Pilar Padilla-Encinas

This study examines magnesium potassium phosphate cements (MKPCs) modified with industrial wastes for extrusion-based 3D concrete printing, evaluating the rheological properties (workability, setting time), mechanical performance and printability of formulations incorporating secondary materials: Mg dross waste (up to 20 wt.%, replacing MgO), calcined sewage sludge (up to 10 wt.%, replacing KH₂PO₄), alternative fillers such as glass from municipal solid waste glass and from construction and demolition waste and ground blast furnace slag, benchmarked against volcanic ash. The baseline MKPC exhibited initial/final setting times of 34/109 min, good workability and compressive strengths of 29 MPa (1 day)/28 MPa (28 days). Optimal low-waste mixes (e.g., using municipal glass or 20 wt.% Mg dross) shortened the initial setting to 19–25 min (decreasing 24–42%), reduced the slump by 9–18% yet remained printable at laboratory-scale and achieved 1-day strengths >23 MPa/28-day >31 MPa (comparable or superior). Glass from municipal waste proved most promising, due to superior workability, lighter aesthetics and strength gains, supporting circular economy goals while substantially reducing material costs; higher waste levels compromised fluidity and buildability. Mineralogical analyses confirmed K-struvite formation alongside residual periclase, validating these formulations for upscaling sustainable 3D printing.

Cementation using Ordinary Portland Cement (OPC) remains the standard method for conditioning low- and intermediate-level radioactive waste, including Spent Ion Exchange Resins (SIERs). This work presents an integrated strategy involving thermal pretreatment to minimize waste volume and eliminate organic constituents, followed by encapsulation within three distinct binders: CEM I, CEM III, and a novel one-part geopolymer. The one-part geopolymer system represents a significant operational innovation, enabling safe and simple “just-add-water” processing and avoiding the need to handle alkaline solutions. The proposed geopolymer, synthesized from metakaolin, blast furnace slag, and solid sodium silicate, was systematically benchmarked against conventional OPC matrices (CEM I, CEM III) by assessing their capacity to immobilize thermally treated SIER ashes under accelerated leaching conditions. For benchmarking, leaching indices for Cs and Sr were determined following the ANSI/ANS 16.9 standard protocol in three representative environments simulating operational and long-term repository scenarios, providing a quantitative evaluation of radionuclide retention and matrix durability. Results indicate that the one-part geopolymer improved leaching indices for Cs and Sr compared to both cementitious binders and complied with regulatory waste acceptance criteria. The comparative results highlight the potential of geopolymer technology to increase waste loading efficiencies and improve long-term safety, establishing a robust framework for future radioactive waste management approaches.

This work shows how the presence of calcium carbonate and sodium carbonate (5% and 20%) affects the hydration of a commercial calcium sulfoaluminate clinker (KCSA). For this study, water-hydrated pastes were prepared and the mechanical compressive strength and hydration rate were determined. The hydration products were characterised by XRD, DTA/TG, FTIR and SEM. The incorporation of CaCO₃ can have a beneficial effect on the development of the mechanical strength of KCSA, especially at 90 days. It does not significantly alter the hydration kinetics and the hydration products formed are mainly ettringite and AH₃. However, sodium carbonate has a detrimental effect, slowing down the hydration kinetics and reducing the development of mechanical strength, especially at early ages. The 20% Na₂CO₃ favours the formation of calcium aluminate, gaylusite and thenardite over ettringite. These phases are metastable in the presence of sodium and decompose to form calcite, alumina gel and a large amount of thenardite, which leaches out as efflorescence, causing microcracks and loss of strength in the material.

The use of the fillers wollastonite and volcanic ash for the formulation of magnesium phosphate cements prepared at magnesium-to-phosphate molar ratios of 2, 3 and 4 has been investigated, with the objective of evaluating these formulations for the encapsulation of aluminium radioactive waste. The workability, mechanical strength, dimensional stability, pH, chemical composition and mineralogical properties of cement pastes and mortars were examined. All cement pastes presented fast setting, and the workability was only good at 3 and 4 M. The cement mortars presented high compressive strength and dimensional stability. K-struvite was confirmed as the sole reaction product of the reaction for all formulations. The pH of the cement pastes, measured in suspensions, achieved values in the range of 7.8 to 9.5 after the first days of setting, exceeding pH 8.5 for the 2 and 3 M formulations. pH values below 8.5 are theoretically preferred to avoid potential aluminium corrosion. Both fillers presented adequate characteristics (good workability, chemical compatibility) to be used in the formulation of magnesium phosphate cements. The increasing magnesium-to-phosphate molar ratio prevented unwanted efflorescence and increased the mechanical stability of the cement.

This paper examined how the amount (5% or 20%) and type (CaSO₄ or Na₂SO₄) of sulphate salt affect the hydration of calcium sulphoaluminate clinker (KCSA). The mechanical behavior of the pastes was determined at 1, 3, 28, and 90 days, the heat flow and total heat were measured with isothermal conduction calorimetry, and the reaction products were characterized using X-ray diffraction (XRD), differential thermal analysis/thermogravimetry (DTA/TG) and scanning electron microscopy (SEM). The results obtained indicated that both the amount of sulphate salt (5% or 20%) and its type (CaSO₄ or Na₂SO₄) affect the hydration kinetics, type of reaction products formed, and development of mechanical strength. The incorporation of CaSO₄ has a positive effect on the development of the mechanical strength of KCSA. However, Na₂SO₄ can also produce adverse side effects at older ages. The presence of Na₂SO₄ increases pH values, which partly destabilizes the ettringite formed, thereby favoring carbonation and thenardite precipitation, which can cause the specimens to crack and break.