Estimating Fractal Dimension of Cement Matrix for Predicting Chloride Ingress into Cement-Based Materials

The durability performance of cement-based materials in service environments is affected by numerous factors, many of which involve attacks due to ionic transport, leading to reduced service life. The durability must be ensured in both an economically and environmentally responsible manner. Chloride-induced steel corrosion is a serious threat to the durability of reinforced concrete structures in marine environments, and the diffusion is the dominant transport mechanism of chloride ingress into concrete. Therefore, clear understanding of chloride transport mechanism, particular the diffusion path, is important for designing the durability performance of reinforced concrete structures. The purpose of this study is to determine tortuosity of cement-based materials and to predict the chloride ingress using the tortuosity values. The pore-structure model to obtain capillary pore was extended by introducing fractal dimension which represents microstructural complexity. The fractal dimension was determined by fitting experimental data to simulation results considering two types of C-S-Hs (low and high density) or two types of products (inner and outer), and it was used as tortuosity to determine effective diffusion coefficient of chlorides in the reactive transport model. The chloride ingress was simulated using the transport model and verified with experimental data for hydrated cement having water to cement (W/C) ratio of 0.5 and cured for 28 days. A good agreement between experimental data and simulated chloride profiles demonstrates that the diffusion path is influenced by presence of C-S-H types (HD-CSH and LD-CSH) and their pore structure characteristics

Zonal Isolation Material for Low-Temperature Shallow-Depth Application: Evaluation of Early Properties Development

Shallow-depth cementing presents unique challenges due to its low temperature and low pore pressure characteristic. The curing process of the cementitious material is typically prolonged at low temperatures resulting in a delayed curing process. The use of a low-density slurry to mitigate low pore pressure introduces another challenge, as it leads to a reduction in the final compressive strength. On the other hand, the operation requires the material to develop enough strength swiftly to be able to efficiently continue the next drilling operation. In addition, the presence of flow zones such as shallow gas and shallow water flow increases the complexity of the cementing process. There have been many developments in cementitious materials for shallow-depth cementing such as rapid-hardening cement and gas tight cement. However, there is little research focusing on the performance evaluation of each material at low-temperature conditions. This paper aims to present a thorough material evaluation for low-temperature shallow-depth cementing. The incorporated materials are American Petroleum Institute (API) Class G cement, rapid-hardening cement, gas tight cement, and geopolymer. Geopolymer is included to evaluate its potential as the green alternative to Portland-based cement. The sets of characterization were conducted during the liquid, gel, and solid phases. The samples were prepared under wide-ranging low temperatures and typical bottomhole pressures for shallow sections. The result shows different performances of each material and its behavior under low temperatures such as prolonged strength development and low reactivity, which necessitates further development of these materials.publishedVersio

Cesium incorporation in metakaolin-based K-geopolymer

Recently, considerable attention has been paid to using synthetic zeolites and titanates for cleanup of the waste water containing Cs and Sr radionuclides from Fukushima Daiichi Nuclear Power Plant. It has been considered that geopolymers have high potential for immobilization of Cs- and Sr-loaded zeolites and titanates, but more studies are needed to validate the geopolymers for radioactive waste disposal. The interaction of cesium with metakaolin-based K-geopolymer is studied in this paper. Geopolymers with composition of SiO2/K2O: Al2O3/K2O: H2O/K2O = 1:1:11 were synthesised and characterised based on ref. [1]. The binding of Cs and release of K in varying CsOH concentration were determined using ICP-AES (Figure 1). At very low concentration, the same amount of K is released for the binding of Cs, but the release of K is much higher than binding of Cs at high concentration of Cs. It is suggested that CsOH solution may promote the dissolution of geopolymer at high concentration. The results of zeta potential measurement indicate that there is no specific adsorption of Cs on geopolymer because the absolute value of zeta potential is increasing slightly with Cs concentration (Figure 2). Thus, the primary mechanism for Cs incorporation in geopolymer is exchange with K. Please click Additional Files below to see the full abstract

