Showing papers on "Portlandite published in 2009"

Initiation of chloride‐induced corrosion of steel in concrete: role of the interfacial zone

Abstract: This paper provides a brief review of research aimed at characterising the steel-concrete interfacial zone (SCIZ) and its influence on the susceptibility of the metal to pitting corrosion when concrete is exposed to environments that cause ingress of chloride ions accompanied by leaching of hydroxyl ions. For reinforced concrete made from Portland cements, exposed to aqueous solutions of sodium chloride, the buffering effect of solid calcium hydroxide (portlandite) at pH ~12.6 has been shown to restrain the gradual decline in the hydroxyl ion concentration of the concrete pore solution phase at depths corresponding to the embedded steel. When the concrete is produced under laboratory conditions that are carefully controlled to exclude macroscopic defects from the SCIZ and the steel is cleaned before being embedded, this can lead to observed chloride threshold levels being consistently greater than 1% chloride by mass of cement. The buffering action of cement hydration products formed in the SCIZ is believed to be partly responsible for this high tolerance to chloride-induced corrosion because it counters the generation of 'anodic acidity' that is a necessary condition for stable growth of pits to occur. Translating this behaviour of laboratory specimens to the performance of full-scale reinforced concrete structures has often proved difficult in the past and there is a need for further research in this area, particularly in relation to the role of non-traditional cements.

Alkali-aggregate behaviour of alkali-activated slag mortars: Effect of aggregate type

Abstract: The alkali–silica reaction in waterglass-alkali-activated slag (waterglass-AAS) and ordinary Portland cement (OPC) mortars was evaluated using three types of (siliceous and calcareous) aggregates. The tests were conducted to the ASTM C1260-94 standard test method. The mortars were studied by volume stability, mechanical strength and Hg intrusion porosity. The ASR products were studied with XRD, FTIR and SEM/EDX techniques. According to the results obtained, under the test conditions applied in this study, waterglass-AAS mortars are stronger and more resistant to alkali-aggregate reactions than OPC mortars. When the mortars were made with a reactive siliceous aggregate, expansion was four times greater in the OPC than in the AAS material. When a reactive calcareous (dolomite) aggregate was used, no expansion was detected in any of the mortars after 14 days, although the characterization results showed that the dolomite had reacted and calcareous-alkali products (brucite) had in fact formed in both mortars. These reactive processes were more intense in OPC than in AAS mortars, probably due to the absence of portlandite in the latter. When the calcareous aggregate was non-reactive, no expansions were observed in any of the mortars, although a substantial rise was recorded in the mechanical strength of AAS mortars exposed to the most aggressive conditions (1 M NaOH and 80 °C).

Novel Use of Kaolin Wastes in Blended Cements

Abstract: This paper describes the influence of different thermally activated clay wastes (ACW) on the hydration phases in cement pastes containing two percentages of addition (10% and 20%). Results show that the main products obtained during hydration of cement pastes containing ACW were portlandite, calcium aluminate hydrates, calcium silicate hydrates, and hydrotalcite-type compounds. Portlandite formation decreases when addition percentage is 20%, contrary to tetracalcium aluminate hydrate, which increases in similar conditions. The ACW that showed the most portlandite consumption was ACW1 (700°C, 2 h) according to thermogravimetric data.

Multi-scale Study of Calcium Leaching in Cement Pastes with Silica Nanoparticles

Abstract: Calcium leaching is a degradation process consisting in the progressive dissolution of the cement paste as a consequence of the migration of the calcium ions to the aggressive solution. Although the most important changes take place at the nano- and micro-scale, their consequences are observed at every length scale. Within this work, a multi-scale approach combining a wide variety of experimental techniques was used to study such phenomenon in cement pastes with silica nanoparticles. The experimental results proved that the pozzolanic reaction induced by the nanoparticles resulted in a C-S-H gel more stable chemically and with longer silicate chains. In addition, the reduction of the amount of portlandite gave place to pastes with improved microstructure. As a consequence, the performances of such pastes were greatly enhanced both before and during the degradation process.

Relationship Between Engineering Properties, Mineralogy, and Microstructure in Cement‐Based Hydroceramic Materials Cured at 200°–350°C

Abstract: Cement-based materials used to seal geothermal or deep oil wells are exposed to severe conditions. Optimizing engineering properties such as strength and permeability is therefore very important. We have synthesized hydroceramic materials for such applications based on the CaO-Al 2 O 3 -SiO 2 -H 2 O (CASH) system and cured them over a range of temperatures (200°-350°C). Depending on initial composition and curing temperature, hydroceramics of complex and diverse mineralogy and microstructure are formed. The minerals found include portlandite, jaffeite, xonotlite, gyrolite, 11 A tobermorite, truscottite, hydrogarnet, and calcium aluminum silicate hydrate. These cement-based hydroceramic materials develop complicated pore structures, which strongly affect bulk properties. We report the compressive strength and permeability of these materials and show how these bulk engineering properties are related to microstructure. The compressive strength was found to be in the range 2-52 MPa and the intrinsic permeability in the range 0.5 × 10- 17 to 3300 × 10 -17 m 2 . Scanning electron microscopy (SEM) was used for imaging the hydroceramic microstructures. Further, we have computed the intrinsic permeability from 2-D SEM images by using the Stokes equation solver, Permsolver, applied to reconstructed 3-D images and the results are shown to be in good agreement with experimentally determined values.

