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Heavy metals escaping from industrial processes and industrial waste are a significant environmental hazard all around the globe. Since they cannot be destroyed, the only way to manage the pollution is to capture and contain the hazardous components. This study focuses on stabilization-solidification of a highly toxic industrial filter slag that contains a range of different heavy metals in extremely high quantities. The stabilization-solidification treatment was executed with a mayenite and gypsum based solid cementitious binder. The major components that were monitored during the study were selenium, lead and sulphate. Different ratios of gypsum addition were tested in order to observe the effect of total sulphate content in the system to the immobilization efficiency of anionic selenium and sulphate. During the experiments, all cationic heavy metals were immobilized efficiently, most of them achieving the immobilization efficiencies of more than 99.9%. By adjusting the sulphate content of the system, the immobilization efficiency of selenium was improved from 95.669% to 99.925% and the immobilization efficiency of sulphate raised from 96.069% to above 99.964%. Controversially, the immobilization of lead was at its highest (99.999%) with high sulphate content and dropped to 98.162% as the sulphate rate decreased. The results show clearly that mayenite has good potential for stabilization-solidification applications.

This study aims to develop a robust strain-hardening ultra-rapid-hardening mortar (URHM) with high-volume cementitious materials and polyethylene (PE) fibers. To achieve this, the combined effect of cement kiln dust (CKD) and silica fume (SF) on the initial hydration process of ultra-rapid-hardening cement and the tensile performance of URHM was analyzed. Optimum amounts of CKD and SF of 0.15 and 0.2, respectively, by weight ratios to cement, were determined to develop the strain-hardening URHM containing 2% PE fibers. As a result, the tensile strength of 7.3 MPa, strain capacity of 5.12%, and energy absorption capacity prior to tension softening of 297.5 kJ/m3, respectively, were achieved at a very early age (4 h) of air-drying curing. The tensile performance of URHM deteriorated when the CKD content was 0.4 or greater, regardless of the SF content. A lower SF content of 0.2 was effective in terms of the tensile performance enhancement compared with the higher content of 0.4 up to the CKD content of 0.2, but they became similarly lower at higher CKD contents due to insufficient initial hydration.

Tailings wastewater contains a large number of harmful ions which damage the hydraulic concrete structures and seriously affect the safety of water conservancy systems. This paper studied the dam concrete of a medium-sized reservoir in southeastern China which was corroded by tailings wastewater. To this end, a long-term follow-up investigation was conducted into the corrosion of the dam concrete during the period from 2013 to 2019. A multiscale analysis was also carried out on the long-term influence of the tailings wastewater on the mechanical performance and microstructure of the dam concrete. The results revealed that the corrosion by the tailings wastewater containing H+ and SO 4 2 − caused a significant decline in the mechanical performance of the dam concrete in terms of the increase in the carbonation depth and the reduction of the compressive strength. Compared to the time period from 2006 to 2013, the rate of corrosion slowed down during 2013–2019. Furthermore, the main reason for the deterioration of the dam concrete was that the attack of H+ and SO 4 2 − resulted in the dissolution erosion and expansion erosion of the dam concrete. The dissolution erosion of the concrete led to the increase in the average pore diameter, median pore diameter, and porosity of the dam concrete, while the expansion erosion improved the formation of gypsum and ettringite. Moreover, the dam concrete experienced various degrees of corrosion in different zones. Specifically, the dissolution erosion was most serious in the tidal zone, while the expansion erosion was severest in the submerged zone. Finally, this paper can provide a frame of reference for the research on the corrosion mechanism and service safety of hydraulic concrete structures eroded by various types of tailings wastewater.

