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Portlandite dissolution: Part 1. Mechanistic insight by Molecular Dynamics (MD)
Published in Günther Meschke, Bernhard Pichler, Jan G. Rots, Computational Modelling of Concrete and Concrete Structures, 2022
K.M. Salah Uddin, Bernhard Middendorf, Mohammadreza Izadifar, Neven Ukrainczyk, Eduardus Koenders
The portlandite (Ca(OH)2) is a major by-product of the cement hydration reaction, which results in the passivation of steel reinforcement. The dissolution of portlandite leads to the carbonation that accelerates the corrosion of the reinforcement by dropping the pH value of the pore solution by approximately three units. Which plays a vital role in the reduction of the service life of the concrete (Taylor 1997). The carbonation of portlandite is also played a critical role during the setting of the concrete mixture, since the calcite formed by carbonation is comparatively less soluble than the portlandite itself. During the hydration of cement, portlandite is not only precipitated in the hardened cement paste but also forms a thin crystalline layer between the steel reinforcement and aggregate. Those interfacial layers of portlandite influence the resistance of the reinforced concrete (Lea 2004).
The Chemistry of Concrete Biodeterioration
Published in Thomas Dyer, Biodeterioration of Concrete, 2017
Another important constituent is calcium hydroxide (portlandite, Ca(OH)2). This compound acts as the main source of hydroxide ions, and consequently the high pH of water in the pores of hydrated Portland cement. It should be noted, however, that the solubility of portlandite is relatively low. Thus, the high pH is achieved through the release of OH− ions from portlandite to balance the charge of potassium and sodium ions in solution which were originally present in the clinker as sulfate salts.
Properties of sustainable self-compacting concrete made with rice husk ash
Published in European Journal of Environmental and Civil Engineering, 2022
Ravinder Kaur Sandhu, Rafat Siddique
Figure 4 shows the variation of compressive strength with RHA at 7, 28, 90, and 365 days. At 28 days, the control mix attained a compressive strength of 59.16 MPa. However, with the inclusion of 5%, 10%, 15%, 20%, 25% and 30% RHA the percentage change in compressive strength was 3.06%, 7.20%, 1.64%, −5.17%, −6.69% and −12.31% respectively. It was observed that SCC with 10% RHA achieved maximum compressive strength. Thereafter, strength values started decreasing but remained higher than the control mix up to RHA15. The reason for increased strength was due to RHA properties. RHA being a pozzolanic, finer, higher silica content material with a more specific surface area that increases the strength of concrete when added to the SCC mix. In this process, cement during its hydration reaction produces Calcium silicate hydrates (CSH) and calcium hydroxide (CH) also known as Portlandite. When RHA, being pozzolanic, is added it consumes Portlandite produced during hydration reaction of cement that forms additional C-S-H gel thereby increasing the packing and strength of the SCC mix (Molaei Raisi et al., 2018).
Coupled effect of sulphate and temperature on the reactivity of cemented tailings backfill
Published in International Journal of Mining, Reclamation and Environment, 2021
The above explanation is supported by the measurements of the degrees of saturation for mature CPT samples with different types of binders (PC, PS and PF), as shown in Figure 9, which shows variation in the degrees of saturation. The degree of saturation was calculated based on the standard soil phase relationship equation for degree of Saturation (Sr.: Vw/Vv = wGs/e; where Vw = volume of water; Vv = volume of void, w: water content; Gs: specific gravity; e: void ratio). In this figure, the PF-CPT sample has the highest degree of saturation as compared to the others, thus explaining for its lower reactivity. With respect to the other factors (e.g. pore structure, porosity, and binder type), the degree of saturation is a key factor that controls the oxygen diffusion rate through the CPT samples. This is due to the fact that the diffusion of gases (e.g. oxygen) in the water phase is about 10000-folds less than in the gas phase [45,46]. Furthermore, the results of the XRD analyses concur with the fact that higher curing temperatures reduce the reactivity of CPT with no sulphate or a low sulphate content, as shown in Figure 10. Figuress. 10(a,b) show the results of the XRD performed on 150-day PS-CPT without sulphate cured at 20°C and 35°C, respectively. Portlandite (mineral name of CH) is one of the main hydration products which dominate the properties of cement paste, and therefore can be used to determine the hydration rate of cement. By comparing the intensity of CH at 18 degrees 2-theta in these figures, it can be observed that the sample cured at 35°C has a higher intensity (around 95 CPS), than that of the sample cured at 20°C (75 CPS). This high intensity (CH peak) indicates that the sample cured at 35°C produces more hydration products as compared to the sample cured at 20°C. This is mainly due to the fact that there are more hydration products due to a higher curing temperature [7]. Therefore, a higher curing temperature leads to the refinement of the pore structure and densification of the sample matrix which enhance its transportability. Consequently, the rate that oxygen diffuses through the sample is reduced and thus reduces the reactivity.