Calcium Sulfoaluminate Clinker Research Articles

Stabilization and Solidification of Beryllium Waste: Influence of the Cement Composition on the Corrosion of Be Metal

Beryllium metal is used as neutron moderator and reflector or multiplier in certain types of fission or fusion reactors. Dismantling of these reactors will produce radioactive beryllium waste, classified as low- or intermediate-level waste, that will need to be stabilised and solidified before being sent to disposal. The cementation process is under consideration because it may offer a good compromise between simplicity of implementation, cost, and quality of the final cemented wasteform. Nevertheless, knowledge of the corrosion behaviour of Be metal in a cement-based matrix is still limited, partly due to the high toxicity of Be that complicates testing. This study thus investigates Be corrosion in cement suspensions using potentiometry, voltammetry, and electrochemical impedance spectroscopy. Among the five different investigated systems (Portland cement blended without or with 40 wt.% silica fume, calcium sulfoaluminate clinker blended without or with 15% anhydrite, and calcium aluminate cement), Portland cement blended with 40% silica fume and calcium sulfoaluminate cement comprising 15% anhydrite are the most effective in mitigating beryllium corrosion. They allow reduction in the corrosion current by factors of 4 and 50, respectively, as compared to Portland cement.

Synthesis of hydraulic waste-derived belite-rich calcium sulfoaluminate clinker: Feedstock design, component calculation, and clinkering optimization

Solid waste-derived belite-rich calcium sulfoaluminate (BRC) clinker was successfully synthesized, which incorporated incineration bottom ash, recycled concrete powder, and phosphogypsum as the feedstock. The mineral components of BRC clinker were predicted by theoretical formula and thermodynamic modelling, which aligned well with experiments. The effect of clinkering temperature on viscosity, Gibbs free energy, phase equilibrium, and real mineralogy was thoroughly investigated by thermodynamic predictions and experimental characterizations. Optimal clinkering at 1200 °C for 3 h yielded BRC clinker predominantly consisting of 46.3 wt% belite, 22.5 wt% ye'elimite, 16.6 wt% fluorellestadite, and 7.1 wt% brownmillerite. Hydrates AFt and amorphous (AH3 and C–S–H gel) formed the network, playing a crucial role in developing a 60 d paste strength of 42.2 MPa. Utilizing lower temperatures and low-carbon embodied solid wastes for producing this BRC clinker promised to reduce CO2 emissions and preserve natural resources.

Development of calcium sulfoaluminate cements from rich‐alumina bauxite and marble wastes: Physicochemical and microstructural characterization

AbstractThis work discusses the effect of rich alumina bauxite on the mineralogical composition of calcium sulfoaluminate (CSA) clinker cement on their performances. After preparation of different local raw materials (rich alumina bauxite and marble), they were mixed with 15 wt% of commercial gypsum and pressed at 2 MPa. The obtained pellets were thermally treated at 1200°C to produce clinker, which is use to synthesize the CSA cements. The raw materials as well as products were characterized by many analyses, such as FT‐IR, X‐ray fluorescence, X‐ray diffraction, differential scanning calorimetry (DSC)/thermogravimetry analysis (TGA), isothermal calorimetry (ICC), scanning electron microscope, and physico‐mechanical tests. ICC analysis of the powder of clinker cement showed that the reactions are strongly affected by gypsum content. XRD results revealed that clinker and cement present a strong intensity of ye'elimite as main mineral phase. Moreover, the hydrated cement exhibited ettringite and monosulfate as the new phases formed. The compressive strength of hydrated cement reached ∼21 and 28 MPa after 1 and 28 days of curing, respectively. From the aforementioned results, this local rich alumina bauxite can be used to produce high‐strength cement for self‐leveling materials, which allow their use in engineering and building applications.