The aim of this research is to interrogate the feasibility of using Waste brick powder (WBP) successfully in concrete as a substitute of cement. The replacement levels were kept at 5% and 10%, compared to the reference concrete. The tests consisted of slump, density, compressive, flexural and splitting tensile strengths, Schmidt Hammer, Ultrasonic Pulse Velocity and microscopic analysis. The results revealed an increase in workability, which is attributed to the particle size and shape of WBP. WBP particles have round edges and smooth surfaces, which lubricate the mix and give a ball bearing effect. The decrease in density is attributed to the lower density of WBP. The increase in compressive, split tensile, and flexural strengths highlighted an indication of the improvement of concrete quality by WBP inclusion: This is attributed to compact structure of the specimens owing to the location of pozzolanic products in concrete voids. Rebound Hammer and Ultrasonic pulse velocity also validated enhancement of the quality of concrete. Scanning Electron Microscopy (SEM) confirmed the formation of primary ettringite and enhanced CSH content in WBP-replaced samples. Energy Dispersive Spectra (EDS) showed highest peaks of Ca and Si (enhanced quantity of CSH) and low peak of Al (Ettringite) in WBP-containing concrete specimens.

The variability in characteristics of concrete material, such as pore size and chemical composition, has significant impact on the long-term performance of concrete structures under sulfate attack. This paper presents a reliability-based numerical model for assessing the long-term corrosion behaviour of concrete subject to sulfate attack. Using a reliability approach, the model can describe the reactive transport behaviour of sulfate ions and concrete degradation process. Specially, the variables in pore characteristics in interfacial transition zone (ITZ) and concrete chemical composition were quantified using a large amount of experimental data to determine the time-dependent reliability index (β) under long-term sulfate attack. The results show that the filling of ettringite and monosulfate after sulfate attack could modify the physical pore structure of ITZ at early stage. The time for concrete to reach the reliability index limit state shorten remarkably with the increase of sulfate concentration. For example, a four-fold increase of sulfate concentration (e.g. from 57 mol/m3 to 14.25 mol/m3) could shorten the time to reach the reliability index limit (i.e. β = 4.7) by 70 %. In addition, aggregate surface treatment especially with silica fume could improve the reliability of deteriorating concrete significantly by prolonging the time to reach the reliability index limit by 40 % in this study.

The present study focuses on exploring the effects of reactivity and degree of dispersion of BaCO additions in the manufacture of sulfate-resistant OPC cements. A new electrochemical deposition method is attempted to effectively disperse BaCO particles (studying two different materials with particle size: D = 11.45 and 2.37 μm) on cement to enhance their reactivity and favour sulfate immobilisation in the form of BaSO. The barium carbonate additions, particularly the finest, activate cement hydration to a greater extent. Electrodeposition is also observed to improve early age reactivity (2 d–7 d) in fine BaCO. Cement paste bearing 15 wt % BaCO is more resistant to sulfate attack by a 5% (w/v) solution of NaSO (180 d at 23 °C) than a commercial sulfate-resistant cement, although secondary ettringite and gypsum precipitated in all cases., This study was funded by the Spanish Ministry of the Economy and Competitiveness and the European Regional Development Fund, BIA2015-73237-JIN project (MINECO/ERDF; EU) and MAT2017- 86450-C4-1-R. The authors wish to thank Grupos Cementos Portland Valderrivas and in particular Germ´an García P´erez for the supply of the OPCs used.

The autogenous healing of cementitious materials allows for the self-healing of cracks in concrete structures. It is known to be caused by (a) further hydration occurring when water penetrating the crack reacts with the unreacted binder on the crack face and (b) calcite precipitation resulting from the reaction between Ca2+ diffused from the cement paste and CO32− in penetrated water. Herein, isothermal calorimetry was used to analyze the further hydration of unreacted binder in hardened pastes containing ordinary Portland cement (OPC), supplementary cementitious materials, and crystalline admixtures. The amount of heat produced decreased with increasing sample age because of the reduction in the unreacted binder amount. Long-term further hydration in samples containing ground granulated blast-furnace slag and silica fume caused them to show increased heat production compared to OPC. However, no particular difference in comparison to OPC was detected for samples containing fly ash. When calcium sulfoaluminate was used as an expansion agent, heat production increased for a material age of 7 days and decreased after 28 days (compared to the case of OPC). Finally, self-healing products were analyzed by scanning electron microscopy with energy dispersive spectrometry. The results showed that the products contained calcite and an amorphous material regardless of the binder, while ettringite was observed to form in calcium sulfoaluminate and crystalline admixtures containing samples. Keywords: Further hydration, Isothermal calorimetry, Cementitious materials, Healing potential, Scanning electron microscopy

