Effect of decalcification on C-S-H gel microstructure in cement paste

Effect of decalcification on microstructure of C-S-H gel in ASTMⅠcement paste were studied by X-ray fluorescence spectrometer(XRF),X-ray diffraction(XRD),infrared spectroscopy with Fourier transform(FTIR) and ~(29)Si MAS NMR with deconvolution technique.It is shown from test results that the decalcification process can be divided into two stages.When the molar ratio of CaO to SiO_2 in the pastes at 3,28 d is lowered from～2.78 to～2.00,almost all of CH is removed and C-S-H gel is partially decalcified, the average chain length(ACL) of C-S-H is increased from 2.4 to 4.4 for 3 d paste and from 2.9 to 5.8 for 28 d paste respectively,and I(Q~2)/I(Q~1) is also increased.While the molar ratio of CaO to SiO_2 in the pastes comes to～1.83,decalcification of C-S-H gel occurs.C-S-H polymerizes and I(Q~2)/I(Q~1) increases largely in this stage,which results in further increase of ACL to 6.2 for 3 d paste and 9.9 for 28 d paste.The mechanism of increase of C-S-H polymerization could be transformation of dimeric C-S-H toward highly polymerized C-S-H.However,Al doping could affect the stability of C-S-H microstructure. In molecular level,the existence of the pure bridging silicon-oxy tetrahedrons in C-S-H is beneficial to the optimization of the microstructure.