Fabrication and Properties of Blended Calcium Sulfoaluminate Cements Based on Thermally Treated Reservoir Sediments

In 2021, approximately 4.1 billion tonnes of cement were globally produced and the annual CO2 emissions from cement plants reached almost 2.8 billion metric tonnes. In recent years, many efforts have been made to manufacture low-CO2 cements. In this regard, great consideration has been given towards calcium sulfoaluminate (CSA) binders for both their technical features and sustainable properties, principally connected to their industrial process. The use of blended cements composed by CSA binders and supplementary cementitious materials (SCMS) can be an effective way to (a) reduce the CO2 footprint and (b) produce greener binders. This scientific work studied the utilization of different amounts (15–35 wt%) of calcined reservoir sediments (RS) as SCMS in blended CSA binders, where the binders were cured for up to 56 days and characterised by various analytical techniques. It was found that thermally treated RS were particularly noteworthy as their utilization allowed for a dilution of the CSA clinker, thus implying a decrease in CO2 emissions and a reduction in costs related to their production. However, compared to a plain CSA cement, the blended systems showed rather similar volume stability levels, whilst their compressive strength and porosity values were, respectively, lower and higher at all the investigated aging periods.

Effect of Na versus Ca Sulfate Salts on the Hydration of Calcium Sulfoaluminate Clinker.

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.

On the hydration of ternesite and the formation of thaumasite

The calcium sulfosilicate phase ternesite, previously considered as an undesired product in cement kilns, has recently gained attention due to its potential replacement of ye'elimite in calcium sulfoaluminate clinkers and the contribution of ternesite hydration products to the strength development of such clinkers. However, the potential formation of thaumasite during ternesite hydration, in particular in the presence of ettringite, has not yet been fully explored. In the present study synthesized ternesite was hydrated in the presence of ettringite, carbonated ettringite, portlandite and calcite. Thaumasite formed, together with gypsum and C-S-H, only in samples initially containing ettringite, both at 6.5 °C and at ambient temperature. Portlandite and calcite did not have a noticeable impact on the hydration of ternesite and the formation of thaumasite. The dissolution and/or carbonation of ettringite, together with its surface template effect, is suggested to be key to initiate thaumasite nucleation via the solid solution route.

Hydrate assemblage stability of calcium sulfoaluminate-belite cements with varying sulfate content

To assess the feasibility of employing calcium sulfoaluminate (CSA)-belite cements for radioactive waste encapsulation purposes, a series of 40 mixes were conducted on a 3-litre scale. This series of tests incorporated two commercially available CSA-belite clinkers, gypsum or anhydrite addition at 15, 25 or 35 wt%, a water to solids ratio of 0.5, 0.6 or 0.7, and a high or low mixing shear regime. CSA clinkers chosen contain predominantly ye’elimite, with some belite, and a small quantity of calcium sulfate as anhydrite. This paper outlines the effect of these parameters upon the phase composition and structure of these CSA samples tested at 7, 28, and 90 days. The development of the hydrate assemblage was monitored in order to assess the suitability of these grouts for use in encapsulation processes. The trends in phase development and evolution are consistent across both clinkers tested. The levels of ettringite formed as a hydration product are dependent upon the levels of calcium sulfate addition and the availability of water, with ettringite production evident, though at a decreasing rate, from between 7 and 90 days. Throughout the entire catalogue of mixes, no ettringite decomposition was detected. The majority of belite within both clinkers appears to remain unreacted, with a small increase in mixes producing less ettringite. Simulations also show similar phase assemblages to those encountered experimentally, with little phase development after 90 days.

Fundamental design of low-carbon ordinary Portland cement-calcium sulfoaluminate clinker-anhydrite blended system