Municipal solid waste incineration bottom ash fractions ≤4 mm are the most contaminated ones in terms of potentially toxic elements (PTEs). In order to estimate potential environmental impacts, it is important to understand the association of the PTEs with the mineral phases. Large area phase mapping (SEM/EDX) using “PhAse Recognition and Characterization - PARC” software in combination with quantitative X-ray powder diffraction has been used to characterize amorphous and crystalline BA phases for the first time. The results show that one of the main incineration products was melilite and an amorphous phase with a melilitic composition. The ratio of crystalline to amorphous melilite was 1:2. They formed an inhomogeneous layer around BA particles and contained a high percentage of the PTEs, i.e., Cu, Zn, Ni and Cr. Other major sources of PTEs (especially Ni and Cu) were iron oxides produced during incineration and the weathering products, such as calcite and ettringite (Cu and Zn). After extensive characterization of BA, a sequential extraction procedure (SEP) was performed, which exposed bottom ash to different chemical environments designed to dissolve specific phases and release their PTEs into solution. The extracted solutions and solid residues generated from the extraction procedure were analyzed to identify the association between PTEs and dissolved phases of BA. By combining SEP results with information obtained via large area phase mapping it is shown that SEP can be used for studying the association of PTEs with the phase that cannot be investigated with XRD/EDX, such as organic matter and Fe-Mn-hydrous oxides. Furthermore, according to SEP results a high percentage (40–80 wt%) of each investigated PTE can be considered immobile and not susceptible to leaching in the environment.

This study evaluates the sulphate resistance of blended cement derived from the addition of thermally activated coal waste as pozzolan, at replacement levels of 0%, 20% (CMW20) and 50% (CMW50) with a multi-method approach. The pastes were prepared at a w/b ratio of 0.5, using a water-reducing admixture in the blended pastes to compensate for their high water demand. Sulphate resistance is reduced in CMW20 pastes, while longterm benefits are observed for CMW50 pastes, although moderate signs of deterioration are observed at the beginning of the exposure. The initial damage in CMW50 pastes is attributable to the precipitation of products (mainly ettringite and monosulphoaluminate) in the small capillary pores (0.05 ¿m). The greater long-term resistance of CMW50 pastes was explained by thermodynamic calculations, which suggests that the lower calcium leachability of these cements prevented gypsum formation. This, together with the higher content of carboaluminate phases, leads to lower supersaturation levels in the pore solution with respect to ettringite, reducing stresses derived from sulphate exposure., This work was supported by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (MINECO/FEDER) [grant number BIA-2015-65558-C3-1,2,3-R] and as well as by the Spanish Training Program, and the European Social Fund (MINECO/FSE) [grant number BES-2016-078454]. The authors are also grateful to the Sociedad An´onima Hullera Vasco-Leonesa, SIKA (Madrid, Spain), the Spanish Cement Institute (IECA) and the Scientific Research Support Services of the University of A Coru˜na for their assistance. The sample preparation of the SEM samples by A. M. Rzepkiewicz-Hansen is gratefully acknowledged.

The converter steel slag is a by-product during the steel-making process, which usually acts as an inert ingredient in construction due to the relatively low reactivity. However, its mineral composition results in a high reactivity in a CO2 rich environment. This study reveals the modification of converter steel slag by an ambient CO2 pretreatment, and the application of modified converter steel slag as the supplementary cementitious material (SCM) in the design of eco-friendly ultra-high performance concrete (UHPC) by using a particle packing model. The results show that converter steel slag after ambient CO2 pretreatment is feasible to be used in eco-friendly UHPC design, which achieves a relative higher compressive strength over 150 MPa with the cement substitution from 15% to 45%, compared to non-carbonated steel slag. The ambient CO2 pretreatment can modify the physical and chemical properties of converter steel slag particles, which produces a rough and porous surface of slag particles because of the precipitation of calcium carbonate and amorphous silica gel as carbonation products. Consequently, the incorporation of carbonated steel slag improves the cement hydration, enhance the formation of ettringite and C-S-H, while densify the microstructure compared to non-carbonated slag in eco-friendly UHPCs. The leaching of potentially hazardous elements, e.g. Cr and V from carbonated steel slag can be efficiently reduced below legal limits in UHPC mixtures.

