High performance pipe piles are widely used in precast industry. In order to improve the production efficiency and performances of the products, steam curing and mineral admixtures are usually applied. This paper evaluates the performances of HPC pipe piles produced with metakaolin-based admixtures. Quantitative phase composition analysis shows that the addition of 10 wt% metakaolin reduces the portlandite content by up to 57.1%, while increases the content of ettringite to some extent. The addition of metakaolin or metakaolin-slag/limestone blends improves the mechanical properties, especially the hybrid usage of metakaolin and limestone filler. Pore structure and microscope analyze confirm the remarkably-refined pore characteristics caused by the addition of metakaolin-based admixtures, in which case the gel pores accounts up to 85% of the total pores. Those remarkably improved properties caused by the addition of metakaolin-based admixtures indicate a promising future for their applications in producing high performance pipe piles with further modified properties.
Pre-stressed high performance concrete (HPC) pipe pile is widely used in various engineering projects such as building, highway and railway, port and wharf constructions, because of its advantages in mechanical properties, density, bearing capacity and convenience of construction [1,2]. Due to its increasing demand, the effective utilization of the molds is required in order to improve the economic efficiency of the production of HPC pipe piles. In order to shorten the cycle of the mold, the HPC pipe pile is usually required to gain a high strength at early age.
Among the approaches of achieving high early strength, accelerating the hardening process of cementitious materials by heat treatment such as steam curing, is an efficient manner of providing the desired early strength. Elevated temperature is known to improve the early stage hydration rate of the cement clinkers [3-8], and the relations between temperature/energy and hydration kinetics was also well established by the Arrhenius equation :
[mathematical expression not reproducible]
where [alpha] is the degree of hydration, Ea is the apparent activation energy (J/mol). R is the gas constant (8.31243 J x [mol.sup.-1] x [K.sup.-1]) and T is the absolute temperature. The acceleration mechanism includes both physical and chemical procedures that speeding up the dissolution, precipitation and diffusion of the hydrates [10,11]. However, one negative effect of elevated temperature curing on the hydrated matrix is the increased capillary porosity and reduced gel porosity [5,12], which will further results in the corresponding performance degradations. Kim  showed that concrete specimens subjected to a high early temperature (40 [degrees]C) attain higher early-age strength but eventually attain a lower later-age strength than cured at 20 [degrees]C.
The heterogeneous structuration of the matrix with a dense hydrated phase develops rapidly around the remaining cement grain and involves a slow-down of the diffusion process, which would result in a majority of less dense hydrated phase surrounding the denser one and weakening the whole matrix, therefore a reduced strength is shown at later stages [13-15]. These influences should be carefully considered and handled when...