Calcium sulfoaluminate (CSA) cements have lower carbon dioxide (CO2) emissions during production compared with conventional ordinary Portland cement (OPC). However, the high cost of production is a significant limitation of the application of CSA cements. This paper aims to improve the use efficiency of CSA clinker by adding OPC and anhydrite. The low-carbon OPC-CSA clinker-anhydrite system is expected to contain less CSA clinker while showing excellent performance. A design methodology was developed to optimize the component of the OPC-CSA clinker-anhydrite blended system. The hydration mechanism of the ternary system was investigated using isothermal conduction calorimetry, X-ray diffraction (XRD), and thermogravimetric analysis (TGA), and thermodynamic modelling was used to predict the amount of hydration products. The results showed that ettringite is the main hydration products of the OPC-CSA clinker-anhydrite system, and the addition of OPC provides portlandite for the formation of ettringite. The ternary system is characterized by rapid development of strength and low CO2 emissions. The compressive strength at 4 h and the CO2 emissions are about 10 MPa and 500 kg/t, respectively. In addition, the ternary system, composed of 27.9 wt% CSA clinker, 32.5 wt% anhydrite, and 39.6 wt% OPC has the highest compressive strength at 28 d and the lowest CO2 intensity.

Managing the Heat Release of Calcium Sulfoaluminate Cement by Modifying the Ye’elimite Content

Nowadays, calcium sulfoaluminate cement (CSA) is garnering a large amount of attention worldwide and is being promoted as a sustainable alternative to Portland cement for specific applications. This study aimed to control the heat release of CSA cement paste by choosing the appropriate composition. For this purpose, different calcium sulfoaluminate clinkers with up to 75 wt. % of ye'elimite were synthetized. Then, a reactivity study on the synthesized clinkers was conducted while varying the amount of gypsum added. The heat of hydration was measured by isothermal calorimetry. The influence of the ye'elimite content on the heat release and on the compressive strength was investigated. According to the findings, the amount of ye'elimite in the cement has a direct relationship with the heat release. The heat release as well as the mechanical performance increase with the increase in the ye'elimite content in the CSA cement. An equation allowing the prediction of the total heat release after 24 h is provided. Such data can be of particular interest to consultants aiming at the reduction of thermal cracking in massive concrete.

Recycling of bone ash from animal wastes and by‐products in the production of novel cements

AbstractThe purpose of this research is to demonstrate the utilization of animal wastes and by‐products in the production of low‐energy and low‐CO2 clinkers and cements in order to preserve natural resources, such as limestone, while reducing CO2 emissions released from the cement manufacturing process and reducing potential health risk to the world population (such as bovine spongiform encephalopathy or other health issues…). Pure calcium sulfoaluminate clinker was produced with calcium hydroxide, aluminum hydroxide, and calcium sulfate hemihydrate; followed by additional clinkers produced from substituting calcium hydroxide with bone ash (from 0 to 100% of the calcium hydroxide replaced). The final clinkers contained various amounts of ye'elimite, calcium aluminate phases, as well as tricalcium phosphate, depending on the firing temperature. Finally, some preliminary results on the hydration process and compressive strength are provided for the production of these binders.

Effect of Class C and Class F Fly Ash on Early-Age and Mature-Age Properties of Calcium Sulfoaluminate Cement Paste

To promote the sustainable development of eco-efficient calcium sulfoaluminate (CSA) cements through the partial replacement of the CSA clinker with supplementary cementitious waste products, the effects of coal fly ashes on the early-age and mature-age properties of a calcium sulfoaluminate (CSA)-based cement paste were investigated. The impacts of both Class C and Class F fly ashes on the rheological properties, hydration kinetics, and compressive strength development of CSA cement paste were studied. Rheology-based workability parameters, representing the rate of loss of flowability, the rate of hardening, and the placement limit, were characterized for the pastes prepared with fixed water-to-cement (w/c) and fixed water-to-binder (w/b) ratios. The results indicate a slight improvement in the workability of the CSA paste by fly ash addition at a fixed w/b ratio. The isothermal calorimetry studies show a higher heat of hydration for the Class C fly ash-modified systems compared to the Class F-modified systems. The results show that fly ash accelerates the hydration of the calcium sulfoaluminate cement pastes, chiefly due to the filler effects, rather than the pozzolanic effects. In general, ettringite is stabilized more by the addition of Class F fly ash than Class C fly ash. Both fly ashes reduced the 1-day compressive strength, but increased the 28-day strength of the CSA cement paste; meanwhile, the Class C modified pastes show a higher strength than Class F, which is attributed to the higher degree of reaction and potentially more cohesive binding C-S-H-based gels formed in the Class C fly ash modified systems. The results provide insights that support that fly ash can be employed to improve the performance of calcium sulfoaluminate cement pastes, while also enhancing cost effectiveness and sustainability.

