Investigations On The Relationships Between Compressive Strength Of Cement Pastes And Mortars And Their Pore Structure Of Multiscale Characteristics

Cement-based material is a kind of porous material,its pore structure has multi-scale,and its compressive strength has a decisive influence.Previous studies have failed to recognize the great difference between pore structure of cement-based materials under dry and saturated conditions,and the research on the relationship between pore structure and compressive strength under saturated conditions has been missing.Net in this paper,the research object to white Portland cement paste and mortar,the use of low field mri(LF-NMR)and mercury intrusion method(MIP),respectively,to test the saturation of pore structure and dry conditions,comparison of the groups of specimens under dry and saturated pore structure,total porosity,aperture parameters such as porosity and characteristics,the qualitative analysis of the difference of pore structure and the influence of different size of pore against compressive strength.The existing models were tested with the experimental data,the applicability of the models under saturated state was emphasized,the advantages and disadvantages of the models were analyzed,some assumptions in the models were extracted,and the mathematical models of compressive strength and pore structure parameters under dry and saturated states were established respectively.The main research results of this paper are as follows:(1)In the saturated state,the large pore with an equivalent diameter of more than 1000 nm had the most significant weakening effect,while the interlaminar pore and gel pore with an equivalent diameter of less than 10 nm also had a certain weakening effect.The increase of transition pore and capillary pore content at 10-1000 nm will reduce the strength,but the effect is not significant.In the dry state,the pore resistance and compressive strength greater than 1000 nm play a major role in weakening,while the large capillary pores of 100-1000 nm are mainly distributed in the interface transition zone,which weakens the bonding force between the cement slurry matrix and the aggregate interface and reduces the compressive strength.Holes of less than 100 nm will reduce the strength of cement paste matrix.(2)The critical pore size and percolating pore size are not directly related to the overall compressive strength.When the water-cement ratio is the same and the total porosity is similar,the median pore diameter and logarithmic average pore diameter,the higher the compressive strength.There is no direct correlation between water-cement ratio and compressive strength.(3)According to Griffith fracture theory and strength criterion,variables such as fracture energy and elastic modulus are recombined,and logarithmic average aperture is adopted to replace fracture length,and then a new semi-empirical fracture theoretical model is combined.It is found that the model can be used to describe the relationship between pore structure and compressive strength when the relationship between elastic modulus and fracture energy is exponential.For the saturated state,if the partial porosity of pores in the gel less than 10 nm is considered and the influence of cement mass fraction is introduced to improve the combined semi-theoretical model,it can effectively describe the relationship between pore structure and compressive strength in the saturated state.