Study On Performance Of Alkali/Gypsum Slag Composite Gelling System

Gypsum slag cement is a green hydraulic cementitious material with slag as the main component.It has the advantages of low hydration heat,good anti-seepage and anti-erosion performance,good anti-alkali aggregate reaction performance,etc.,but its slow setting speed and low early strength,The surface is easy to carbonize and sand.Combining the characteristics of alkali-activated materials with fast setting speed and high early strength,this paper systematically studies the alkali/gypsum slag cement cementing system,which has the advantages of both gypsum slag cement and alkali slag cement,in order to be used in the preparation of concrete precast parts.First,the mechanism of action of different stimulants on gypsum slag cement was studied.The slag cement with different contents of gypsum was stimulated by Portland cement,and the optimal content of gypsum was determined to be 12wt%of the total cementitious material.The experiment compares the excitation effects of P·O 42.5R,CaO,MgO,and Na₂CO₃ on the alkali/gypsum slag cement cementing system.The cement group is used as a reference,and the Na₂CO₃ group is used as a strong soluble alkali control group.The results show that the specimen prepared with magnesium oxide as the stimulant has the best compressive strength,followed by the cement group,and the calcium oxide group has the lowest strength.The strength of the gypsum slag cement is most sensitive to the sodium carbonate content.When the amount of magnesium oxide is 7.5%by weight,the compressive strength of the 28-day test piece is 92.8MPa and the flexural strength is12.5MPa.SEM test shows that the structure of the hydration products of the magnesium oxide group is the most dense;XRD test results show that the four groups of hydration products are CASH,AFt,CaCO₃,etc.Secondly,the action rules and mechanisms of magnesium oxide on gypsum slag cement were studied.The effects of different activities of light magnesium oxide and light burned magnesium oxide on the strength of gypsum slag cement were compared and analyzed:the compressive strength of the two 28-day-olds was 92.8 MPa and 65.6 MPa respectively,and the corresponding 28-day flexural strength was 11.7 MPa and 12.9MPa;the compressive strength of both 56 days of age is 96.3MPa and 80.9MPa respectively,and the corresponding 56-day flexural strength is 12.5MPa and 14.1MPa respectively.SEM test shows that the light magnesium oxide group has only a few local microcracks,while the lightly burned magnesium oxide group can see more developed local and penetrating microcracks,and even the penetrating cracks have a tendency to communicate;EDS test results shows that the degree of hydration of lightly burned magnesium oxide is lower than that of light magnesium oxide.Introducing silica fume components with large specific surface area into the alkali/gypsum slag system,comparatively analyzing the mechanism of the specific surface area of light magnesium oxide on the gypsum slag system,the results show that the light magnesium oxide has an important influence on the early strength of the gypsum slag system;The compressive strength of light magnesium oxide and silica fume system 7d is 63.7MPa and 49.7MPa respectively,and the corresponding 7-day flexural strength is 9.7MPa and 7.2MPa respectively.The strength development law of gypsum slag cement mortar stimulated by different contents of magnesium oxide shows that when the magnesium oxide content is higher than 7.5wt%,its strength development is a rapid growth period within 7 days,a slow growth period within 14-28 days,and after 28 days.Stable development period.Under the experimental conditions,the mortar specimen prepared by the light magnesium oxide/gypsum slag system is better than ordinary portland cement in seawater erosion resistance,and its strength fluctuation is minimum when the amount of magnesium oxide is 7.5wt%.Finally,the effect of fly ash on the properties of lightly burned magnesium oxide/gypsum slag cement was studied.By changing the ratio of slag and fly ash to study the effect of fly ash on alkali/gypsum slag cement,When the content of fly ash is in the range of 8wt%-32wt%,the mortar strength decreases with the increase of the content of fly ash;when the content of fly ash is 40%,the appropriate amount of silica fume can replace the fly ash and slag respectively to improve the group Early and late intensity.When the amount of fly ash is 40%,the appropriate amount of silica fume to replace fly ash and slag can improve the early and late strength of this group.When the amount of silica fume is2.4%,the amount of fly ash is 37.6%,and the amount of slag is 40%,the 28d compressive strength and flexural strength of cement mortar are 59.86MPa and 12.33MPa,respectively.The improvement effects were 106.01%and 113.15%respectively.;after increasing the sodium hydroxide content to 1.3%,the fluidity of the system slurry decreases sharply,and there is no obvious improvement effect on the demolding time and early strength.The curing results at 40?and 60?show that the two curing systems are not good for the later strength.