ITZ Strengthening And Chloride Ion Penetration Resistance Of Recycled Aggregate Concrete

With the rapid development of construction in China, demolished building waste increases sharply, which has reached 30-40% of municipal refuse. The waste concrete can be recycled and reused as aggregate. The reuse of old concrete as aggregate not only conserves the limited raw materials, but also reduces energy and resource consumption and therefore the overall construction costs.Due to the inferior mechanical properties of recycled aggregate than those of natural aggregate in general, it is generally accepted that for concrete made with recycled aggregate, the compressive strength is in the range of 0 to 30% lower than that of concrete made with natural aggregate, and other properties of interest like the tensile strength, flexural strength, shear strength, and elastic modulus are also typically inferior, while the creep and shrinkage tend to be higher. In recent years, several new special technologies have been developed to obtain high-quality recycled aggregate or to prepare recycled aggregate concrete at home and abroad. However, the equipments used in these technologies are all rather complicated and expensive. By using a double mixing method, i.e., by coating aggregate with cement and admixture, the interfacial transition zone (ITZ) between the recycled aggregates and hardened cement paste can be effectively improved to a certain extent and thus enhance the properties of recycled concrete. However, there still exists a weak ITZ according to the mechanism of the method. In this paper, a novel mixing method is developed through coating aggregate with admixture to adsorb the CH crystal enriched and forms C-S-H gels in the ITZ during hardening of the recycled concrete, which can be helpful to reduce or even eliminate the effect of the ITZ on the properties concrete. The influence of various mixing methods on the mechanical properties and chloride ions penetration resistance of recycled aggregate concrete are studied. The mechanism of improving ITZ through coating aggregate with admixture is also investigated by determining the bonding strength of recycled aggregate and hardened cement paste and analyzing microstructure of the ITZ between them. The results show that:(1) The mechanical strength and chloride ions penetration resistance of recycled aggregate concrete can be effectively improved through the novel mixing method of coating aggregate with admixture, compared to the mixing method of coating aggregate with cement and admixture. The mechanical strength and chloride ion penetration resistance of the recycled aggregate concrete at 28 days can be further improved over 10% and 40%, respectively, than that of the recycled aggregate concrete prepared by the mixing method of coating aggregate with cement and admixture. Whether using either of the two mixing methods, increasing the slump of the fresh concrete by using superplasticizer can help to improve the mechanical properties, but has minor effects on the chloride ion penetration resistance of the recycled aggregate concrete, especially for the recycled aggregate concrete by using the mixing method of coating aggregate with cement and admixture. The mechanical properties and the chloride ion penetration resistance of the recycled aggregate concrete can be improved by using slag as admixture more than that by using fly ash as admixture.(2) The adhesive strength between the recycled aggregate and the hardened cement paste can be effectively enhanced by using the mixing method of coating aggregate with admixture, even at early curing ages. Without coating the recycled aggregate with cement or admixture, CH crystals start to accumulate in the ITZ, but there is no preferential orientation at 1 day curing age. At 3 and 28 days, large quantities of CH crystals aggregate and show obvious preferential orientation in the ITZ. While coating with admixture such as slag and fly ash, no obvious accumulation of CH crystals is observed and large quantities of admixture micro-particles are distributed evenly in the ITZ at 1 day curing age. Simultaneously, some micro-particles with minor size and high pozzalanic reactivity have reacted with the accumulated CH and formed C-S-H gel in the ITZ, which can effectively enhance the adhesive strength between the recycled aggregate and the hardened cement paste, thus improving the properties of the recycled aggregate concrete. At 7 and 28 days, the admixture existed in the ITZ continues to react with the accumulated CH to further improve the properties of the recycled aggregate concrete. However, if there exist micro-particles with large size or low pozzalanic reactivity in the ITZ, the accumulation of CH crystals still can be observed between the recycled aggregate and the large particles, which can be further adsorbed by the large admixture particles to improve the properties of the recycled aggregate concrete.