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The very early hydration behavior of a hydraulic binder phase, ye’elimite, Ca4Al6O12SO4, in the absence and in the presence of calcium sulfate, has been investigated. A time-resolved in situ multin...

The paper addresses the carbonation of CSA based binders from the mechanical, chemical and mineralogical point of view. Three different binders have been investigated, revealing that many hydrated phases can protect ettringite against carbonation. In particular, the presence of AFm phases gives a significant contribution to buffer the pH in alkaline range and to preserve the protective film of steel reinforcement bars. In a carbonated environment AFm phases act as ionic exchangers: carbonate ions replace sulfates in the interlayer and in these conditions the released sulfate ions contribute to the formation of new ettringite, with a beneficial effect on compressive strength performances. Experimental data are compared with models and carbonation mechanisms are described.

The identification of affordable healing agents for cement based materials, able to promote autonomous crack healing, is a challenge to improve the durability of building structures. In this study, a thorough investigation of the reactivity between a hydrated Portland cement and sodium silicate solutions, as healing agents, has been carried out. The goal is to quantitatively assess the chemical reactivity and actual binding capacity of sodium silicate. Mechanical recovery was evaluated by means of a healing agent strength test on hydrated cement treated with sodium silicate. XRPD and Solid-state NMR allowed the definition of reaction times, the involved species, and the nature and stability of the reaction products. Highlights show that sodium silicate reacts not only with Ca(OH)2 (namely portlandite), but also with calcium aluminate phases (AFt, AFm, TAH) to extract calcium and/or aluminum ions, with the formation of crystalline/semi-crystalline C-S-H/C-A-S-H tobermorite phase.

The hydration of four sulfoaluminate cements have been studied: three sulfoaluminate systems, having different content of sulfate and silicate, and one blend Portland-CSA-calcium sulfate binder. Hydration was followed up to 90 days by means of a combination of X-ray diffraction and solid state MAS-NMR; Differential scanning calorimetry and Scanning electron microscopy were also performed in order to help the interpretation of experimental data. High amount of amorphous phases were found in all the four systems: in low-sulfate cements, amorphous part is mainly ascribed to monosulfate and aluminium hydroxide, while stratlingite is observed if belite is present in the cement; in the blend system, C-S-H contributes to the amorphous phase beyond monosulfate.

The study addresses the hydration behavior of different sulfoaluminate clinkers produced in a pilot kiln at burning temperature comprised between 1150 and 1300 °C. The clinkers contain different relative amount and distribution of sulfates and silicates. The hydration behavior from 4 h up to 3 months has been investigated by means of a multi-technique approach. The results indicate that two contributions regulate the different reactivity of these sulfoaluminate clinkers. On one hand, the presence of a readily-available sulfate source allows the faster development of ettringite, whereas, the reactivity of silicate component takes the advantage of the lack of sulfate. Moreover, the first effect influences the early reactivity, while the second one is observed only when medium to long term hydration behavior is considered. Low-temperature sulfoaluminate clinker showed similar performances in comparison with a high-temperature clinker, however, a significant reduction in burning energy (temperature is 150 °C lower) and CO2 emissions (limestone is 5% less) are envisaged.

Construction and demolition waste (CDW) recycling is generally limited to the use of the coarser fraction as aggregate for new concrete. The recovery of fine aggregates requires a cleaning by removing the hydrated cement waste (HCW). In this paper, the possibility to use HCW extracted from CDW as alternative component for the production of new clinker is explored. A pure HCW sample was prepared and used in partial replacement of natural materials in raw admixtures for new clinker production. At a replacement degree of 30%, a new Portland clinker containing almost 50% of C3S could be produced with a huge spare in the release of CO2 (about 1/3 less). At higher HCW dosage a non-Portland clinker containing almost 80% of C2S has been obtained: its use as supplementary cementing material in blended cements revealed satisfying long term performances.

Four different binders based on calcium sulfoaluminate cements have been submitted to accelerated chlorination through ionic exchange on hydrated pastes, in order to investigate their ability to chemically bind chloride ions that might reduce chloride penetration. The composition of hydrated cements before and after the treatment was evaluated by means of an X-Ray Diffraction– 27 Al Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy combined study, allowing to take into account even partially amorphous phases and to make quantitative assumption on the relative abundance of the different aluminium-containing phases. It was found that low SO 3 Sulfoaluminate–Portland ternary systems are the most effective in binding chloride ions and the active role played by different members of the AFm family in chloride uptake was confirmed. Moreover, a peculiar behavior related to the formation of Friedel's salt in different pH conditions was also established for the different cements.

