The Rheological and Shrinkage Behaviour of Fe-rich Slag-based Binders

The global anthropogenic CO2 emission, which is considered the main contributor to global warming, is exponentially increasing yearly. The proportion of CO2 emission from construction is around 8-9% and is expected to become more significant in future, as the demand for raw materials in construction, such as ordinary Portland cement (OPC), increases every year. The production of OPC is environmentally unfriendly and the availability of supplementary cementitious materials is limited. Therefore, it is interesting to address the vast amount of residues from the non-ferrous metallurgical industry, i.e. Fe-rich slags, which are not yet valorised. These slags are interesting to be used as an alternative precursor for binder systems to obtain more environmentally friendly construction materials as secondary resources are incorporated. The Fe-rich slag can dissolve upon alkali activation and can form a binder. The binder formed of solely alkali-activated Fe-rich slag is called an inorganic polymer (IP) binder, while a binder formed from an alkali-activated blend of mainly Fe-rich slag and a small amount of OPC is called a hybrid. Both binders are suitable for construction as the IP is fire resistant and can reach high strength properties, while the hybrid can be easily adjusted and is easier to implement in conventional production processes. These binders can qualify for construction when their rheology is thoroughly investigated. Identifying and tailoring the rheology is important for production processes and is so far not investigated yet for Fe-rich slag mixtures. This research identified that the rheological behaviour of the IP is strongly driven by the solid volume fraction, the interaction forces between the slag particles and the chemistry of the activating solution. Apart from the fresh properties, the volumetric stability of the binder is another important aspect, which is challenging to control in alkali-activated materials. A volumetric unstable binder can lead to the formation of cracks and warping, which can harm durability and can pose difficulties at installation. This research identified that IPs are volumetric unstable (5.1 mm/m), initiated by the capillary pore pressure during drying. Various shrinkage mitigation strategies were applied on IPs, such as the introduction of reactive and non-reactive shrinkage reducing agents. A volumetric stable IP was reached (1.4 mm/m) without compromising compressive strength (67 MPa) when introducing 2-methyl-2.4-pentanediol and by applying heat curing. Hybrid binders are more functional compared to IPs because superplasticizers can be effectively used in hybrids, suggesting that these binders can be suitable for high-performance materials. However, literature does not report a hybrid binder containing Fe-rich slag. Therefore, current research developed a hybrid binder, on a performance-based approach, with optimal proportions of Fe-rich slag (80 wt%), OPC (10 wt%), fine limestone (8 wt%), superplasticizer and bassanite (0.3 wt%). This hybrid binder formulation was designed to be self-compacting and to reach a high early strength (38 MPa in 2 days). Based on this hybrid formulation, another type of high-performance material is developed, i.e. a 3D-printable mortar. Extrusion-based 3D printing is an emerging technology that, once implemented in construction, can automate the so far low innovative construction industry. This research is novel as up to this moment no Fe-rich slag hybrid mortar is developed and is printed on a semi-large scale. In order to obtain a 3D-printable hybrid mortar certain print criteria need to be reached, such as pumpability, extrudability and buildability. The hybrid mortar was developed in a performance-based approach and a 3D-printable OPC-based mortar was used to identify the print criteria. First, the particle packing was improved by introducing, with the appropriate proportion, silica fume, fly ash, fine limestone and quartz sand to the paste. Second, the effect of each raw material on the reactivity, the stiffness development and the rheology of the hybrid mortar was investigated. Finally, the amount of superplasticizer was decreased and microfibers were added, according to a specific amount, to obtain a hybrid mortar that exhibited an accelerated early-age stiffness development, high yield stress and strong shear-thinning behaviour. This research proved that the Fe-rich slag reacts and contributes to the strength of the hybrid binder, of which the latter is a complex system of different chemistries depending on the nearby precursor source. The effect of each admixture on the shrinkage, strength and creep of the hybrid binder was investigated, and indicated that shrinkage and creep values were low and not affected, while the strength is significantly affected by the admixture proportion. Several structures were printed from the developed hybrid mortar, indicating that the printed material was pumpable, extrudable and reached excellent buildability, confirming that the preliminary tests were effective in developing a printable mortar. Not only was a complete analysis of the fresh state properties provided in this research, but the printed material was also subjected to an exhaustive investigation of its hardened properties. Micro and macro porosity analysis was performed and indicated an increase in porosity due to air entrainment during pumping. The porosity of the interlayers was reduced due to compaction. The compressive and flexural strength of the printed mortar, up to 80 and 9.3 MPa respectively, outperformed the load-bearing criteria. The durability of the printed material was assessed and showed that the freeze-thaw resistance was poor during the first freeze-thaw cycle, but reached at a later cycle an outstanding resistance. The leached concentrations fell within the limits of Flemish regulation (VLAREMA). Last but not least, a cost and environmental analysis was made, indicating that the printed hybrid mortar was cheaper and three times more CO2-friendly compared to an OPC-based 3D-printable mortar. This research proved that Fe-rich slag from the non-ferrous metallurgical industry can be used as a precursor to form a binder (with or without OPC), in which the rheology of the binder can be tailored according to the application. The binder is volumetric stable and a minimum in strength can be guaranteed. A self-levelling high early-strength mortar and a 3D-printable mortar were successfully developed from Fe-rich slags and its hardened properties were identified. This research can increase the chance of success to implement these Fe-rich slags as an alternative precursor for binders in construction. In this way, the ultimate goal can be reached, which is reducing the environmental impact of construction and transforming society where a circular economy is embraced.

Jaar van publicatie:2021

Toegankelijkheid:Closed