Structure And Properties Of C-(A)-S-H With Different Calcium To Silicon Ratios And Their Interactions With Erosion Ions

Calcium silicate(aluminum)hydrate gel,namely C-(A)-S-H(including C-S-H and C-A-S-H),as the main hydration product of cement-based materials,fundamentally affects the engineering properties of concrete.Faced with ionic erosion under severe environmental conditions,the use of mineral admixtures improved the durability of concrete,but affected the composition and structure of C-(A)-S-H gel in cement paste.The most obvious change is the calcium to silicon ratio(Ca/Si).However,at present,the changes in the structure and properties of C-(A)-S-H and the interaction mechanism with erosion ions under different conditions of Ca/Si have not been clarified.Therefore,C-(A)-S-H molecular models with different Ca/Si were established based on the chemical composition,the structure and properties of synthesized C-(A)-S-H gel were characterized.Combined tobermorite and jennite models,Al³⁺doped Al-tobermorite and Al-jennite models,C-(A)-S-H nanopore channel models with different Ca/Si were established.The molecular dynamics method was used to simulate the interactions of C-(A)-S-H combined models,Na Cl solution and Na₂SO₄solution.Finally,four kinds of cement-based materials,namely ordinary portland cement,fly ash,slag powder and silica fume,were selected to prepare paste matrix containing C-(A)-S-H gel with different Ca/Si.The paste microstructure changes during Cl^-and SO₄^2-ion erosion were studied.The important innovative results obtained in this paper were:(1)The Clay FF molecular force field parameters of the bridge oxygen in the silicon-oxygen tetrahedral dimer structure were fine-tuned to distinguish between the framework calcium ions and interlayer free calcium ions in the C-S-H model;the atomic charges of the tobermorite and jennite models were redistributed to strictly meet the requirements of charge neutrality,which laid a foundation for the establishment of C-(A)-S-H models with different Ca/Si.(2)Based on the continuous Ca/Si increasing process,a modeling method that evolved from tobermorite to jennite to C-S-H molecular model was proposed.On this basis,amorphous C-(A)-S-H models with different Ca/Si were established.The anisotropic elastic modulus of C-S-H with different Ca/Si was significantly different.The isotropic elastic modulus of C-S-H was about 65 GPa.The anisotropic elastic modulus of C-A-S-H with different Ca/Si was relatively consistent.The isotropic elastic modulus of the high Ca/Si C-A-S-H was close to that of the C-S-H model,and the low Ca/Si C-A-S-H ratio was increased by about 8%.In the same way,C-(A)-S-H defective crystal models with different Ca/Si were established.The complete removal of interlayer calcium ions would cause the instability of C-S-H layer structure,while the partial removal can reach the convergence.Establishing Q³structure through translation operation can improve the stability of C-S-H layer structure.Changing Ca/Si or replacing Si⁴⁺with Al⁴⁺had no significant effect on the cell parameters of the C-(A)-S-H model,and the ordered layer structure can be maintained.(3)The C-S-H morphology with gel state was observed using transmission electron microscopy and freezing technology(Cryo-TEM),which had two typical characteristics of fibrous and parallel straight stripes.Grinding and drying process would destroy the gel structure of C-S-H.Selected area electron diffraction results showed that nano-crystallites exist in the C-S-H gel structure,which can be described by defective crystals.C-S-H films with different Ca/Si were prepared by physical vapor deposition magnetron sputtering.The nano-dynamic mechanical analysis method(Nano-DMA)was introduced in the nano-indentation test to obtain the eigenvalues elastic modulus of the C-S-H films with film thickness less than 100 nm.The static elastic modulus of C-S-H films with different Ca/Si was about 60 GPa and the storage modulus range was 50-80 GPa,corresponding to the calculated elastic modulus of the C-S-H amorphous model.The adsorption capacity of C-(A)-S-H gel to Cl^-and SO₄^2-ions was affected by the combined effect of Ca/Si and Al content,and it was not significantly affected by Cl^-ion penetration,but would produce decalcification by SO₄^2-ion attack.(4)When C-(A)-S-H contacted with Na Cl solution,Na⁺ions replaced the surface free Ca²⁺ions and were adsorbed on the surface.The adsorption amount was larger in C-A-S-H with higher electronegativity.Cl^-ions can directly bind to the high Ca/Si surface.Al substitution improved the Cl^-ion adsorption on the low Ca/Si surface and decreased the Cl^-ion adsorption on the high Ca/Si surface.In the C-S-H mixed phase,Cl^-ions tended to adsorb on the high Ca/Si surface and stay away from the low Ca/Si surface,while in the C-A-S-H mixed phase,the difference between the different Ca/Si surfaces was little.When C-(A)-S-H contacted with Na₂SO₄solution,compared with the Na Cl group,a larger amount of Na⁺ions were adsorbed on the surface.C-(A)-S-H surface indirectly adsorbed SO₄^2-ions through cations as bridges.Al substitution enhanced the binding effect of the surface to SO₄^2-ions.In the C-S-H mixed phase,high Ca/Si surface adsorbed SO₄^2-ions more easily,and the C-A-S-H mixed phase was quite the opposite.This paper presented a statistical method to calculate the dynamic states of Cl^-and SO₄^2-ions,analyzed the proportion of ion adsorption and diffusion states in the entire time domain of the simulation process,validated the results of ion adsorption experiments,and proposed a high-durability reinforced concrete microstructure design and a sulfate resistant concrete material design.(5)When the Ca/Si of hydration product C-(A)-S-H gel in the outer exposed paste matrix was higher than that of the inner non-exposed paste matrix,the diffusion of erosion ions can be delayed,which conformed to the aforementioned molecular dynamics simulation.The Cl^-ion penetration process promoted the formation of more C-(A)-S-H gels;the change of polymerization degree was related to Al doping.The formation of C-A-S-H gels could increase the polymerization degree.The SO₄^2-ion attack process also promoted the formation of more gels in the paste matrix,and caused the decalcification of C-(A)-S-H gels,resulting in fragmentation of the gel structure and a general decrease in polymerization degree.Under severe environmental conditions,the combination of the outer slag cement paste(Slag-PO)and the inner fly ash cement paste(FA-PO),that is,the g A system,was a better choice to reduce the effect of erosion ions.The g A paste matrix can absorb more free Cl^-ions and delay the further penetration of Cl^-ions;during the SO₄^2-ion attack process,it can stimulate the secondary hydration reaction to produce later-stage products such as C-A-S-H gel,which was helpful to resist the erosion damage of SO₄^2-ions.The research results of this paper can provide theoretical and experimental basis for the design of high-durability concrete microstructure.