Influence Of Recycled Fine Aggregates On Shrinkage Of Cement Mortar And Its Mechanism

A large number of construction wastes have become a major problem to be solved urgently in the construction industry and even in the whole society.Therefore,it is an inevitable requirement for the construction of ecological civilization to recycle and reuse the construction wastes.Recycled aggregate crushed from recycling construction waste,can be applied to preparing cement-based materials,and it has a good application prospect.Recycled fine aggregates(RFA)have the characteristics of high water absorption,complex composition and instability.It is significant to study the shrinkage properties of cement-based materials with RFA and its mechanism for the comprehensive utilization of construction waste.In this paper,specialized researches were carried out on the water absorption and releasing behavior of RFA in cement paste,and its influence on the autogenous shrinkage and drying shrinkage of mortar.The shrinkage mechanism of mortar with RFA was analyzed through the capillary stress theory.(1)A rapid and accurate method of measuring the water absorption after immersing for 24 h(WA24h)at atmospheric pressure of RFA was established.Because capillary force of water at the outer gap of RFA stockpile is different from that in the inner pores,they can be separated by a centrifugation method.From the water content change of RFA with the centrifugation speed and time,when the centrifugation radius is 6-10 cm,the water content of RFA tends to be stable after centrifugation for 10 min at 2000 rpm,and RFA can reaches the saturated surface dry state.The water absorption of RFA in paste(WAp)can be evaluated by determining the free water content and effective W/C((W/C)eff)of paste after separating RFA in plastic stage.The results show that the WAp is less than the absorption in pure water,and its value at 1 h is about 72.13%-80.18%of WA24h.WAp is related to the WA24h of RFA,W/C and S/C of mortar.Therefore,considering these factors,the(W/C)eff of mortar with dry RFA in different mixtures can be estimated with the function of(W/C)eff=W/C-0.605×WA24h-0.032(S/C-1),and the additional water(AW)can be calculated by the formula of AW=[(W/C)eff+0.605×WA24h+0.032(S/C-1)]×C-W,which can be benefit for analyzing the performance of mortar and mixing additional water accurately.(2)A dynamic vapor sorption analyzer can be used to draw an isotherm desorption curve of water from saturated RFA,so as to investigate the release behavior of water from the saturated RFA.As RH decreases from 100%to 10%,the development of water release of RFA undergoes four different stages.Firstly,the water is lost rapidly until RH reduces to 85%.Then,the water is lost slowly until RH reduces to 35%.And then,when RH reduces from 35%to 30%,there is a short stage of rapid water loss process again.Finally,the water is released gradually until RH reduces to 10%.When RH reduces to 93%,the water release is less than 40%,which indicates that RFA has a certain internal curing effect.The RH development of mortar at an early age was discussed by capillary pressure,whose starting time and development rate can be delayed by water release from pores of saturated RFA with the self-desiccation.(3)The shrinkage behavior of RFA cement mortar was studied by stages in the whole process.And the influences of particle size,dosage,WA24h and initial saturation degree of RFA were analyzed.When S/C is 1,compared to NFA mortar,RFA can reduce the autogenous shrinkage of mortar by 50.34%-74.84%before 1 d,and with the decrease of RFA particle size and the increase of WA24h,the effect of reduction is more obvious.The autogenous shrinkage can also be significantly reduced with the increase of RFA.The relieving effect of saturated RFA on the autogenous shrinkage of mortar is gradually weakened with the curing age.The autogenous shrinkage of RFA mortar is 1.60%-40.13%lower than that of NFA mortar before 28 d.After 28 d,the autogenous shrinkage of RFA mortar is less than that of NFA mortar only when using R-C-1.18 with a WA24h of 12.41%.With the same W/C and S/C,the drying shrinkage of RFA mortar is larger than that of NFA mortar,and it increases with the decrease of RFA particle size and the increase of WA24h.When S/C is 1,the drying shrinkage of RFA mortar at 180 d is 12.64%-50.27%higher than that of NFA mortar.When S/C increases from 1 to 2 and 3,RFA mortar decreases by 19.48%and 38.63%respectively at 180 d.The water absorption capacity of aggregate in paste varies with the increasing of RFA saturation degree,which affects the(W/C)eff and final water content of RFA,resulting in the difference of effect on the mortar shrinkage.When the total water is the same,the autogenous shrinkage and drying shrinkage of mortar increases gradually with the increase of saturation degree.At 180 d,the autogenous shrinkage increases by 2.41%-10.38%,and the drying shrinkage increases by 0.84%-10.30%.When using the equal amount of mixing water,with the increase of saturation degree,the autogenous shrinkage of RFA mortar decreases by 3.16%-11.48%,while the drying shrinkage increases by 2.73%-20.16%at 180 d.(4)The present shrinkage model of cement paste is based on the capillary tension as the shrinkage force,which can be as the equation ?=?(1/3K-1/3Ks)Svp.The mathematic model of shrinkage of RFA mortar is put forward on the basis of a composite material model:?m=?p(1-Vagg)n,in which n is the function of the elastic modulus of aggregate and mortar.The modulus of elasticity of mortar in the model is mended by micromechanics method,in which the W/C,the hydration degree and the volume fraction and modulus of elasticity of RFA are introduced.The(W/C)eff is taking into account for revising W/C,which is obtained by the test method and prediction formula of proposed in this paper.The calculation model of mortar autogenous shrinkage is established by considering the water absorption of RFA.The water loss of RFA mortar under drying condition was investigated by continuous weighing method.The finite difference method was employed to realize the moisture transportation in mortar under drying condition through the diffusion equation in the time field and space field.Then a calculating model based on the capillary stress mechanism was proposed for the drying shrinkage.The development of relative humidity,elasticity modulus,hydration degree and pore size distribution of RFA mortar with hydration ages was investigated.The test results show that RFA has positive and negative effects on the mortar shrinkage.On the one hand,the saturated RFA releases curing water with the self-desiccation of mortar,so as to alleviate the humidity decline,and decrease autogenous shrinkage at the early stage.On the other hand,because of the porous characteristics of RFA,the elastic modulus of mortar decreases,and the internal water will be evaporated gradually under drying condition,resulting in the increase of drying shrinkage.The model is validated by substituting different RFAs and mortar mixtures into the model,which agrees well with the experimental value.(5)The expansive and shrinkage reducing agent was put forward to reduce the shrinkage of RFA mortar on the basis of the shrinkage model.The results show that CaO expansive has a better effect on autogenous shrinkage at an early age than shrinkage reducing agent,but the effect became weaker at the later age.Shrinkage reducing agent could make a larger reduce on drying shrinkage than that of CaO expansive.