The effect of admixtures on the mechanical properties and durability of Fe-rich alkali-activated materials

Abstract. Alkali-activated materials (AAM) as a potential alternative to ordinary Portland cement (OPC) could decrease CO2 emissions and increase valorization of various metallurgical wastes and side streams in the construction sector. Ferich precursors are potential materials to be utilized in AAM production. Unlike usual AAM precursor materials in which the constituents and the main reaction products are known the role of iron in AAMs is not fully understood. This limits the utilization of Fe-rich precursors in AAMs since the optimal activation parameters, such as activating solution composition or curing regimes, remains unclear. The goal of this thesis is to study the effect of low molarity of NaOH solution as a sole activator and different additives on the mechanical and durability properties of alkali-activated Fe-rich slag. NaOH molarities used in this study were in a range of 1–9 M. To enhance the performance of the AAM, Ca(OH)2, CaO, MgO, and silica fume additives were used in different dosages up to 20 wt.%. Mechanical properties of the produced mortars were determined with unconfined compressive strength (UCS) test at different curing periods of 7–28 days. The durability of selected mortars was studied with free-thaw and chemical resistance tests using 3 wt.% NaCl, MgSO4, H2SO4, and HCl solutions and water as a reference medium with 14 and 28 days of treatment time. The composition and microstructure of the produced mortars and pastes were studied with XRD, FT-IR, and SEM. Environmental leaching tests were carried out to study the solubility and content of harmful substances. The results indicate that the increasing molarity of NaOH solution from 3M to 9M did not significantly increase the UCS. Ca(OH)2 and MgO additions did not enhance the mechanical performance of the AAM. In contrast, CaO and silica fume additions were found to be beneficial to the UCS development of the mortars. The addition of CaO affected the compressive strength even when using 1M NaOH, while higher molarity up to 3M was needed to incorporate silica fumed into the reaction product. The positive effect of using CaO at low molarities can be attributed to the formation of katoite observed by XRD. The higher compressive strength of the samples with silica fume is due to the more uniform microstructure observed by SEM and the higher degree of polymerization observed by FT-IR. Resistance to a salty environment was found to be good on average the UCS decreased only by 4%. Sulphate resistance of the mortars against MgSO4 caused strength loss on all mortars on average UCS loss being 15%. Both acid treatments had a significant degradation effect on the mortars UCS loss on average was 18% and 39% with H2SO4 and HCl, respectively. All of the mortars retained rigidity and were not brittle. With and without additives, regulatory leaching limits of some elements from mortars were exceeded in the environmental leaching tests. Interestingly salt and MgSO4 treatments reduced Mo leaching and acid treatments reduced V leaching below the regulatory limits. The results shed light on the alkali-activation of Fe-rich materials and the effect of mineral additives. The obtained durability results offer great insight into the possible utilization of Fe-rich materials in industrial applications. The results should help future research on this topi