Preparation And Performance Of Internal Curing Natural Hydraulic Lime Based Grouting Materials

Chinese grotto temples are long-standing,widely distributed,large in scale,and systematic,condensing splendid achievements in religion,culture,art,and technology.Due to long-term exposure to the natural environment,grottos face diseases such as surface weathering,salt damage,cracking,and peeling,which urgently need restoration and protection.However,the use of inappropriate restoration materials has caused much protective damage to the grottos.Hence,natural hydraulic lime has become the best material for the restoration of grottos due to its high compatibility with the physical and chemical properties.But how to achieve its complete hydration with good properties has become a challenge for this material.Therefore,in this study,natural hydraulic lime(NHL)was used as the matrix material,commercial superabsorbent polymer(SAP)and in-situ superabsorbent polymer(in-situ SAP)were chosen for the internal curing of NHL,and polycarboxylate superplasticizer(PCE)was chosen for the modification of pastes.To study the effects of SAP,in-situ SAP,and PCE on the hydration process,microstructure,and properties of NHL.Various methods were used to analyze the phase,microstructure,rheological properties,and performance of NHL.The major studies and conclusions are as follows.(1)Internal curing of NHL was carried out with commercial SAP particles as internal curing agents.The effects of SAP on the rheological properties,hydration process,phase changes,and microstructure of NHL paste were investigated by using XRD,FT-IR,TG-DSC,SEM,ultra-deep field microscopy and universal testing machine.The results indicated that the different chemical structures contributed to the differences in the water absorption/release characteristics of SAP,and the absorption of SAP in NHL filtrate was significantly reduced compared with that of deionized water due to the ionic shielding effect and the stable complexation of Ca2+ with carboxyl groups.The release of stored water in SAP resulted in a decrease in the plastic viscosity and yield stress of the paste over time,but the rheological properties of the paste did not fully match the changes in flowability.In addition,SAP promoted the early hydration of NHL and counteracted the selfshrinkage of NHL,the hydration products showed "gradient growth" on the SAP surface,as well as the formation of the NHL/SAP interface transition zone.However,the release of stored water in SAP causes self-shrinkage and leaves pores,leading to an increase in the total porosity of NHL and the amount of macropores,which also provides access for CO2 into the matrix,promoting carbonization while decreasing compressive strength.(2)An in-situ SAP modified NHL method in paste was developed by using in-situ polymerization technology.The relations between hydration process,structure and properties of in-situ SAP modified NHL composites were investigated using XRD,FT-IR,microcalorimetry,TG-DSC,SEM,TEM,XPS and universal testing machine.The results show that in-situ SAP has moderate water absorption and excellent water storage capacity,therefore it has excellent internal conservation properties.The in-situ SAP is uniformly distributed in the NHL matrix in a fibrous or reticulated form,which complements the combination with the hydration product network through chemical bonding,fills the defects in the matrix,making the SAP/NHL composite denser and changing its microstructure,while transforming the microscopic morphology of the NHL hydration products from disordered distribution to regular arrangement.As a result,SAP/NHL exhibited excellent compressive strength(7.66 MPa at 7-day age and 13.63 MPa at 28-day age)and flexural strength(1.26 MPa at 7-day age and 2.94 MPa at 28-day age).In addition,the in-situ SAP provides the matrix with good flexibility,scouring resistance and bonding properties.(3)Using PCE modified SAP/NHL to improve the paste flowability to meet the grouting reinforcement needs.The effects of PCE on the rheological properties,hydration process,structure and performance of SAP/NHL were investigated using TOC,zeta potential meter,super depth-of-field microscope,rotational rheometer,microcalorimeter,TG-DSC,SEM,TEM and universal testing machine.The results show that PCE adsorbs on the surface of SAP/NHL particles as multilayer adsorption,and the first layer adsorption causes the decrease of particle zeta potential,while the subsequent adsorption has a weak effect on the potential.The multilayer adsorption is due to the formation of a stable complex layer of Ca2+ on the surface of the first PCE layer,which provides sites for new PCE adsorption.Under the synergistic effect of electrostatic repulsion and spatial site resistance,PCE disintegrates the flocculation structure in SAP/NHL paste,the agglomeration phenomenon is weakened,the particles are dispersed and homogeneous,and the number of small particles increases.the critical dosing of PCE is 1%,the flow rate can be increased to about 335 mm,and the flow rate tends to be stabilized after exceeding the critical dosing.PCE decreases the yield stress and plastic viscosity of the paste,so that the paste changes from plastic fluid to a near-Newtonian fluid.In addition,PCE will delay the early hydration of C2S and make the hydration products develop towards crystal homogenization and dense network structure,which will accelerate the hydration of SAP/NHL paste in the later stage.Meanwhile,the right amount of PCE will also promote the in-situ polymerization of SAP solution in the paste to densify the overall structure,and the hydration products will be closely and regularly arranged,which will eventually lead to the increase of compressive strength of the paste,while the excess will cause the decrease of compressive strength.