Study On The Effect Of Natural Mineral Powder On The Properties Of Cementitious Materials At Micro And Nano Scale

Cement is one of the main raw materials in the construction industry,but its production process is accompanied by high energy consumption,high pollution and high CO₂ emissions,which seriously affect the environment.Using mineral admixtures to replace part of cement in concrete can effectively alleviate the environmental problems caused by cement production.The commonly used mineral admixtures include rock powder,silica fume,fly ash,etc.In recent years,natural minerals such as diatomite powder（DP）,shell powder（SP）and coral powder（CP）are also used as admixtures to replace cement.In this paper,the effects of diatomite,shell powder and coral powder on the rheology,hydration,pore structure,drying shrinkage and strength of cementitious materials are investigated at micro and nano scale.Firstly,X-ray diffraction and laser particle size analysis were conducted on diatomite,shell powder and coral powder.The interaction of these particles with ions in the pore solution of cement paste was then investigated by zeta potential test.Rheological test and flocculation observation were performed to investigate the effect of these powders on the rheological properties of cement paste.Their effect on the nucleation and growth of hydration products was explored.Then the pore size distribution and drying shrinkage of the cement paste mixed with these powders were studied.Finally the interface properties between these powders and hydration products were studied with SEM observation of the fracture surface,and the compressive strength was tested parallelly.The results show that:（1）Quartz powder and diatomite powder have similar surface charge properties,and Ca²⁺adsorption on their particle surfaces is driven by electrostatic interaction.Shell powder and coral powder belong to calcite phase,which can chemically adsorb Ca²⁺in cement paste pore solution and have a stronger adsorption capacity for Ca²⁺.（2）The surface charge and particle size of natural mineral powders affect the rheological properties of cement paste.Quartz powder and diatomite powder have similar surface charge properties and particle size as cement particles,but diatomite powder has a large number of nanopores on its surface,which can absorb water in cement paste,reducing the fluidity of cement paste.Shell powder and coral powder have smaller particle size and larger specific surface area,leading to less free water between the particles and thinner water film,and reducing the space between the particles,which in turn deteriorates the rheological properties.（3）The adsorption of Ca²⁺on the surface of natural mineral powders affects the nucleation and growth of C-S-H.The adsorption capacity of Ca²⁺on the surface of quartz powder and diatomite powder is weak,but the porous structure of diatomite powder is favorable to the nucleation and growth of C-S-H,resulting in more C-S-H on its surfaces.Shell powder and coral powder can chemically absorbs Ca²⁺,which is favorable to the nucleation and growth of C-S-H.（4）Diatomite powder has a large number of nanopores,which is conducive to the generation of hydration products,resulting in a lower porosity compared to the hardened cement paste with quartz powder.The smaller particle size of the shell and coral powder can fill the voids between cement particles,resulting in lower porosity.In addition,the drying shrinkage of the hardened cement paste is related to its pore structure,and the drying shrinkage and mass loss of blended hardened cement paste are larger than that of the pure hardened cement paste.（5）Compared with quartz powder,the interface between diatomite and hydration products is stronger.The interface between shell or coral powder and hydrates is weak at early ages,but it is enhanced as the hydration proceeds.（6）The compressive strength of the hardened cement paste is related to the the pore structure.The cement paste mixed with 15%quartz powder has a higher accumulated pore volume of capillaries and macropores;while the cement paste mixed with 15%diatomite,shell powder and coral powder has a smaller accumulated pore volume of capillaries and its compressive strength is higher.