Pore Solution Chemistry Research Articles

Further insights into calcium sulfoaluminate cement expansion

There is still an ongoing debate on the mechanisms of expansion of calcium sulfoaluminate (CSA) cements, which can be either favourable, for example, in shrinkage compensating construction materials, or deleterious in cases of excessive expansion. In order to expand on previous studies, CSA cements with molar ratios (M) of calcium sulfate/ye'elimite of 0·1, 1, 1·5 and 2 were investigated from 30 min to 182 d. All systems showed expansion, and expansion was found to increase with increasing gypsum content. The sample with M = 2 showed a high expansion and severe cracking after 1 d of hydration. Based on the pore solution chemistry, crystallisation pressures were calculated, which revealed that not only ettringite, but also the other hydrate phases present (aluminium hydroxide (AH3), strätlingite, calcium aluminate decahydrate (CAH10), monosulfate) may contribute to the total crystallisation pressure. The percolation pore radii obtained by mercury intrusion porosimetry were used to calculate hydrostatic tensile stresses. It was found that in all investigated systems the hydrostatic tensile stresses exceeded the 1 d tensile strength, which is in line with the fact that all samples showed expansion.

Pore Solution Chemistry of Calcium Sulfoaluminate Cement and Its Effects on Steel Passivation

Calcium sulfoaluminate cement (CSA) is a type of low-CO2 binder which has been widely applied in the production of concrete. To investigate the protection capability offered by CSA to keep steel from corrosion, the pore solution chemistry of CSA on steel passivation was investigated in this study. The pore solution of CSA pastes, extracted by an ex situ leaching method, was studied and compared with ordinary Portland cement (OPC). The results show that the alkalinity of the CSA pore solution is not only much lower than that of OPC, but also that a new type of ion, Al(OH)4−, and high concentration of SO42− were detected in the liquid phase of CSA. Based on pore solution chemistry analysis, a simulated pore solution (SPS) system was designed to assess the comprehensive impact of alkalinity and ion composition, featured as properties of CSA, on steel passivation. The results of the corrosion potential evolution highlight the importance of alkalinity in passivation. SO42− can cause depassivation when there is not enough hydroxyl, but Al(OH)4− is able to maintain the alkalinity of the system, enhancing the stability of the passive film.

Impact of carbonation on the chloride diffusivity in concrete: experiment, analysis and application

This paper addresses the impact of carbonation on the chloride diffusivity in concrete through experimental and theoretical analysis. Chloride ingress tests were performed on concretes with OPC and complex binders (SZC) with/without carbonation, and the apparent chloride diffusivity was regressed with an enhanced diffusion model. Then, the impact of surface carbonation on the chloride ingress was investigated in terms of such influential factors as the pore structure, the chloride sorption, and the pore solution chemistry. Finally the results are applied to a design case of composite slabs exposed to marine atmosphere. The study shows that: (1) after carbonation, chloride sorption of OPC concretes is more affected than SZC concretes with complex binders and about 50% of sorption capacity is left for SZC concretes after carbonation; (2) the carbonation promotes the chloride diffusion, increasing the apparent chloride diffusivity by up to 80%, highlighting the impact of pore structure change on chloride diffusivity; (3) the durability requirements should consider the influence of surface carbonation for concrete exposed to marine air-borne salts but sheltered from natural precipitation.

Hydration of C3S in presence of CA: Mineral‐pore solution interaction

AbstractC3S and CA are the main phases of OPC and Fe‐rich CAC, respectively. The objective of this research was to investigate the influence of CA on C3S hydration, representing an under sulfated OPC‐rich binder, and to shed light on the underlying hydration mechanisms. To this end, C3S was blended with 1‐30 wt‐% CA and the pastes (w/c 0.5) were investigated by heat flow calorimetry, in situ X‐ray diffraction and analysis of the pore solution chemistry. CA additions ≥5 wt‐% reveal a separation into three distinct heat flow maxima, whereas additions ≤3 wt‐% just retard the start of the main reaction. The silicate reaction (dissolution of C3S and precipitation of C–S–H with or without CH) can be retarded for 4 to ≥22 hours in comparison to pure C3S depending on the admixed CA content. The start of the silicate reaction seems to be related to a decrease in Al‐ and increase in Ca‐concentration in the pore solution. However, it can be shown in this study that C3S is able to dissolve even at high Al concentrations in the pore solution.

