Reduction of water content in calcium aluminate cement with/out phosphate modification for alternative cementation technique

Cementation of the secondary aqueous wastes from TEPCO Fukushima Daiichi Nuclear Power Plant is challenging due to the significant strontium content and radioactivity, leading to a potential risk of hydrogen gas generation via radiolysis of water content. The present study investigates the reduction of water content in calcium aluminate cement (CAC) with/out phosphate modification by a heat-treatment during the solidification. The reduction of water in the CAC was found restricted by the rapid formation of crystalline hydration phases, whereas the phosphate-modified system allowed the gradual reduction of water, achieving the reduction of 60% water content at 95 °C. Curing at 60–95 °C also eliminated the significant cracks found at 35 °C in the phosphate system. The possible difference in the amorphous products, NaCaPO4∙nH2O type at 35 °C and Ca(HPO4)∙xH2O type at 60–95 °C, may have contributed to the improvement in the microstructure together with the change in the pore size distribution

Una aproximación estadística al estudio de la carbonatación del hormigón

Carbonation is one of the factors that conditions reinforced concrete durability, while porosity is one of the parameters that determines the carbonation rate: as a rule, the greater the porosity, the higher the rate. While many papers have been published on the effect of CO2 penetration in the pore solutions of concretes prepared under different experimental conditions, the literature has yet to address the joint effect of the factors considered in concrete design, such as the water/cement (w/c) ratio, type of cement, type of aggregate and presence of admixtures. The present paper discusses the findings of a statistical study of the impact of the aforementioned factors on both system porosity and carbonation rate. The type of cement, individually and in its interaction with the rest of the factors, proved to be the major determinant in concrete carbonation.La carbonatación es uno de los factores que supedita la durabilidad del hormigón armado, siendo la porosidad uno de los parámetros que más condicionan la velocidad de carbonatación. Son muchos los trabajos que estudian el efecto de la penetración del CO2 en la solución de los poros de hormigones preparados bajo distintas condiciones experimentales, sin embargo, no se encuentran referencias que analicen de forma conjunta el efecto de ciertos factores como son la relación agua/cemento (a/c ratio), el tipo de cemento, el tipo de árido y la presencia de aditivos, normalmente consideradas a la hora de diseñar un hormigón. En este trabajo se discuten los resultados obtenidos tras realizar un estudio estadístico del efecto que tienen los factores previamente mencionados, tanto en la porosidad de estos sistemas como en su nivel de carbonatación. El cemento tanto de manera individual como en sus interacciones con el resto de los factores es el factor que mas afecta a la carbonatación del hormigón

Influence of mixing solution on characteristics of calcium aluminate cement modified with sodium polyphosphate

This study investigated characteristics of a calcium aluminate cement modified with a phosphate (CAP) by changing an amount and concentration of mixing solution with sodium polyphosphate. When the amount of mixing solution was increased with a constant amount of sodium polyphosphate, an enhanced consumption of monocalcium aluminate was observed compared with gehlenite in calcium aluminate cement. Formation of gibbsite, Al(OH)3, was also increased as a hydration product in the CAP and a reduction of water in the amorphous gel phase. When the amount of mixing solution was increased with a constant concentration of sodium polyphosphate, the enhanced consumption of monocalcium aluminate was not observed. Neither gibbsite nor any other crystalline hydration products were identified in this series. In addition, unreacted sodium polyphosphate remained in the system. The increased formation of gibbsite and the possible reduction of water from the amorphous gel phase appears to contribute to the improvement of the microstructure in the products

Mineralogical and microstructural changes in alkali-activated and hybrid materials exposed to accelerated leaching

Alkali-activated materials (AAM) and hybrid cements (HC) have become sustainable alternatives to Portland cement (PC) due to their low carbon footprint. The main differences between AAM and HC lie in their content of clinker (none in the AAM and usually lower than 30% for HCs) and the type of activator used (strong alkaline solutions for AAM and small amounts of solid alkalis for HC). Durability problems related with microstructural changes due to decalcification and leaching of the cementitious paste have been well researched for PC pastes, but it is still not well known for AAM and HC. The present work aims to study the leaching process for cement pastes of both types of sustainable pastes. Blast furnace slag (BFS) was selected as a precursor to manufacture hybrid slag (HS) pastes and alkali-activated slag (AAS) pastes. A commercial CEM IV was selected as reference material. A 6 M NH4NO3 solution was used to accelerate leaching kinetics. After 28 days of immersion, the mineralogical and microstructural changes were evaluated. Results show that AAS pastes exhibited the highest leaching resistance of all the pastes under study, due to the absence of portlandite and the high level of polymerization of silicate chains. In HS pastes, the presence of portlandite (due to PC in the material) and gypsum (due to the activator) explains their intermediate performance, in between CEM IV and AAS.The authors have been able to carry out the present research thanks to financial support from the Ministerio de Ciencia, Innovación y Universidades of Spain (RTI2018-096428-B-I00 and PID2020-116738RJ-I00 projects) and Madrid Government (Comunidad de Madrid) under the Multiannual Agreement UC3M in the line of "Fostering Young Doctors Research" (HORATSO-CS-UC3M) in the context of the V PRICIT (Regional Programme of Research and Technological Innovation)

