Corrosion Mechanisms Of Artificial Reef Reinforced Concrete Attacked By Biological Sulfuric Acid

Artificial reef reinforced concrete can repair marine environment and improve fishery production.However,its defects are large alkalinity and high cost.Meanwhile,it is suffered from biological corrosion in marine environment.Based on the cost,enviromental protection and durability of concrete,a new artificial reef concrete with sulphoaluminate cement,marine sand and sea water et al.(namely NARC)was proposed,then sea water curing was adopted.Fist,the effects of cement type(namely ordinary Portland cement and sulphoaluminate cement),sand type(namely marine sand and river sand),mixed water type(namely sea water and fresh water)on the performance of the artificial reef concrete(ARC)were studied.Then,a device and chemical sulfuric acid soaking were made to simulate the biological sulfuric acid attack on artificial reef reinforcement concrete in marine environment.The parameters for sulfuric acid type(namely biological sulfuric acid and chemical sulfuric acid),sulfuric acid concentrations(name-ly pH=1.1,1.6 and 2.0)were changed.The change of appearance,mass,compressive strength,Ca2+ releasing concentration,chloride ion concentration and pH values of pore solution at the position of steels in concrete,corrosion potential,corrosion current and corrosion rate of steel in concrete were tested.The thermogravimetry,SEM-EDS,XRD,IR and NMR were used to study the corrosion products of ARC.The theory of liquid crystal degree of supersaturation,chemical reaction thermodynamics theory,diffusion theory and Faraday’s first law were used.The degradation depth model of concrete,evaluation method about degradation degree of concrete attacked by biological sulfuric acid,threshold values equation of steels corrosion before bacteria not involved in and initial steel corrosion rate model were proposed and veried.The experiment results and theoretical analysis were combined to reveal the influence of corrosion mechanisms.The main research results are as follows:1.The workability of NARC can meet the requirement of construction(namely slump,slump loss,cohesiveness and water retention).The mechanical properties,anti-permeability and ecological effect of NARC are superior to those of ordinary Portland cement concrete(OPC).2.The concrete attacked by biological sulfuric acid is transport controlled.The corrosion damage of concrete attacked by biological sulfuric acid is the result of joint action by H+,SO42-and bacteric effect.Fistly,CH,C-S-H and AFt are dissolved and transformed by H+,and then Ca2+ reacts with SO42-in pore solution to produce the corrosion expansive products consist of CaSO4.2H2O.T.f likes to grow in rough and loose concrete surface filled with gypsums and produces acid in concrete surface,which reduces the path of biological sulfuric acid to concrete(namely bacteric effect).3.A degradation depth model of concrete attacked by biological sulfuric acid considering diffusion coefficient dependent on time,an evaluation method about degradation degree of concrete caused by biological sulfuric acid referred to quanlity loss,the threshold values equation of steel corrosion based on pH values and chloride ion before bacteria not involved in is proposed and veried,respectively,which provides theoretical support for studying corrosion mechanisms.4.The main hydration products of OPC is C3S2H3,which is completely converted to silicone gel and aluminum gel under the action of biological sulfuric acid.The main hydration products of sulphoaluminate cement concrete(SCC)is AFt,which is not completely dissolved under the action of biological sulfuric acid.Compared with corrosion area of SCC,micro-cracks appear in corrosion area of OPC and corrosion area of OPC has more bacteria effect.Therefore,the corrosion rate and damage degradation degree of SCC is less than those of OPC,respectively.The pore structure and interfacial transition zone(ITZ)of SCC is more compact than those of OPC,respectively.The pH reduction rate of OPC is higher than that of SCC.Therefore,the corrosion current and corrosion rate of steel in SCC at the age of 140 d is less than those of steel in OPC,respectively.5.Compared with river sand and fresh water,marine sand and sea water can improve the pore structure and ITZ of SCC,which increases the difficulty of medium diffusing into concrete.The surface roughness and porousness of concrete with river sand and fresh water is more serious than those of concrete with marine sand and sea water,respectively.Bacteria effect in corrosion area of concrete with river sand and fresh water increases.The dissolution rate of hydration product,gypsum generation amount and pH reduction rate of concrete with river sand and fresh water is more than those of concrete with marine sand and sea water,respectively.Therefore,compared with SCC with river sand and fresh water,the corrosion rate and damage degradation degree of SCC with marine sand and sea water increases respectively.The corrosion potential and corrosion current of steel in concrete with marine sand and sea water mixing inhibitor is easier to shift positive and less than those of steel in concrete with river sand and sea water not mixing inhibitor,respectively,which has better steel corrosion resistance.6.With the increase of biological sulfuric acid concentration,the dissolved Ca2+,gypsum generation amount,increase rate of diffusion coefficient of biological sulfuric acid,bacteria effect,damage degradation degree and corrosion rate increases,respectively.The pH values reduction rate and Cl-released rate in concrete increases,repectively.The chloride corrosion threshold of steel in concrete decreases.The corrosion potential of steel in concrete shifts to negative direction.The corrosion current and corrosion rate of steel in concrete increases,respectivly.7.Gypsum of corrosion products acts as a barrier to the penetration of chemical acid to concrete.However,for the biological sulfuric acid,gypsums of corrosion products build a suitable environment for bacteria to grow and more biogenic sulfuric acid are produced,which accelerates corrosion.The corrosion rate,damage degradation degree and corrosion current of SCC and OPC attacked by biogenic sulfuric acid is more than those of SCC and OPC attacked by chemical sulfuric acid,respectively.The corrosion potential of steel in concrete attacked by biological sulfuric acid is more easy to shift negative than that of steel in concrete attacked by chemical sulfuric acid.The resistance to biological sulfuric acid of NARC is superior to that of OPC.The corrosion rate of steel in NARC is less than that of steel in OPC without inhibitor.Therefore,NARC has a good prospect in engineering.