Design And Preparation Of An Anti-carbonation Coating For Slag-based 3D Printing Concrete

Extrusion 3D printing technology enables mold-free molding of building structures,reducing the waste of human resources and raw materials.The processing of solid waste slag has low energy consumption and low CO₂emissions.It is a high-quality cement substitute and can be used as cementitious material for 3D printing.However,there is a weak structure between the layers of 3D printed concrete.Its anti-carbonation is weaker than that of casting sample.And the anti-carbonation of slag is weaker than that of ordinary concrete.Therefore,it is necessary to improve the anti-carbonation of 3D printed slag-based concrete.Polymer emulsion coating can improve the anti-carbonation of concrete,but it has the problem of easy aging.Graphene（Gr）is introduced to form a barrier layer in the emulsion coating,and cement was introduced to improve the mechanical properties of the coating.The effect of the content Gr and cement on the durability and protective performance of the coating is investigated.This research relies on the national key research and development plans—Design and High Performance of Solid Waste Based Concrete Materials for 3D Printing（subproject 2019YFC1907105-03）.The cement-graphene emulsion coating with good durability and protective effect are prepared to enhance the anti-carbonation of 3D printed slag concrete.The results show that:The styrene butadiene emulsion（SBR）has good compatibility with Gr.The Gr encapsulated within the coating blocks the entry of erosive species into the coating.With the increase of Gr content,the durability of SBR coating improve.When Gr content is 0.5%,the tensile strength of the coating is increased by 53.8%.The addition of Ordinary Portland Cement has a significant effect on the improvement of the tensile strength,water resistance and alkali resistance of SBR coating,but it does not improve the resistance to UV aging.The coating layer of the raised position of the hydration product is thin,and it is easy to degrade and crack UV aging.The synergistic use of Gr and cement enhances the effect better.The modification effect of adding 0.3%Gr with cement is close to that of adding 1%Gr alone.The hydration products are encapsulated within the surface layer,where sheet-like Gr exists,reducing the presence of weak regions.When the specimen is initial setting,the coating is applied,and the carbonization depth of the specimen is 8.5 mm.After hydration for seven days,the coating is coated,and the carbonization depth of the specimen is 8 mm,which decreased by 5.88%.Emulsions will penetrate the interior through the loose surface during specimen initial setting,resulting in incomplete coating structure.The coating formed by brush forming has a complete structure,and the protective effect is between the coating formed by soaking and spray forming.Considering the actual construction situation,brush forming is the most suitable coating forming method.After carbonation under standard conditions for 28 days,the unprotected specimen is completely carbonation（carbonation depth is 20 mm）.The carbonation depth of SBR coating protective specimen is 11.66 mm.The protective effect of the coating increases with the increase of Gr content.When the Gr content is 1%,the carbonation depth reaches the lowest value of 6.7 mm.With the increase of cement content,the protective effect of the coating first increases and then decreases.When the ratio of cement to SBR is 0.5,the carbonation depth is 8.7 mm.On this basis,when 0.3%Gr is added,the carbonation depth of the specimen is reduced to 7.1 mm.Compared with the addition of 0.3%Gr alone,it decreases by 12.57%.The cement,Gr and emulsion are mixed to finally form the structure in which the emulsion layer coats the Gr to form surface layer,and then the hydration product is coated in the surface layer.This structure strengthens the weak surface layer where the hydration product is coated,which not only ensures good coating performance but also controls the cost of the coating.