Interaction of strontium chloride solution with calcium aluminate phosphate (CAP) system

Processing of contaminated water in Fukushima Daiichi Power Plant results in various aqueous secondary wastes. A significant strontium ( 90 Sr) contamination represents one of the key challenges for their management, and we have been developing a new cementing matrix based on calcium aluminate phosphate (CAP) system with reduced water content for such aqueous secondary wastes. In the present study, the interaction of CAP with SrCl2 was investigated to gain insight into the capability of CAP system in Sr immobilisation. It was found that the immobilisation is possible, and the incorporation of Sr appears to be associated with the formation of insoluble phosphate salts. The obtained results also suggest that 1 kg of CAP powders can incorporate 200 g of Sr under tested condition

A Concerted Kinase Interplay Identifies PPARγ as a Molecular Target of Ghrelin Signaling in Macrophages

The peroxisome proliferator-activator receptor PPARγ plays an essential role in vascular biology, modulating macrophage function and atherosclerosis progression. Recently, we have described the beneficial effect of combined activation of the ghrelin/GHS-R1a receptor and the scavenger receptor CD36 to induce macrophage cholesterol release through transcriptional activation of PPARγ. Although the interplay between CD36 and PPARγ in atherogenesis is well recognized, the contribution of the ghrelin receptor to regulate PPARγ remains unknown. Here, we demonstrate that ghrelin triggers PPARγ activation through a concerted signaling cascade involving Erk1/2 and Akt kinases, resulting in enhanced expression of downstream effectors LXRα and ABC sterol transporters in human macrophages. These effects were associated with enhanced PPARγ phosphorylation independently of the inhibitory conserved serine-84. Src tyrosine kinase Fyn was identified as being recruited to GHS-R1a in response to ghrelin, but failure of activated Fyn to enhance PPARγ Ser-84 specific phosphorylation relied on the concomitant recruitment of docking protein Dok-1, which prevented optimal activation of the Erk1/2 pathway. Also, substitution of Ser-84 preserved the ghrelin-induced PPARγ activity and responsiveness to Src inhibition, supporting a mechanism independent of Ser-84 in PPARγ response to ghrelin. Consistent with this, we found that ghrelin promoted the PI3-K/Akt pathway in a Gαq-dependent manner, resulting in Akt recruitment to PPARγ, enhanced PPARγ phosphorylation and activation independently of Ser-84, and increased expression of LXRα and ABCA1/G1. Collectively, these results illustrate a complex interplay involving Fyn/Dok-1/Erk and Gαq/PI3-K/Akt pathways to transduce in a concerted manner responsiveness of PPARγ to ghrelin in macrophages

Modification of calcium aluminate cement with phosphate for incorporation of strontium chloride

Large quantities of aqueous secondary waste are generated from the processing of contaminated water after the nu-clear accident in Japan. Cementation of these wastes is challenging because their significant radioactivity may cause the radiolysis of water contents in cement, posing a risk of hydrogen gas generation. The application of calcium aluminate cement modified with phosphates (CAP), as an alternative cementing system, is interesting because this system is based on acid-base reaction, and its water content can be reduced by mild heating once the system is mixed. The present study focused in the use of Secar 71, a calcium aluminate cement with a high alumina and low silica compositions, and its effects of on the production of CAP system at elevated temperatures. The modification of Secar 71 with phosphates was successful, and the reduction of water content by about 35% was achieved in the CAP system containing SrCl2 after curing the system at 80°C for 7 days. The micro cracks, typically observed in the CAP system cured at lower temperature, was significantly reduced by curing at 80°C. The obtained results show a potential of Secar 71 to prepare CAP for cementation of aqueous secondary wastes

Some durability aspects of hybrid alkaline cements

Blended cements that contain a high content of fly ash and a low content of Portland cement typically suffer from low early strength development and long setting times. Recently, one method of overcoming these problems has been to use an alkali activator to enhance the reactivity of fly ash particles at early ages. Such cements can be grouped under the generic term “hybrid alkaline cements”, where both cement clinker and fly ash, encouraged by the presence of alkalis, are expected to contribute to cementitious gel formation. The work presented here examines some of the durability aspects of high fly ash content hybrid alkaline cement. Specifically, the aspects investigated were: exposure at high temperatures (up to 1000°C), resistance to immersion in aggressive solutions and susceptibility to the alkali aggregate reaction. All tests were repeated with a commercially available sulfate resistant Portland cement for comparison. When exposed to high temperatures, the hybrid alkaline cement showed strikingly different behaviour compared to the control Portland cement, showing fewer micro-cracks and maintaining residual compressive strengths at least equal to original strengths. Beyond 700°C, the hybrid alkaline cement began to sinter, which resulted in shrinkage of around 5% and a 100% increase in residual compressive strengths. No such sintering event was noted in the control Portland cement, which showed a drastic loss in residual compressive strengths upon heating. In immersion tests, the hybrid alkaline cement possessed excellent resistance to sulfate and seawater attack, similar to the control sulfate resistant cement. Both cements were however severely degraded by immersion in 0.1M HCl for 90 days. Both binders complied with the accelerated alkali-aggregate test but when this test was extended, the hybrid alkaline binder showed much greater dimensional stability. Possible reasons for the differences in durability behaviour in both cements are discussed, based on experimental evidence provided