Calcium sulfoaluminate cement as binder for structural concrete
The use of calcium sulfoaluminate (CSA) cement as a concrete material can save energy by 25% .and reduce CO2 emissions by 40%. The potential of using ggbs, pfa, bottom ash, pyrite ash and other by-product and waste materials to produce the CSA cement can result in further environmental benefits. The research undertaken in this investigation aimed to explore the potential of CSA cement as the main binding material for structural grade concrete, identify the limitation of this material in this context and suggest possible applications for the resulting concrete. The experimental study covered a number of variables, anhydrite content, OPC and lime Inclusion, water/cement ratio and curing regimes. The investigation encompassed the preparation of CSA cement in the laboratory and the use of a commercially produced CSA. The systems investigated included paste specimens prepared with laboratory produced CSA and commercially manufactured CSA and concrete specimens prepared with the commercially manufactured CSA cement. The investigation in paste included hydration product identification using X-ray diffractometry and scanning electron microscopy, expansion and compressive strength development. Setting time of CSA cement paste was determined using samples made with the commercially manufactured CSA cement. The properties of fresh and hardened concrete investigated were setting time, workability using both slump test and Tattersall's two-point test, expansion, compressive strength, indirect tensile strength, flexural strength, oxygen permeability, water absorption and rapid chloride permeability. The research carried out in this investigation on CSA cement paste established that ettringite was the main product of hydration. The hydration reaction occurred at a fast rate, with hydration being almost complete within a week after casting. Formation of ettringite as a result of CSA and anhydrite hydration did not cause expansion but in the presence of calcium hydroxide in the system, resulted in expansion. In systems where expansion was evident after long-term water storage, it is suggested that this had resulted from the ettringite imbibing water and expanding. This expansion was found to be controlled by the presence of internal constraints, such as unhydrous particles or aggregates (in the case of concrete). The use of low water/cement ratio and the resulting low water absorption can further reduce such expansion. Concrete workability was improved in CSA cement and anhydrite systems over that of control OPC concrete resulting in lower water demand. The use of OPC as cement replacement in CSA concrete adversely affected the workability and accelerated the initial setting time. The compressive and flexural strength of concrete made with CSA cement and anhydrite were considerably superior to those of control OPC concrete but, in general, were comparable with respect to their indirect tensile strengths. However, compressive strength was found to degrade by 10-20% with prolonged water storage and the OPC, as cement replacement, did not contribute significantly to strength. The need for water for CSA cement hydration was generally higher than the mixing water required for workability. As a consequence, CSA concrete is expected to have lower capillary porosity than OPC concrete. This fact was manifested in the lower water absorption value found for CSA concrete. High oxygen permeability found for CSA and the inconclusive results of the rapid chloride permeability test suggest that further research into the durability performance and durability related properties is required. The high early-age flexural strength of CSA concrete is an advantage in rigid pavements and pre-stressed concrete. The low pH of CSA concrete is another advantaged when glass or vegetal fibres are used. The concrete, however, needs to be of low permeability to safeguard against steel corrosion.