Effect Of Hardening Accalerating Admixture On The Early-age Shrinkage And Cracking Of Mortar

Early-age cracking continues to be a significant problem for concrete construction.It is a serious and direct factor on the durability of concrete. In current civilengineering, in response to some special construction environment, concreteadmixtures are more and more favored. The major ingredient of concrete admixture isinorganic salt. Consequently, it is meaningful for the durability of concrete to researchthe effects of inorganic salt on the early-age cracking of concrete.In this work, the effect of calcium formate, calcium acetate and sodiumthiocyanate in different amount on the early-age cracking and strength of mortar indifferent water-cement ratio was studied. The amount of calcium formate, calciumacetate and sodium thiocyanate was ranging from0.25%to2.0%of the mass ofcement. The water-cement ratio of the mortar was0.30,0.35and0.40. The heat ofhydration and chemically combined water of cement paste inside24h were also studiedin this work. And the setting time of the cement mixed with calcium formate, calciumacetate and sodium thiocyanate in the amount above was studied as well.The results showed that, the area of early-age crakes increased along with theamount of calcium formate and calcium acetate adding inside a certain range, while theamount was outside the range, the area decreased. And crack width was increasinglyconcentrated, which increased a lot in the range of smaller width. The area of early-agecrake's increased and more narrow cracks appeared, when sodium thiocyanate wasincreased in the mortar. In other words, calcium formate and calcium acetate promotedthe early-age cracking during a certain amount. And when the amount was increased, itwould bring an opposite effect. Sodium thiocyanate promoted the early-age cracking.And with the increase of the amount, this effect was more and more prominent.Calcium formate improved the strength of the mortar, while its amount was not toomuch. But when the water-cement ratio of the mortar was small, a large amount ofcalcium formate would weaken the strength. The effect of calcium acetate on thestrength of the mortar was nearly the same with calcium formate. A little amount ofcalcium acetate would increase the strength, a large amount would weaken it. Sodiumthiocyanate had little effect of the strength. Calcium formate shorten the setting time ofthe cement paste. The more amount of calcium formate there was, the shorter thesetting time was. And it was the same effect of calcium acetate, which was just a little more weak than calcium formate. Sodium thiocyanate extended the setting time whichincreased with the amount of it.Although calcium formate and calcium acetate can be used as early strength agentand accelerant, it will bring the risk of early-age cracking as well, which isdisadvantage to the concrete durability. Sodium thiocyanate can be used as retarder,which has little effect on the early strength, but will increase the later strength. It alsowill bring the risk of early-age cracking. In conclusion, more attention should be paidto the amount of the calcium formate, calcium acetate and sodium thiocyanate used inthe concrete construction.