•A proposal of hydration degree calculation for blended cement pastes is presented.•The method is based both on the contributions of various authors and on DTA–TG results.•Paste and mortar specimens ...with BFS, FA and SF mineral admixtures were used.•The evaluation of CH gives information on hydration and pozzolanic reactions.•The assessment of α provides an insight into future strength evolution.
The degree of hydration assessment of cement paste from differential thermal and thermogravimetric analysis data has been performed by several authors that have offered a number of proposals for technical application to blended cements. In this paper, two calculation methods are studied in detail. Then, a proposal of the degree of hydration calculation for blended cements, based on the analysis of experimental results of DTA–TG, is presented. The proposed method combines the contributions of the authors and allows straightforward calculation of the degree of hydration from the experimental results. Validation of the methodology was performed by macroscopic and microstructural tests through paste and mortar specimens with blast furnace slag, flying ash and silica fume mineral admixtures bei(g)ng used. Tests of scanning electron microscopy with an energy dispersive analyser on paste specimens, and of mechanical strength on mortar specimens with the same percentages of substitution, were performed. They showed good agreement with the information derived from the differential thermal and thermogravimetric analysis data.
Raman spectroscopy combined with Confocal microscopy is a non-destructive technique that provides relevant structural information in materials. In this study, we present non-destructive Raman image ...and structural analysis of anhydrous cements with carbonation evidences by means of Confocal Raman Microscopy (CRM). The results obtained by CRM have been contrasted with the techniques commonly used for this purpose as FTIR and DTA/TG. CRM shows the main cement phases distribution (C2S and C3S) reveals their degree of weathering. The results obtained by CRM evidence a surprising coexistence of carbonate with amorphous carbon indicating that weathering mechanism in more complex than expected. Moreover, the size of sulphate particles contributes kinetically to weathering reaction. The weathering of cement particles by the atmospheric agents requires thus the combined action of amorphous carbon and sulphate. This study opens new analytical possibilities applied to commercial cements due to the combined chemical and spatial high resolution of CRM.
Cement chemistry notation: C: CaO, S: SiO2, H: H2O, A: Al2O3, F: Fe2O3, $: SO3.
► Chloride profiles from an offshore oil platform are studied and interpreted. ► Modelling chloride transport considering only diffusion provides incorrect results. ► Considering diffusion and ...convection allows a better interpretation of the results.
The aim of this paper is to explain the chloride concentration profiles obtained experimentally from control samples of an offshore platform after 25years of service life. The platform is located 12km off the coast of the Brazilian province Rio Grande do Norte, in the north-east of Brazil. The samples were extracted at different orientations and heights above mean sea level. A simple model based on Fick’s second law is considered and compared with a finite element model which takes into account transport of chloride ions by diffusion and convection. Results show that convective flows significantly affect the studied chloride penetrations. The convection velocity is obtained by fitting the finite element solution to the experimental data and seems to be directly proportional to the height above mean sea level and also seems to depend on the orientation of the face of the platform. This work shows that considering solely diffusion as transport mechanism does not allow a good prediction of the chloride profiles. Accounting for capillary suction due to moisture gradients permits a better interpretation of the material’s behaviour.
In this paper, the influence of additions of nanosilica (nSi) and microsilica (mSi) on the behaviour of binary and ternary mixtures in chloride environments is studied. The main objective is to ...obtain high-performance self-compacting concrete (HPSCC) with a high durability which can meet specific demands in such aggressive environments. Ten blends were manufactured using Portland cement (CEM I 52.5 R) and additions of nSi and mSi in binary and ternary mixtures. The results of three tests frequently used to evaluate resistance to chloride penetration– electrical resistivity, migration and chloride diffusion –were studied and compared. Both binary and ternary mixtures presented significant improvements in chloride resistance, generally in proportion to the total content of the addition. In all the ternary mixtures, high resistivity is obtained, which indicates that such mixtures have a notably low chloride penetrability. Furthermore, these mixtures provided extremely low chloride diffusion coefficients even at small addition ratios.
•New mortars prepared by electrostatic dry deposition of nanosilica and microsilica.•Optimal hydration degree and C-S-H gels percentages for mortars with 10% of addition.•Enhanced compressive ...strength of the cement mortars with additions.•Lower chloride migration coefficients and higher electrical resistivity values.•Microstructurally, the role of ettringite in durable properties is evaluated.
This work reports on the preparation and characterization of cement-based materials with additions of nano and micrometric silica. Their microstructure, mechanical properties and resistance to penetration of aggressive agents were studied. Mixtures of cement pastes and mortars with different percentages of nano and microsilica additions were prepared, with such additions being electrostatically incorporated into all cases in the anhydrous cement particles before the mixing process. The samples were characterized by the following experimental techniques: thermogravimetric analysis/differential thermal analysis (TGA/DTA), compressive strength tests, determination of the chloride migration coefficient, scanning electron microscopy (SEM), electrical resistivity measurement and mercury intrusion porosimetry (MIP). A relevant improvement in the mechanical and durable properties of the additivated samples when compared with the reference material was obtained.
