The Influence Factors And The Mechanism Of The Carbonation Of Hardened Cement Pastes

Steel corrosion is one of the main reasons of causing concrete damage, in china,the loss of concrete structural damage caused by steel corrosion is up to100billion eachyear. Concrete carbonation, reinforcement depassivation induced by chloride ion andacidic media are three main reasons causing reinforcement corrosion. Because CO2iseverywhere, so carbonation ubiquitous; and, in recent years, the concentrations of CO2in the atmosphere rose gradually, which exacerbates the concrete carbonation further, sothe study of concrete carbonation is imminent. Cement is the only component ofconcrete can occur carbonation directly, so researching the cement carbonation centrallyis the key to solving the problem of carbonation.In this paper, the chemical analysis method is used; An exclusive measuringcarbonation installation be used, the content of calcium carbonate in carbonating cementpastes was measured in each layer. The influences of water to cement ratio, carbonatinghumidity, CO2concentration, curing condition, Superplasticizer and carbonation time,etc. on the carbonation of cement pastes were investigated and the carbonationmechanism were researched. The main conclusions are as follows:1) With the water-cement ratio is increased, the degree of cement carbonizationincreases, the width of partially carbonized region is gradually increased. Water-cementratio of0.45is the turning point in the performance of cement carbonation: When thewater-cement ratio greater than or equal to0.45, as the water-cement ratio increases, thedegree of cement carbonation increases significantly, increasing the partially carbonizedarea; When the water-cement ratio less than0.45, the partially carbonized area is veryshort, and the cement has a low level of carbonation.2) Carbonation humidity has significant effects on cement carbonation resistance.59%is cement carbonation critical humidity: When the humidity is higher than or equalto59%, the partially carbonized area is narrow, at this time the carbonation rate iscontrolled by the diffusion rate of CO2;and when the humidity is below59%humidity,With the humidity decreases, the degree of cement carbonation increase significantly,and the width of partially carbonized zone increases, at this time the carbonation rate iscontrolled by the rate of chemical reaction.3) The higher the CO2concentration, the more serious the cement carbonates.From the experiment also found that the impact of CO2concentration on the performance of cement carbonation seems to have some relationship with thecarbonation humidity, the difference of the level of cement carbonation is not very largewhen carbonation humidity is higher; when the humidity is low, the higher of CO2concentration, the degree of cement carbonization increased significantly, but still moretests to verify;4) Curing methods affect the carbonation resistance of hardened cement paste.When cured in saturated Ca(OH)2solution, partly of Ca(OH)2into the cementstructure,which is conducive to increased cement alkali reserves, so compared with thewater curing,for the use of saturated Ca(OH)2solution can improve conservationcement carbonation resistance.5) When a low water-cement ratio, the impact of superplasticizer on cementcarbonation on the short-term is not obvious. At higher water-cement ratio, mixingsuperplasticizer can improve the performance of anti-carbonation of cement. Thechange of superplasticizer dosage has not much impact on the performance of cementcarbonation.6) With the increase of carbonation time, the degree of carbonization increase.When a low water-cement ratio, the increases of carbonation depth with carbonationtime increases is more slowly and when the water-cement ratio is0.35and higher, itstarts carbonation faster, then gradually slowed. The function relation is found aboutcarbonation time and carbonation depth.7) The method of chemical analysis can characterize the true extent of carbonationmore accurately than phenolphthalein spray test.