Research On Type Evolution Of Calcium Silicate Hydrate In Ultra-high Performance Concrete

Ultra-high performance concrete is increasingly used in various engineering cases due to its superior mechanical properties and durability.For UHPC,ordinary curing regime are difficult to significantly activate mineral admixtures,but dry air curing,combined curing and autoclave curing can work well in this aspect.When the curing temperature reaches 200℃,xonotlite will be formed in UHPC,which can be detected by microscopic testing techniques.In 2018,our previous research proposed a combined curing regime,namely hot water+dry air combined curing,i.g.90℃hot water curing for 2d+250℃dry air curing for 3d.Under this curing regime,the mechanical properties,thermally explosive spalling resistance and microstructure of UHPC were greatly improved.The needle-shaped xonotlite was observed by scanning electron microscope after combined curing.Based on the research results of our research group,this thesis studies the influences of various factors on the formation of xonotlite in ultra-high performance concrete including three aspects such as curing regime,mineral admixtures and intentionally added Ca（OH）₂.In this thesis,the effect of curing regime on the formation of xonotlite in UHPC was verified firstly.Then,the effect of mineral admixtures on mechanical properties and microstructure of UHPC was studied based on the hot water-dry air combined curing.Finally,on the basis of 10%silica fume（mass fraction,the same below）+10%fly ash+10%ground granulated blast furnace slag+20%quartz powder,Ca（OH）₂powders were added to study the effect of Ca（OH）₂on the mechanical properties and microstructure of UHPC.The main conclusions of this thesis are as follows:（1）When the dry air curing temperature reaches 200℃,xonotlite appears in UHPC with a small quantity.When adopting combined curing,the properly increased curing temperature（dry air curing temperature increases from 200°C to 250°C）can increase the quantity of xonotlite（from 0.7%to 1.0%）,but the quantity of tobermorite decreases（from 1.2%to 0.9%）.This phenomenon has not been reported in previous studies.Xonotlite mostly exists in the round pores,and usually appears to intersect with tobermorite and other crystals.At the same time,it is found that tobermorite transforms into xonotlite gradually with temperature increasing.In a specific round pores,this process develops from the edge to the center.（2）The best combination of mineral admixtures is 10%silica fume+10%fly ash+10%ground granulated blast furnace slag+20%quartz powder.Properly increasing the dosage of quartz powder（from 15%to 20%）promotes the formation of more xonotlite,but there is a balanced point for the transformation of tobermorite to xonotlite.Excessively doped quartz powder（≥30%of the quartz powder）will inhibit this transformation process,which is the latest research finding of this thesis.In addition to cement and quartz powder,if there are also active mineral admixtures such as silica fume in the cementitious materials,tobermorite and xonotlite will be formed in UHPC.Otherwise,jennite will be formed.（3）With incorporating intentionally added Ca（OH）₂,the mechanical properties and microstructure of UHPC were both improved.This way of adding Ca（OH）₂——unlike in ordinary cement-based systems where Ca（OH）₂is derived from hydration of cement,the added artificial pure Ca（OH）₂powders（content≥95%）——has not been reported in any previous literature.When the Ca（OH）₂dosage is 5%,the 7d compressive strength reaches 191.3 MPa.With increasing the Ca（OH）₂dosage,tobermorite,xonotlite and other crystalline phases are formed increasingly,while the hydration of cement is restrained to a degree.On the basis of 5%Ca（OH）₂dosage,it is found that increasing the dry air curing time will have little impact on the quantity of xonotlite formed,and almost all Ca（OH）₂has been consumed by the chemical reaction after one day of dry air curing.（4）In addition to standard curing and hot water curing,UHPC under dry air curing and combined curing encountered a retrogression of long-term compressive strength.This thesis speculates that this was due to the uneven distribution of hydration products in UHPC after dry air curing,which hindered the subsequent hydration.However,after dry air curing or combined curing,the hydration process of UHPC is basically completed.This may explain that UHPC exhibits a slight retrogression of long-term compressive strength.Moreover,the compressive strength under dry air curing and combined curing is 30 MPa higher than that under hot water curing.The strength reduction is more serious after adding 7%Ca（OH）₂,where compressive strength was 186.5 MPa at 7d and 167.6 MPa at 90d.Thus the compressive strength decreased by 10.1%.It is speculates that the strength retrogression is related to the addition of Ca（OH）₂which inhibits the further hydration of cement.The mechanism of retrogression of long-term compressive strength needs to be further studied.