By mixing several binder materials and additions with different degrees of fineness, the packing density of the final product may be improved. In this work, ultrafine cement and silica fume mixes ...were studied to optimize the properties of cement-based materials. This research was performed in mortars made of two types of cement (ultrafine Portland cement and common Portland cement) and two types of silica fume with different particle-size distributions. Two Portland cement replacement ratios of 4% and 10% of silica fume were selected and added by means of a mechanical blending method. The results revealed that the effect of the finer silica fume mixed with the coarse cement enhances the mechanical properties and pore structure refinement at a later age. This improvement is somewhat lower in the case of ultrafine cement with silica fume.
•The transport process of chloride ions in non-saturated concrete is studied.•The influence of the silica fume and blast furnace slag is studied.•High-mountain environment with presence of deicing ...salts is experimentally reproduced.•Numerical modelling of chloride ion transport considers diffusive and advective flux.•Modelling takes into account pore network, chloride binding and precipitation.
The durability of reinforced concrete (RC) is a challenge. One of the most aggressive and frequent ions is chloride, which frequently affects RC structures by steel corrosion. Many studies have concluded that the use of mineral admixtures improves concrete durability. This paper examines the transport process of chloride ions in non-saturated concrete. Three types of concrete have been designed and tested: the first one with only CEM I 52,2 R (CP), the second with 10% of silica fume (CPHS) and the third with 20% of blast furnace slag (CPEAH). An experimental programme was designed with the aim of studying the influence of external condition variation in a high-mountain environment with presence of deicing salts. The total time duration of the programme was 210days. The specimens were conditioned at different temperatures, relative humidity and chloride surface concentrations in five phases to reproduce the seasons annually. The experimental chloride concentration profiles were obtained at the end of each phase for each concrete. These profiles were mainly influenced by two transport mechanisms: diffusion and capillary suction. The chemical chloride binding and physical adsorption of chlorides in the concretes with silica fume (CPHS) and blast steel furnace (CPEAH) were two decisive factors in the chloride penetration processes. A numerical model for simulating chloride ion transport in non-saturated concrete is presented. It considers the diffusive and advective flux. In addition, the concrete pore network and the associate properties, such as chloride binding capacity and chloride precipitation, are also taken into account. The model has been implemented in a finite element programme. The chloride concentration profiles obtained by the model at the end of each phase for each concrete properly fit the experimentally obtained.
This paper presents the results of a thorough microstructural characterisation of a set of cement mortars (made with three w/b ratios and five different cement types), degraded by the ammonium ...nitrate method. Both destructive and ultrasonic non-destructive techniques were used to characterise the samples. The initial calcium content of the samples plays a dominant role in both the advance of the degradation process and the degradation grade of the samples. In addition, the decalcification process kinetics and the degradation grade are modelled, using Fick’s second law of diffusion and the shrinking unreacted-core model. Two parametric equations are presented and used to estimate the decalcification process kinetics and the degradation grade using accessible microstructural parameters of the native mortars, such as the initial CaO content, the open porosity, and the cementitious matrix volume fraction. Furthermore, the degraded depth in the samples is measured using ultrasonic testing with an average error of about 10%.
•We compared concretes cured in saturated calcium hydroxide versus sodium chloride solution.•The change in the mechanical and microstructural properties was not significant.•The free, bound and total ...chloride content was computed experimentally.•The proposed model, called Langmuir–Freundlich, showed the best fit to the experimental results.•The concretes with blast furnace slag cement and with silica fume showed the best performance.
The aim of this work is to study the influence of chloride ion in the mechanical, microstructural and chemical properties of concrete. For this purpose, four concrete mixes were designed, with two types of sulfate-resistant cement and two mineral admixtures (silica fume and fly ash). Furthermore, a chloride binding isotherm is offered that is compared with Freundlich and Langmuir models. This provides certain advantages in reproducing experimental results. According to such results, the properties do not present significant changes in all cases. The concretes with blast-furnace slag cement and with silica fume are those that present the best performance.
Concrete in marine environments is exposed to chemical mechanisms of deterioration, most of them involving chloride and sulfate ions. The principal motivation of this study is to try to minimize the ...expansive reactions between the aggressive ions and the cement matrix. However, the most effective protection will undoubtedly be the one that prevents the penetration of aggressive substances. The ingress of any aggressive substance in concrete is determined by concrete’s porous structure, especially the accessible connected porosity. This porosity is defined by the composition of the concrete and the chemical characteristics of the cement. The results clearly show that concrete that includes silica fume is significantly less porous and less permeable. In the rest of the mixtures studied, the porosity is higher, and the pore radius is the most decisive factor in defining the permeability.
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