Effect of calcium nitrate on buildability and structural build-up in a set-on-demand 3D concrete printing setup

Abstract

The economic and environmental advantages of extrusion-based 3D concrete printing have made it a frontrunner in large-scale buildings. Nonetheless, this approach suffers from a contradictory rheological requirement and a high percentage of Portland cement (PC) in its mixture, which challenges its sustainability. It is claimed that the addition of supplementary cementitious materials (SCMs) to the mixture might solve this issue.
Fly ash (FA), silica fume (SF), Ground Granulated Blast furnace Slag (GGBS), limestone, and calcined clay are examples of such materials. However, the production of certain of these SCMs, such as FA, SF, and GGBS, is restricted, while others, such as limestone and calcined clay, are plentiful. To address these challenges, the purpose of this research was to investigate two developed mixtures: limestone calcined clay cement (LC3-based) and limestone slag cement (slag-based) activated with limestone-based accelerator slurry (with calcium nitrate as an accelerator).
The first phase of this work was the formulation of flowable and pumpable mixtures. The flowability of two cementitious materials (LC3 and slag-based) and limestone-based accelerator slurry was evaluated. For each mixture, the optimum superplasticizer/ accelerator dose was determined to ensure optimum flowability and pumpability. The optimal dose of superplasticizer for the LC3- and slag-based mixtures, as determined by flowability, pumping, and flow curve tests, was 0.6% and 0.3% of the binder’s mass, respectively. The recommended Ca(NO3)2 dose for limestone-based accelerator slurry was 7% of the cement weight.
Part two of this research looked at how combining limestone-based accelerator slurry with cementitious materials affected the mixture’s fresh qualities. Here, the initial setting time and buildability of the formulated mixes were investigated. At last, a printable mixture of LC3 was developed containing just 275 kg/m3 of PC with compressive strength of more than 30 MPa at 28 days of curing.
The third section of this research was devoted to material properties. Here, the development of the mixture’s compressive strength, heat evolution, and hydration product was investigated. The compressive strength development of all mixtures was assessed at 7 and 28 days of curing. In general, the compressive strength of LC3-based mixtures with various accelerator dosages was greater than that of slag-based mixtures with the same accelerator content.
Isothermal calorimetry was employed to investigate the hydration of the mixtures. The
findings for LC3- and slag-based mixtures demonstrated that a larger calcium nitrate dose significantly accelerated the hydration of the fresh mixtures. This acceleration was shown by a quicker induction time, a shifted primary hydration peak to an earlier age of hydration, increased intensity of the primary hydration peak, and greater cumulative heat. Within the first 7 days, LC3-based mixes had a greater cumulative heat evolution than slag-based mixtures.Thermogravimetric analysis (TGA) was used to analyze the amount of calcium hydroxide and hydration water in the investigated mixes at various curing periods, including 1h, 4h, and 168h. In general, the results of the TGA test and isothermal calorimetry were comparable. The final objective of this study was to examine the adaptability of the developed mixture to an actual printing structure. So, the design of a cycle arch bridge in Zaanstad was explored. The results indicate that the developed mixture will be helpful for utilizing in the 3DCP method since the mixture showed encouraging buildability and sustainability behavior.