Supplementary Cementitious Materials

Supplementary cementitious materials, usually shortened to SCMs, are mineral powders used in place of part of the Portland clinker in a mix. Some of them react hydraulically or pozzolanically and contribute directly to the binding phases; others are close to inert and work mainly by filling and packing. They are the main practical route to lowering the clinker content of a mortar, and in 3D concrete printing they are also a tuning instrument, because every one of them changes water demand, flow, stiffening rate and admixture response in a way that the printer feels immediately.

What it is / Why it matters

Siliceous fly ash (EN 450-1) is the fine residue captured from the flue gas of coal-fired boilers. It is a pozzolan: its glassy aluminosilicate phases react with the calcium hydroxide — portlandite — released by clinker hydration to form additional calcium silicate hydrate. Its particles are largely spherical, which gives the well-known "ball bearing" effect: at equal water content the mix flows more easily. The reaction is slow, so early strength drops and strength continues to climb well past 28 days. Its availability is declining across Europe as coal plants close, and quality varies between sources.

Ground granulated blast-furnace slag (EN 15167-1) is rapidly quenched iron-making slag, ground to cement fineness. It is latent hydraulic: it hydrates on its own once activated by the alkalinity of the pore solution, so high replacement levels are chemically possible. Slag mixes set and gain early strength more slowly, particularly in cool conditions, but develop a denser, finer pore structure with better resistance to chloride ingress and sulfate attack. Supply follows primary steelmaking.

Silica fume (EN 13263-1) is a condensed by-product of silicon and ferrosilicon production, with particles roughly two orders of magnitude finer than cement. It is strongly pozzolanic and also acts as a filler in the interfacial zone, densifying the paste. The cost is rheological: it sharply raises water demand, plastic viscosity and thixotropy, so even a small addition changes pumping pressure and the feel of the fresh mortar.

Limestone powder is primarily a filler that improves particle packing and provides nucleation sites for clinker hydration. Its chemical contribution is real but limited: carbonate reacts with the aluminate phases to form carboaluminates, which stabilise ettringite and add a modest amount of solid volume.

Calcined clay is kaolinitic clay heated to roughly the temperature range where kaolinite loses its structural water and becomes reactive metakaolin. It is a strong pozzolan, and it is complementary to limestone: the alumina it supplies reacts with carbonate to form carboaluminates, which is the synergy exploited in LC3, limestone calcined clay cement, developed and documented by the LC3 project. Suitable clays are geologically widespread, which is why LC3 is treated as a scalable option rather than a by-product-limited one. The penalties are high water demand, strong admixture adsorption, and a colour shift towards red or pink depending on the iron content of the clay.

Natural pozzolans — volcanic ashes, tuffs, diatomaceous earths — behave broadly like a slower fly ash and are locally important. Rice husk ash, when burned under controlled conditions, is a highly reactive amorphous silica with a porous structure and a correspondingly high water demand. Recycled fines from crushed concrete are partly reactive, partly filler, and highly variable; they are a subject of active research rather than routine practice.

How it is measured

Reactivity is conventionally assessed with the strength activity index of EN 450-1 and EN 15167-1, measured on mortar prisms to EN 196-1: the compressive strength of a blended mortar is compared with a control at fixed ages. It is a comparative index, not a chemical measurement, and it is slow. Isothermal calorimetry tracks heat release and shows directly how the replacement shifts the induction period and the main hydration peak; it is standard laboratory practice but not a product conformity test in the same sense.

Fresh-state effects need their own measurements. Flow table to EN 1015-3 and, for more fluid mixes, EN 12350-8 give a repeatable consistency comparison before and after replacement. Vicat setting time to EN 196-3 captures the shift in setting. A rotational or vane rheometer gives yield stress and plastic viscosity, and repeated shear cycles show how thixotropy changes — the property that silica fume and calcined clay affect most. Hardened properties are checked with EN 12390-3 for compressive strength and, where durability is the concern, EN 12390-8 water penetration or chloride migration testing.

For printing specifically, much of the assessment is non-standard: uniaxial unconfined compression on fresh specimens, static yield stress growth at rest, and simple stacking tests on printed walls are research methods reported in the 3DCP literature rather than codified procedures. Whatever the method, each delivery and each source has to be qualified locally, because an SCM specification defines a class, not a fixed material.

Typical ranges

All figures above are indicative orders of magnitude drawn from standards and published studies, not measured values for any specific mix. Replacement levels that work in cast concrete are not transferable to a printable mortar without testing.

What changes it

Replacement level is only the first variable. Fineness governs water demand and early reactivity as strongly as chemistry does. Source variability is decisive for by-products: two fly ashes meeting the same class can behave differently in the same mix. Temperature amplifies the slow reaction of fly ash and slag, so a blend qualified in summer may behave badly in an unheated workshop in winter.

For printing, three coupled effects matter. Replacement changes open time and setting, and therefore the safe interval between layers. It changes structural build-up, the rate at which yield stress recovers at rest, which is what allows a printed wall to stand. And it changes superplasticiser demand: clays in particular adsorb polycarboxylate ether molecules, so a mix that reached target flow at one dosage may need a substantially higher one, or a different polymer, after a calcined clay is introduced.

Failure modes

The common failures are predictable. Excessive retardation from a high fly ash or slag content leaves the lowest layers too soft, and the object slumps or buckles during printing. Over-fluid mixes appear when the ball-bearing effect of fly ash is not compensated by reducing water, producing layers that spread and lose their edge. Viscosity spikes from silica fume or calcined clay show up as pump pressure rising, pulsing extrusion, and eventually blockage. Admixture starvation with clay-rich binders looks like an underdosed mix even though the dosage is unchanged. Batch-to-batch drift in a by-product SCM produces unexplained changes in setting or flow between deliveries. Finally, applying a prescriptive replacement rule written for cast concrete — including the EN 206 k-value approach, which was calibrated for conventional mixes — and assuming it transfers to a printable mortar is a design error rather than a material one.

Concreef context

Concreef is a Bulgarian 3D concrete printing company based in Sofia. It prints on a Crane WASP system and currently uses a commercial premix while developing its own dry mix, and it runs material trials in its Sofia workshop. SCMs are directly relevant to that development work: the binder composition of a dry mix determines its open time, its stiffening behaviour and how much superplasticiser it needs, so any change in the mineral additions has to be evaluated on the printer and not only in a mortar prism. Concreef has no published test data, does not publish formulations or dosages, and makes no claim that any mix it is developing is certified, standard-tested or commercially available.