Alum salts are commonly used as coagulants in the purification of surface water for potable supplies. The resultant waste alumina-rich sludge is currently landfilled in the UK. This study aimed to valorise sludge by using heat-treated alum sludge as an additive in cement. Alum sludge was calcined at 475–1100 °C and then characterized to correlate physical and mineralogical changes with cementitious activity and engineering performance. Alum sludge calcined at 825 °C transforms to poorly crystalline η-alumina (eta) and has cementitious activity. The calcined sludge rapidly reacts with gypsum to form ettringite which leads to a shortening of the induction period and onset of alite hydration. Gypsum depletion leads to undersulfated C3A hydration which consumes ettringite to form monosulfoaluminate thereby inhibiting further alite hydration. The poorly crystalline η-alumina is metastable, transforming to highly crystalline α-alumina at 1100 °C. In pastes containing more crystalline sludges, alite hydration is enhanced because the undersulfated C3A reactions are avoided, leading to improved performance.

The elevated level of selenium (Se) in the aqueous system presents long-term ecological risks. Hydroxylated calcined dolomites (HCDs) are considered as potentially sustainable lime sources due to their lower production temperatures and resourceful than lime. In this study, HCDs were developed for the removal of Se contaminated water. The HCDs exhibited a better performance to co-precipitate with Se oxyanions than pure Ca(OH)2. Considering that HCDs consisted of Ca(OH)2 with MgO and Mg(OH)2, the function of Mg compounds was also elaborated by various characterization techniques. Mg compounds were proved to enhance the precipitation process and Se removal. STEM-EDX observation revealed that SeO32– was incorporated into the ettringite structure with Mg compounds, whereas SeO42– exhibited a affinity to Mg compounds. Extended X-ray absorption fine structure (EXAFS) results for the MgO/Mg(OH)2 after the reaction proved that both SeO32– interacted with Mg(OH)2 and MgO via inner-sphere complexes whereas SeO42– formed outer-sphere complexes with these Mg compounds. Besides, Mg compounds also influenced the surface charge of solid residues and thus enhanced SeO32– removal. This study provides a fundamental understanding of the roles of Mg compounds in the removal of Se oxyanions during environmental remediation. Besides, the HCDs were proved to be sustainable Ca sources for waste/wastewater remediation.

Chemical compatibility with a wide range of materials is among the features that has driven the use of alkoxysilanes as consolidants in built structures. Such compatibility is particularly important in cementitious materials where the reaction with portlandite may generate C-S-H gel, one of the main hydration phases of OPC. The cementitious matrix is a complex system, however, and the reaction of its many phases with alkoxysilanes, while poorly understood, may determine treatment efficacy. This article describes a detailed study of the individual interactions between an oligomeric alkoxysilane-based impregnation treatment previously shown to interact with the portlandite present in cement paste and the cementitious phases generated in ordinary portland cement hydration. The findings show that both portlandite and C-S-H gel interact with the silicon oligomers in the hydrolysed impregnation treatment to generate a C-S-H gel (in the case of portlandite) and a rise in C-S-H gel mean chain length (MCL). Ettringite is also altered in the presence of alkoxysilanes, transforming to gypsum and AH. Its transformation generates a tetrahedral aluminium that is taken up into a high silicon gel sourced from the treatment to form an amorphous aluminosilicate gel. Monocarboaluminate and katoite also partially decompose in the interaction with the product, whereas gibbsite remains unaffected., This research was funded by the European Union’s Horizon 2020 - Research and Innovation Framework Programme under grant no. 760858 (Innovaconcrete project), by the Regional Government of Madrid (TopHeritage-CM; S2018/NMT_4372) and by the Spanish Gov- ernment/FEDER-EU under research project MAT2017-84228-R. Dr. Paula M. Carmona-Quiroga’s participation was supported by the Span- ish Ministry of Economy and Competitiveness and the European Regional Development Fund under project BIA2015-73237-JIN (MINECO/ERDF; EU). Authors thank the support of the PTI-PAIS plat- form

