Modification of Hydration Properties of CaO-activated Slag Cementitious Materials

Author:Wang Jun Xiang

Supervisor:lv xian jun

Database:Doctor

Degree Year:2017

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Pages:127

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Alkali-activated slag(AAS)binders are characterized by low energy consume,CO2 emission and sustainble production due to the large consumption of by-product of the iron production,which are considered as the most reliable alternatives to ordinary Portland cement.This study investigated the modification effect of slag fineness and compounds activators on the hydration properties of CaO activated slag binders,providing basic data and theoretical support for the application of AAS binders.It was found that CaO can be used as a potential activator for slag.However.the CaO一activated slag binders suffered from long induction period,slow strength development at the early age and low strength values at the late age.CaO content played a vital role in activating the hydration reactivity of CaO-activated slag binders.Mechanical test results showed that the optimal dosage of CaO was 5%by mass ratio of the total binder.The compressive strength of the sample with 5%CaO addition and specific surface area of 353 m2/kg which prepared at a water to binder ratio of 0.5 and a binder to sand ratio of 1:3 reached 6.5,14.6 and 22.3 MPa at 3,7,and 28 days,respectively.The pH value ot pore solution increased with CaO content up to 5%,which raised the salg reaction degree and thereby increased the amounts of calcium silicate hydrate(C-S-H)gels and hydrotalcite-hke phases(Ht)and the compressive strength of the sanpes.TGA and SHM results showed that the excess of Ca(OH)2 resulted in lower density and compressive strength when the CaO content was higher than 5%.In addition,NMR results revealed that the increment in CaO content was benificial for the extension of mean chain length ot C-S-H gels when the CaO content was lower than 5%.The variation of slag fineness also played significantly modification effect on the hydration of CaO-activated slag binders.The volume of particles less than 5 μm and 20 μm increased with the specific surface area of slag,which accelerated the slag reaction rate and the formation of C-S-H gels and Ht,resulting in higher compressive strengtl.Test results showed that the slag disolution rate hardly changes when the specific surface area of slag reached 513m2/kg.The compressive strength of the sample prepared by the slag with a specific surface area of 513m2/kg reached 15.0,22.9 and 33.2MPa at 3,7 and 28 days respectively and increased by 131%,57%and 49%compared to the sample prepared by the slag with a specific surface area of 278m2/kg.In addition,although the increase in specific surface area of slag had little effect on the type of hydration products,the mean chain length of C-S-H gels and the Al proportion in C-S-H gels both increased with the specific surface area of slag,which were benificial for the strength development.The utilization of CaSO4 can also affect the hydration properties of CaO-activated slag binders.Mechanical test results showed that the optimal dosage of CaSO4 was 5%mass ratio of the total binder when the CaO content was 5%.The compressive strength of the sample reached 20.4MPa,32.8MPa and 43.1 MPa at 3,7 and 28 days,respectively when the specific surface area of slag was 513m2/kg,increased by 36%,43%and 30%compared to the sample without CaSO4.Test results showed that large amounts of ettringite formed and occupied the pore space between the slag particles and played reinforcing effect similar to that of fiber when CaSO4 was added.However,the amounts of C-S-H gels decreased when the CaSO4 content was higher than 5%and consequently reduced the compressive strength.In addition,the presece of CaSO4 inhibited the fonnation of Ht and increased the Ca/Si ratio of C-S-H gel.The mean chain length and the Al proportion of C-S-H gels were also reduced.Based on the test results,the methods which could modificate the hydration propertiesof CaO-activated slag based binders were provided,revealing the effet of slag fineness and compound activators on the CaO-activated slag based binders and providing the theoretical support for the application of AAS binders.