DOI: 10.20350/digitalcsic/17477
title: Dataset for the paper “Criado, M., Puentes, J., Torres, E., Ramallo, L., Juan Ruiz, J., Pachón-Montaño, A. 2025. Immobilisation of decontamination sludge in low-carbon cement: a study of early-age physico-mechanical behaviour of two alkali-activated slag-based waste forms.”

description: The fresh properties of the three formulations: reference cement (R) and sodium carbonate-activated slag (BFS-C) and sodium silicate-activated slag (BFS-S) pastes with and without the decontamination sludge studied were evaluated by means of mini-slump and Vicat needle tests. The mini-slump test is performed to study the variation in flowability of the cementitious matrix with time. This test consists of measuring the slump diameter of the paste with a calliper. For this, a truncated cone mould and a methyl methacrylate board are used, starting to measure from time 0 and continuing every 30 min until reaching 2 hours. On the other hand, an automatic Vicat needle (Auto-Vicat, Ibertest, Spain) was employed to determine the initial and final settings, according to the European Standard UNE-EN 196-3. The needle periodically penetrates the mixture and allows analysis of its specific resistance to penetration. The flexural and compressive strengths and the pore size distribution and pore volume of the prismatic (1×1×6 cm3) pastes made of the R, BFS-C, and BFS-S formulations in the absence and presence of the decontamination sludge at 28 days are determined according to European standard UNE-EN 196-1 [32] in a mechanical testing machine (Autotest-200/10—SW, Ibertest, Spain), and a porosimeter (Autopore IV 9500, Micromeritics Instrument Corporation, USA), respectively. For this last, Washburn’s law was used, where D = (-4cosθ)γ/P, being D: the pore diameter (µm); θ: the contact angle between the fluid and the pore mouth (141.3º); γ: the surface tension of the fluid (485 N/m); and P: the applied pressure to fill the pore with mercury (maximum was 227 MPa). The results obtained from compressive strength tests were studied using two-factor statistical models with interaction and replications. The first factor, denoted as “paste”, had three levels: R, BFS-C, and BFS-S. The second factor, named “sludge”, indicated the presence of decontamination sludge incorporated in two states: a liquid state with three ratios (l/s= 0.45, 0.5, 0.55), and 5% in a solid state with l/s=0.45. In total, there were 3 x 4 = 12 treatments, with six experiments (replicates) conducted for each combination. The fundamental assumptions of homoscedasticity and normality of the random errors, which support the statistical models used, were thoroughly analysed. The analysis included the sequential application of two hypothesis tests: the F-test, as displayed in the Analysis of Variance (ANOVA) tables, followed by a post-hoc test for comparing treatment means. The Tukey procedure was utilized for this post-hoc mean comparison when significant differences were detected in the ANOVA. This statistical method pinpointed what means differ from each other by adjusting for multiple comparisons. This adjustment was crucial to minimise the risk of Type I errors—false positives—thereby maintaining the accuracy of claims about the differences between group means in all comparisons. The analysis was conducted using the statistical software R, although it can be replicated with any other software that supports standard analysis of variance models. Subsequently, a more detailed study at early ages was carried out on the transition from a plastic to a pseudo-rigid microstructure of the three types of pastes with the maximum amount of the confined sludge, specifically according to WAC of mechanical properties, the sludge was incorporated in a liquid state and with a liquid/solid ratio of 0.55. This study was developed through induction calorimetry, ultrasound pulse velocity, capillary pressure, and water vapour diffusion to assess the compatibility of waste-matrix and short-term performance for its subsequent final storage. An isothermal calorimeter (TAM Air, USA) was employed to monitor the evolution of the reaction kinetics for 550 h after mixing at 25 ºC. The fresh pastes were performed externally and manually for 2 min and then 5 grams of each paste were transferred immediately into the calorimeter ampoule to record the heat flow and total heat. Ultrasonic pulse velocity (UPV) was monitored on Ultratest IP-8 and BP-700 Pro ultrasonic testers, whose measuring moulds were filled with pastes prepared by mixing 200 g of the three pastes for 3 min. The piezoelectrically excited transmitter applied ultrasonic waves at a frequency of 25 kHz and a voltage of 600 V to 50 mm high. 50 mm diameter specimens bearing two transducers spaced at 40 mm. Primary wave velocity (Pwave,vp, m/s) was recorded at 60 s intervals throughout 550 h under laboratory conditions (T = 23 ºC; RH = 60%). Capillary pressure was measured with pressure sensors Sensortechnics RVA (0–250 kPa) inserted through small tubes filled with water. The sensors were connected to a plastic pipe of 2.8 mm diameter, filled with water. The pipe was embedded in the sample. In each test, four sensors were placed in the sample: two at 15 mm and the other two at 35 mm depth from the surface of the sample. The water vapour diffusion was evaluated by UNE EN 1015-19 standard. Resistance value was measured by the wet cup method with a saturated salt solution. Cylindrical specimens with a diameter of 35 mm and between 20 and 30 mm thick were used.
