Early autogenous shrinkage and dry shrinkage of cement based materials containing ultra-fine fly ash

Autogenous shrinkage and dry shrinkage of cement based materials, produced by lower water/binder ratios, are the key factors resulting in the concrete early cracking. Experiments on ultra-fine fly ash (UFA) replaced 20%-40% equivalent cement and UFA blended cement mortars were conducted. Experimental results show that autogenous shrinkage (before 7-day) of UFA mortar reduces and dry shrinkage increase with increase of UFA amount under the condition of fixed water/binder ratio. On the other hand, the autogenous shrinkage of UFA blended cement mortar reduces and dry shrinkage increase with increase of water/binder ratio under the condition of variable water/binder ratio. The better linear relationship between autogenous shrinkage and dry shrinkage is shown by regression analysis. The addition of UFA reduces water demand of cement based materials under the condition of keeping the same fluidity, so early strength and long-term strength of UFA mortar catch or surpass that of reference cement mortar, solving the problem of early strength of UFA mortar being lower because of no hydrating of UFA. The microscopic test results indicate that UFA has the better effects on dense packing, water reducing, dispersing and strength increasing.