Effect of MgO on chloride penetration resistance of alkali-activated binder

The present study investigated the physicochemical characteristics of alkali-activated fly ash/slag (AAFS) modified by MgO and the influence of MgO on the chloride penetration resistance of AAFS. MgO was added to replace the fly ash and slag at dosages of 0%, 5% and 10% by weight of the binder. Samples were immersed in a 10 wt% sodium chloride solution for 91 days. The results showed that the formation of hydrotalcite was promoted by MgO incorporation. Hydrotalcite was found to be effective in improving chloride penetration resistance. However, there exists a threshold level upon which the amount of hydrotalcite may not further increase the chloride penetration resistance. In addition, the adsorption of chloride in AAFS can be seen not only as the result of effects of hydrotalcite but also as the combined effect of the pore network structure and pore solution chemistry.

Acceleration of OPC by CAC in binary and ternary systems: The role of pore solution chemistry

In dry-mortar-formulations calcium aluminate cement (CAC) is often used as a set accelerator for ordinary Portland cement (OPC). However, a critical amount of CAC is able to delay not only the silicate reaction but also the renewed C3A dissolution of an OPC. This delay can be counteracted by adding an appropriate amount of calcium sulfate (C$) to the mix OPC/CAC. The initial hydration period is dominated by fast ettringite formation from CA (CAC) and CaSO4 from OPC generating early strength. High aluminum and low calcium concentrations in the pore solution probably hinder C-S-H seeding and precipitation, thus preventing alite dissolution. Aluminum has to be initially removed by precipitating hydrate phases to change the pore solution chemistry. The start of the silicate reaction is then induced by increasing the calcium concentration in the pore solution.

Effect of Fly Ash, Sinking Beads and Metakaolin on the Workability, Strength, Free Shrinkage and Chloride Resistance of Concretes: A Comparative Study

The aim of this study is to compare sinking beads (SB) with fly ash (FA) and metakaolin (MK) with regard to their ability to improve the performance of concretes. The workability, compressive strength, free shrinkage and chloride resistance of concretes with and without the above mineral admixtures are investigated. To explicate the differences of above results, the pore structure and pore solution chemistry were also studied. Besides, the Chapelle test was adopted to compare the pozzolanic activity of the mineral admixtures. The water-to-binder ratio of concretes and pastes was 0.35 by mass. The incorporation level of MK was 5 and 10 wt%, while that of FA and SB was 10, 20 and 30 wt%. A systematic improvement in flowability, compressive strength, chloride resistance, free shrinkage is observed for SB concretes compared with FA concretes due to the smaller particle size, more round particle shape and higher pozzolanic activity of SB. MK shows the overwhelming ability to improve compressive strength and chloride resistance ability of concretes compared with SB and FA for the same replacement level, but comparable compressive strength and chloride resistance for SB concretes can be obtained by increasing the content of SB. In addition, inclusion of 20 wt% SB demonstrates remarkable potential to reduce free shrinkage to 25% less than concretes with 10 wt% MK. It may be ascribed to the dense structure of SB, which brings less volume reduction or even some increase during pozzolanic reaction.

Comparison of chloride permeability methods for Alkali-Activated concrete

This paper presents the results of an experimental study of chloride permeability in alkali-activated fly ash, alkali-activated slag, and Portland cement concrete. Test methods include the rapid chloride permeability test (RCPT), AC and DC electrical resistivity, and the 90-day salt ponding test. We hypothesize that differences in pore solution chemistry between alkali-activated and Portland cement binders render electrical methods unable to accurately estimate chloride permeability in alkali-activated concrete. The present study seeks to evaluate this hypothesis by comparing results from electrical tests with diffusion coefficients from salt ponding tests. Contrary to previous claims, the RCPT provided a good estimate of chloride permeability in both alkali-activated slag and alkali-activated fly ash concrete. RCPT results showed excellent correlation with diffusion coefficients determined from salt ponding tests. Resistivity-measurements exhibited poor correlation to diffusion coefficients and overestimated the resistance to chloride ion penetration. Furthermore, AC and DC resistivity measurements showed significant disagreement for alkali-activated concrete. Finally, evidence form salt ponding tests suggests differences in chloride binding potential between alkali-activated and Portland cement concretes.

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.

Steel corrosion in reinforced alkali-activated materials

The development of alkali-activated materials (AAMs) as an alternative to Portland cement (PC) has seen significant progress in the past decades. However, there still remains significant uncertainty regarding their long term performance when used in steel-reinforced structures. The durability of AAMs in such applications depends strongly on the corrosion behaviour of the embedded steel reinforcement, and the experimental data in the literature are limited and in some cases inconsistent. This letter elucidates the role of the chemistry of AAMs on the mechanisms governing passivation and chloride-induced corrosion of the steel reinforcement, to bring a better understanding of the durability of AAM structures exposed to chloride. The corrosion of the steel reinforcement in AAMs differs significantly from observations in PC; the onset of pitting (or the chloride ‘threshold’ value) depends strongly on the alkalinity, and the redox environment, of these binders. Classifications or standards used to assess the severity of steel corrosion in PC appear not to be directly applicable to AAMs due to important differences in pore solution chemistry and phase assemblage.

