The Performance And Hydration Of Cement Composites With Thermally Activated Kaolinitic Coal Gangue

The green and low carbon development of traditional cementitious material is becoming increasingly prominent with the demand to cut down the CO₂ emission to achieve the dual carbon goals.Meanwhile,the recycling of industrial solid waste as raw material or cement substitution to produce green cementitious binder is regarded as a future option.One of the most stocked piled industrial solid wastes in China is coal gangue.There are a huge number of coal gangues underutilized and left in landfills,occupies lands and poses serious environmental threats.The comprehensive utilization of coal gangue is important,while the complex compositions and instable reactivity hinders its industrial application.Recently,the limestone calcined clay cement（known as LC³）has attracted increasing research interest,owing to its wide availability,high cement replacement ratio and good mechanical performance and durability.The excellent performance is mainly attributed to the synergic reaction between metakaolin and limestone.Coal gangue mainly comprises with clay minerals.The contained kaolinite could convert to metakaolin after calcined to a low temperature of 600℃-800℃,which makes coal gangue a potential material for the production of LC³.However,there is lack of systematic investigation on the activation method of coal gangue and impacting factors of blended cement.Therefore,this study firstly investigated the use of complex coal gangue（with varied composition）in blended cement.The activation methods of coal gangue and influencing factors on blended cement were explored.The correlation between kaolinite content of coal gangue and mechanical performance of blended cement was clarified.Based on these results,this study further investigated the hydration mechanism and performance enhancing methods of metakaolin-limestone cement,aiming to overcome its performance weakness and benefits to its reliability in field use.The main objectives and findings are summarized as follows:（1）Coal gangues from different sources were used to elucidate the influence of grinding duration and calcination temperature on the performance and hydration of calcined coal gangue binary cement and calcined coal gangue-limestone ternary cement.Based on the analysis of hydration products,microstructure and thermal dynamic modeling,it is found that that at temperature range of 600℃-800°C,higher calcination temperature improves the reactivity of coal gangue.In binary cement,the increased reactivity of coal gangue enhances the pozzolanic reaction,transfers ettringite to monosulfoaluminate.While in ternary cement,the increased reactivity of coal gangue promotes the synergic effect,transfers the monosulfoaluminate to carboaluminates with lower density,and indirectly stabilized ettringite.Thus,a more profound contribution to microstructure densification and strength improvement is found in ternary cement system with the increase of coal gangue’s reactivity.It is also found that kaolinite content is the main reactivity source of calcined coal gangue and influencing factor of blended cement.（2）The limestone was added in blended cement to enhance the binder performance containing low-active calcined coal gangue and benefit to its practical application in filed.The performance and hydration mechanism of blended cement with 10%-60%cement replacement was studied.Based on experimental results and thermal dynamic analysis,it is found that the synergic effect between calcined coal gangue and limestone can improve the performance of low-kaolinite calcined coal gangue cement.The volume of hydration products is 7%higher in ternary system,and the binder strength of ternary cement is 5%-12%higher than binary cement.The suggested replacement for low-kaolinite calcined coal gangue is 20%for binary cement,and 30%for ternary cement to fulfill the requirement for 42.5 grade cement.（3）The effect of curing temperature（15°C-25°C）on strength,hydration kinetics,hydration products and microstructure of coal-series metakaolin blended cement with and without limestone were studied.The performance degradation tendency of metakaolin contained cement under low curing temperature was observed.It is found that the pozzolanic reaction was inhibited and postponed at lower temperature of 15°C.At the later stage of hydration,the pozzolanic reaction was further inhibited due to the denser pore structure,difficult diffusion and limited growth space,which results in the slow strength development and lower strength at later ages.With the use of limestone,the synergic effect of limestone and metakaolin can transform monosulfoaluminate to thermodynamically more stabled hemi-and mono-carboaluminates,thus promoting the pozzolanic reaction and reduces the adverse effect.In addition,the limestone has higher solubility at lower temperatures and enhances the synergic reaction between limestone and metakaolin,which produces more carboaluminates at later ages,refines the pore structure and compensates for the strength loss.（4）The workability of metakaolin based blended cement was studied and improved by adjusting particle size.It is found that the use of coarser particles of quartz can reduce the static yield stress and viscosity,thus effectively improving its workability over time.In addition,it is found that different particle sizes of quartz（median particle size:5.21μm to 141.53μm）have little effect on the mechanical properties of blended cement.Based on experimental results,it is found that the coarser particles of quartz pose a dilution effect,inhibit the hydration and cause decrease of mechanical performance.Besides,it was interesting to notice that the coarsening of pore structure by using coarse quartz provides more growing space and beneficial to the synergic reaction between metakaolin and limestone,forming increased amount of pore-refining carboaluminates,which compensates for the strength loss caused by the reduced packing with coarser particles.（5）The ground granulated blast furnace slag（GGBS）is incorporated in metakaolin-limestone ternary cement to improve its mechanical performance,and to further increase the limit of cement substitution.It is found that at a high cement replacement level of 60%,the compressive strength of metakaolin-limestone-GGBS quaternary system can reach or even exceed that of pure cement specimens from 7 days onwards.The hydration and the synergic effect of metakaolin-limestone-GGBS were studied.It is found that GGBS was rapidly activated at the early age of 1-3 days,enables the fast strength development of quaternary cement.The addition of GGBS has no significant influence on the synergic effect between metakaolin and limestone.The synergic effect of metakaolin and limestone further refines the pore structure and improves the strength at later ages.Besides,the synergic effect among metakaolin,limestone and GGBS enhances the microstructural bonding and enhances the flexural strength of quaternary cement.