Microstructure and performance of calcium sulfoaluminate cements
The microstructure and performance of calcium sulfoaluminate (CSA) cements are described. CSA cements contain C4A3S, 4CaO.3Al2O3.SO3,and are interground with gypsum/anhydrite. They have been used structurally in China for more than 20 years and elsewhere as special cements in non-structural applications. Their long-term behaviour of depends on ettringite, a relatively fragile substance, and a novel matrix former compared with Portland cement. The mineralogy, evolution of hydrate and microstructure of CSA cement pastes and concretes were studied to relate hydration products and microstructure with performances. Samples taken from in-service structures in China, ranging from several to 25 years old, were investigated. Drying methods for cement pastes affect experimental results, especially for ettringite-based materials and are reported. Calorimetry, X-ray diffraction, microprobe and porosimetry are used to investigate early (<24 hours) and late hydration processes in different curing regimes. A "shrinking sphere" model is proposed to explain hydration processes. At early stages, mineralogy differs between inner and outer products: the former contain mainly AFm and C-S-H while the latter contains mainly ettringite and gibbsite. At 25°C, these differences persist but inner products densify the paste. At 55°C, and at 85°C, substantial siliceous hydrogarnet, forms after ~3 d. It is a major inner product. This results in a volume decrease and coarsening of pores. Long-term exposure to higher temperatures ~85°C is harmful. The impact of water:cement ratios on hydration mechanisms is reported. Depths of carbonation of CSA cement concrete are reported: results show similar resistance Portland cement concrete. Pipe immersed in seawater for 16 years shows that CSA cement has excellent resistance to seawater attack. The steel in the pipe is uncorroded even though chloride ions have penetrated the cover. Long-term exposure to high temperatures, 85°C, of CSA cement should be avoided but under normal in-service conditions its long-term behaviour is satisfactory.