| In recent years,the demand for high performance cement based repairing and reinforcement materials is increasing,but it is mainly used in national key construction projects,but there is little research on repairing and reinforcement materials suitable for existing buildings in villages and towns.With the implementation of the national rural revitalization strategy and the rapid development of new rural construction,the research on repairing and reinforcement materials for existing buildings in villages and towns has attracted widespread attention.The existing buildings in villages and towns are mainly brick-concrete structures,which are mainly characterized by strong water absorption,resulting in rapid water loss and reduced strength of traditional cement based repairing materials.At the same time,due to the repairing interface is too dry to reduce the interface bond strength,resulting in construction difficulties and repairing materials easy to fall off.Moreover,the traditional Portland cement repairing material has high porosity,poor waterproof,impermeability and erosion resistance performance,which is easy to cause a decrease in engineering durability.Based on this,in this paper,the fast hardening and early strength special calcium sulfoaluminate（CSA）cement as the main component,and the introduction of polyvinyl alcohol（PVA）powder,additives and admixtures,effectively improve the repairing mortar comprehensive performance,and reduce the cost.In this paper,the composition of PVA powder modified CSA cement repairing mortar is designed.The effects of PVA powder on mechanical strength,bond performance,waterproof and impermeable performance and durability of CSA cement repairing mortar are studied.The influence mechanism of PVA powder on the hydration of CSA cement is analyzed.The modification mechanism of PVA powder to improve the bond performance,waterproof and impermeable performance of CSA cement repairing mortar is put forward.The feasibility of reducing the cost of repairing mortar by adding sawdust is also explored.And the engineering application of PVA powder modified CSA cement repairing mortar is studied.The main conclusions are as follows:（1）Under the synergistic action of water reducer,defoaming agent and retarder,the best dosage of PVA powder in CSA cement mortar is 1.5%of the CSA cement mass.PVA powder can improve the water retention performance of CSA cement mortar,reduce the drying shrinkage.When the hydration age exceeds 1d,PVA powder can form film in the mortar to increase the bond force between hydration products and improve its mechanical strength.PVA powder can optimize the interfacial transition zone between repairing mortar and old matrix structure,and improve the interfacial bond performance.PVA powder can form a film,in reducing the porosity of hardened structure at the same time,further block the water transmission,and improve the waterproof and impermeability of mortar.The optimal composition of PVA powder modified CSA cement repairing mortar is as follows:PVA powder 1.5%,water reducing agent 0.8%,defoamer and retarder 0.2%.Compared with the Reference,the porosity is reduced by 5.36%.The 28d flexural strength is 8.89MPa,increased by 6.16%.The 28d compressive strength is 41.91MPa,increased by 8.36%.The28d tensile bond strength with clay brick is 2.75MPa,increased by 27.69%.The 3d impermeability pressure can reach 4.0MPa.（2）During the hydration age of 0～60min,PVA powder can inhibit the early dissolution of C4A3$and the precipitation of AFt,and prolong the setting time of CSA cement paste.During the hydration age of 6 h～3 d,PVA powder can promote the hydration of C4A3$and the formation of Aft.In the alkaline environment of CSA cement hydration,the acetate（-OOCCH3）group in PVA powder can hydrolytic reaction,so that O on the branch chain of PVA can bond with Al in CSA cement hydration products and clay brick powder to form C-O-Al molecular chain structure,which can enhance the chemical bond between the polymer film and CSA cement repairing mortar and clay brick,thus improving the bond performance.（3）PVA powder can improve the water immersion resistance and freezing resistance of sulphoaluminate cement repairing mortar,and reduce the heat aging performance.After water immersion and freeze-thaw cycles,the 28d tensile bond strength of the 1.5%PVA repairing mortar and clay brick is 2.11MPa and 0.98MPa,respectively,which is increased by 15.03%and 6.52%compared with the sample without PVA powder,respectively.The polymer film existing in the pore of mortar can reduce the number of connecting holes in mortar,block the transport of water and harmful ions,and improve the freezing resistance and sulfate erosion resistance.In addition,the polymer film can bind the cement matrix together more closely,improve the strength and compactness,and improve the wear resistance.The wear of 1.5%PVA repairing mortar per unit area is 5.19kg/m2,which decreases by 24.21%.The wear depth is 2.66mm,with a reduction of 29.77%.（4）The introduction of sawdust can reduce the cost of PVA powder modified CSA cement repairing mortar.When the content of sawdust is in the range of 0～30.0%,the fluidity of repairing mortar can be improved.When the content of sawdust is low（0～3.0%）,the flexure strength and compressive strength do not decrease,or even increase slightly.With the increase of the content of sawdust,the flexural strength,compressive strength and tensile bond strength decrease.When the content of sawdust is 20.0%and 30.0%,the 28d flexural strength and compressive strength can meet the requirements of the standard of rigid repairing mortar and flexible repairing mortar,respectively.When the content of sawdust is 30.0%,the 14d tensile bond strength of repairing mortar and clay brick can meet the requirements of the standard of rigid repair mortar.Therefore,the cost of repairing mortar can be greatly reduced by adding sawdust.（5）When the cement-sand ratio of repairing mortar is 1:2,and the sand particle size is less than 2.50mm,the repairing mortar has good working performance,easy to spread,bond and hang,and easy to operate in plastering construction.The thickness of repairing mortar is 10±2mm,and there is no cracking after 3 days of water sprinkling curing.At the same time,the workability of repairing mortar can be further improved by adding sawdust,which is more convenient for the operation of plastering and finishing. |