Effect of CaO Sourced from CaCO3 or CaSO4 on Phase Formation and Mineral Composition of Iron-Rich Calcium Sulfoaluminate Clinker.

The performance of iron-rich calcium sulfoaluminate (IR-CSA) cement is greatly affected by mineral composition and mineral activity in the clinker. This study aims to identify the effect of CaO sources, either CaCO3 or CaSO4, on the phase formation and mineral composition of the IR-CSA clinker. Targeted samples were prepared with different proportions of CaCO3 and CaSO4 as CaO sources at 1300 °C for 45 min. Multiple methods were used to identify the mineralogical conditions. The results indicate that the mineral composition and performance of the IR-CSA clinker could be optimized by adjusting the CaO source. Both Al2O3 and Fe2O3 tend to incorporate into C4A3-xFxS¯ with an increase in CaSO4 as a CaO source, which leads to an increased content of C4A3-xFxS¯ but a decreased ferrite phase. In addition, clinkers prepared with CaSO4 as a CaO source showed much higher x value in C4A3-xFxS¯ and higher compressive strength than clinker prepared with CaCO3 as the sole CaO source. The crystal types of both C4A3-xFxS¯ and C2S were also affected, but showed different trends with the transition of the CaO source. The findings provide a possible method to produce IR-CSA cement at a low cost through cooperative utilization of waste gypsum and iron-bearing industrial solid wastes.

Effect of alkalinity on early-age hydration in calcium sulfoaluminate clinker

Early-age hydration with neutral or alkaline (0 M to 8 M) solutions was studied in calcium sulfoaluminate clinker (CSA) using isothermal conduction calorimetry, X-ray diffraction (both continuous and discontinuous, i.e., after detaining hydration with isopropanol) and infrared spectroscopy. Hydration in 0.1 M NaOH was observed to be similar to hydration in water. At the next highest concentration studied (1 M), however, the reaction rate was slower, due primarily to lower AFt stability in that medium. Faster hydration but less total heat was observed in the pastes hydrated with 2 M NaOH, a finding associated with the presence of a metastable compound, U-phase, that subsequently transitioned to AFt, C3AH6 and thenardite. Hydration with highly alkaline (4 M and 8 M) solutions inhibited AFt formation completely, although ye'elimite reacted more vigorously (particularly with 8 M NaOH) to form katoite, AH3 and thenardite.

Control the early-stage hydration of expansive additive from calcium sulfoaluminate clinker by polymer encapsulation

Ye′elimite (C4A3S¯), a main compound in calcium sulfoaluminate (CSA) clinker, is an important ingredient as expansive additive in shrinkage compensating cement. This study proposes to modify the expansive additive by encapsulating it with polyethylene glycol (PEG). The polymer provides a matrix structure, in which the ye′elimite particles are embedded. When the modified expansive additive come into contact with water, the polymer matrix acts as a water barrier, but can dissolve away. This slowly exposed C4A3S¯ to hydration, resulting in gradual early-stage ettringite formation; hence control early expansion in expansive cement. The study compared the ettringite formation between the unmodified and the modified expansive additive using thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) from 1 hour to 3 days. The results show that the unmodified expansive additive generated more ettringite than the modified ones at the same hydration time. The study subsequently investigated the mortar properties with the unmodified and modified expansive additives admixtures. The results showed that the modified expansive cement showed superior flowability and drying shrinkage behaviours, while the compressive strength of the finished products underperformed that of the untreated expansive additives.