Comparison of alkali-silica chemical reaction of reactive glass and chert aggregate

Alkali silica reaction (ASR) is one of the most important factors of deterioration of Concrete. However, there are a lot of unknown points about ASR such as mechanism of ASR and pessimum effect. We investigate to the mechanism of Alkali silica chemical reaction by using two different aggregates, early type expanded aggregate and delayed type expanded aggregate. Experiments were used to investigate ASR by reaction between reactive silica and alkali ions with or without calcium ions. Two phases of reacted samples (liquid and solid) are presented. In the absence of calcium hydroxide (CH), reactive materials with specified sizes (Yorochert, and Pyrex glass) were mixed with sodium hydroxide (1M-NaOH) and kept at temperatures of 70°C. After specified reaction times, liquid samples with or without CH were withdrawn, filtrated, and provided for an ICPAES analysis to investigate the concentration of silica. After the filtration, insoluble product mixed with CH at 70 °C was used to investigate the chemical components and structure using XRD, 29Si-NMR and SEM/EDX. NMR and ICP results show that degree of dissolved SiO2 from Yo+CH samples is higher than that of PG+CH after 5 days of reaction, but the results are in opposite for the case of without CH. Further, the amount of Q3 sites, which contributes to expansion, in the ASR products of Yo+CH is lower than that of Pyrex glass, indicating that expansion from ASR cannot be explained by only the reaction degree of aggregates. However, the pH changes in the solution is related to Q3 sites suggesting that pH around the aggregates significantly affects the expansion

Impact of Portlandite on Alkali-Silica Reaction of Pyrex Glass and Blastfurnace Slag Aggregate

In this study, it is investigated the effect of Ca ion on dissolution and reaction products in alkaline environment using Pyrex glass showing ASR expansion and blast furnace slag fine aggregate suppressing ASR. It is reported that Ca plays a role in the dissolution and polymerization of silica, and is therefore an important factor in ASR. For each sample, calculation of dissolution rate and analysis of solid phase product by 29Si MAS NMR and XRD/Rietveld analysis, and liquid phase analysis by ICP-AES were performed. As a result, it was confirmed that addition of Ca has an influence on dissolution behavior of PG and BFS. In the reaction of PG, addition of Ca promotes the polymerization reaction of silica and increases the amount of N-S-H which has expandable and contributes to deterioration mechanism. On the other hand, in BFS, the dissolved silica did not polymerize and N-S-H did not form. It was suggested that the difference in ASR reactivity between PG and BFS is due to the difference in the reaction behavior

Physical, chemical, and mineralogical characteristics of blast furnace slag on durability of concrete

A partial replacement of Portland cement (PC) by ground granulated blast furnace slag (GGBFS) is an effective method to improve the durability of concrete due to its lower diffusivity and higher chemical resistance compared to PC. Further, the microstructure of GGBFS blended cementitious materials controls the physicochemical properties and performance of the materials in concrete. Therefore, understanding of cement hydration and cementing behavior of GGBFS is essential to establish microstructure property relationship for predicting performance. In this study, hydration, microstructure development, and chloride ingress into GGBFS-blended cement have been investigated. Solid-phase assemblage and pore solution chemistry of hydrating PC and cement blended with GGBFS were predicted using thermodynamic model and compared with experimental data. A mathematical model integrating PC hydration, GGBFS reaction, thermodynamic equilibrium between hydration products and pore solution, ionic adsorption on C-S-H, multi-component diffusion, and microstructural changes was developed to predict chloride ingress into GGBFS blended cementitious materials. The simulation results on chloride profiles for hydrated slag cement paste, which was prepared with 50% of replacement of PC with GGBFS, were compared with experimental results. The model quantitively predicts the states of chloride such as free, adsorbed on C-S-H, and chemically bound as Friedel’s salt

Intrinsic Differences on the Photodegradation Mechanisms between Pigmented and Non-Pigmented Coatings Determined by Multi-Scale Analysis

