Study On Mechanism Of Admixture Suppressing Alkali-silica Reaction In Different Alkali Conditions

In this study,Ceramic polishing powder and fly ash were used as admixtures to replace part of Portland cement.Mortar bars were prepared according to the JGJ52-2006 rapid method of mortar bar.The expansion rate and mechanical properties of benchmark specimens and partly cement specimens replaced by admixtures were tested.The suppressing of ASR by admixtures in different alkali conditions?KOH/NaOH?were compared and analyzed.The microstructure,width,elements distribution and their contents of the interfacial transition zone were compared and analyzed by SEM and EDS,the products of ASR were analyzed by XRD,to explore the general rule and inherent mechanism of admixture suppressing ASR.The main conclusions are as follows:?1?The expansion rate of pure cement benchmark specimens in KOH conditions is smaller than that in NaOH conditions,Under the same conditions,the effect of K⁺on ASR is smaller than that of Na⁺.Fly ash and Ceramic polishing powder can effectively inhibit ASR in these two alkali conditions,and the general trend is that the expansion rate decreases with the increase of substitution rate.?2?Replacing part of cement with fly ash and Ceramic polishing powder can effectively improve the strength of mortar bar in alkali conditions,but excessive substitution will reduce the strength.The strength of mortar bar specimens replaced by mineral admixtures under alkaline conditions is related to the composition and structure of hydration products,ASR expansion and interfacial transition zone.Considering the effects of fly ash or ceramic polishing powder on ASR inhibition and the strength,it is recommended that the replacement levels of cement with fly ash or ceramic polishing powder should be 20%³0%.?3?The structure of interfacial transition zone of cement specimens replaced by admixtures is more compact than that of pure cement specimens,and the width of interfacial transition zone is reduced.In both KOH and NaOH conditions,when mineral admixtures are added,the content of Si increases,the content of Ca decreases,the content of Na decreases and tends to distribute evenly in the interfacial transition zone,while K and Al are enriched in the interfacial transition zone.This is the general rule obtained by mixing fly ash or polishing slag.?4?The inhibition mechanism of mineral admixtures on ASR is as follows:One side,the fineness of the admixture is smaller than that of cement and the secondary hydration reaction produces cementitious gel and other hydration products,which can fill pores and improve the microstructure of the interfacial transition zone,so that moisture and alkali are not easy to circulate.On the other hand,Al₂O₃ in the admixture plays a key role in inhibiting ASR.After two hydration reactions,C-A-S-H gel with low Ca/Si and Ca/Al is formed,which has strong adsorption and fixation ability to alkali.The radius of K⁺ion is larger than Na⁺.Therefore,K⁺is easier to combine with Al₂O₃,and it is enriched in the interface region,Na⁺reacts with excessive silicon in the admixture and is detained in the far aggregate region.K⁺and Na⁺are easier to combine with Al₂O₃ to form alkali aluminosilicate gel than Ca²⁺.K⁺is combined with mineral admixtures and high content SiO₂ and Al₂O₃ in the aggregate.A stable volume of alkali aluminosilicate gel is formed in the interfacial transition zone.Finally,it crystallize into a framework like silicate mineral with stable volume and is not easy to expand,thus inhibiting the occurrence of AAR.