Initial hydration and setting were continuously monitored in calcium sulfoaluminate clinker (KCSA) mixed with water or 2 M NaOH with isothermal conduction calorimetry, ultrasonic pulse velocity (UPV) measurement, the Vicat apparatus, and (in- and ex-situ) X-ray diffraction. P-wave acceleration (ap) proved highly useful to detect very rapid initial reactions, not generally perceptible with other techniques. Setting in the water-hydrated KCSA pastes was mediated by ettringite formation. Flash setting due to calcium aluminate hydrate precipitation was observed in the 2 M NaOH-hydrated clinker. Ye'elemite reacted most intensely at later times in both pastes, inducing AFt and AH precipitation and the formation of smaller proportions of other calcium aluminate hydrates that densified the matrix., This research was developed thanks to the support of the Min-istry of Science and Innovation and Universities and FEDER fundsthat subsidize the Project BIA2016-76466-R and the aid BES-2017-082022. In addition, the PhD student Pilar Padilla-Encinas, thanks to the University Autónoma of Madrid, to be able to carryout her thesis in the Applied Chemistry program

This study aims to obtain insights into the retardation mechanism of citric acid in an ettringite-based binder from ladle slag and gypsum. The hydration kinetics and phase assemblage of the binder were experimentally investigated and thermodynamically modelled. Additionally, the effects of citric acid on synthetic ettringite were studied to obtain further understanding of the interaction between this organic ligand and the crystal. Experimental results reveal that citric acid works as an inhibitor of ettringite's formation leading to the precipitation of monosulfate and gypsum; the ettringite surface blockage by citrate ligand effectively prevents precipitation of this crystal. This leads to an overestimation in the precipitation of ettringite in the thermodynamic model due to this kinetic barrier imposed by the ligand. Thermodynamic modelling suggests ettringite, monosulfate, aluminum hydroxide, and stratlingite as main hydrates in this binder, whereas an intermixed C-(A-)S-H gel was observed experimentally instead of stratlingite. This work is a part of the FLOW Project (project number 8904/31/2017) funded by Business Finland in the ERA-MIN 2 Innovation program (EU Horizon 2020 program). SSAB Europe Oy is acknowledged for providing ladle slag. The authors thank Dr. Dmitrii Kulik (Paul Scherrer Institute) with the help in creating citrate ligand in GEMS and valuable instruction, Prof. Jorgen Skibsted (Aarhus University) for fruitful discussion on 29Si MAS NMR, and Viljami Viinikka and Durgaprasad Ramteke for the support during lab work. Hoang Nguyen gratefully acknowledges financial support from travel grant of UniOGS (the University of Oulu Graduate School) for his research visit in Technical University of Denmark. P.K. acknowledges financial support from Academy of Finland (grants 322085, 329477 and 326291). A part of material characterization was carried out with the support of the Centre for Material Analysis, University of Oulu, Finland. Nguyen, H (corresponding author), Univ Oulu, Fibre & Particle Engn Res Unit, Pentti Kaiteran Katu 1, Oulu 90014, Finland. hoang.nguyen@oulu.fi; wolku@byg.dtu.dk; katrijn.gijbels@uhasselt.be; pieter.samyn@uhasselt.be; valter.carvelli@polimi.it; mirja.illikainen@oulu.fi; paivo.kinnunen@oulu.fi

In this study, the effect of chloride environment containing various concentrations of chloride ion (Cl−) on self-healing performance of pre-cracked cementitious composite containing crystalline admixture (CA) and MgO expansive agent (MEA) was investigated under wet-dry cycles in chloride solutions. The results revealed that the Cl− changed the mineralogy of self-healing products, and consequently, affected the crack closure ratio and mechanical strength recovery. When self-healing occurred in distilled water, a large amount of ettringite (AFt) were detected, whereas in Cl− solution, monosulfate (AFm) was consumed by Cl− to form Friedel's salt (Fs), and then the Fs was decomposed to Al(OH)3(AH3) due to carbonation. During the multiphase conversion process, hydroxide (OH−) was released into crack solution, therefore the dissolved carbon dioxide (CO2) concentration was increased. The carbonation of the crystals formed in cracks was accelerated with the volume expansion, which achieved rapid crack sealing but contributed little to the mechanical performance recovery.