Synthesis of MgO-Belite calcium sulfoaluminate cement from phosphate mine waste rock and phosphogypsum

MgO-Belite calcium sulfoaluminate low carbon clinker (BCSAC) was successfully synthetized from a raw meal containing more than 77 wt% of phosphate by-products: 67 wt% of phosphate mining waste rocks and 10 wt% of phosphogypsum (PG: CaSO4·2H2O), while the remaining portion was Kaolin clay. The obtained clinker could be considered as an environmental friendly material. In fact, it can be synthetized at lower temperature (1250 °C) and it is physically softer than Portland cement. Ye’elimite and belite were the major cement phases in the obtained clinker. The hydration of the synthesized cements was assessed by microcalorimetry, compressive strength tests, X-ray diffraction and thermogravimetric analysis. Ettringite was identified as the main crystalline hydrate phase, which is in accordance with thermodynamic modelling results. The presence of P2O5 and the residual acidity in PG negatively influenced the strength development. However, the compressive strengths of the obtained belite calcium sulfoaluminate clinkers was higher than Portland cement at 3 and 7 days. It was explained by the high quantity of ettringite in sulfate-rich cement pastes.

Analysis of theoretical carbon dioxide emissions from cement production: Methodology and application

Alternative low-carbon cements could contribute significantly to CO2 reduction in the cement industry. However, the existing methods require the input of the actual production data, and thus cannot be used to calculate CO2 emissions of alternative low-carbon cements that are not yet produced in cement plants. This study aims to develop a model to analyze the theoretical CO2 emissions of alternative low-carbon cements. The novelty of this model is that it can be used to calculate the fuel consumed in the production of new low-carbon cements based on the theory of heat balance, and then predict their CO2 emissions. The model is used to calculate CO2 emissions of several low-carbon binders and ordinary Portland cement (OPC). The results show that the direct CO2 emissions of OPC clinker calculated by this model approach the value derived from the cement plants and the Cement Sustainability and Initiative (CSI). Calcium sulfoaluminate (CSA) clinker has the direct CO2 emissions of 0.540 kg/kg, 34% lower than OPC clinker, while its cost of raw materials is over four times that of OPC clinker. High-belite calcium sulfoaluminate (HB-CSA) clinker, with the direct CO2 emissions comparable to CSA clinker, lowers its cost of raw materials by half as it requires less expensive bauxite. Moreover, CO2 emissions from HB-CSA cement production can be further reduced by the use of high volume of supplementary cementitious materials (SCMs). The knowledge gained provides a valuable reference for the design of new low-carbon binders.

Recycling phosphogypsum as the sole calcium oxide source in calcium sulfoaluminate cement production and solidification of phosphorus

Because the disposal of phosphogypsum (PG) can lead to serious contamination of the air, soil, and water, recycling of PG has attracted wide attention. This study investigated the effect and solidification of phosphorus in the production of calcium sulfoaluminate (CSA) cement using PG as the sole CaO source. The effects of three phosphorus impurities (Ca3(PO4)2, CaHPO4, Ca(H2PO4)2) on the decomposition of CaSO4, formation of minerals, microstructure of the clinker, and the hydration and mechanical properties of the cement were studied. Experimental results show that Ca3(PO4)2 and Ca(H2PO4)2 promoted the decomposition of CaSO4 and the formation of clinker minerals with the increase in P2O5 content, whereas CaHPO4 showed a promoting effect only when the P2O5 content was more than 1.5 wt%. The increase in phosphorus incorporation in Ca2SiO4 leads to the transformation of β-Ca2SiO4 to α′-Ca2SiO4 and then to Ca7Si2P2O16. The presence of three phosphates in the clinker enhanced the growth of crystal grains and the generation of a liquid phase. Compared with Ca4Al6SO16 without phosphorus, the hydration reaction of phosphorus-bearing Ca4Al6SO16 started later and ended earlier, and the reaction time was shorter. The presence of phosphorus impurities reduces the 1-day strength of CSA cement but does not affect the development of the 3-day and 28-day strengths. Considering environmental aspects, the solidification of phosphorus in the production of CSA clinker were quantified by measuring the distribution of elements. The results indicated that phosphorus is solidified by Ca4Al6SO16, Ca2SiO4, and Ca4Al2Fe2O10, and Ca2SiO4 has a stronger ability to solidify phosphorus than the other two minerals. Ca3(PO4)2 is more difficult to solidify than CaHPO4 and Ca(H2PO4)2. This study is of great significant to guide the large-scale clean utilization of PG in the production of CSA cement.

