Selective Paste Intrusion: Stability of Cement Paste Mixtures Towards Changing Ambient Temperature

The Selective Paste Intrusion (SPI) is an additive manufacturing method in which aggregates in a particle bed are selectively bonded layer-by-layer with cement paste to build complex, free-formed concrete elements. To ensure both sufficient layer bonding and shape accuracy, the adjusted paste yield stress (τ₀ ) needs to be almost constant during the entire printing period. The τ₀ depends beside others on the ambient air temperature. Temperature changes, as common in the precast plants, are transferred to the raw materials, mixing tools and printer peripherals. Thus, the fresh cement paste will vary in temperature. A change especially during the production process can either lead to insufficient layer bonding and thus strength of the built component or excessive spreading of the cement paste in the particle bed and thus poor shape accuracy. We therefore investigate the stability of the paste τ₀ at temperatures between 16 ℃ and 26 ℃ in steps of 2 ℃ each, starting at 20 ℃, which is the reference temperature. We found a continuously increasing τ₀ for increasing temperatures. This worsens the printing process i.e. the penetration depth of the paste, shown in simulations based on Darcy’s law. To ensure consistent component quality at varying ambient temperatures, the worsened penetration depth needs to be compensated by adjustments in either the mixture composition or the process parameters, e.g. velocity of the print head.