Experimental study on drying shrinkage and microscopic characteristics of borax-sodium silicate alkali-activated slag mortar
[Objective]Alkali-activated materials(AAMs)present numerous advantages,such as high strength and a low carbon footprint.However,their wide application is hindered by significant shrinkage.This study addresses this key challenge by developing a composite activator to examine the drying shrinkage and microscopic characteristics of borax-silicate sodium alkali-activated slag(AAS)mortar.[Methods]The samples were tested and characterized using mercury intrusion porosimetry(MIP),X-ray diffraction(XRD),scanning electron microscopy(SEM),and Fourier-transform infrared spectroscopy(FTIR).These methods helped to analyze the reduction mechanism of the composite activator on drying shrinkage.MIP is a significant technique for investigating the structure properties of porous materials.Numerous studies have established a connection between the drying shrinkage of cement-based materials and their porous structures.Given that both AAMs and cementitious materials are classified as porous materials,MIP could also be applied to AAMs.SEM enabled the observation of the mortar microstructure,while XRD was instrumental in characterizing the hydration products of the mortar.FTIR analysis provided insights into the strength of chemical bonds in the gel of the AAS mortar,particularly after the introduction of new B—O bonds resulting from borax hydrolysis.[Results]The experimental results showed that:1)B10-N6.3 exhibited the highest fluidity(183 mm),which was 5%higher than that of C-N7.0.2)The drying shrinkage of B20-N5.6 at 7 days was 34%and 25%lower than that of C-N7.0 and C-N5.6,respectively.This reduction further increased to 12%and 8%at 28 days,respectively.3)MIP indicated fewer pores of less than 50 nm in B20-N5.6(3.38%)compared to the control group C-N7.0(5.78%).4)SEM images showed that borax addition lessened the microcracks in the AAS mortar.XRD detected the presence of the ulexite phase(NaCaB5O6(OH)6(H2O)5)in the AAS mortar.5)FTIR analysis showed a significant enhancement in the Si—O—T(T represents Si,Al,or B)band of the AAS mortar under the optimal borax proportion(20%).[Conclusions]Increasing the borax proportion in the borax-silicate sodium composite activator initially increased and then decreased the fluidity of the AAS mortar.This fluidity was influenced by both the alkali equivalent and free water content.The composite activator effectively reduced the drying shrinkage of the AAS mortar,mainly due to the decrease in the proportion of mesopores(<50 nm)and the retarding property of borax,which slowed down the early gel polymerization rate.However,excessive borax addition weakened the Si—O—T absorption peak.This is attributed to a significant reduction in the sodium silicate dosage,which led to a decrease in the dissolution of[SiO4]and[AlO4]in slag.Consequently,the C-(A)-S-H gel content in the AAS mortar was reduced.The low alkalinity of the solution hampered the development of the gel network,weakening the gel skeleton′s ability to resist shrinkage stress.Therefore,the borax proportion should not exceed 20%.These findings provide a foundation for reducing the drying shrinkage of AAMs,expanding their application range,and contributing to the carbon emission reduction of the cement industry.
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