| The production of Portland cement accounts for about 8%of global CO2 emissions and contributes to global warming.The application of amorphous industrial solid waste in cement-based materials can significantly reduce the consumption of silicate clinker and thus reduce CO2 emissions.The results show that hydration reaction and alkali-activation take place in the admixture cement pastes of Portland cement and vitreous silica-aluminate（calcium）mixed with water,and the main gel products are C-（A）-S-H gel and C-A-S-H gel generated by reaction between by-product Ca（OH）2 and vitreous;Alkali-activation is the main reaction in the alkali-activated pastes prepared by mixing amorphous or vitreous aluminosilicate materials with alkali activator solution,and the main gel product is N-A-S-H gel.The above two kinds of cementitious materials made from amorphous materials have been thoroughly studied and widely used in engineering field.However,the pastes of vitreous aluminosilicate materials mixed with clinker phase mineral and alkali solution are called alkali-activated hybrid binder materials,but the reaction of the system and the formation of gel products have been poorly studied.Research shows that:Based on the pseudo-equilibrium relationship between active species in pore solution and gel products,the pore solution of alkali-activated pastes was separated by mechanical method,and the chemical composition and structure of pore solution and pastes were monitored synchronously by ATR-FTIR technology.The results provided direct evidence for the formation of gel products.The content of Q2 silica tetrahedron species in alkali-activated fly ash paste decreases gradually with the increase of curing time,while the content of Q3 unit increases,indicating that the formation of alkali aluminosilicate gels is a process of continuous polymerization of silicate oligomer or monomer.In the pore solution of alkali-activated steel slag paste,the content of Q2 unit decreases rapidly with time,while the weak Q3 unit appears in the middle stage of the reaction,and the"main band"of the paste is always located at the lowest wavenumber,which mean that the main reaction product is C-S-H gel.In the alkali-activated fly ash and steel slag hybrid binder materials,Q3 and Q2units exist simultaneously in pore solution,and the content of Q3 unit decreases with the increase of steel slag,indicating that C-S-H and alkali aluminosilicate gels coexist in the hybrid pastes.In addition,the changes of wavenumber of"main band"and OH-concentration of pore solution indicate that the steel slag can accelerate the alkali-activation kinetics,compensate the OH-necessary for the alkali-activation,and significantly improve the mechanical properties and microstructure of the alkali-activated hybrid binder materials.Subsequent curing experiment shows that the addition of steel slag can significantly improve the mechanical properties of waterglass-activated fly ash paste.At room temperature,the specimens with less than 70 wt.%steel slag are suitable for sealing curing,otherwise,the performance of water curing specimens will be better.The compressive strength of alkali-activated steel slag specimen cured in water without extra heat for 1 year is up to 72MPa.The results of XRD and SEM show that the clinker phase in the paste is completely hydrated,the matrix structure is very smooth and compact,and no cracks can be observed.The early high temperature curing is beneficial to the rapid development of paste strength,but inhibits the alkali-activation of the precursor in the later stage,which is not conducive to the further formation of gel products,resulting in a small contribution to the strength of the later stage.The activation effect of Na OH activator on alkali-activated fly ash and/or steel slag binder materials is obviously weaker than that of waterglass activator,especially on the hydration reaction of steel slag.The hydration degree of clinker phase in Na OH-activated steel slag paste is low,and no dense matrix structure is formed.Different from the reaction when waterglass activator was used,the hydration by-product Ca（OH）2 appeared in addition to the formation of C-S-H gel in the hydration reaction of Na OH-activated steel slag paste during sealing curing.However,the substitution of steel slag for fly ash is still beneficial to the improvement of the strength of alkali-activated binder materials,and C-S-H and alkali aluminosilicate gels coexist in Na OH-activated fly ash/steel slag hybrid pastes. |
