Study On The Hydration And Hardening Mechanism Of High Belite Calcium Sulfoaluminate Cement

Rapid setting and hardening high belite calcium sulfoaluminate cement revealing excellent properties like rapid setting and hardening,high early and late strength,microdilatancy,low shrinkage,frost resisting and anti-permeability,corrosion resistant and so on relied heavily on the hydration and hardening process to gain the above performance.Therefore,Investigation on hydration performance,hydration hardening process and microstructure morphology change of rapid setting and hardening high belite sulfoaluminate cement are of great significance to production cost reduction and usability improvement.Therefore,the hydration properties,hydration and hardening process and microstructure change of rapid setting and hardening high belite calcium sulfoaluminate?HBCSA?cement clinker,HBCSA cement clinker–anhydrite binary system,HBCSA cement clinker–cement admixture-anhydrite triple system were studied.Impact of anhydrite and cement admixture on the hydration of HBCSA cement were also confirmed,with normal consistency water,setting times,mechanical strength being used to explore macro-properties,isothermal conduction calorimetry?ICC?,X-ray diffraction?XRD?,Thermogravimetric–Differential scanning calorimetry?TG-DSC?,pH,Inductively coupled plasma atomic emission spectroscopy?ICP-AES?,Mercury intrusion porosimetry?MIP?and Scanning electron microscope?SEM?employed to investigate micro-properties,making it possible to discuss the hydration and hardening mechanism of HBCSA cements.The results show that:?1?The early hydration can be assigned to the quick dissolution of f-CaO and f-CaSO₄ in order to promote the hydration of C₄A₃S?and C₂S to form ettringite?AFt?,aluminum hydroxide gel?AH₃?and calcium silicate hydrate gel?C-S-H?.The late hydration is primarily attributed to the dissolution of C₂S and the transformation of AFt,followed with the precipitation of monosulfate phase?AFm?and C-S-H phases;Structurally,needle-like,column-like and tubular like AFt phase overlaps and interlocks with each other,hexagonal-platelet morphology AFm phase and clusters of C-S-H gel filling and cementing AFt,thus leading to a low porosity,dense structure of hardened cement.?2?The hydration acceleration facilitated by anhydrite was confirmed,like 10%anhydrite beneficial to early strength and 20%anhydrite to later strength.The additive anhydrite can dissolve a certain concentration of SO₄^2-in hydration environment to prevent AFt from transforming to AFm phase.In the later age hydration,SO₄^2-from anhydrite together with Ca²⁺,OH^-,Al?OH?₄^-would continue to form AFt,consuming CH phase at the same time to promote C₂S hydration,leading to the reduction of liquid phase and increase of pH value;With the effect of filling and cementation,hardened cement paste would gain a denser,lower porosity microstructure and get the compressive strength further improved simultaneously.?3?In the ternary hydration system,Portlandite,generated along with C-S-H gel from belite hydration,can be promoted by enough concentration of SO₄^2-ions in hydration environment,which in turn facilitates the dissolution of vitreous bodies in the cement addition materials.Among them:activated Al₂O₃ in Ground Granulated Blast Furnace Slag?GGBFS?is low,which restricts the ions dissolution to form hydration products precipitating out;Fly Ash?FA?dissolution can be accelerated in OH^-and SO₄^2-ions environment;Steel Slag?SS?can hydrolyze out Ca?OH?₂,which promotes expansive AFt formation and cause effect to volume stability,which is much easier to get hardened cement pastes cracked in hydration environment of SO₄^2-ions;On the microscopic structure,cement addition materials'particle size is much smaller than cement clinker,which can accelerate the nucleation of hydration products to precipitate out and fill the residual pore structure after hydration to form lower porosity and more denser structure.