Carbonation,Chloride Binding And Sulfate Resistance Of Alkali-Activated Slag/fly Ash Cements

The environmental problems caused by the production and application of Portland cement are increasingly serious.As a relatively green and low-carbon material,alkali-activated cement?AAC?has been widely studied.However,the durability of AAC is not clear and needs syetemtatic research.In this paper,the effects of slag/fly ash ratio,Na₂O concentration,modulus?Ms?and water/binder ratio?w/b?on carbonation,chloride binding and sulfate resistance of alkali-activated slag/fly ash cement were studied,and compared with Portland cement.A more reasonable test method to study the carbonation,chloride ion binding and sulfate resistance mechanism of AAC was improved to provide a theoretical basis for the establishment of durability model and the improvement of test specifications.At first,the effects of different factors on natural and accelerated carbonation of alkali-activated slag/fly ash cement were studied.The results showed that at the early carbonation stage of Portland cement,Ca?OH?₂ and C-S-H gel are carbonized simultaneously.After Ca?OH?₂ is completely consumed,carbonation occurrs mainly on C-S-H gel.However,the carbonation mechanism of AAC is different from that of Portland cement.The accelerated carbonation test method of Portland cement cannot be directly applied to AAC,especially for the high-Ca AAC?slag content?60%?.Carbonation products of alkali-activated slag/fly ash cement are mainly related to the characteristics of C-A-S-H gel,and hydrotalcite acts as CO2 adsorbent,especially under the natural condition.Appropriate slag/fly ash ratio can effectively reduce the carbonation rate of alkali-activated slag/fly ash cement.After carbonation,the decalcification of C-A-S-H phase will lead to the production of calcium carbonates and silica gel,while N-A-S-H phase is basically unchanged.The increase of Na2O concentration can increase the C-A-S-H phase in AAC,and more calcium carbonates generated after carbonation,but it has little effects on the carbonation rate of gel.The increase of Na₂O concentration will also promote the activation of fly ash,and produce more N-A-S-H phase that can reduce the gel carbonation rate.The reduction of w/b?0.5 to 0.3?can reduce the gel carbonation rate,especially for the high-Ca AAC,and more C-A-S-H will be carbonated,which can lead to producing more calcium carbonate.In the high-Ca alkali-activated system,although the increase of Ms?0 to 1.5?can reduce the gel carbonation rate,the amount of calcium carbonates decreases.Therefore,the appropriate Ms also requires specific analysis on the changes of porosity and pore structure before and after carbonation.Chloride penetration is the main factor affecting the corrosion of concrete reinforcement,and the chloride binding capacity of cement will affect the transport of chloride ion.In this paper,the adsorption equilibrium method was used to study the factors on binding capacity of alkali-activated slag/fly ash cement,which provided a theoretical basis for the establishment of chloride penetration model.The results of adsorption equilibrium test of Portland cement and alkali-activated slag/fly ash cement showed that the Langmuir isotherm can well characterize the chloride binding properties,and the chloride binding capacity increases with the increase of free chloride concentration.The chloride binding of alkali-activated slag cement is mainly related to the physical adsorption of C-A-S-H phase.With the incorporation of fly ash,the chloride binding capacity of alkali-activated cement increases with the increase of N-A-S-H,but no Friedel's salt is detected.In addition,the chloride binding capacity of AAC increases significantly with the increase of w/b.However,the effects of Ms and Na₂O concentration on chloride binding are not clear,because the adsorption equilibrium method used in AAC is not perfect and affected by multiple factors,especially the effect of internal and external OH^-concentration.Therefore,a further study and improvement for the influencing factors of chloride binding and the test methods are needed.Sulfate attack is also an important factor causing the degradation of concrete structure.In this paper,a new test method was proposed to study the erosion effect of Na₂SO₄ and MgSO₄ solution on alkali-activated slag/fly ash cement,providing a theoretical basis for the application of AAC in sulfate environment.The results indicated that ettringite but no gypsum is detected in Portland cement exposed to 5%Na₂SO₄ solution.Due to the action of Mg²⁺,decalcification of C-S-H in Portland cement after MgSO₄ attack resulted in a large amount of gypsum and ettringite,and the degradation of sample is much more serious than Na₂SO₄ attack.The sulfate resistance mechanism of alkali-activated slag/fly ash cement is different from that of Portland cement.In alkali-activated slag cement,a small amount of ettringite and gypsum can be observed after Na₂SO₄ attack,the structure of C-A-S-H gel is basically unchanged.Furthermore,Na₂SO₄ has an activation effect on alkali-activated slag cement.In addition,the incorporation of fly ash can inhibit the formation of ettringite and gypsum.After MgSO₄ attack,the C-A-S-H gel of AAC is decalcified and decomposed,and a large amount of gypsum is formed,but the main expansion product,ettringite,cannot be detected.Therefore,in terms of the reaction products,alkali-activated slag/fly ash cement has a better sulfate resistance than Portland cement.In alkali-activated slag/fly ash cement,Ms has little effects on Na₂SO₄ reaction products.Compared with NaOH as the activator?Ms=0?,Na₂SO₃-activated slag cement has a better MgSO₄ resistance,because a small amount of ettringite is detected after MgSO₄ attack in NaOH-activated slag cement.The influence of Na₂O concentration and w/b on the types of reaction products after Na₂SO₄ and MgSO₄ attack on AAC is not significant.Therefore,the influence of porosity and pore structure should be considered.