Hydrothermal Synthesis Hydrochemical Phase Growth And Strengthening Mechanism Of Reactive Powder Concrete

Reactive powder concrete(RPC)is a new type of super high strength concrete.The main objective of this paper is to study the HV hardness and hydration of the RPC matrix,as well as the phase and morphology of hydration products with different water cement ratio and curing time,mainly the growth and transformation of hydration products at high temperature for a long time.Investigating of the enhancement mechanism of RPC is another goal of the study.The RPC matrix was mainly composited with cement,Silica Fume,Fs1002 and water reducing agent,whose proportion was 1:0.3:0.37:0.03.The RPC sample of 0.21 water cement ratio and 0.30 water cement ratio were molded by vibrating,while the RPC sample of 0.16 water cement ratio was molded by the way of cold isostatic pressingThe curing methods used in the experiment included standard curing,hot water curing and high temperature autoclave curing at 200?.After specific periods of curing,several kinds of properties of the RPC matrix were characterized.The mechanical properties were characterized by vicker's hardness method.The reaction extent of hydration was quantitatively evaluated by the content of acid-insoluble residues.The pore structure of RPC matrix was characterized by nitrogen adsorption volume method.DSC test,X-ray analysis(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM)were also used to characterize the type and morphology of hydration products.The experimental results are as follows:Firstly,the properties of samples autoclaved at 200? with different water cement ratio were studied.The results showed that HV hardness of the samples with 0.21 water cement ratio was higher than that of 0.16 water cement ratio.However,the HV hardness of samples with 0.30 water cement ratio was the lowest.It reflects that the decrease of water cement ratio can reduce the defections such as pores,but excessively low water cement ratio would affect the hydration of the matrix which could result in the decrease of the HV hardness.The examinations of the content of acid-insoluble residues in the RPC matrix suggested that it decreased by a large margin during the first 24 hours and then decreased slowly.And with the increase of the water cement ratio,the content of acid-insoluble residues would lessen.The XRD patterns showed that hydrated calcium silicate gel(C-S-H gel)was produced during the hydration,and the strength of characteristic peak of the C-S-H gel would be stronger if the water cement ratio increased.The SEM images of samples with two different water cement ratios showed that the amount of TOB had a positive correlation with the value of water cement ratio.Comparing the properties of RPC matrix under different curing conditions,it could be found that samples autoclaved at high temperature exhibited higher HV hardness and more content of acid-insoluble residues than that of samples under standard curing.The XRD analysis results showed that there were still no characteristic peaks of C-S-H gel after standard curing for 168 hours,while samples autoclaved at high temperature for 96 hours appeared a peak of 3.04A which belongs to C-S-H gel.It indicated that conditions of autoclave curing at 200?can accelerate the hydration,improve reaction efficiency in a short period,and greatly reduce the curing periodicity.Finally,the growth of the hydration products of samples with 0.21 water cement ratio autoclaved at 200? for a long period was studied.The results showed that HV hardness of the RPC matrix increased greatly during the first period of 24h and then increased slowly.The content of acid-insoluble residues in the RPC matrix decreased obviously after curing for 48h and then slowly decreased,which eventually tended to stability.It suggested that the hydration of the samples was more intense during the early periods of autoclaving at high temperature,and the reaction was still carried out during the later periods.The most probable pore size of the RPC matrix was about 10nm by nitrogen adsorption volume method,and the size of the pore did not decrease with the increase of curing time.The specific pore volume of the RPC matrix decreased and the specific surface area increased gradually when the curing time increased.From the results of XRD and SEM,it could be clearly seen that tobemorite(TOB)formed in the pores during the hydration with the increase of autoclave curing time at 200?.After long periods of autoclave curing,the TOB almost filled the entire pore.The cement mineral phase of C3S and C2S in the RPC matrix gradually transited to C-S-H gel in the initial stage of curing,which generated in the position of calcium hydroxide and appeared lamellar structure.With the increase of curing time,the lamellar structure developed into stripe like structure which crowded in multiple directions and finally grew along one direction because of the lack of space.TEM results showed that the diffusion of C-S-H gel in the matrix caused the formation of TOB,which generated in several directions in the initial stage of curing.Afterwards,the growth of TOB turned into the same direction as the space for the growth was quite limited.The transformation from C-S-H gel to the TOB resulted in the expansion of the volume on the meta-microscale,which enhanced the properties of the matrix.