Influence of the amount of calcium sulfate on physical/mineralogical properties and carbonation resistance of CSA-based cements

Binary and ternary binders, Calcium SulfoAluminate (CSA) clinker plus anhydrite (A) and blends of Portland cement, CSA clinker, and A, respectively, are examined in order to understand the effect of the A/CSA clinker ratio (A/CSA) on hydration process, physico-mechanical properties and durability against carbonation. The study revealed that: (i) in binary systems the increase of the A/CSA has a positive influence on the physical performance, but worsens the resistance against carbonation (ii) in ternary systems a non-linear behavior related to the physical aspects is observed, being the system with the intermediate A/CSA the best performer in terms of dimensional stability and carbonation resistance.The effect of carbonation on the hydrate phase assemblage was studied through a multi-technique approach and results were supported by thermodynamic models, indicating that the optimal dosage of sulfates is strictly dependent on the carbonation exposure and the ternary systems are more durable than the binary ones.

Effect of Iron Phase on Calcination and Properties of Barium Calcium Sulfoaluminate Cement.

In this paper, the effect of iron phase content on the calcination and properties of clinker and barium calcium sulfoaluminate cement was studied. The compressive strength of the samples was tested and combined with an XRD and SEM-EDS analysis, and the microstructure and composition of the barium calcium sulfoaluminate clinker and hydrated samples were characterized. The results showed that the oval-shaped particles were C2S minerals, and the hexagonal plate-shaped or rhombohedral dodecahedral particles were C2.75B1.25A3. The Ba element was mainly distributed in the barium calcium sulfoaluminate region, and some of it was dissolved in C2S; the Fe element was distributed between C2.75B1.25A3 and C2S crystal grains in the form of an iron phase solid solution, which acted as a solvent. When the iron phase composition was C4AF and the iron phase content was 5%, the early hydration and later strength were better, and the compressive strength after curing for 1, 3 and 28 days was 73.2 MPa, 97.9 MPa and 106.9 MPa, respectively. A proper amount of the iron phase can reduce the eutectic point of the sintered mature material system, increase the amount of liquid phase, reduce the viscosity of the liquid phase, effectively accelerate the migration of mineral ions and promote the formation and growth of minerals.

Strength Development and Shrinkage of Superabsorbent Polymer Concrete with Calcium Sulfoaluminate Clinker and Shrinkage Reducing Admixture

In this study, we analyzed the strength and shrinkage properties of concrete with three additives, superabsorbent polymer (SAP), calcium sulfoaluminate (CSA) clinker, and shrinkage-reducing admixture (SRA), to verify the internal curing and shrinkage reduction effects. According to compressive strength tests, the use of SAP as an additive resulted in a slight decrease in compressive strength, whereas using 10% CSA clinker as an additive resulted in a compressive strength 8 MPa higher than that of ordinary concrete. In the shrinkage tests, we observed the shrinkage behavior at the surface and in the middle of the concrete while exposing the surface to ambient air for 80 days. According to the results, SAP and SRA had greater shrinkage reduction effects on the concrete than CSA clinker. In particular, the shrinkage reduction rate achieved by adding SAP to the mixture was approximately 32% compared with ordinary concrete. Based on this result, we concluded that the shrinkage of the mixture reduced due to the internal curing effect (humidity adjustment within the concrete) of the SAP. In addition, the shrinkage reduction effect was maximized when we added these materials simultaneously. In particular, the shrinkage reduction rate achieved by adding SAP and SRA together was found to be approximately 69% compared with ordinary concrete. When we added CSA, SAP, and SRA to the concrete mixture, the shrinkage reduction rate was approximately 96% compared with ordinary concrete, making this the best shrinkage reduction effect achieved.