Cement paste leaching and concomitant decalcification is a common cause of concrete structure deterioration. Such alterations entail microstructural changes that condition concrete durability. This study analysed those changes in cement pastes exposed to accelerated decalcification, layer-by-layer, from the surface in contact with the leaching agent, 6 M NH4NO3, to the unaltered area. The microstructural changes were analysed with BSEM, MIP and BET whilst the mineralogical and nanostructural alterations in each layer were assessed with XRD, FTIR, 29Si and 27Al MAS NMR and BSD. Decalcification affected not only portlandite and CSH gel, but also the anhydrous clinker and ettringite, inducing major micro- and nano-structural changes, particularly in the two layers closest to the leaching agent. CSH gel decalcification was more intense in the outer layers where long chain gels co-existed with silicon rich gels. BDS, in turn, revealed differences between the amount of bound water in the surface interfacing with the leaching agent and in the leaching front. More specifically, larger numbers of water molecules were tightly bound to silanol groups in the outer, whereas hydroxyl group distribution was more orderly in the inner layers., This study was funded by the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No. 760858. Funding was also received from the Regional Government of Madrid, (S2018/NMT-4372 TOP Heritage-CM Programme).

Supplementary cementitious materials (SCMs) have been shown to have a beneficial impact on carbonate-bearing mortar and concrete resistance to sulfates in general and thaumasite sulfate attack (TSA) in particular, although the effect is limited in some cases. In pursuit of improved sulfate-resistance formulations that inhibit thaumasite formation, this study added barium carbonate, both as mineral addition and as aggregate, to cements blended with SCMs. The aim was to immobilise sulfates via conversion to barite, a practically insoluble form of barium sulfate. The study was conducted on standardised mortars prepared with CEM I 52.5 N-SR 5 (reference, Ref) cement, laboratory-blended CEM III/C-SR (with slag) or CEM IV/B-SR (fly ash-bearing) cements. Mixes were also prepared with the latter two cements in which the slag or fly ash filler was partially replaced with barium carbonate. Barium or calcium carbonate sands were used as aggregate in the respective mortars, all of which were soaked in a NaSO solution at 8 °C for 6 months and monitored for variations in mass, compressive strength, ultrasonic velocity, soundness (expansion) and mineralogy. The findings showed that at such low temperatures the amount of Ba solubilised did not suffice to effectively immobilise the external sulfates in the blends containing SCMs. The fly ash-bearing samples, particularly where the addition was partially replaced with barium carbonate, deteriorated severely or very severely (substantial expansion and mass loss) because thaumasite precipitated even in the presence of the barite formed. The low cement content, slag-bearing blends exhibited greatest sulfate resistance, irrespective of the presence of barium carbonate as addition or aggregate. In mortars made with commercial, low CA content (Ref.) sulfate-resistant cement, barium carbonate sand inhibited gypsum and ettringite formation, whereas both products precipitated in analogous and less porous mixes bearing limestone aggregate., This research was funded by the Spanish Ministry of the Economy and Competitiveness and the European Regional Development Fund (MINECO/ERDF; EU project BIA2015-73237-JIN; Spain). Supply of the OPCs by Grupos Cementos Portland Valderrivas and of the limestone aggregates by Omnya Clariana is gratefully acknowledged. The authors wish to thank Germán García Pérez and Isabel Pajares, respectively, for making the necessary arrangements. The professional support provided by the CSIC’s ‘Open Heritage: Research and Society (PTI-PAIS)’ Interdisciplinary Theme Platform is also gratefully acknowledged.

