Study On The Modification Of BCT Cement Synthesized At Low Temperature By Admixture

Belite-ternesite-calcium sulphoaluminate（BCT）cement,as a low-carbon cement,has fast setting time and fluidity loss in practical applications.Based on the influence of chemical admixtures on the properties of high belite-calcium sulphoaluminate（HBCSA）cement,a suitable admixture was selected to modify the self-made low-temperature BCT cement from industrial waste slag coal gangue in order to improve the working performance and mechanical properties of BCT cement.The effects of water reducing admixtures,accelerating admixture and retarder on the macroscopic properties of BCT cement such as setting time,fluidity and strength were investigated.Effects of admixtures on the hydration process and products of cement were analyzed by hydration heat,X-ray diffraction and scanning electron microscopy.The corrosion resistance of BCT cement to sulfate and chloride ions was further explored,and BCT cement repair materials were prepared.Firstly,the effects of suitable water reducing agent,early strength agent and retarder types and dosing variation on HBCSA cement were studied.The results showed that polycarboxylate superplasticizer（PCE）,calcium nitrite（Ca（NO₂）₂）and sodium citrate（SC）could improve the mechanical properties of HBCSA cement while delaying the setting and hardening.In particular,the retarding effect of SC is significant,in which the initial and final setting time of the slurry can be extended by 149 min and 219 min at most.Ca（NO₂）₂could promote the adsorption of PCE on the surface of cement particles and improve the improvement of slurry fluidity by PCE.When 0.7%PCE was compounded with 1.2%Ca（NO₂）₂,the 3 d and 60 d compressive strengths of HBCSA cement could reach 95.7 MPa and 110.0 MPa.Moreover,the formation of ettringite（AFt）was promoted,a hydration product,when hydrated for 3 d,and AFt crystals was refined,which improved the microstructural densities.Next,the low-temperature BCT cement clinker was prepared by promoting the mechanical force chemically activated industrial waste gangue as the main raw material at 1200°C for 2 h.The 3 d and 60 d compressive strengths of BCT cement can reach 61.9MPa and 88.4 MPa,respectively,at a water-cement ratio of 0.3.The performance study of BCT cement mixed with PCE,Ca（NO₂）₂ and SC found that all three could inhibit the generation of AFt at 3 d and improve the fluidity performance of the paste,and the appropriate admixtures were 0.4%,0.8%and 0.7%,respectively,for which the 60 d compressive strength of the cement reached 99.8 MPa,116.3 MPa and 105.9 MPa.When0.4%PCE was compounded with 0.8%Ca（NO₂）₂,the 60 d compressive strength of cement could reach 121.1 MPa;the synergism of PCE and Ca（NO₂）₂ could further enhance the flowability of the paste,reducing the exothermic rate at the initially stage of hydration of BCT and prolonging the setting time of the paste.The amount of AFt generation was reduced in the 3 d hydration sample,but the length-to-diameter ratio of AFt crystals was decreased,and the microstructure of the crystal skeleton was mostly built by columnar AFt.When 0.7%SC was incorporated,the initial hydration process of BCT was delayed and the generation rate of AFt was reduced,which led to the increase of columnar AFt in the 3 d hydrated samples and the uniform distribution of intertwined with needle-like AFt with increased microstructure density.Finally,the study found that BCT and HBCSA cements outperformed ordinary portland cement in corrosion resistance under the action of sulfate ions,chloride ions and multi-ion erosion in artificial seawater respectively.The incorporation of PCE compounded Ca（NO₂）₂ and SC could hinder the ion intrusion,and the 28 d corrosion resistance coefficient of BCT cement tended to 1 in 5%Na SO₄/Mg Cl₂solution and artificial seawater,respectively.