Multi-scale analysis of photodegradation are conducted for pigmented coating containing acrylic urethane + TiO2 pigment and for non-pigmented coating containing only acrylic urethane. We discuss the intrinsic differences in the photodegradation mechanism between the pigmented and nonpigmented coatings and the effect of the interface between the pigment and the binder. Photo-aging tests are conducted using artificial ultraviolet (UV) irradiation under conditions of 60 °C and dry atmosphere. The results of Fourier transform infrared spectroscopy (FTIR), solvent swelling experiments, ultrasonic measurements of elastic moduli, and colourimetry used for material characterisation before and after photo-aging. Although the behaviour of E and the carbonyl index (CI) show common trends for both samples, the overall trends of yellowness index (YI) and swelling degree (Q) differ significantly between the pigmented and non-pigmented samples. The results reveal that changes in macroscopic properties may not necessarily correspond with the CI behaviour and that characteristic interfacial effects exist between the pigment and the binder. The onsets of coating erosion and chalking are observed in the pigmented coating as surface topological changes. The different behaviour of YI and Q between the sample types can be attributed to the interfacial effect at pigment/binder interface based on the photocatalytic effect from TiO2 pigmen

Engineering Properties and Microstructural Performance of Low Energy Super-Sulfated Cement Using Industrial Waste Anhydrite

This study aims at proposing the mix proportions of low energy super-sulfated cement (SSC) concrete from industrial waste anhydrite from circulating fluidized bed combustion (CFBC) fly ash (CFA) as an alternative sulfate activator of ground granulated blast furnace slag (GGBFS/slag). The optimized mix proportion of the SSC was carried out by using mixture of different amounts of CFA in range of 25—45 wt.% and various quantities of ordinary Portland cement (OPC) in range of 0 — 10 wt.% to trigger the hydration of slag. Experimental results showed that with the expected slump at values of 190 — 220 mm, the 28-day compressive strengths of the concrete with low energy SSC reached 43.69 MPa which can be feasibly applied for widely advanced construction materials. The OPC in range of 3 — 5 wt.% and 25 wt.% of CFA were considered as the optimum ingredients of the activator and was suggested to be used for fabricating the low energy SSC concrete with the good performance on compressive strength, dynamic Young’s modulus, UPV measurement, and stabilized change of length. The OPC additive up to 10 wt.% was encouraged to be used for producing the SSC concrete with significant reduction on creep

Adsorption behaviour of simulant radionuclide cations and anions in metakaolin-based geopolymer

Geopolymers are a class of alkaline-activated materials that have been considered as promising materials for radioactive waste disposal. Currently, metakaolin-based geopolymers (MK-GPs) are attracting interest for the immobilisation of radionuclides in contaminated water from the Fukushima Daiichi Nuclear Power Station. However, the associated chemical interaction mechanisms and the theoretical prediction of the adsorption behaviour of MK-GP in response to cationic radionuclides have not been thoroughly studied or fully understood. In addition, there is a lack of studies on the adsorption capacity of MK-GP for anionic radionuclides. In this study, two types of metakaolin-based (Metastar501 and Sobueclay) geopolymers were synthesised at a K2O:SiO2:H2O ratio of 1:1:13. The binding capacity and interaction mechanism of MK-GP with Cs+, Sr2+, Co2+, I-, IO3-, SeO32-, and SeO42- were evaluated based on the zeta potential, radionuclide binding, and alkali leaching. The results showed that MK-GP does not have the ability to incorporate anionic radionuclides irrespective of the metakaolin source used, but both types of geopolymers have a high capacity to immobilise cationic radionuclides. The uptake of Cs+ was observed as a one-to-one exchange between Cs+ and K+ whereas both one–two and one–one ion exchanges are possible in the case of Sr2+ and Co2+ with K+. The formation of cobalt blue (CoAl2O4) also contributed to the binding of Co2+. Thermodynamic modelling was conducted according to the ion exchange mechanism which predicts the binding of Cs+ and Sr2+ at low concentrations