Flow-through reactor experiments on basalt-(sea)water-CO2 reactions at 90 °C and neutral pH. What happens to the basalt pore space under post-injection conditions?

Recent publications on the successful mineralisation of carbon dioxide in basalts in Iceland and Washington State, USA, have shown that mineral storage can be a serious alternative to more mainstream geologic carbon storage efforts to lock away permanently carbon dioxide. In this study we look at the pore solution chemistry and mineralogy of basaltic glass and crystalline basalt under post-injection conditions, i.e. after rise of the pH via matrix dissolution and the first phase of carbonate formation. Experimental findings indicate that further precipitation of carbonates under more alkaline conditions is highly dependent on the availability of divalent cations. If the pore water is deficient in divalent cations, smectites and/or zeolites will dominate the secondary mineralogy of the pore space, depending on the basalt matrix. At low carbonate alkalinity no additional secondary carbonates are expected to form meaning the remaining pore space is lost to secondary silicates, irrespective of the basalt matrix. At high carbonate alkalinity, some of this limited storage volume may additionally be occupied by dawsonite −if the Na concentration in the percolating groundwater (brine) is high. Using synthetic seawater as a proxy for the groundwater composition and thus furnishing considerable amounts of divalent cations to the carbonated solution, results in massive precipitation of calcite, magnesite, and other Ca/Mg-carbonates under already moderate carbonate alkalinity. More efficient use of the basaltic storage volume can thus be attained by promoting formation of secondary carbonates compared to the inevitable formation of secondary silicate phases at higher pH. This can be done by ensuring that the pore water does not become depleted in divalent cations, even after carbonate formation. Using seawater as carbonating fluid or injection of CO2 into the basaltic oceanic crust, where saline fluids percolate, can reach this goal. However, such an approach needs sophisticated reactive transport modelling to adjust CO2 injection rates in order to avoid too rapid carbonate deposition and clogging of the pore space too close to the injection well.

Influence of limestone on the hydration of ternary slag cements

The hydration kinetics, microstructure and pore solution composition of ternary slag-limestone cement have been investigated. Commercial CEM I 52.5 R was blended with slag and limestone; maintaining a clinker to SCM ratio of 50:50 with up to 20% slag replaced by limestone. The sulphate content was maintained at 3% in all composite systems. Hydration was followed by a combination of isothermal calorimetry, chemical shrinkage, scanning electron microscopy, and thermogravimetric analysis. The hydration of slag was also followed by SEM image analysis and the QXRD/PONKCS method. The accuracy of the calibrated PONKCS phase was assessed on slag and corundum mixes of varying ratios, at different water/solid ratios. Thus, the method was used to analyse hydrated cement without dehydrating the specimens. The results show that the presence of limestone enhanced both clinker and slag hydration. The pore volume and pore solution chemistry were further examined to clarify the synergistic effects. The nucleation effects account for enhanced clinker hydration while the space available for hydrate growth plus the lowering of the aluminium concentration in the pore solution led to the improved slag hydration.

Comparing ion diffusion in alternative cementitious materials in real time by using non-destructive X-ray imaging

Alternative cementitious materials (ACMs) are receiving increasing attention worldwide but there is a lack of knowledge around the resistance of these materials against harmful ion intrusion. In addition, current accelerated test methods that measure ionic diffusion under an electric field are not reliable when comparing binders with vastly different pore solution chemistry. This paper overcomes these issues by using laboratory transmission X-ray microscopy (TXM) and micro X-ray fluorescence (μXRF) imaging to make real-time measurements of ion diffusion in paste and mortar samples for five commercially available ACMs and an ordinary portland cement.The results compare the apparent ion diffusion rate, quantify the change in the ion diffusion rate over time, and give insights into ion binding. The results show that after 42 d of ion exposure that the samples made with calcium aluminate cement had the lowest rate of ion penetration while the samples with alkali activated and calcium sulfoaluminate cement had the greatest rate of ion penetration. The portland cement had an ion penetration level that was between these two. Also, both the alkali-activated and calcium sulfoaluminate samples showed a decrease in the rate of penetration over time. These measurements are important to quantify the sustainability of ACMs, better justify their uses where durability are a concern, and guide future durability testing for these promising materials.

