Project progress
Year 2021
Objective: Determining the effect of accelerators on the hydration process, porosity, flexural and compressive strength of cement paste
Phase 2021 of the project was dedicated to the investigation of the effects introduced by an accelerator on the hydration process, porosity and strength development of cement paste. This stage represents a first step in achieving the main objective of the project, that of developing a new cement-based material, suitable in 3D printing applications. The effects introduced by the accelerator have been comparatively studied with the nuclear magnetic resonance (NMR) relaxometry technique and classical techniques commonly used by civil engineers (Vicat apparatus, mini cone slump tests, mechanical tests, X-rays, SEM). From the combined use of these techniques, a continuous acceleration of the hydration process of the cement paste was observed by the progressive addition of calcium nitrate (0-3% of the cement mass). In the case of the largest amount of accelerator (3%) used in the preparation, it was observed the disappearance of the dormancy phase and the fact that the hydration process is continuous. Unlike the acceleration of hydration introduced by the increase in temperature, in the case of accelerators the final mechanical strength is not significantly affected. Also, the pore distribution of the material is not influenced by the presence of the accelerator. Furthermore, the X-ray analysis has shown that, despite that there are hydration products that form faster in the presence of calcium nitrate, they do not affect the porosity.
Year 2022
Objective: Determining the influence of silica fume on the hydration process, porosity, flexural and compressive strength of the cement mixture
Phase 2022 of the project was dedicated to the investigation of the effects induced by the addition of silica fume ( SF) together with calcium nitrate (CN) accelerators on the hydration process, porosity, and flexural and compressive strength of cement paste. This is an important step in achieving the main objective of the project, which is to develop new cement-based materials for use in 3D printing, as silica fume is one of the key additives that, when used in conjunction with superplasticizers, favorably modify the rheological as well as the mechanical properties of cement. The effects of the additives have been studied both by special nuclear magnetic resonance (NMR) relaxometry techniques and by X-ray diffraction (XRD), optical microscopy (OM), and classical techniques commonly used by construction engineers (Vicat apparatus, mini-cone, and compressive and flexural strength tests). It was observed, by the combined use of these techniques, that the SF additive (representing 5% of the cement mass) does not interfere considerably with the acceleration of the setting and hardening process of the cement paste induced by CN (representing 2% of the cement mass). Further analysis indicates a slight decrease in mechanical performance with increasing accelerator concentration (the minimum in compressive strength was observed for samples with 2% accelerator). On the other hand, it was noted that the inclusion of SF in the composition can counteract this effect. Results are correlated with the pozzolanic effect revealed by the X-ray diffraction technique and with a 5% increase in solid mass percentage respectively with variations in the proportion of capillary pores.
Objective: Determining the influence of silica fume on the hydration process, porosity, flexural and compressive strength of the cement mixture
Phase 2022 of the project was dedicated to the investigation of the effects induced by the addition of silica fume ( SF) together with calcium nitrate (CN) accelerators on the hydration process, porosity, and flexural and compressive strength of cement paste. This is an important step in achieving the main objective of the project, which is to develop new cement-based materials for use in 3D printing, as silica fume is one of the key additives that, when used in conjunction with superplasticizers, favorably modify the rheological as well as the mechanical properties of cement. The effects of the additives have been studied both by special nuclear magnetic resonance (NMR) relaxometry techniques and by X-ray diffraction (XRD), optical microscopy (OM), and classical techniques commonly used by construction engineers (Vicat apparatus, mini-cone, and compressive and flexural strength tests). It was observed, by the combined use of these techniques, that the SF additive (representing 5% of the cement mass) does not interfere considerably with the acceleration of the setting and hardening process of the cement paste induced by CN (representing 2% of the cement mass). Further analysis indicates a slight decrease in mechanical performance with increasing accelerator concentration (the minimum in compressive strength was observed for samples with 2% accelerator). On the other hand, it was noted that the inclusion of SF in the composition can counteract this effect. Results are correlated with the pozzolanic effect revealed by the X-ray diffraction technique and with a 5% increase in solid mass percentage respectively with variations in the proportion of capillary pores.
Year 2023
Objective: Determining the influence of clay on the sample characteristics and formulation of an optimal combination of ingredients for a 3D printing mortar
Phase 2023 of the project was dedicated to the investigation of the effects induced by the addition of clay powders (metakaolin, MK) together with other additives (calcium nitrate - CN, silica fume - SF, superplasticizer - SR) and the formulation of an optimal composition of 3D printable mortars. Initially, cement-MK-water compositions were investigated, then a comparative study of the effects induced by MK and SF respectively on the properties of cement-based pastes or mortars having CN and SR additives included in the composition was initiated. The composites were investigated both by special nuclear magnetic resonance (NMR) relaxometry techniques and by X-ray diffraction (XRD), Vicat tests, workability tests and mechanical strength tests in compression and bending. Additional studies such as extrudability and constructability tests and UPV concrete testing were carried out. This step was essential to achieve the main objectives of the project: (I) the development of a new cement-based material usable in 3D printing and (II) the establishment of a method for the rapid determination of the "optimal deposition time" (open time) for cement-based materials used in 3D printing applications. It was observed, that the MK additive alters the paste extrusion behaviour due to adsorption of water on the surface of the clay lamellae and delays the cement curing process. Printable mortars with improved mechanical performance were obtained by substituting 5% cement with SF, respectively adding 5% MK/SF mixture in 1/1 mass ratio. The optimum deposition time for the mortars was between 25-60 min depending on the content of SF, accelerator and superplasticizer. The acceleration effect in the presence of calcium nitrate and superplasticizer was confirmed, which can ensure both a long deposition time and a high structuring speed. Using NMR relaxometry techniques it was possible to follow the hydration stages of cements and mortars, allowing the determination of curing time, latent stage with increased accuracy. The latest results indicate that NMR techniques can even provide information on the speed of mortar structuring and the time in which mortars can be extruded. The results highlight NMR relaxometry techniques as techniques of great interest for the development of composites for 3D printing applications.