Synthesis and Fundamental Properties of 3CaO·CaCl_2·15H_2O Causing CaCl_2 Attack,and Its Formation in Hardened Cement Paste Exposed to CaCl_2 Solution

【Author】

Ryuichiro Kuga;Hiroaki Mori;Shoichi Ogawa;

【Abstract】

<正>3CaO·CaCl_2·15H_2O is the main factor for the chemical deterioration of concretes caused by CaCl2.In this study,thefundamental properties of 3CaO·CaCl_2·15H_2O were studied through morphological analysis,X-ray diffraction(XRD),and Thermogravimetric/Differential Thermal Analyzer(TG-DTA) by using a synthesized 3CaO·CaCl_2·15H_2O singlecrystal.Moreover,the condition of 3CaO·CaCl_2·15H_2O formation using hardened cement paste and variousconcentrations of CaCl_2 and NaCl solutions in various temperatures was studied through XRD and thermodynamiccalculations.As a result,the synthetic sample is significantly unstable, and forms a needle-shaped crystal.The heatcharacteristics and chemical reactions accompanied with heating are explained Additionally,it was shown that 3CaO·CaCl_2·15H_2O has a strong preferred orientation.In the reaction between CaCl_2 solution and hardened cement paste,under the condition of high CaCl_2 concentration and low temperature,Ca(OH)2 peaks disappeared and 3CaO·CaCl_2·15H_2O peaks appeared in XRD patterns of the cement pastes.These results by experimental methods corresponded tothe change of mineral components calculated by the thermodynamic method From the results of calculations,the total volume of cement paste was markedly increased by 3CaO·CaCl_2·15H_2O formation,and this volume change may leadto deterioration. Originality 3CaO·CaCl_2·15H_2O(3-1-15) single crystals were synthesized,absent of other known phases such as Ca(0H)2,whichwas found in prior studies.This allowed us to measure the fundamental properties of 3-1-5,including its preferredorientation,and relative stability,and examine the basic outcomes of concrete deterioration by CaCU attack. Althoughthe mechanism is still unknown,we have determined,by calculations,the range of conditions where 3-1-15 may form inhydrated cement and inevitably cause damage to the structure of the cement.The damage was caused by exposure toCaCU,which in turn resulted in deterioration and volume expansion.Because de-icing salts,NaCl and CaCl_2, areregularly used to maintain the safety of roads during cold seasons,we hope that our findings may help with futurestudies on maintaining the longevity of roads and understanding the effects of salts used.

【Keywords】

Chemical deterioration;;Calcium chloride;;3CaO·CaCl_2·15H_2O;;XRD;;Thermodynamic calculations

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