Hydrothermal synthesis of calcium sulfoaluminate–belite cement from industrial waste materials

Portland cement industry has been looking for compulsory alternative cement to replace traditional cement binder, ordinary Portland cement due to energy consumption and carbon dioxide emission.

Belite or dicalcium silicate is an example of a special cement which is both useful for strength development and zinc encapsulation. However they undergo hydration slowly resulting to low early strength of the set cement and concrete. Calcium sulfoaluminate-belite containing dicalcium silicate (C₂S), calcium sulfoaluminate (C₄A₃Ŝ), tetracalciumaluminferrite (C₄AF) and calcium sulfate (CŜ) was thereby developed.

Researchers at Chiang Mai University-Khon Kaen University-National Metal and Materials Technology Center in Thailand synthesized calcium sulfoaluminate-belite cement from a mixture of industrial waste materials viz: fly ash, aluminum rich sludge and flue gas desulfurization gypsum coupled with commercial grade hydrated lime using hydrothermal-calcination method. The study is published in Journal of Cleaner Production.

All phases in Calcium sulfoaluminate-belite have various advantages in terms of stable temperature at 1250⁰C, good dimensional stability, low alkalinity and low permeability, comparable compressive strength to Portland cement and high resistance to chemical attack. Clinkerization method of Calcium sulfoaluminate-belite cement from reagent-grade chemical was previously used but its high temperature requirement over 1200⁰C does not make it attractive. Hence, the hydrothermal synthesis of producing Calcium sulfoaluminate-belite cement was developed and showed several advantages over clinkerization method because of its low temperature requirement and better morphological control. Previous research in hydrothermal synthesis have proven operating temperature less than 1000⁰C in production of belite cement by hydrothermal-calcination method.

In Rungchet et al. (2016) research, the designed cement was based on a calculation of Bogue equation which was commonly applied for cement synthesis using solid state reaction and effects of type of hydrothermal solvent, time of hydrothermal process and calcination temperature of hydrothermal products or intermediate phases on cement phase function was investigated.

Raw materials used to prepare the mixture included industrial waste material consisting of class F fly ash   and flue gas desulfurization gypsum from Mae Moh power plant in north of Thailand with aluminum rich sludge from aluminum anodizing or power coating industrial process of Zodiac Aircatering Equipment, Thailand. Particle size (d_4,3) of fly ash, flue gas desulfurization gypsum and aluminum rich sludge were 75, 50 and 110μm respectively. Mineralogical composition using X-ray diffraction (XRD) of fly ash consisted of anhydrite, calcite, hematite, magnesioferrite and traces of quartz with some of amorphous phases.’

All industrial wastes were dried in an oven at 60⁰C for 24h which were later grounded to obtain particles passed sieve No. 100. Theoretical phase composition were predicted from each batch using a modified Bogue’s equation for Calcium sulfoaluminate-belite cement and the phases were C₄AF, C₄A₃Ŝ, C₂S, CŜ  and CaO.

Proportional raw mixes were dispersed in deionized water with a liquid to solid ration of 7 by stirring at 250rpm for 1h to allow homogenous mixing and pre-dissolving of starting materials. Simultaneous thermogravimetric and differential scanning calometric (TG/DSC) analysis (Netzsch STA 409C) was used to follow the physical and chemical changes with temperature of hydrothermal products in order to design calculation temperature.

From results obtained, non-alkaline activation hydrothermal treatment led to complete dissolution of anhydrite and precipitation of ettringite from reactions between dissolved anhydrites and aluminum species containing aluminum rich sludge and fly ash while alkaline hydrothermal treatment (1 M NaOH) resulted in formation of new hydration phases called Katoite. The use of water without hydrothermal treatment had similar phase development as that of non-alkaline activation hydrothermal treatment.

Thermogravimetric and differential scanning calometric analysis of hydrothermal products showed two endothermic peaks between 50⁰C and 120⁰C corresponding to dehydration of ettringite. At 160-180⁰C an endothermic effect due to the decomposition of high crystallization degree of formed C-S-H was observed.

X-ray diffraction patterns of compounds obtained after calcination (1050⁰C) showed C₄A₃Ŝ  and -C₂S phases were obtained in all cements but intensities depend on treatment used.  C₄A₃Ŝ was formed in alkaline activation condition not by transformation of ettringite similar to non-alkaline activation condition, but via the reaction between katoite, gibbsite, portlandite and thenardite.

Result from calcination temperature in case of C₃A synthesis, a high temperature product (about 1300⁰C for clinkerization method) could be obtained at only 1150⁰C. At 950⁰C, C₄A₃Ŝ and C₄AF phases stated to form with an increase in -C₂S and reduction of C₁₂A₇ content. At 1050⁰C, there were significant changes of C₄A₃Ŝ, -C₂S and C₄AF contents with a gradual reduction of C₁₂A₇. At 1150⁰C, C₁₂A₇ lost its stability and converted to tricalcium aluminate. Specific area for non-alkaline Calcium sulfoaluminate-belite and alkaline Calcium sulfoaluminate-belite were 1331 and 1355m²/Kg respectively which was higher than clinkerization method.

28 day comprehensive strength of ordinary Portland cement, non-alkaline Calcium sulfoaluminate-belite and alkaline Calcium sulfoaluminate-belite cements were 41.6, 30.5 and 23.3MPa. Non-alkaline Calcium sulfoaluminate-belite cement comprehensive strength of synthesized cements increased per day as ages of 1, 7, 28 and 90 days were 12.9, 21.1, 30.5 and 41.1MPa respectively. Hydration product in a form of ettringite was mainly responsible for strength development of C₄A₃Ŝ phase.

For non-alkaline activated cement, besides ettringite, formation of stratlignite phase, one of the building phases took place via the chemical reaction between Calcium sulfoaluminate  and  belite rather than via the reaction between  belite and gibbsite.

Results of this study on recycles of high alumina and silica containing waste materials such as aluminum rich sludge and fly ash could reduce the use of expensive alumina-silica bearing materials such as bauxite and clay mineral.