and dry pumice block pull water out of the mortar within seconds. At the interface the water/cement ratio collapses, hydration stops in that thin layer and adhesive strength (EN 1015-12) falls away. Meanwhile the specimen cast from the same mortar cures with enough water and reaches its compressive class (EN 1015-11) comfortably. The measurement is correct; what misleads is that it does not represent the field problem.

The second constraint is sand grading. In washed crushed sand the fraction below 0.063 mm leaves with the wash water, and what remains is a skeleton full of voids that bleeds. This is what the mason means when he calls a mortar harsh. The reflex is to add cement: workability improves a little, but elastic modulus and shrinkage rise and the render shows hairline cracking in the first week. What is missing is not binder content but the fine fraction.

Third is the rate limit on carbonation. The reaction Ca(OH)₂ + CO₂ → CaCO₃ only proceeds where a thin water film sits on the pore wall. Below 40 % relative humidity there is no film for CO₂ to dissolve into; above 90 % the pores are water-filled and CO₂ diffusion is some four orders of magnitude slower than in the gas phase. In a closed, unheated space in winter a lime render can stay chalky for weeks — the problem is the curing environment, not the lime class.

Fourth is synchronisation on autoclaved lines. In an AAC cake the slaking exotherm of quicklime and the hydrogen evolution from aluminium powder must fall in the same time window. A very reactive lime (t60 below one minute) stiffens the mix early: gas bubbles are trapped, the cake never completes its rise and tears at cutting. A very slow lime releases its heat after the rise is over, so the cake enters the wire cutter before reaching green strength and deforms. In both cases the available CaO figure from the laboratory looks impeccable.

In factory mortar and render the decisive product is hydrated lime. A CL 90-S grade to EN 459-1 holds mix water physically, thanks to its high specific surface (typically 15–25 m²/g) and the plate-like morphology of portlandite. Water retention measured to EN 1015-8 sits at 75–82 % for a plain cement mortar and rises above 90 % once lime is added at 10–25 % of cement mass. In practice that means hydration does not stall at the interface on an absorbent substrate.

Lime's second contribution is rheological and mechanical, and it works against strength. A lime-bearing mortar has a low elastic modulus; it accommodates the moisture- and temperature-driven movement difference with the substrate without accumulating stress. Its vapour diffusion resistance is low (µ ≈ 6–12, EN 1015-19), so it does not seal the wall. This is why in EN 998-1 external renders the target is not maximum compressive strength but staying in the CS II–CS III band while meeting adhesion and the capillary absorption class (W1/W2, EN 1015-18).

On the filler side calcium carbonate does three jobs at once. A carbonate with D50 in the 3–20 µm range fills the voids left by cement and sand, raises packing density and lowers the water needed for the same consistence. Second, the particles provide surface for C-S-H nucleation, so early strength develops faster. Third, the effect is chemical: carbonate forms monocarboaluminate with C₃A, stabilises ettringite, delays its conversion to monosulfate and steadies volume behaviour. Typical use in dry mixes is 10–40 % of the dry blend.

In AAC the main calcium source is quicklime, and the selection criterion is reactivity rather than purity. Used at 15–25 % on a dry-solids basis, the lime should show a t60 of 1.5–4.0 minutes to EN 459-2, available CaO ≥ 90 % and CO₂ ≤ 2 %. Slaking releases roughly 1160 kJ per kilogram of CaO; that heat lifts the casting bed from 38–45 °C to 70–80 °C and directly governs the gassing rate of the aluminium. In the autoclave, at 180–195 °C and 10–12 bar of saturated steam, the CaO–SiO₂–H₂O system converts to tobermorite; holding the Ca/Si ratio at 0.8–1.0 decides the pore structure and the final strength.

For calcium silicate brick the recipe is leaner: 88–92 % silica sand, 6–10 % quicklime and water. The mix slakes in a reactor silo for 1–4 hours, is pressed at 20–60 MPa and held in the autoclave at 200–203 °C and 16 bar for 4–8 hours (EN 771-2). Here sand grading matters as much as the binder: the fine fraction supplies the reactive silica surface, the coarse fraction gives green strength after pressing. The same grading logic governs aggregate selection and is a close relative of the filler discussion in road infrastructure and asphalt.