Effect Of White Mud On Properties Of Alkali Activated Slag

White mud(WM)is a solid waste from paper industry.It mainly contains CaCO3 and residual alkali metal ions(e.g.,Na+,Mg2+).In order to explore the feasibility of using WM in preparation of alkali activated slag(AAS),the physical and mechanical properties,reaction mechanism and carbonization resistance of alkali activated white mud/slag was carried out.The main research objectives and conclusions are as follows:Firstly,effect of WM inclusion on physical and mechanical properties of AAS with different alkali activator:taking the WM inclusion as the experimental variable,the flash AAS paste samples of NaOH activated system and waterglass activated system were made,respectively,and their workability,setting time and compressive strength were tested.The experimental results show that the workability of fresh paste was reduced and the setting time was shortened when slag was replaced with WM.With the increase of WM content,the compressive strength increases first and then decreases.The strength of waterglass system is generally higher than that of NaOH excited system,and the strength of the former and the latter reach the highest when the content of 15 wt%is added,which are 84.15 MPa and 134.73 MPa,respectively.Secondly,effect of WM on hydration products,microstructure and reaction mechanism of alkali activated slag:the hydration products and microstructure of AAS containing white mud were analyzed by XRD,FTIR,TGA and SEM.The results show that the main reaction products are C-(N)-A-S-H gel,the participation of Na+ and Ca2+in the geopolymerization accelerates the hydration reaction rate,increases the Ca/(Si+Al),and decreases the degree of cross-linking.Mg2+ and CO32-promoted the precipitation of Mg-Al and formed layered hydroxide hydrotalcite.However,excessive WM inclusion reduces the content of active precursors,leading to the decrease of Si and Al content in the system,reduce the formation of gel products,and reduce the density of microstructure.Thirdly,effect of WM on carbonation mechanism of AAS.The carbonization resistance of AAS cement containing white mud was evaluated by XRD,FTIR,TGA,SEM and image processing,taking carbonization depth,compressive strength and microhardness of samples as carbonization degree indexes.The results show that the main factors affecting the carbonation resistance of the materials are the amount of gel product and porosity.The carbonization products are CaCO3 and decalcified silica gel,and no Na-crystallization phases are formed.CaCO3 exists in three different forms:calcite,vaterite and aragonite.On the one hand,Mg2+ in the white mud can migrate to the calcite lattice,increasing the solubility of calcite,promoting the formation of metastable aragonite and vaterite with better filling effect,promoting the generation of Mg-Al hydrotalcite to absorb carbon dioxide and reduce the degree of carbonization.On the other hand,after C-(N)-A-S-H decalcification,Ca2+ in the WM will migrate to balance the charge,reducing the decalcification sensitivity.Therefore,adding proper amount of white mud(15 wt%)can improve the carbonation resistance of the material.