Chloride Binding Capacity Of Paste Research Articles

Effects of a novel carbon nanomaterial on hydration, mechanics, and chloride binding of cement composites

Nanomaterials have made significant progress in the field of cementitious materials, but suffer from primary problems of high cost and poor dispersity. Carbon dots (CDs) as a novel low-cost and high-dispersity nanomaterial preliminarily demonstrate their potential for cement. Nevertheless, it lacks systematic research on their effects on cement properties. For the first time, this work comprehensively investigates the effects of CDs on the hydration, mechanics, and chloride binding of cement composites. More importantly, the relevant mechanisms are revealed in depth. Specifically, cost-efficient ($0.013/g) and well-dispersed (dispersed in simulated concrete pore solution for 7 d) CDs are directly prepared by a facile microwave approach. The CDs dramatically retard the early hydration, improve later mechanical performance, and obviously enhance chloride binding. In detail, 0.2 wt% CDs reduce the second exothermic peak value by 75%, but improve the chloride binding capacity of paste by 51%. And 0.1 wt% CDs enhance the compressive/flexural strength of mortar by 17%/21%. Crucially, related influence mechanisms are uncovered. Namely, CDs create a protective barrier to retard hydration through the complexation of their abundant surface groups with Ca2+; While CDs exert the nucleation effect to boost the formation of C–S–H and Friedel's salt, thereby playing a positive impact on chloride binding; Meanwhile, CDs with suitable dosages carry out nucleation effect to counteract their air-entraining effect, thus improving later mechanical properties of cement composites. This study is expected to provide a novel low-cost and high-dispersity CDs nanomaterial to advance the development of high-performance cementitious materials.

Chloride binding behavior of calcium silicate hydrates and Friedel's salt of pastes

Understanding the chloride binding capacity of cement pastes is crucial to investigating the concrete durability. To characterize the chloride binding behavior of calcium silicate hydrates (C–S–H) and Friedel's salt (Fs), this study investigates the chloride binding capacity of four types of cement paste using chloride binding isotherms, analyzes the correlation between phase composition and chloride binding capacity, and discusses the evolution law of the chloride binding capacity of Fs and C–S–H. The results indicate that the influence of mineral admixtures on the chloride binding capacity of C–S–H is basically the same as that of pastes. Slag (SL) has a significant positive effect, while silica fume (SF) generally plays a negative role, and the effect of fly ash (FA) depends on the concentration of NaCl solution. As to the influence of chloride concentration, the chloride combined by per unit mass of C–S–H and Fs presents different evolution trends, and a concentration break-even point exists between C–S–H and Fs combined chloride. Notably, this point provides the insight into which substance occupies the majority of the total bounded chloride. Moreover, the impact of chloride concentration on the chloride binding capacity of pastes cannot be solely determined by the content of C–S–H and Fs, and the chloride combined by per unit mass of C–S–H and Fs at different chloride concentrations offers more relevance.

Effect of carbonation on chloride binding capacity of limestone calcined clay cement (LC3) and binary pastes

Carbonation of hydrated cement paste not only directly impairs the durability of reinforced concrete structures, but also alters the chloride binding capacity of cement-based materials. Besides, incorporating supplementary cementitious materials (SCMs) may influence both the durability against carbonation and the chloride binding capacity of binders. In this study, the chloride binding capacities of Limestone Calcined Clay Cement (LC3) pastes, prepared with four different kaolinite clays from local resources, have been investigated. In addition, binary pastes containing calcined clays (CC), silica fume and limestone powder (LS) as SCMs were designated to compare their pore structures, compressive strengths and durability properties with LC3 blends. The durability against carbonation was investigated via carbonation depth measurement, determination of hydroxide ions reduction and qualitative evaluation of phase assemblage changes through XRD analysis. Additionally, the chloride binding capacity of pastes was measured by the equilibrium method and the effect of carbonation on chloride binding capacity has been studied. The thermogravimetric analysis was also conducted to quantify the portlandite and Friedel's salt content of paste specimens. Based on the results, LC3 mixtures had lower bound chloride and Friedel's salt contents than binary blends prepared with CCs, while the chloride binding capacity of carbonated binary and ternary CC incorporated pastes were nearly similar. Besides, although LC3 pastes had higher chloride binding capacity than binary SF and LS containing blends, they were more susceptible to carbonation. Moreover, the CO3-AFm phases had generally lower capability to bind chloride ions than hydration products of LS-free binders.

Chloride binding capacity of pastes influenced by carbonation under three conditions

This research investigates the influence of carbonation on chloride binding capacity under three conditions varying in the sequence and way of carbonation-chloride contact: I, hardened pastes first carbonated then contacted with chloride; II, hardened pastes first contacted with chloride then carbonated; III, pastes inner-introduced with chloride during casting, hardened and then carbonated. The results indicate that, before carbonation, the bound chloride content of pastes inner-introduced with chloride is slightly higher than that of pastes first hardened then contacted with chloride because more Friedel's salt is formed through the former way. And during the carbonation process, the remaining bound chloride content mainly depends on the content of un-carbonated C-S-H gel. And based on the content of it before total carbonation, the content of residual bound chloride in samples under condition I is higher than that under the other two conditions after the same carbonation time. After complete carbonation, the bound chloride content under the three conditions all approximates to zero, which indicates that carbonation makes paste lose chloride binding capacity completely.