Expansive concretes based on the formation of ettringite from calcium sulfoaluminate agents are presented as an effective alternative to minimize the problems derived from shrinkage and low tensile strength at the early ages of the concrete. However, determining the mechanism responsible for the expansion is still a controversial issue today. In this study, the influence of three doses of expansive agent on fresh state and on compressive strength and magnitude of free expansion under two curing environments was evaluated on a reference mixture of self-compacting concrete (SCC). The microstructural evolution of the hydrates and anhydrous related with the expansive phenomena was also studied. From our observations it seems that an eminently amorphous ettringite is formed and that this ettringite would be strongly related to expansion. As a result, the influence of the ratio w/cementitious material and curing environment on the magnitude of the expansion as well as the influence of this on the mechanical properties of the SCCs was identified., Authors gratefully acknowledge the Spanish Ministry of Economy and Competitiveness for the financial support given to this research in BIA2015-64363-R project.

Recent studies on the harmful effects of sulfates on OPC have shown that these salts can be immobilised by conversion to insoluble BaSO4. The response of blended OPC-BaCO3 cement to thaumasite sulfate attack is still poorly understood, however. This study explored the effect of BaCO3 on sulfate resistance in fairly high C3A (>10%) OPC mortars and pastes in the presence and absence of CaCO3. Further to the findings, due to low BaCO3 solubility at 8 °C, the temperature that favours thaumasite precipitation, neat OPC sulfate resistance was only slightly improved. Specimens, particularly mortars with 12.5% BaCO3 and no added CaCO3 (12.5B), deteriorated due to gypsum, ettringite and thaumasite precipitation, although less intensely than the samples bearing other blends. None of the BaCO3 or BaCO3 + CaCO3 blends performed better than the commercial sulfate-resistant cement used as the reference., Funding from the Spanish Ministry of the Economy and Competitiveness and the European Regional Development Fund, BIA2015-73237-JIN project (MINECO/ERDF; EU), is gratefully acknowledged.

The main objective of the work presented in this paper is to evaluate the effectiveness of using three inorganic expansive agents in developing expansive cementitious grouts for sealing concrete cracks. Two of the agents were ettringite-based (K and NK) and one was CaO-based (G). Nine expansive cement grouts and one non-expansive cement grout were developed, and their expansive and mechanical performance were characterized. Additionally, an original method based on the water penetration test is proposed to evaluate the sealing capability of the produced grouts which has demonstrated high feasibility in determining the sealing effectiveness of the cement grouts designed. After sealing the simulated cracks, a strong decrease (≈80%) of the water penetration was detected when using 5% in weight cements of G or NK agents. An important but lower decrease (≈50%) was detected by using 5% of K agent. Moreover, the results obtained in the Brazilian test indicated that sealing with expansive cement grouts allows a high integration of the sealing material within the surrounding concrete element, achieving good mechanical bonding, so the sealing material and concrete act as a compact sole material. BSEM-EDAX analyses also showed slight evidence of chemical interaction between the sealing expansive grout and the surrounding concrete when using NK agent, but strong evidence of physical interaction since the penetration of some kind of amorphous hydrates from the grout side to the concrete side was detected., Authors gratefully acknowledge the Spanish Ministry of Economy and Competitiveness for the financial support given to this research in BIA2015-64363-R project.

The behavior of sulfoaluminate (CSA) cements hydrated in neutral or alkaline solutions (0 M, 0.1 M, 1 M, 4 M and 8 M NaOH) was studied. 2-d, 28-d and 90-d mechanical strengths were determined, as well as the reaction rate by isothermal conduction calorimetry. The reaction products were characterised with XRD, DTA and SEM. The findings revealed differences in performance depending on whether hydration transpired at pH ≤ 14 (0.1 and 1 M) or pH > 14 (4 M and 8 M). To 0.1 M and 1 M, there is an improvement of mechanical strength than in water-hydrated pastes. The majority reaction products in these conditions were ettringite and microcrystalline aluminium hydroxide. Mechanical strength was lower in the high alkalinity (4 M and 8 M) than in the reference pastes, mainly due to the formation of different reaction products. Ettringite was unstable at 4 M and failed to form at 8 M, whilst thenardite and U-phase were detected in the 28d and later materials hydrated at both concentrations., This study was partially supported by the Brazilian National Council for Scientific and Technological Development (CNPq) under project 208380/2017-5, and CAPES. Financial support for this study was also furnished by the Spanish Ministry of the Economy and Competitiveness and FEDER under research project BIA2016-76466-R.

