Research On Preparation And Performance Of Alkali-activated Low Activity Fly Ash Composite Cementitious Materials

Fly ash is one of the largest industrial wastes in China.At present,the utilization rate of fly ash in China has been hovering around 70%,mainly focusing on the application of dry fly ash with fine particle size and excellent quality.Low activity fly ash can not be used directly as mineral admixture because of its low content of vitreous body,high impurity content,unstable carbon content,unstable layer and composition,complex processing technology,slow reaction and high water content.At present,low activity fly ash is mainly piled in the open air,occupying large amounts of land resources,causing serious air and soil pollution.Therefore,it is urgent to find a practical way to raise the effective utilization rate of low activity fly ash.Three kinds of alkali activated low activity fly ash cementitious materials were prepared with low activity fly ash as raw material,water glass as activator and calcareous admixture(hydrated lime,slag and steel slag).The influence of fly ash grinding time,admixture content,modulus and dosage of water glass,water to solid ratio and other external factors such as curing system and ambient temperature on the cementitious properties of low activity fly ash compound cementitious material were studied.The microstructural evolution law of alkali activated low activity fly ash was explored with XRD,FTIR,SEM and other microcosmic testing methods.The internal relationship with the strength of the pulp was analyzed,and the reaction mechanism and key factors were preliminarily explored.The results of the study show that:The three different low activity fly ash composite systems have similar rules: the setting time is shortened with the increase of the content of calcareous admixture,which increases with the increase of the alkali equivalent of the system,while the compressive strength of the pulp increases with the increase of calcareous admixture and the alkali equivalent of the system.Excessive base system and modulus of water glass will adversely affect the later strength of the system.Considering the comprehensive setting time and strength development,the better ratio of the three systems is: in hydrated limefly ash system,15% of hydrated lime proportion,1.4 of water glass modulus,10% of system alkali equivalent,0.4 for water cementitious material ratio;in slag-fly ash system,25% of slag proportion,1.6 of water glass modulus,10% of system alkali equivalent,0.4 for water cementitious material ratio;in steel slag-fly ash system,40% for steel slag proportion,1.6 of water glass modulus,10% of system alkali equivalent and 0.4 of water cementitious material ratio.Wet heat curing can greatly improve the mechanical properties of alkali activated fly ash composite system,especially its early strength.But too high curing temperature will make the late strength of the system obviously shrink.Considering the development of comprehensive mechanical properties,the better curing systems of the three systems are: 60℃moisture heat curing for 9h.At this time,alkali activated hydrated lime-fly ash system,alkali activated slag-fly ash system,alkali activated steel slag-fly ash system 3d compressive strength can reach 16.4 MPa,28.4 MPa,18.8 MPa,respectively,and 56 d compressive strength was 20.4 MPa,30 MPa,and 26.6 MPa respectively.The microanalysis shows that the main reaction products of alkali activated low activity fly ash compound cementitious material are amorpHous gels such as aluminosilicate and C-S-H,and these amorpHous gels are the main factors that make up the strength of low activity fly ash compound cementitious material.In addition,no new crystalline pHase was formed during the reaction.