Effect Of Alkali Content On The Properties Of Portland Cement In Early Age

Due to concerns about the alkali aggregate reaction of concrete,the adverse effects on the compatibility of cement and additives as well as on the cracking sensitivity of concrete,the content of alkali in cement is restricted strictly in recent years.This goes against the full use of low-grade raw materials and the sustainable development of the cement industry.Because the compositions and properties of cements and concretes vary greatly recent years,the effecting mechanism of alkali on the hydration process of cement needs to be examined renewedly and recongnized comprehensively,for a reasonable use of cements with different alkali content.In this thesis,Portland cement clinkers with different alkali content produced by different manufacturers were used,a certain dosage of potassium sulfate were added to prepare a series of Portland cements with a large range of alkali content.The influence of alkali on cement hydration process was studied systematically.The thesis focuses on the hydration kinetic,hydration rate,hydrates and pore solution composition of cements with different alkali content,as well as their influenced factors.Moreover,the influence of alkali content on the compatibility of cement and additives,the development law of cement strength were studied.According to the analysis results of Portland cement clinkers produced by different manufacturers,high alkali content in clinker depress the development of C₃S crystal in clinker.The distributions of alkali in different mineral phases of clinkers are different,the segregation of K is more significant.The alkali sulfates in the cement clinkers dissolve rapidly after the mixing of cement and water,the concentrations of alkali metal ions tend to be stable after 10 minutes.The dissolved ratio of K is much higher than that of Na.The higher the ratio of sulfur to alkali is,the higher the dissolved ratio of alkali in cement is.The dissolution of soluble alkali in cement changes the composition of pore solution directly,influences the dissolution of C₃A and C₃S,the precipitation of CSH and the formation of CH and AFt through its effect on the saturation of related substances.High alkali content in cement depresses the dissolution of C₃A and the formation of AFt,promotes the dissolution of C₃S and the formation of CH,and leads to a higher hydration degree of cement paste at early age.With the increase of alkali content,the effect on the precipitation of CSH changes from promoting its growth rate to promoting its nucleation simultaneously,which corresponds to the acceleration of cement hydration reaction and the shortening of induction period.High alkali content has an adverse effect on the compatibility of cement and polycarboxylate superplasticizer.The alkali addition increases zeta potential of cement paste,resulting in some of the polycarboxylate superplasticizer is adsorbed onto the diffusion layer on the surface of silicate phase,which restrains its adsorption-dispersion effect.Moreover,the competitive adsorption of SO₄^2-in pore solution and polycarboxylate superplasticizer on the surface of aluminate phase restrains the adsorption-dispersion effect of polycarboxylate superplasticizer ulteriorly.The compatibility of cement with low C₃A content and polycarboxylate superplasticizer is more sensitive to the change of alkali content.The moderate increase in alkali content in cement delay the setting of cement with low alkali content.The large increase in alkali content(K-base)lead to the formation of syngenite crystal in the paste,promoting the setting of cement.High alkali content can promote the refinement of the pore structure of the hardened paste and improve the strength of cement in early age,but it is not conducive to the optimization of the pore structure of the hardened paste and the strength development of cement in later age.