Release of heavy metals from different industries and industrial waste is a major global threat for as well humans as ecosystems. In this study, immobilization of an industrial filter sludge (FS) with an extremely high content of several heavy metals (24.6 wt% Pb, 21.7 wt% Hg, and 9.00 wt% Se) and sulfate via calcium sulfoaluminate-belite (CSAB) cement was tested. The ratios of 25%, 50% and 75% of CSAB addition were tested, and the target was to achieve immobilization of the hazardous components. The leaching of Pb, Hg, SeO4, SO4, Ni, Cd, Cu, and As was monitored, and the structure of the immobilized materials was examined via X-ray powder diffraction (XRD) and field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS) analysis. It was observed that Hg, Cu, As, Cd, and Ni were immobilized completely and leaching of Pb was reduced by 69% from the theoretical release. On the other hand, the leaching of SeO4 and SO4 experienced major increase when CSAB was added. XRD indicated significant ettringite formation as the amount of added CSAB increased, and the formation of gypsum as the amount was decreased. FESEM-EDS indicated that the immobilization was largely based on encapsulation into the CSAB binder, but chemical immobilization into the ettringite binder was also observed. It was concluded that the increased release of SO4 and SeO4 might have resulted from an excess amount of sulfates (added gypsum) during hydration.

In this investigation, ettringite-based mortars were synthesized from ladle slag (LS) and phosphogypsum (PG), promoting the concept of a circular economy. However, the reuse of naturally occurring radioactive materials (NORM), such as PG, requires radiological investigation. Also, the immobilization degree for contaminants contained in PG should be evaluated. The former was investigated using gamma spectroscopy and radon exhalation/emanation tests, while the latter was assessed using an up-flow percolation column test according to the CEN/TS 16637‐3. The produced mortars comply with current legislation on naturally occurring radionuclides (NOR) in building materials, proving that they can be safely used for building purposes. The radon emanation decreased upon increasing the Polish PG content, which was mainly determined by the microporosity. The specific surface areas were 20‐30 times lower than conventional cement, and the immobilization degree for contaminants was generally high (>90%). This investigation demonstrates high potential for PG reuse in ettringite-based mortars.

This paper aims to investigate the effect of volcanic scoria (VS) when added at micro or nano level on some concrete properties. Syria is rich in volcanic scoria with estimated reserves of nearly 1 billion tonnes. Three concrete properties, namely compressive strength, porosity and sulfate resistance, have been examined. Six percentages of micro-VS ranging from 0% to 50%, and nano-VS ranging from 0% to 5% have been used in making the investigated concretes. Test results revealed that the low early compressive strength of micro-VS-based concretes could be overcome by grinding VS to a nano size. In addition, significantly improved durability properties were obtained when VS was used either at micro or nano level. Much lower porosity and expansion values were clearly noted in the VS-based concretes. The efficiency factor (k) using the Bolomey equation and the durability indicator were evaluated to compare the behaviour of the VS-containing concrete with the reference concrete. The efficiency factor decreased with the increase of micro-VS/binder ratio and increased with the increase of nano-VS/binder. The calculated durability indicators showed that micro-VS content of 30% and nano-VS contents of 3 and 4% had approximately the highest indicator values. The replacement level of 30% at micro level and (3 & 4)% at nano level met k value of about 0.44 and 6, respectively, which would be useful for the designers of concrete mix. Based on the analyzed data, some relationships among the investigated properties were extracted. Further, scanning electron microscopy (SEM) technique and thermal gravity analysis (TGA) were employed in the current study to observe the resulting modifications in the microstructure of some VS-containing pastes. Large crystals of ettringite were observed in the pastes containing VS0 versus much smaller ones in the 3% nano-VS containing paste, after 52 weeks of exposure to 5% Na2SO4. Keywords: Volcanic scoria, Nanopozzolan, Compressive strength, Concrete durability, Sulfate attack, Efficiency factor