Hydration Performance And Carbonation Of Clay Brick Powder-cement Complex Cementitious Material

With the continuous development of urbanization in China,old buildings are demolished,resulting in an increase in the number of solid building wastes year by year.And the amount of waste clay bricks accounts for approximately 50%in solid wastes,so it is very important to treat and utilize waste clay bricks rationally.At present,the main way to dispose of solid wastes in China is open-air stacking or direct landfill,which not only occupies a large number of land resources,but also causes secondary pollution to the environment.Clay brick is made of clay by high temperature sintering.Its composition mainly includes Al₂O₃,SiO₂ and a small amount of glass phase.It is a kind of artificial pozzolanic material.The clay brick has good grindability,and recycled clay brick powder can be prepared by physical milling.Therefore,cement is replaced by partial clay brick powder?CBP?prepared from waste clay brick,which be used as supplymentary cementitious material to prepare cement pastes and mortars contained CBP in this paper.Through the measurement of physical and chemical properties of CBP and optimum design of mix proportion,cement pastes and mortars with good working performance are obtained.The pozzolanic reaction of CBP,hydration heat of cement pastes contained CBP,chemical bonding water content,mechanical strength of mortars,hydration products,micro morphology and carbonization resistance of mortars are systematically studied,which provides theoretical and technical basis for the application in practical engineering.The characteristic hydration properties of cement composite cementitious materials with CBP were studied experimentally.The results show that the addition of CBP does not produce new hydration heat peak,but reduces cumulative hydration heat of cement and delays the emergence of heat release rate peak.The hydration heat of cement paste mixed with 40%CBP is 35.4%lower than that of the controller after72h.The addition of NaOH makes the heat release from contained CBP cement larger than that of non-activated cement,and shortens the time of appearance of heat release rate peak.With the increase of CBP content,the content of chemical bound water in pastes gradually decreases,and the chemical bound water content increases rapidly at early stage of hydartion.At 28 days,the bound water content in pastes has approached or exceeded to that at 180 days.The CBP did not take part in the reaction at 3 days witout NaOH and the reaction degree of CBP increased gradually after 28 days.The reaction degree of CBP in cement paste mixed with 10%CBP was achieved the maximum 17.82%at 180 days.The reaction of CBP in samples began at 3 days with NaOH,and the reaction degree of CBP in alkali-activated samples was relatively lower within 28 days,but it was similar to that of pastes without NaOH at 180 days.With the increase of the amount of CBP,the compressive strength of mortars decreases gradually within 28 days.The pozzolanic reaction of clay brick powder made the strength of cement mortars increase faster than that of control group.The compressive strength of specimens with 10%and 20%CBP at 180 days is 69.5 MPa and 68.1 MPa,respectively,which exceed the value of the controller.The hydration products of pastes contained CBP were tested.The results showe that the hydration products of pastes contained CBP,including CH,Aft and C-S-H gel,were the same as those of pure cement at 28 days.With the increase of curing time,CH was consumend by CBP to generate C-A-S-H gel and new C-S-H gel.The CH was produced by cement hydration gradually within 90 days,and the content of CH in pastes with 20%and 40%CBP were 3.81%and 2.73%,respectively at 90 days,however the content of CH were only 3.30%and 2.11%at 180 days,which indicated that the reaction degree of CBP was greater after 90 days.The average pore size and total porosity of cement pastes increased with the incorporation of CBP at 28 days.The pozzolanic reaction of CBP produced new solid phase filling pore,which gradually reduced the average pore size of pastes with the curing time increasing.At180 days,the total porosity of cement paste mixed with 20%CBP was 17.16%,and the average pore size was 10.7 nm,which were smaller than the controller.The CBP mainly acts as physical filling effect in early hydration stage.With the continuous pozzolanic reaction,the pore size of the pasets is refined and becomes uniform and dense gradually.The carbonation of cement mortars with CBP was studied.The results show that the carbonation depth of mortar increases gradually with the increase of the content of CBP.The maximum carbonization depth of specimens with 40%CBP is 13.8 mm at28d,but it is not completely carbonized.Accelerated carbonization makes the compressive strength of mortar samples increase obviously within 7 days,but the increase of compressive strength slows down after 7 days.At 28 days,the compressive strength of controller,mortars with 20%and 40%CBP is 62.3 MPa,52.4MPa and 45.7 MPa,respectively.But the effect of carbonization time on the flexural strength of mortars is not obvious.With the continuous carbonization,the diffraction characteristic peaks of unreacted C₃S,C₂S and partial CH gradually disappear,resulting in appearance of a large number of characteristic peaks of CaCO₃.And the diffraction characteristic peaks of different crystalline of CaCO₃ are determined.