Molecular Dynamics Simulation On The Microstructure And Performance Of Calcium Aluminosilicate Hydrate

Low-energy consuming and environmentally friendly concept makes cement-based materials usually designed with high volume of aluminum-rich mineral admixtures(fly ash,mineral powder and etc.).The Al³⁺ions released from the pozzolanic reaction of these admixtures are easily to be incorporated into calcium silicate hydrate(C-S-H)gel structure,resulting in the formation of calcium aluminosilicate hydrate(C-A-S-H)gel.As primary hydration production and main strength-giving phase in cement-based materials,the microstructure and performance of C-A-S-H gel are the basic units to determine the service performance of cement-based materials.However,the structure and properties of C-A-S-H gel at molecular scale and their evolution mechanism under erosive environment are by far not fully comprehended.By using experimental and simulation methods,this paper aims to study the influence mechanism of Al substitution on the nanostructure and properties of C-A-S-H gel,to investigate the Cl^-immobilization mechanism of C-A-S-H gel,and to reveal the nanostructure and mechanical properties evolution of C-A-S-H gel under sulfate attack.The conclusions of this paper are as follows:(1)At molecular scale,C-A-S-H is layered material structurally similar to Tobermorite crystal.When Al/Si is less than 0.1,almost all added aluminum can incorporat into C-A-S-H structure.When Al/Si is higher than 0.1,Al species forms Katoite(C₃AH₆)phase apart from entering the C-A-S-H structure.Al[4]in C-A-S-H structure can bridge the defective silicate chains to form longer aluminosilicate chains.As compared with bridging silicate tetrahedra,the presence of bridging aluminate species show the same influence on the mean chain length of aluminosilicate chains but increase the basal spacing of C-A-S-H gel.(2)At molecular scale,the C-S-H exhibits mechanical properties characteristics which are typical of layered materials.The chemical bonds within primary layers are intensive and the mechanical properties along the inplane directions are strong.On the contrary,the interaction between primary layers main depends mainly on hydrogen bonds and secondly on ionic bonds,and,consequently,the mechanical properties along interplanar direction are weak.Al incorporation heals the broken silicate short chains to form longer aluminosilicate chains,which help the C-A-S-H resist tensile loading along y direction and improve its mechanical properties along this direction.However,Al incorporation shows little effect on the mechanical properties along x and z directions,and the C-A-S-H still exhibited the mechanical properties of layered materials.(3)When the curing temperature reaches 80°C,aluminum incorporation can facilitate the cross-linking between primary layers in C-A-S-H gel,and hence turn C-A-S-H gel into cross-linked C-A-S-H gel.Al-induced cross-linking significantly changes the structure of calcium aluminosilicate skeleton in the C-A-S-H.The Al-O bonds bridge the defective short silicate chains and lead to the formation of branched aluminosilicate chains.This transforms layered C-S-H structure into network-like cross-linked C-A-S-H structure.Meanwhile,the formation of branched silicate chains reduces the connectivity of interlayer regions,blocks the movement of interlayer water molecules,and enhances the adsorption of water molecules on calcium aluminosilicate skeleton.Therefore,the mobility of interlayer water molecules greatly decreases and thus stability of calcium aluminosilicate skeleton improves.This significantly strengthens the connection of aluminosilicate skeleton.The tensile strength and young modulus of C-A-S-H gel along the interlayer direction increased significantly.When all bridge sites in C-A-S-H are cross-linked,the mechanical properties of cross-linked C-A-S-H along the interlayer direction are equivalent to those along the intra-layer directions.(4)The silicate chains and the charge-balancing Ca²⁺on the surface of C-(A-)S-H nanopore make surrounding water molecules have the glass-like structure and dynamic characteristics.The glassy water molecules have layered structure,preferred orientation,increased dipole moment and decreased diffusion coefficient.Non-bridged oxygen atoms in the silicate chain terminals can act as adsorption sites for cations.The cations adsorbed on the surface of C-(A-)S-H can form ion pairs with Cl^-ions,which enable the C-(A-)S-H to immobilize Cl^-ions.Al incorporation bridges the defective silicate chains in C-S-H structure,weakens it adsorption for Ca²⁺ions,reduces the number of cations and anions on the C-A-S-H surface,and finally weakens the immobilization of C-A-S-H for Cl^-ions.As compared with pure silicate chains of the same polymerization degree,Al substitution for Si increased the negative charge of the primary layer of C-A-S-H,enhances its adsorption for Ca²⁺,promotes the formation of ionic pairs on the surface of C-A-S-H,and,consequently,enhances the immobilization of C-A-S-H for Cl^-ions.Therefore,the Cl^-immobilizing ability can be enhanced by increasing the Ca/(Si+Al)ratio of C-A-S-H or increasing the Al/Si ratio of C-A-S-H at fixed Ca/(Si+Al)ratio.The aqueous cations also affect the Cl^-immobilization of C-A-S-H.Cations which bond closely to both oxygen atoms and Cl^-ions can form more stable chemical bonds with both aluminosilicate chains and Cl^-ions.The stable anion-cation ionic pairs on C-A-S-H surface are helpful in immobilizing Cl^-ions.Therefore,the stronger the interaction between cation and oxygen atom and Cl^-ions is,the stronger ability of cation is to help C-A-S-H immobilize Cl^-ions.The ability of cations to help C-A-S-H immobilize Cl^-ions is ranked as follows:Mg²⁺>Ca²⁺>Na⁺>K⁺.(5)Compositionally,sulfate attack shows decalcifying and dealuminizing effects on the C-A-S-H gel in cementitious pastes,leading to the decrease of Ca/Si and Al[4]/Si ratios for the C-A-S-H gel.The MCL of C-A-S-H gel increases and its microstructure changes,resulting in the breaking of calcium aluminosilicate skeleton and degraded mechanical properties(nanoindentation modulus and hardness)of C-A-S-H gel.When the cementitious paste is incorporated with slag,the Al³⁺released from the pozzolanic reaction of slag can react with invaded SO₄^2-ions and mitigate the decalcification and dealuminization of C-A-S-H gel under sulfate attack.Therefore,slag incorporation can weaken the destruction of sulfate attack on the calcium aluminosilicate skeleton and relieve the C-A-S-H gel of degradation in mechanical properties.The research results of this paper can provide theoretical basis for the design and preparation of sustainable and environmentally friendly cement-based materials and their long-life modification in marine serving environment.