description: This is the experimental dataset used in a manuscript in which the fresh state properties, setting time, and flowability, as well as the hardened state properties, mechanical strengths, and total porosity were studied to determine in what form the sludge (liquid, or evaporated state) and at what l/s ratio (0.45, 0.50 and 0.55) it can be incorporated into the alkali-activated and Portland-based reference cement matrices to achieve the highest immobilised amount of sludge according to the Waste acceptance criteria (WAC) of achieving a flexural strength of 1 MPa and a compressive strength of 10 MPa. Subsequently, a more detailed study at early ages was carried out on the transition from a plastic to a pseudo-rigid microstructure of the three types of pastes with the maximum amount of the confined sludge, specifically according to WAC of mechanical properties, the sludge was incorporated in a liquid state and with a liquid/solid ratio of 0.55. This study was developed through induction calorimetry, ultrasound pulse velocity, capillary pressure, and water vapour diffusion to assess the compatibility of waste-matrix and short-term performance for its subsequent final storage. This data set includes the results of minislump values, setting times, flexural and compressive strengths and total porosities for the reference cement and sodium carbonate-activated slag and sodium silicate-activated slag pastes in the absence and presence of decontamination sludge incorporated in a) liquid state with l/s= 0.45, b) liquid state with l/s = 0.5, c) liquid state with l/s= 0.55, and d) 5% of solid state with l/s=0.45. Slag cements activated with sodium carbonate and sodium silicate seemed to be suitable confining matrices because they could immobilise the maximum sludge content without compromising the WAC mechanical strength. The sludge had to be incorporated in a liquid condition and with a liquid/solid ratio of 0.55, resulting in an 8.8% (dry sludge) by weight of cementitious material to be conditioned. The setting time and flowability increased in the presence of decontamination sludge regardless of the form in which it was incorporated in the reference cement and sodium carbonate-activated slag and sodium silicate-activated slag samples. The hydration and activation reactions were delayed due to the initial more acidic medium from which the samples started by the presence of the waste. Its presence also increased the total porosity of all the formulations, being the values for the alkali-activated slag pastes being lower than those of the reference paste. Finally, the results of the study of the early ages on the transition from a plastic to a pseudo-rigid microstructure of the cementitious pastes mixed with the liquid sludge with l/s=0.55 through ultrasonic pulse velocity, isothermal calorimetry and capillary pressure are included. These data show that the sodium carbonate-activated slag with sludge sample reached the state of hardening and consolidation earlier, with the highest UPV values and with compressive strength values of 18.4 MPa. This sample was also less permeable to water vapour, with higher percentages of tortuosity, thus performing well against the ingress of water vapour or other external agents. In light of the results, this matrix appeared to be the most suitable for immobilising the decontamination sludge in a liquid state.