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Hydrated Portland cement paste exposed to a NaCl solution was acidified by adding HCl in small steps, gradually lowering the pH. The chloride binding of the cement paste changed as a function of the pH. For the range of pH from 13.2 to 12.2, decreasing pH resulted in a considerable increase in the chloride binding. At a pH of 11, the cement paste showed almost no chloride binding. In order to explain the changes in chloride binding upon lowering the pH, the phase assemblage was investigated with SEM-EDS, TGA and XRD and compared to a thermodynamic modelling.

We developed a simplified coupled model to simulate the chloride ingress of cement paste and mortar fabricated with CEM II/C-M (S-LL) and CEM VI (S-V) cements. Diffusion of chloride, calcium, sodium, potassium, sulphate and hydroxyl ions is considered, resulting in a set of coupled mass balance equations. Source and sink terms are introduced to represent the dissolution/precipitation processes due to leaching, and interactions between free chloride and hydrated phases are considered through binding isotherm curves. Electrodiffusion phenomenon is introduced through a modified Poisson equation to enforce local electroneutrality. The model is applied to simulate bulk diffusion tests with exposure solutions containing NaCl, and NaCl + KOH to reduce leaching. The main features observed experimentally are correctly reproduced: a higher chloride content is predicted for the materials made with CEM II binder, and a much clearer peaking behavior near the exposed surface is predicted in the case of NaCl exposure. [ABSTRACT FROM AUTHOR]

To reduce CO 2 emissions during the production of cement and to cope with increasing demands for concrete, and thereby cement, the cement industry needs to identify new supplementary cementitious materials. These new composite cements should provide, among others, a similar or improved durability of the concrete structures. This study investigated the hydrate phase assemblage in Portland cement pastes containing dolomite or a combination of dolomite and metakaolin after leaching, carbonation, and chloride exposure. The phase assemblage and phase compositions of the exposed samples and the unexposed reference samples were investigated using TGA, XRD, and SEM-EDS. The reaction of dolomite in the cement paste resulted in the formation of hydrotalcite. It was found that, unlike most other hydration phases, hydrotalcite can withstand high degrees of leaching and carbonation. When the samples were exposed to a chloride solution, the formation of a chloride-containing hydrotalcite was observed. [ABSTRACT FROM AUTHOR]

Mortar specimens were exposed to either a 3% NaCl solution or a 3% NaCl+KOH solution for up to 180 days. Exposure to the NaCl solution provoked much more leaching than the NaCl+KOH exposure. Leaching strongly impacted the chloride ingress profiles. The extended leaching led to a maximum total chloride content almost three times higher and a deeper chloride penetration than exposure with limited leaching after 180 days. The higher maximum chloride content seems to be linked to the enhanced binding capacity of the C-S-H and AFm phases upon moderate leaching as determined by SEM-EDS. The total chloride profile appears to be governed by multi-ion transport and the interaction of chloride with the hydration phases. Service life prediction and performance testing both rely on total chloride profiles and therefore ought to take these interactions into account. [ABSTRACT FROM AUTHOR]

This study investigates how the pH in the pore solution influences the chloride binding in Portland cement pastes. Cement paste samples were exposed to NaCl, CaCl 2 and HCl solutions. The mechanisms of changes in chloride binding were investigated using a wide variety of techniques both for the liquid and solid phase. Lowering the pH from 13 to 12 with CaCl 2 or HCl greatly increased chloride binding compared to NaCl, which was associated with increased amounts of chlorides in the AFm-phases. Lowering the pH below 12 lead to reduced chloride binding in part due to dissolution of AFm. Chloride binding approached zero at a pH of 9. The influence of C-S-H on changes in chloride binding could not be conclusively determined. The impact of leaching and low pH on chloride binding is key to explaining the peaking behaviour of chloride profiles in concrete submerged in chloride containing solutions like sea water. [ABSTRACT FROM AUTHOR]

For alkali-silica reaction (ASR) gels, the relationship between swelling expansion, structure and chemical composition, particularly the effect of aluminum, remains unknown. This study investigates the structure, swelling expansion and associated water uptake of synthetic ASR gels with various Al/Si (0–0.1) and Ca/Si (0.1–0.4) ratios. The results show that aluminum incorporated into the gel structure reduces the overall swelling expansion and the leaching of silicate species during the swelling test. Moreover, they revealed that water in the Al-ASR gels is more tightly bound, reducing the overall water uptake compared to the Al-free ASR gels. Additionally, there is a linear correlation between the maximal swelling results and the ASR gel composition. However, no direct correlation emerged between the amount of water uptake and the free swelling of the ASR gels, which indicates that other factors, like the type of water bonding and pore size of the gels, are decisive for the swelling mechanism. [ABSTRACT FROM AUTHOR]