Although engineered cementitious composites (ECC) exhibit tensile strain hardening and multiple microcracking characteristics, whether the performance of ECC changes obviously under freezing-thawing conditions is important for the design and maintenance of buildings in the areas with freeze-thaw exposure. Results indicates that, as the number of freeze-thawing cycles increase, the PVA fiber/matrix interfacial bonding strength decreases, more fibers can be pulled out, thereby resulting in an increasing deformation on ECC. The results of water capillary absorption, chloride penetration and carbonation on ECC reveal that the frost damage has little effect on ECC. In addition, the steel bar-normal concrete interfacial ultimate bonding strength decreases linearly with the increase in the number of freeze-thawing cycles, the specimen splits failure. However, for the ECC, the steel bar-ECC interfacial ultimate bonding strength decreases with the increase in the number of freeze-thawing cycles, the pull-out failure occurs.
This paper discusses the hydration process of Portland cement (PC) and Calcium Sulphoaluminate cement (CSA) composite material at the early age. The setting time and mortar strength were tested, the hydration process was analyzed by isothermal calorimetry tests, and the hydration products were characterized by XRD, SEM analysis. The results show that: CSA can significantly improve the hydration rate of ultra-early age (2 h and 6 h) and improve the mechanical properties. But there are still interactions between the hydration of PC and CSA, including that the gypsum in CSA will inhibit the hydration of C3A in PC and the CH generated by PC will promote the rehydration of AH3. The inhibiting or promoting effect of CSA on hydration process changes with its content. The design method of PC-CSA composite material was proposed, and the reasonable mixing amount of PC or CSA was also suggested.
Due to lightweight, high-strength, and high toughness properties, Basic magnesium sulfate cement (BMSC) can be widely used in producing various structural and decorative products. Most current research only focuses on the performance of BMSC under room temperature. However, low temperature environments may have a serious impact on the performance of BMSC, such as during cold seasons or in salt lake cold regions with abundant magnesium resources, so the performance of BMSC in low temperature environment needs to be explored to expand its application. This study investigated the influence of different low temperatures (−5 to 20°C) on the compressive strength and water resistance of BMSC cement. The phase composition, microstructure and pore structure of BMSC has been analyzed using XRD, QXRD, TG/DTA, SEM, BSEM/EDS Mapping and MIP. The Results indicate that the low hydration rate, precipitation of magnesium sulfate, increase in porosity and decrease in 517 phase content are the main reasons for the low strength of BMSC under low temperature curing. Although hardened BMSCs cured at low temperatures can continue to hydrate in water to form strength phases, corrosion and cracking may occur. Replacing light burned magnesite with high active magnesium oxide can significantly improve the mechanical properties of low temperature cured BMSC. However, low temperatures reduced the crystallinity of the 517 phase, thereby increasing its water solubility and lead to poor water resistance of BMSC.
Air voids in concrete are sometimes observed to be blocked by white precipitates. In this study, the authors conducted freeze-thaw tests on concrete mixed with lime-based expansion material and then performed microscopic observations using scanning electron microscopy, elemental analysis using energy-dispersive X-ray spectroscopy. The size distributions of air voids in the concrete during the curing period (28 days after placement) and before, during, and after the tests were also measured in order to investigate the effects of the expansion material on air-void blockage as well as the underlying processes. During the curing period, no air-void blockage was observed. However, as the concrete aged, precipitates formed on the inner surfaces of the air voids. During the freeze-thaw tests, the number of air voids with diameters of ≤0.15 mm decreased, and the air-void spacing factor increased as the relative dynamic modulus of elasticity decreased. It was found that freeze-thaw cycles caused calcium-based precipitates to form inside the air voids of concrete mixed-with lime-based expansion material, and that the air voids became blocked.
In this study, the well-established service life design code defined in fib Bulletin 34 is adapted to predict the time to initiation of reinforcement corrosion in alkali-activated concretes submerged in marine conditions. The model approach is based on the probabilistic calculation of the time needed for a critical concentration of chloride ions, migrating from the external environment towards the rebar, to accumulate at the surface of the steel reinforcement and initiate the corrosion reaction. The information required to define the parameters of the model is derived from literature data, relating the concrete mix designs with accelerated laboratory test results. The findings indicate that alkali-activated concretes with high calcium content can exhibit promising characteristics as a construction material applied for structural application in chloride-rich corrosive environments. The probabilistic approach adopted in this model provides the opportunity to assess the influence of variability in mineralogical composition and reactivity of the precursors with the alkaline activating solution, that influence the chemical evolution and microstructure of the binder matrix. The predicted service life is quite sensitive to these factors, with very high service lives predicted for some alkali-activated concretes but rather short service lives predicted for others, and this must be incorporated into any engineering assessment of future material performance.