Organic Fluorescent Molecules For Cement Parameter Detection And Carbonation Characterization

Cement-based materials are widely used in the construction field.Cement hydration consists of two coupled processes:dissolution of clinker phases and hydrate formation,accompanied by constantly changing of physicochemical parameters（e.g.pH,Ca/Si ratio,etc.）.As a high alkalinity material,the OH-concentration in cement affects not only the composition of the hydration products（C-S-H gels,calcium alumina,etc.）,but also the stability of the products and the interaction between cement components（e.g.admixtures）.The hydration of cement is related to the mechanical properties of cement/concrete,durability,and the performance level of the building structure.In addition,the hardened cement material is never free from carbonation,i.e.CO₂ reacts with Ca（OH）₂/C-S-H gels,resulting in a decreasing alkalinity of the hardened cement.For reinforced concrete,this process destroys the protective film and leading to corrosion of the reinforcement and eventually causing deterioration of the concrete properties.Therefore,real-time and rapid detection of the pH value during cement hydration and the carbonation depth of hardened cement is very important for researchers to explore the cement hydration and carbonation processes.In this paper,we designed and synthesized a series of pH-responsive fluorescent probes,characterized and analyzed their structure and fluorescence properties,and realized the detection of pH values of cement hydration and carbonation depth of hardened cement,respectively.It solves the problem of accurate measurement of pH values during cement hydration and fine characterization of pH change during carbonation of hardened cement,which provides a new research avenue for cement research.The specific research of this paper is as follows:1.Synthesize two organic fluorescent probes achieve pH detection of multisystem cements.One is a non-aromatic amide small molecule（CA-BAPP）prepared from citric acid（CA）and 1,4-bis（3-aminopropyl）piperazine（BAPP）by low-temperature chloride method.The structures were characterized by nuclear magnetic spectroscopy(¹H NMR,¹³C NMR)and Fourier transform infrared spectroscopy（FTIR）.The fluorescence spectra showed a good linear response（R²=0.994）in the pH range of 9.0-13.5.CA-BAPP was applied to detect the pH of leachate of ordinary Portland cement（OPC）for different hydration time periods.The sensitivity of CA-BAPP based on fluorescence intensity for cement pH measurement was low,and the leachate needed to be treated with the metal chelator EDTA-2Na to eliminate the effect of interfering ions before measurement.The second one is a ratiometric pH fluorescence probe（CPR）formed by physically mixing rhodamine B with a polymer spot synthesized from CA and 1,3-propanediamine（PDA）.CPR showed a sensitive linear response（pH 12.00-13.25）and was largely unaffected by interfering ions.The pH measurement of cement leachate with high/low water-cement ratios,in-situ slurries and fly ash（FA）blended samples was better achieved by CPR.Combined with isothermal calorimetry,mineral composition and micromorphological analysis,the change of pH value is consistent with the change of mineral composition phase during hydration.2.T-Series fluorescent indicator enables pH marking of carbonation of hardened cement.Four fluorescent indicators（DDP（T1）,CA-Aphen（T2）,Et-Curcumin（T3）and Julolidine-Chalcone（T4））with phenolic hydroxyl,diethylamino and Schiff-base bonds,respectively,were selected to characterize the carbonation state of accelerated/naturally carbonated cement specimens（OPC,calcium sulfoaluminate cement（CSA））.T1-T4 showed different pH response thresholds（T1:pH 7.0-7.6,9.0-10.4;T2:pH 6.2-6.7,9.0-9.9;T3:pH 8.3-8.8,9.5-10.8;T4:pH 6.8-8.0,9.0-10.0,12.0-12.5）.Compared with phenolphthalein（pH 8.2-8.5）,these four fluorescent indicators can not only achieve fine labeling of partial carbonation zone of OPC,but also applied to fine labeling of carbonation process of low alkalinity CSA cement.The UV-Vis absorption spectra and zeta potential indirectly show that the cause of discoloration is its different protonation states at different pH.Combined with FTIR,X-ray diffraction（XRD）,scanning electron microscopy（SEM）and thermogravimetric analysis（TGA）,the discoloration ranges of these indicators were shown to correspond to the products of different carbonation stages of the cements.In addition,the pH range for producing reinforcement corrosion was clarified by electrochemical methods,and thus the reliability of the indicators applied to mark the pH of producing reinforcement corrosion in concrete was evaluated.