A Research On The Formation,Evolution And Regulation Mechanism Of Thaumsite In Cement-based Materials

Thaumasite form of sulfate attack?TSA?is a new type of sulfate attack that C-S-H gel in hardened cement paste exposed in sulfate environment transforms into non-cementitious and mud-like substance,which is undiscoverable and destructive.the TSA has become a hot topic in the field of concrete engineering.At present,a large number of major projects would be built with the implementation of"One Belt and One Road"and Western development strategy in China.However,the resources,geology and climate along the Northwest part of China make the concrete structure with great risk of TSA.Hence,it has important strategic significance and guiding value to study the formation,evolution and regulation mechanism of thaumasite.This paper is funded by the National Natural Fund?511786363?and the National Key Research and Development Project?2016YFC0701003-05?to study the formation mechanism and control factors of thaumasite,and the features of Al phase and C-S-H gel during the formation of thaumasite.The following conclusion can be drawn:1.The formation condition and evolution mechanism of thaumasite based on synthesis system of CaO-Na₂SiO₃-Na₂SO₄-NaAlO₂-Na₂CO₃?1?When S/Al is 3 and Ca/Si is 3,after 6 months of curing age in the sulfate environment,the main hydration was ettringite and the thaumasite was not found.However,with higher Ca/Si ratios of 6 and 9,the thaumasite was formed at 14 days and 3 days,respectively,indicating that the thaumasite was more prone to be formed with increased Ca/Si ratio.With higher S/Al ratios?6 and 9?,the thaumasite was not formed in the solution with Ca/Si=3 after 6 months but was found in the solution with Ca/Si=6 at 7 days.To sum up,higher S/Al ratio could promote the formation of thaumasite,while Ca/Si ratio is the dominant factor on the formation of thaumasite.?2?At low Ca/Si ratio of 3,Al[4]phase could be found in the solution,which may prevent the replacement of Al in ettringite by Si and thus prevent the formation of thaumasite.2.The formation condition and evolution mechanism of thaumasite in the system of C₃S-Na₂SO₄-NaAlO₂-CaCO₃ and C₃S-C₃A-Na₂SO₄-CaCO₃?1?In the system of C₃S-Na₂SO₄-NaAlO₂-CaCO₃,after the curing age of 14months,the ettringite was found but the thaumasite did not formed in the specimens,which present a similar phenomenon that found in the synthesis system with Ca/Si ratio of 3.Namely in spite of the S/Al ratio,the plentiful Al[4]phase prevents the transformation of ettringite to thaumasite.?2?In the system of C₃S-C₃A-Na₂SO₄-CaCO₃,after reaction for 14 months,the ettriginte present as the main product.No thaumasite was produced in the system with S/Al ratio of 3.While in the case of higher S/Al ratio?6 and 9?,both ettriginte and thaumasite were formed.3.The formation threshold and temporal spatial evolution rule of T salt in cement paste system.?1?Obvious TSA damage was observed in control sample after soaking for 12months,and it is characterized by cracking,falling off and sliming.The specimens with 30%Aluminum-Silicon Micro-Bead?ASMB?also showed obvious skin peeling,cracking and shedding.However,the erosion is typical erosion damage of ettringite type sulfate rather than TSA.With the increase of ASMB content?45%and 60%?,no deterioration occurred in the specimens.?2?Microscopic test results show that,with the increase of ASMB content,the average molecular chain length?MCL?and Al[4]/Si ratio of C-S-H gel increase gradually,indicating that the TSA resistance ability increased with the increasing of polymerization degree of C-S-H gel.In addition,the content of Al[4]in the aluminum phase also increased,which confirmed that the presence of Al[4]coordination aluminum phase inhibited the transformation of ettringite to T salt.?3?The spatiotemporal evolution rule of C-S-H gel and Al phase in TSA process was analyzed by layer-by-layer sampling method.With the increase of erosion age?1months,6 months and 12 months?,the MCL of the C-S-H gel in the surface layer0-1mm of the control sample increased from 4.540 to 5.266 and then decreased to3.621,and the Al[4]/Si ratio increased from 0.031 to 0.058 and then decreased to0.032.In addition,the content of Al[4]in aluminum phase decreased from 53.03%to0,and the content of Al[6]increased from 46.97%to 100%.The results showed that the C-S-H gel decalcified and Al[4]bridged to the silicon chain at the initial stage of erosion,resulting in an increase in the ratio of MCL to Al[4]/Si.With the age of erosion extended to 12 months,the Al[4]transferred to Al[6],resulting in the breakage of silicon chains by C-S-H gel removing the bridging Al[4],reducing the ratio of MCL to Al[4]/Si,resulting in the decrease of the polymerization degree of C-S-H gel and thus lowing its resistance to TSA erosion.The MCL of C-S-H gel increased from 2.882 to 3.251 in the layer of 1-2mm,and Al[4]continuously changed to Al[6]but not bridged into C-S-H gel,which reduced the Al[4]/Si ratio?from 0.028to 0.009?.?4?With the extension of erosion age?1 months,6 months and 12 months?,The MCL and Al[4]/Si ratios of C-S-H gel in the surface of 0-1mm and 1-2mm of60%ASMB paste showed an increasing trend,indicating that the degree of polymerization of C-S-H gel increased,resulting higher TSA resistance ability of C-S-H gel.In addition,the Al[4]and Al[6]contents in the aluminum phase of 0-1mm layer decreased from 44.30%to 26.50%and increased from 31.25%to 50.91%,respectively.The content of Al[4]in aluminum phase of 1-2mm layer decreased from33.90%to 24.93%,and the content of Al[6]did not change significantly?from47.29%to 54.79%?.4.Erosion characteristics and prevention strategy of thaumasite in cement based mortar system.In TSA environment,the mass and compressive strength of control mortar specimens showed a decrease trend during the whole erosion period?1 to 18 months?withfinal mass loss and strength lossof 3.36%and 38.6%,respectively.However,the surface of ASMB-30,ASMB-45 and ASMB-60 mortar specimens remained intact throughout the erosion period without any significant quality change.Their compressive strength increased by 43.49%,89.18%and 117.92%with the extension of erosion time,respectively.Therefore,when there is a potential risk of TSA in the service environment of concrete,adding a large number of active ASMB could effectively prevent and control TSA damage in concrete.