Fundamental And Application Research On The Utilization Of Acetic Acid Modified Recycled Aggregate

With the rapid development of construction industry,the production of concrete consumes a lot of natural resources such as sand and gravel.On the one hand,excessive exploitation has gradually exhausted natural aggregate resource during the rapid development of construction industry over the past 30 years in China.On the other hand,the rapid development of construction industry has generated a large amount of construction and demolition(C&D)waste.The main component of C&D waste is waste concrete.It is economical and environmentally friendly to prepare recycled concrete aggregates(RCAs)from crushed waste concrete.RCAs obtained by crushing and sieving waste concrete have larger porosity,higher water absorption,lower density and more micro cracks compared with natural aggregates(NAs)because of the adhered cement paste.In order to improve the quality of RCAs,a zero-waste process to treat RCAs was carried out through immersing them in acetic acid solutions.Three different acid concentrations(0.1,0.2 and 0.3M)and three different immersion durations(12h,24 h,and 36h)were adopted to investigate the effects of different dilute acetic acid concentration and soaking time on the performance of RCAs.The quality,water absorption,apparent density and crushing index of the RCAs before and after acid soaking were tested.The test results showed that the treatment with acetic acid solution can decrease the water absorption of RCAs.The RCAs immersed in acid with a concentration of 0.2M for 24 hours showed the lowest water absorption of 3.5%,25.5% lower than the untreated RCAs.In addition,the treatment with acetic acid solution increase the apparent density and decrease the crushing value of RCAs.Combined with the results of water absorption,apparent density and crushing index,the acid concentration of 0.2M and immersion duration of 24 hours was found to be the best regime among all the immersion conditions.Several ions commonly existing in cement and its hydration products such as Na,Mg,Si,S,and K were detected in the soaking solution obtained by filtration.The pH value of the soaking solution is between 5.50 and 7.17 and the major ions in the solution is Ca.The soaking solution was used as mixing water substituting for the ordinary mixing water to prepare cement paste and mortar samples then.After mixing and curing,setting time,fluidity,heat evolution,strength,pore structure and shrinkage over time were tested.Results showed that the waste soaking solution can slightly prolong the setting time and increase the initial fluidity and fluidity loss of the produced cement paste.The use of soaking solutions as mixing water can improve the flexural strength of mortars at the age of 3 and 28 d,reaching up to 35% and 16.9% higher than that of reference group,respectively.The compressive strength of mortars at 28 d increases as well,reaching up to 35.3% higher than that of reference group.It is found that both the flexural and the compressive strengths of the mortars at 28 d increased with the immersion treatment duration,and were higher than that of the reference group.The decrease of the porosity and the refinement of the pores in the cement paste with the waste soaking solution account for this increase of the strength with immersion treatment duration.However,the strength of the mortars decreased when the acid concentration increased from 0.2M to 0.3M.This is because more pores larger than 100 nm were produced in the pastes by the acid with higher concentration.At the same time,the decrease of porosity and the refinement of the pore size of the soaking solution increased the shrinkage of samples prepared with soaking solutions.The sediment obtained by filtration is a kind of coarse powder with complex composition.Three different grinding times were adopted for the grinding of sediment.Results showed that that only 3.97% of the slag have particle size over 100μm after grinding for 60 s.The main components of the sediment are silica and calcium acetate,and the sediment also contains a small amount of calcium carbonate and impurities.The sediment after grinding for 60 s was used as recycled cementitious material to replace the cement with substitution rates of 5%,10% and 15%.The setting time,compressive strength,hydration products and micro-morphology of cement paste samples prepared with sediment were tested.Results showed that the initial and final setting time of cement paste samples decreased with the increase of replacement ratio.It is found that the compressive strength of cement paste specimens at the ages of 3d,7d and 28 d decreases with the increase of replacement ratio due to the extremely low hydration activity of the sediment.