Experimental Study On Properties Of Micro-Aggregates And Their Influence On Concrete Performance

Fine particles less than 75μm in concrete aggregates significantly affect the performance of concrete.They not only impact the initial workability of the concrete mix but also influence its time-dependent work stability,namely slump retention.The mineral composition of these fine particles is complex,including primary minerals,i.e.,stone powder,consistent with the parent rock of the aggregate and formed through physical actions,and secondary minerals,i.e.,clay powder,formed through various chemical actions.The impact of clay powder on concrete performance is significant and is a critical issue in aggregate quality control.However,the types and sources of clay powder are diverse.In practical concrete production,testing the types of clay powder and controlling its content in the aggregate are particularly important for improving and enhancing concrete quality.This study used the methylene blue titration method to detect the MB values of nine types of fine particles under 75μm,including pure stone powder,montmorillonite,kaolin,illite,gold tailings,iron tailings,washed manufactured sand,and natural river sand.The physical property indicators such as microscopic morphology,mineral composition,specific surface area,particle size distribution,and swelling volume of these fine particles were tested,and their water absorption and adsorption characteristics were studied to establish an MB value characterization of these properties.The research shows that due to differences in mineral composition,particle size,and surface features of micro-aggregates,their water absorption and adsorption properties significantly vary.The swelling volume test results show that clay particles such as montmorillonite,illite,and kaolin have higher water absorption;the zeta potential test reveals that montmorillonite and kaolin have greater adsorption for polycarboxylic acid admixtures.The order of MB values is montmorillonite,kaolin,and illite.Based on the study of clay powder properties and their MB value characterization,different types and contents of micro-aggregates were added to cement paste,mortar,and concrete to investigate the impact of micro-aggregate types and contents on paste,mortar,and concrete performance.The study focused on the fluidity,thickening,and viscosity of the paste;the flow-to-strength ratio and strength of the cementitious sand;and the workability and hardened strength of the concrete mix.The research shows that without using admixtures,the main factors affecting paste,mortar,and concrete mix are the water absorption and content of micro-aggregates.In the presence of admixtures,besides water absorption and content,the adsorption of micro-aggregates on admixtures is the primary factor affecting the concrete mix,and the impact becomes more significant with a decrease in the water-cement ratio.Although illite is also a sticky particle,its adsorption capacity for admixtures is relatively small,mainly due to its water absorption.Therefore,its impact on the workability of concrete with added polycarboxylic acid admixtures is relatively small.In contrast,montmorillonite and kaolin not only have higher water absorption but also higher admixture adsorption,significantly affecting the workability of the concrete mix,especially montmorillonite.The MB value mainly reflects adsorption,not water absorption.Both montmorillonite and kaolin have higher MB values,indicating stronger admixture adsorption and significantly affecting the performance of the concrete mix.Comparative research shows that due to the different types and contents of clay particles in micro-aggregates from different sources,their impacts on the performance of paste,mortar,and concrete mix are also different.However,the impact pattern is mainly controlled by the types and contents of the clay particles they contain,consistent with the research results of pure clay particles.As the MB value mainly reflects the adsorption of micro-aggregates on polycarboxylic acid admixtures,and adsorption is the primary factor affecting the workability of the concrete mix,the MB value can effectively reflect the influence of micro-aggregates on the workability of the concrete mix.To limit the clay content in sand and gravel aggregates,the current quality standard for sand and stone provides the MB value and its content limit,but it does not establish the relationship between the MB value and content and the workability of the concrete mix.Through extensive experiments,this paper reveals the impact pattern of micro-aggregates with different MB values on the workability of the concrete mix,which has some practical significance for concrete quality control.