Bottom ashes from a north Italian municipal solid waste incinerator (MSWI) were vitrified at 1450 °C without adding any vitrifying agent, then ground and sieved to different granulometry (ranging from 50 μm to 20 mm), and used as filler, sand, or aggregate for concrete. Samples were characterized via slump tests (UNI 9418), alkali-silica reactivity (UNI 8520/22 and ASTM C 298), and compression strength tests (UNI 6132, 6132/72, 6686/72), and compared to reference samples obtained without vitrified bottom ashes (VBA). Our results show that vitrified bottom ashes are unsuitable as a sand substitute; however, concrete containing up to 20 wt.% of VBA filler used as a substitute for cement and up to 75 vol.% of VBA as a substitute for natural aggregate retains the same mechanical properties as reference samples. Alkali-silica or other detrimental reactions were not observed in VBA-containing concrete samples after a period of two years. The results of this work demonstrate that vitrified bottom ashes from MSWI can be used instead of natural aggregates in mortar and concrete production.

Calcium Looping (CaL) is recognized as one of the most promising emerging technology for CO2 capture in cement plants. The highly integrated Calcium Looping process configuration enables CO2 capture with an efficiency target over 90% and high-energy efficiency. The core activity of the CLEANKER project is the design, construction and operation of a CaL demonstration system including the entrained-flow carbonator (the CO2 absorber) and the entrained-flow oxyfuel calciner (the sorbent regenerator). This demonstration system, connected to the Buzzi Unicem kiln of the Vernasca cement plant (Italy), will capture the CO2 from a portion of the flue gases of the kiln, using as CO2 sorbent the same raw meal that is used for clinker production. The CLEANKER implementation plan spans four years and half, from October 2017 to March 2022 (an extension of six months has been requested due to the pandemic situation). The first two years have been devoted to the (i) detailed design of the CaL demonstration system, the (ii) characterization of raw meals as CO2 sorbents and the (iii) erection of the demonstrator. The pilot plant has been commissioned in October 2020 and then will be demonstrated by means of several short and long steady-state tests (during spring and summer 2021), with the aim of optimizing the operational parameters governing the CO2 capture process and of bringing the integrated CaL technology at TRL7. The pilot plant and the conventional kiln operations will be deeply integrated: the CaL calciner will be fed by the same kind of raw meal used in the kiln for producing clinker, whereas the carbonator will treat the effluents coming from the cement plant. The CaL calciner will be fired in a recirculated oxy-fuel combustion mode, where a heavy fuel oil will be burnt with oxygen and a fraction of the CO2-rich exhausts will be recirculated to control the oxidant composition. Before being recycled to the CaL calciner inlet, this CO2-rich stream will be properly cooled in a regenerative riser-cyclone stage, designed to preheat the fresh raw meal-based sorbent fed to the pilot, minimizing the additional fuel supplied to run the calcium looping process. In the carbonator, the amount of sorbent (FCa/FCO2) will be tuned either by increasing the raw meal exchanged with the CaL calciner or by internally recycling a fraction of sorbent from the carbonator outlet to the inlet. First short campaigns will be carried out by specialized Buzzi Unicem operators (assisted by CLEANKER partners such as, Politecnico di Milano, Laboratorio Energia e Ambiente Piacenza, IKN, University of Stuttgart and VDZ) to test the process performances as a function of the type of raw meal, the amount of sorbent (solid to gas ratio in the carbonator) and the carbonation/calcination operating temperatures. The experimental activity will prove the stability and the effectiveness of the process by running over hundreds of hours the optimal experimental configuration, capable of achieving a constant carbon capture rate higher than 90%. Besides the continuous monitoring of process temperatures, flow rates and gas compositions, the experimental campaigns will be supported by post-processing sorbent analysis, in order to evaluate the capture capacity of the raw meal and the extent of side reactions. Test results will be exploited to validate simulation models and to improve the future scale-up and the design of an industrial size CaL plant.