The Water Stability And Mechanism Of Low Water Binder Ratio Cement Based Materials

Low water binder ratio cement based materials( LWBRCM) is a new cement-based matrix composite material, which has strategic important significance in the defense, nuclear power, offshore platforms and other special projects. LWBRCM usually exerts various problems, such as a high content of unhydrated cement particles, because of its low water-binder ratio(<0.20) and high proportion of binder. With the humidity condition or in the water, the entrance of the external water would lead to the rehydration of unhydrated cement in concrete, as a result the new hydration products would expanse which had a negative influence on its long-term service performance(Water stability).From now on, the water stability of LWBRCM is still a dispute topic, and less attention has been paid on the hazard caused by the unhydrated cement. Currently, high-temperature water bath method is used for evaluating the water stability followed by B. Hillemeier. The accelerated testing temperature is too high(> = 80 ?), which cannot rule out the harm of DEF and may overstate the water stability of concrete. What is more, the assessment used to evaluate the water stability mainly concentrate on the strength loss rate, the relative value of ultrasound, frost resistance,water permeability, and the number and width of the surface crack on concrete. The influence of rehydration of unhydrated cement on the durability of concrete, such as volume stability, resistance to chloride corrosion, and anti- carbonization, need a further research.In this paper, accelerated high-temperature test(20??40??60??90?)were used to investigate the influence of rehydration of unhydrated cement on the mechanical properties,volume stability, resistance to chloride corrosion, and anticarbonization of neat paste with different water-cement ratio and mineral admixtures. The results can be concluded as follows.(1) The damage process of ultra-low water cement ratio cement based materials in the external water environment can be divided into two stages: a. Compaction stage. The hydration products produced by the rehydration of unhydrated cement fill the voids, cracks, pores and other defects, until the expansion stress within the pores of the filler starts to break down the outer wall of the pores; b. Micro crack growth phase. Cracks begin to emerge and expand resulting from uneven expansion, leading to the carbonation of Ca(OH) 2 in cracks. Therefore, surface crack is open, leading to surface corrosion, as a result water pour into the concrete.(2) The lower water cement ratio, the more serious phenomenon of water stability of cement paste in 90 d.Besides, with the decrease of the water cement ratio, the increased percentage of bond water, the expansion value, the flexural strength loss rate and the compressive strength loss rate increase.(3) Under the same curing temperature, the bound water of the same stage of the cement paste increase with the increase of curing time. This demonstrates the further hydration of unhydrated cement. What is more, with the increase of penetration depth, the content of bound water and Ca(OH)2 will decrease,which indicate that there must be moisture migration progress from the outside.(4) According to MIP and BSEM results, unhydrated cement can continue to hydrate, which generate micro-cracks in cement paste, as a result the total porosity and harmful big pore increase.(5) Fly ash and silica fume have a serious influence on the water stability of cement paste. For cement paste contained with fly ash, the change of pore structure of cement paste is responsible for the performance loss of cement paste. However, for cement paste contained with silica fume, change of unhydrated cement paste is the main reason for the improvement of performance of cement paste. Improvement of pore structure, decrease of unhydrated cement paste in cement paste are the main reasons in improving performance in cement paste with both fly ash and silica fume.