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A new kind of high strength cementitious material is made from phosphogypsum (PG), active carbon and fly-ash. Through the orthogonal research, it was showed that the calcination temperature, retention time, dosage of active carbon and fly ash on the compressive strength of cementitious binder are the most important. The result also showed that, in the conditions of temperature 1200°C, time retention 30 min, dosage of active carbon 10%, dosage of fly ash 5%, the compressive strength of the cementitious material for 3d and 28d could reach to 46.35MPa and 92.70MPa, the content of sulfur trioxide was 11.60% accordingly. A lot of active mineral materials, such as dicalcium silicate, tricalcium silicate, tricalcium aluminate were formed in the calcination. The C-S-H gel, calcium hydroxide and ettringite were found in 3d and 28d hydrates. It is found that the lime saturation ratio and silica modulus need to be control between 0.40~0.65 and 4~8 in order to produce high strength cementitious material.

Cracking is a major problem in cementitious systems. It can be caused by loading, creep, shrinkage, chemical attack, or cycles of thermal expansion and contraction. Whatever the cause, cracking inexorably leads to lowered durability in the form of reduced mechanical properties and inability to protect reinforcing steel. The ability of a material to self-heal, that is to heal cracks without resort to external media, has been termed autogenous healing [1]. In a perfect system, autogenous healing would occur automatically upon the creation of a crack and would not require any external influence. The crack would be both sealed (reducing flow of liquids through the material) and, more importantly, healed, (restoring the material's mechanical properties). A cementitious material capable of robust autogenous healing would have a substantially higher durability, longer life, lower overall cost, and would be more environmentally friendly.Autogenous healing of cementitious materials occurs due to one of two mechanisms: either the hydration of unreacted cement particles or the carbonation of portlandite crystals, both of which are exposed during cracking. A number of studies on ordinary cement have shown the production of calcite to be the most important, and sometimes only, mechanism at work. Autogenous healing, via these mechanisms, occurs only in the presence of water, and only in very fine cracks below roughly 50 µm wide. The autogenic healing capabilities of ordinary cementitious materials are poor, as the crack size cannot be controlled or predicted, and is often far too large for autogenous healing to occur ( fig. 1a).Engineered Cementitious Composites (ECCs) are a class of ultra-ductile, micromechanically designed, fiber-reinforced cementitious composites [2]. The high ECC tensile strain capacity of above 3% is due to distributed microcracking produced by low loadings (< 2%) of polymer fibers ( fig. 1b). These microcracks are small enough to be autogenously healed, and can be custom tailored depending on application.Much work on autogenous healing in ECC has focused on mechanical properties. It is known that healing products not only seal cracks, but substantially improve stiffness and ultimate tensile strength upon reloading [3,4]. Chemical characterization of the healing products themselves, however, has received limited attention.A number of microanalysis techniques, such as FTIR, Raman, and energy dispersive spectroscopies complimented by x-ray diffraction have been used to characterize self healing products in ECC. The data show that the product is a mixture of both hydrated cement and calcite. The FTIR results in particular suggest that the calcite is not being produced by carbonation of portlandite, but rather by CO 3 2-ions in solution stripping calcium from the calcium silicate hydrate (C-S-H) phase of the 1220