Setting Accelerator Engineering in (2K) Two-Component 3D Concrete Nozzle Systems

Two-component 3D concrete nozzle:

Prescription Accelerator Chemistry and Dynamic Mixing Engineering

3D concrete printing technologies are often evaluated based on mechanical structure, number of axes, or machine scale. However, the real technical threshold begins at a much more critical point:

The ability to control the setting behavior of concrete at the nozzle outlet, both instantly and at high flow rates.

This article discusses why the dual-component, motorized, and dynamically mixed concrete nozzle system developed by Jet Robotik, the first company in Turkey to solve this problem with an original engineering approach and create a working system, represents an extremely challenging engineering problem in terms of chemical, mechanical, fluid mechanics, and software aspects.

What is a Shotcrete Accelerator?

Set accelerators are highly reactive chemicals used to accelerate hydration reactions in cement-based systems.

In terms of 3D concrete printing, the setting accelerator determines the following:

• How long does it take for concrete to stop flowing?

• When does the layer start supporting itself?

• Will surfaces at right or reverse angles collapse or not?

In other words, the prism accelerator is the chemical input that determines the fate of the shape .

Fresh Concrete Rheology – The Basic Framework

Fresh concrete does not behave like classic liquids. It does not flow below a certain threshold stress; it begins to flow when this threshold is exceeded. This threshold value is called the yield stress .

The behavior of fresh concrete in engineering is often described using the Bingham model .

BINGHAM RHEOLOGY MODEL

τ = τ₀ + μₚ · γ̇

Here:

• τ: shear stress

• τ₀: yield stress

• μₚ: plastic viscosity

• γ̇ : shear rate

In 3D concrete printing, the critical parameter is τ₀ (yield stress), because shape retention is directly related to the time-dependent increase in yield stress.

The Effect of Power Supply Accelerators on Rheology

When a setting accelerator is added, the yield stress of concrete increases over time. This relationship can be simplified and expressed as follows:

TIME-DEPENDENT INCREASE IN YIELD STRESS

τ₀(t) = τ₀,initial + kᵣ · α(t)

Here:

• τ₀,initial: initial yield stress

• kᵣ : reaction-rheology coefficient

• α(t): degree of hydration

This process is irreversible . Incorrect dosage, incorrect timing, or delay results in a plug-in at the nozzle.

Plug Accelerator Chemistry – What Are We Accelerating?

The main phase determining the early setting behavior is the C3A (tricalcium aluminate) phase in cement. Shotcrete-type setting accelerators directly activate this phase.

A simplified kinetic relationship expressing the rate of hydration:

HYDRATION RATE – KINETIC RELATIONSHIP

dα / dt = k₍chem₎ · C₍acc₎

This statement explains the following:

• As the accelerator concentration increases

• The rate of hydration increases.

• The reaction starts very quickly.

Therefore, a power supply accelerator requires time accuracy on the millisecond level .

Why a Two-Component System?

Why Two-component 3D concrete nozzle system?

In single-component systems, the setting accelerator is pre-added to the mortar. In this case, the accelerator ratio is constant throughout the entire structure.

SINGLE-COMPONENT STRUCTURE

C₍acc₎ = CONSTANT

This approach:

• Parametric design limits

• It cannot respond to local geometry variations.

• Increases the risk of nozzle plugging.

In a two-component system, the power supply accelerator is:

TWO-COMPONENT – DYNAMIC STRUCTURE

C₍acc₎ = C₍acc₎( x , t )

So the amount of accelerator is:

• Location dependent

• Time-dependent

• Related to geometry

This can be changed during printing .

This difference is the basis of true geometric freedom.

Industrial Scale – The Reality of Fluid Mechanics

In Jet Robotic systems, the main mortar flow rate is on an industrial scale.

MAIN MORTAR FLOW RATE

Q₍m₎ = 10 L/min

Concrete at this flow rate:

• It behaves non-Newtonian.

• Yield stress and viscosity change continuously.

Average flow rate:

v = Q₍m₎ / A

The Reynolds number is:

Re = ρ · v · D / μ₍app₎

Bingham apparent viscosity for fluid:

μ₍app₎ = μₚ + τ₀ / γ̇

Therefore, the flow regime is not constant, and the mixture does not stabilize spontaneously.

Prism Accelerator Ratio in Parametric Structures

In aggressive parametric geometries (street furniture, free forms):

Q₍a₎ = 350 – 400 ml / minQ₍m₎ = 10 L / min

This rate:

C₍acc₎ = Q₍a₎ / Q₍m₎ = 3.5% – 4.0%

At this level:

• The reaction window is measured in seconds.

• Mixing time becomes critical.

• Small inspection errors turn into major surface defects.

Building Construction: The Same System, Lower Dose

These rates are not economical at the building scale. Therefore, outlet accelerators are used at much lower flow rates.

Q₍a₎ = 15 – 35 ml / min

This too:

C₍acc₎ = 0.15% – 0.35%

But the crucial difference is this:

• The accelerator is still liquid.

• It is still delivered via the nozzle.

• It can still be increased or decreased instantly.

Software and Control – The Real Challenge

Because the chemical reaction is irreversible, the software:

• It has no tolerance for delays.

• They must intervene before a mistake occurs.

The software's task is:

• Detecting deviation

• Correcting the dosage

• Adjusting the energy of the mixture

• The goal is to suppress the risk of nozzle plugging before it occurs.

The two-component system will not work without this.

Conclusion

This dual-component, motorized, and dynamically mixed concrete nozzle system was developed by Jet Robotics.

• Checks the power outlet at the moment of pressure.

• It leaves no fault tolerance at high flow rates.

• It invalidates premix and powder additive approaches.

• It combines mechanics, chemistry, and software into a single engineering problem.

This technology:

• It cannot be improved through trial and error.

• It cannot be copied with superficial knowledge.

• It is not a simple nozzle.

This is a high-threshold and multidisciplinary field of engineering .