Mescopic model of drying shrinkage and its application into control of shrinkage cracking

Concrete shrinks when it is subjected to drying, whenever its surfaces are exposed to air of a low relative humidity. In such a situation, cracking is to be expected, since there are various kinds of restraint that prevent concrete from contracting freely. Unless the ambient relative humidity is kept at close to 100%, such shrinkage cracking presents a critical problem in concrete construction, in particular for thin, flat structures such as walls and slabs in buildings, highway pavements, and bridge decks. This paper review the state of art of theory drying shrinkage from focus on the fact that microstructure in cement paste matrix consists of slit like pores as well as cylindrical shape pores. According to this fact, a new mesoscopic model of drying shrinkage is proposed. Our model can reproduce the experimental hysteresis loop of drying shrinkage for hardened cement paste, and reveals that drying shrinkage is closely related to the interaction potential between C-S-H nano-scale particles in cement systems. Meanwhile controlling cracking is essentially accomplished by reducing drying shrinkage. This paper also describes the efficiency of shrinkage reducing admixtures (SRAs) in controlling the restrained shrinkage cracking of concrete based on the mechanism of drying shrinkage

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

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

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

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

Characteristics of Ferrite-Rich Portland Cement: Comparison With Ordinary Portland Cement

The cement industry is an energy-intensive industry, and improving the energy efficiency of cement has become necessary to reduce its carbon footprint and to compete in the global market. Clinker production consumes more than 90% of the total energy used in the cement industry. Therefore, a reduction in the burning temperature of the cement clinker can reduce the energy consumption; however, it alters the mineralogy of the clinker composition. Ferrite-rich Portland cement can be produced by lowering the burning temperature by ~100°C (i.e., at 1,350°C), which can reduce the energy consumption by ~5% in comparison with ordinary Portland cement (OPC) clinker. In this study, the hydration reaction and properties of the ferrite-rich Portland cement were examined by experimental techniques and thermodynamic modeling approach, and the results were compared with that of OPC. The produced ferrite-rich cement has almost twice the amount of ferrite phase and half the amount of belite phase contents present in the OPC. The hydration reaction and the composition of hydrates were studied by the X-ray diffraction (XRD)/Rietveld analysis and thermogravimetry (TG) and differential thermal analysis (DTA). The different proportions of the ferrite and belite phases in ferrite-rich cement change their hydration reaction from that of the OPC, but not the total hydration of cement. The XRD results reveal similar phases in both the cements, and the analysis could not identify the new phases formed in the ferrite-rich cement. An equal degree of hydration and quantified hydrates at the early age results in almost identical initial and final setting times in both the cements. The ferrite-rich cement demonstrates a high early strength and relatively slower strength development; however, it can develop adequate strength at 28 days. The thermodynamic model predicts the hydration of ferrite-rich cement and shows comparatively high amount of Fe-containing phases, mainly Fe-ettringite and Fe-siliceous hydrogarnet. Model predictions of the hydrates compositions agreed with the experimental results, and a relationship between the predicted total porosity and the measured compressive strength was derived

Improving the Quality of Recycled Fine Aggregate by Selective Removal of Brittle Defects

Crushed recycled aggregate contains particles with brittle defects such as cracks, pores, and voids. This study presents a method for improving the quality of recycled fine aggregate by selectively removing these defects. Fourteen recycled fine aggregates were manufactured by three types of processors including a jaw crusher, ball mill, and granulator. The influence of the recycled fine aggregate on the flowability and strength of the mortar was evaluated by multivariate analysis. The results showed that flowability was mainly affected by the filling fraction of the recycled fine aggregate and the content of components passing through a 0.075-mm sieve. Both the compressive and flexural strengths of the recycled mortars were unaffected by the filling fraction, but they were affected by the fraction of defects in the aggregate and its surface smoothness. In addition, the results clearly showed that polishing involved in ball mill or granulator processing is effective both for increasing the filling fraction of recycled fine aggregate and reducing the fraction of defects in the aggregate. Moreover, it was determined that the grain surface of grains was more irregular with the granulator than that with the ball mill, resulting in higher strength of the mortar subjected to the granulator. The fracture configuration resulting from flexural stress on the recycled fine aggregate in the mortar differed according to the type of aggregate. Furthermore, the calculated amounts of emitted CO2 and the compressive strength of the recycled mortar showed that the recycled fine aggregate should not be polished excessively

Effect of fly ash on the kinetics of Portland cement hydration at different curing temperatures

This paper describes the effect of fly ash on the hydration kinetics of cement in low water to binder (w/b) fly ash-cement at different curing temperatures. The modified shrinking-core model was used to quantify the kinetic coefficients of the various hydration processes. The results show that the effect of fly ash on the hydration kinetics of cement depends on fly ash replacement ratios and curing temperatures. It was found that, at 20℃ and 35℃, the fly ash retards the hydration of cement in the early period and accelerates the hydration of cement in the later period. Higher the fly ash replacement ratios lead to stronger effects. However, at 50℃, the fly ash retards the hydration of the cement at later ages when it is used at high replacement ratios. This is because the pozzolanic reaction of the large volumes of fly ash is strongly accelerated from early in the aging, impeding the hydration of the cement

Electrical Conductivity and Chloride Ingress in Hardened Cement Paste

The transport properties of hardened cement paste (HCP) have been investigated in many studies; the AC impedance method (ACI) is a non-destructive technique employed for this purpose and has been used in investigations of the electrical characteristic and mass transport properties of HCP. However, there are relatively fewer studies investigating chloride ingress in HCP and using the ACI. In this study, the relationship between the electrical conductivity measured by the ACI and chloride ingress was investigated. Backscattered electron image analysis and mercury intrusion porosimetry and water porosity were used to measure the pore structure of HCP, and the chloride ingress depth was measured by an electron probe microanalyzer. There was a high correlation between the porosity and conductivity and between the conductivity and diffusion coefficient of the chloride ions in HCP. This implies that the diffusion coefficient of chloride ions could be estimated by the conductivity measurements