Effect of Pre-wetting Lightweight Aggregates on the Mechanical Performances and Microstructure of Cement Pastes

The water absorption and desorption processes of different types of lightweight aggregates were studied.Subsequently,the influences of pre-wetting lightweight aggregates on compressive strength,microhardness,phase composition,hydration parameters and micromorphology of the cement pastes were investigated.The results showed that the water absorption and desorption capacities of the lightweight aggregates increased with the decrease of the densification degree.With the addition of pre-wetting lightweight aggregates,the compressive strength of the cement pastes would increase.Moreover,the enhancement effect was more obviously with the desorption capacity of pre-wetting lightweight aggregates increasing.Especially,sample S1 with pre-wetting red-mud ceramisites had the highest compressive strength,of which increased to 49.4 MPa after 28 d curing age.The reason is that mainly because the addition of pre-wetting lightweight aggregates can promote the generation of C–S–H gels in the interfacial zone,and the hydration degree of the interfacial zone increases with the water desorption of pre-wetting lightweight aggregates increasing.It is contributed to optimize the microstructure to enhance microhardness of the interfacial zone,resulting in the compressive strength of the cement-based materials improving.Therefore,the pre-wetting lightweight aggregates with high porosity and strength are the potential internal curing agents for high-strength lightweight concretes.

Effect of Lightweight Aggregate Pre-wetting on Microstructure and Permeability of Mixed Aggregate Concrete

The influence of lightweight aggregate （LWA） pre-wetting on the chemical bound water and pore structure of the paste around aggregate as well as concrete permeability were investi-gated. The results show that, in early age the dry LWA has significant effect on the formation of dense paste around it and improving the concrete impermeability. However the prewetted LWA has strong water-releasing effect in later age, which increases the hydration degree of the paste around it, and makes the adjacent paste develop a structure with low porosity and finer aperture, furthermore the concrete impermeability can be improved. It is suggested that, as for concrete with low durability requirement, the LWA without pre-wetting treatment can be used as long as meet the workability re-quirement of fresh concrete, the good impermeability of concrete can be gained as well. As for con-crete with high durability requirement, the prewetted LWA should be used, and the pre-wetting time should be extended as long as possible, in order to optimize the concrete structure in long term, and improve the concrete durability.

Effect of Pre-wetted Light-weight Aggregate on Internal RelativeHumidity and Autogenous Shrinkage of Concrete

This research indicates that the gradient of internal relative humidity （IRH） decreases rapidly within 7-day curing age in HPC. The amount of water imported by pre-wetted light-weight aggregate can regulate IRH of concrete. By importing a proper amount of water, the process of the decline of IRH can be delayed and the autogenous shrinkage can be reduced. The relationship among the amount of water imported by pre- wetted lightweight aggregate, IRH and AS was established. The result provides a new method of reducing early AS and enhancing early cracking resistance of HPC.