The effect of synthetic zeolite on hardened cement paste microstructure and freeze-thaw durability of concrete

Abstract

Tests were done with synthetic zeolite obtained by synthesizing alumina fluoride production residue, sodium hydroxide and alumina hydroxide. The synthesis lasted for two hours at 95°C temperature. The obtained product was a mix of synthetic zeolite of modifications A and X. The binding material was cement CEM I 42.5R. The test results revealed that substitution of cement with 10% of synthetic zeolite reduces water absorption of hardened cement paste ∼23.8%; the addition of air entraining admixture does not change the rate of water absorption. With the increase of zeolite content in concrete with and without air entraining admixture increases the volume of closed pores from 1.6% to 2.1% and from 4.1% to 7.6% respectively. The open porosity determined by water absorption of concrete with and without air entraining admixture changes insignificantly. The average distance between pores reduces from 360μm to 190μm when part of concrete is replaced by zeolite addition without air entraining admixture and from 190μm to 130μm in the case of using air entraining admixture. The addition of 10% of synthetic zeolite results in reduced content of portlandite and development of hydro aluminate compounds C3AH6 (cubic crystals) in hardened cement paste. Rose-shaped plates of calcium hydrosulphoaluminates (mono sulphate form) were also detected. These crystals together with hydroaluminates fill in the pores of hardened cement paste and thus reduce the open porosity of concrete. Substitution of 10wt% of cement with synthetic zeolite addition increases the freeze-thaw resistance of concrete, i.e. reduces the loss of surface mass after 28 freeze-thaw cycles more than 1.6 times without the air entraining admixture and up to 3.5 times with air entraining admixture.