Computational Molecular Analysis of Chloride Transport in Hydrated Cement Paste

Abstract: Concrete in transportation infrastructure is constantly subjected to the ingress of chloride, which could cause reinforcement corrosion and significant deterioration of concrete structures if not well controlled. Major types of reinforcement corrosion, such as pitting corrosion and concentration-cell corrosion, are usually initiated with localized chloride concentrations whose behavior is closely related to the transport characteristics of chloride ions in hydrated cement paste. However, unless studied at the molecular level, chloride transport behavior cannot be properly understood and controlled in view of the high heterogeneity of cement paste. The findings are presented from a research study recently conducted with a molecular dynamics (MD) approach to investigate the physicochemical nature of the various interactions between chloride ions and cement hydration products, which might significantly influence chloride transport in cement paste. Six hydrated compounds including portlandite, C-S-H phases (t...

Ion Mobilisation and Transport Through Cement Mortars Blended With Thermally Activated Paper Sludge in Natural Climatic Conditions

Abstract: One of the problems to affect Portland cement matrices is low resistance to aggressive agents, due principally to the presence of a high content of portlandite in the hydrated cements. Pozzolanic materials have, for decades, played an important role in improving the durability of cement-based materials. This work studies the behaviour of cement mortar matrices blended with 10% calcined paper sludge (source for metakaolin, MK) and exposed to different environmental conditions (marine and tableland environments). The results obtained using X-ray diffraction and scanning electron microscopy/energy-dispersive X-ray analyser techniques show that the ions present speed of different penetration as well as various phases and/compounds in the matrices following exposure over 1 year.

Slurry Stabilization and Reaction Chemistry of Cement-Treated Soils

Abstract: This project focused on evaluating important parameters for stabilizing soil with cement slurry and also investigated the chemistry of cement-modified soil at curing times up to 24 hours. Cement slurry usage is increasing due to concerns over dusting, and numerous companies and types of equipment are available to apply the slurry. By investigating the impact of cement slurry age, percent solids, and compaction delay time on the performance of cement-stabilized soils in the lab, researchers discovered that slurry age (up to 2 hours) does not affect performance, and slurry percent solids can impact performance in some cases. However, compaction delay time plays a much more significant role in the strength and modulus characteristics of the end product, and this delay time should be accounted for during the design stage. Investigations of the chemistry of cement-modified soils using scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction suggest that after only 10 minutes of curing both C-S-H and portlandite are present. As curing time increases, the amount of C-S-H decreases and the amount of portlandite increases. After 2 hours cure, coatings are developing on the clay, and after 24 hours these coatings are more extensively developed. Additionally, a shift in the montmorillonite basal spacing and a change in the montmorillonite recrystalization peak suggested the cement produced a structural modification. After 24 hours, a measurable amount of cement clinker existed, meaning unreacted cement is available for further reaction.

Influence of SiO2 crystallinity and impurities on the reactivity of lime-silica mixtures

Abstract: Several industrial by-products (Hi-Silica, thermal silica densified) were characterized from the chemical and mineral viewpoint and investigated their interactions with portlandite at 180 and 200 oC. The molar ratio of primary mixture was CaO/SiO2 = 0.66. The duration of the reaction varies from 24 to 168 hours. The phase composition and properties of calcium silicate hydrates is strongly affected by crystallinity and impurities in SiO2. It was determined that silica fume – Hi-Sil contains a large quantity of absorbed water which retarded the reaction between lime and SiO2. Low-base calcium silicate hydrates form more heavily comparing with pure system. Carbon impurities in silica thermal densified influences reactivity of initial mixture and impede reaction between Ca2+ and Si4+ ions. It was found that reactivity of silica thermal densified significantly increased after burning it at 900 oC and became suitable as raw material for the synthesis of calcium silicate hydrates. Meanwhile, in the lime–quartz– H2O system, due to low quartz solubility rate even in 168 hours of synthesis at 200 oC the main products are 1.13 nm tobermorite and xonotlite. Obtained results were confirmed by XRD, DSC, IR and SEM analysis methods.

Original Contribution AN INVESTIGATION INTO THE EFFECT OF MAGNESIUM SULFATE IN ZEOLITE-SUBSTITUTED CEMENTS

Abstract: This study comparatively examines the effects of MgSO4 in terms of the rate of zeolite admixture in paste specimens produced with PC42.5 R cement substituted with different ratios of zeolite. An examination of the chemical and physical changes of the specimens cured in water and MgSO4 solution reveals that 15% zeolite admixture is appropriate. Compression strength of control specimens, 15% zeolite-substituted superplasticizers cured in water, was slightly higher than that of the specimens cured in MgSO4 solution. This indicates that the pozzolanic activity was high and 15% admixture rate made the zeolite-substituted specimens resistant against MgSO4 solution. Consequently, zeolite binds the portlandite, Ca(OH)2, in cement and subsequently, is converted into Calcium Silicate Hydrate (C-S-H) composites; thereby, it prevents corrosion in cements, decreases permeability of concrete, and gives resistance against sulfate environments.