Effect of alkali dosage on alkali-silica reaction in sodium hydroxide activated slag mortars

Alkali-silica reaction (ASR) is one of the durability factors limiting the commercial use of alkali-activated slag (AAS) concrete. The effect of alkali dosage is of great importance and barely studied for ASR in AAS system. This study investigates the effect of alkali dosage on ASR in sodium hydroxide (NaOH) activated slag mortars. In addition to ASR expansion, the changes in sample weight, compressive strength, pore structure, and pore solution chemistry are also investigated for better understanding the factors controlling the ASR in AAS mortars. The present study shows that the pore solution alkalinity increases with increasing the alkali dosage of the activator. However, the ASR expansion decreases with increasing the alkali dosage. The SEM/EDX results show no significant difference in both morphology and chemical composition of the ASR products. The lower ASR expansion at higher alkali dosage may be attributed to the relatively higher concentration of aluminum in the pore solution due to higher degree of slag hydration, which is expected to mitigate the dissolution of silica from the reactive aggregate.

Early hydration of SCM-blended Portland cements: A pore solution and isothermal calorimetry study

In this study the hydration kinetics and the development of concentrations in the pore solution of cement pastes containing different supplementary cementitious materials (blast-furnace slag, Si-rich fly ash, limestone, quartz) at a cement replacement of 50wt.% were investigated during the first 6h of hydration. The results indicate that the degree of undersaturation with respect to alite is the primary factor driving the early hydration kinetics. The accelerating effect of the limestone is related to a higher undersaturation. The data reveal also that high aluminium and sulfate concentrations, which depend on the type and chemical composition of the used mineral addition, retard the reaction. High calcium concentrations had no adverse effect on hydration kinetics. The investigations underpin that the mechanisms controlling the early hydration are a complex combination of the filler effect, the pore solution chemistry as well as intrinsic reactivity of the mineral additions and their surface characteristics.

Effect of supplementary cementitious materials (binder type) on the pore solution chemistry and the corrosion of steel in alkaline environments

The pore solution compositions of paste samples produced with Ordinary Portland Cement (PC), slag 25%, 50% and 75%, fly ash 30%, condensed silica fume (SF) 7%, and a ternary blend of 50% PC, 43% slag and 7% SF were determined. Not only are there significant variations in the concentration of the major cations and anions but also, and equally important from the perspective of development of the passivity of steel in solution, in the level of dissolved oxygen and redox potential.Further, the impact of changes in the pore solution chemistry of cement pastes with SCMs on the passivation and corrosion of steel was investigated with mild steel in simulated pore solutions (SPS). Sulphides and thiosulphates, typically found in slag bearing pastes, appeared to reduce the chloride threshold concentration and increase the rate of corrosion in SPS, which has potential implication for the long term performance of reinforced concrete structures.

Electrical Properties of Cementitious Systems: Formation Factor Determination and the Influence of Conditioning Procedures

Abstract The number of people wanting to use electrical tests to determine the transport properties of concrete has increased with advancements in the portability of hand-held testing devices. Electrical measurements are an attractive test method to quantify transport properties of cement-based materials since they can be performed rapidly. There is a high potential for using these tests in quality control or mixture qualification. However, electrical measurements can be significantly influenced by curing and storage conditions, which can impact the degree of saturation, degree of hydration, sample temperature, and pore solution chemistry. This study proposed a general equation that described the electrical resistivity measurements in cementitious systems and possible methods to account for some of these conditioning-induced changes. It is proposed that these tests are useful in the determination of the formation factor, a numerical quantification that describes the microstructure. A comparison of the formation factor obtained from rapid electrical measurements using the Nernst-Einstein relationship was compared to a migration test with the goal of proposing a curing methodology for rapid electrical tests that allows for the determination of a true transport property.

Chloride transport and the resulting corrosion of steel bars in alkali activated slag concretes

As the relative performance of alkali activated slag (AAS) concretes in comparison to portland cement (PC) counterparts for chloride transport and resulting corrosion of steel bars is not clear, an investigation was carried out and the results are reported in this paper. The effect of alkali concentration and modulus of sodium silicate solution used in AAS was studied. Chloride transport and corrosion properties were assessed with the help of electrical resistivity, non-steady state chloride diffusivity, onset of corrosion, rate of corrosion and pore solution chemistry. It was found that: (i) although chloride content at surface was higher for the AAS concretes, they had lower chloride diffusivity than PC concrete; (ii) pore structure, ionic exchange and interaction effect of hydrates strongly influenced the chloride transport in the AAS concretes; (iii) steel corrosion resistance of the AAS concretes was comparable to that of PC concrete under intermittent chloride ponding regime, with the exception of 6 % Na2O and Ms of 1.5; (iv) the corrosion behaviour of the AAS concretes was significantly influenced by ionic exchange, carbonation and sulphide concentration; (v) the increase of alkali concentration of the activator generally increased the resistance of AAS concretes to chloride transport and reduced its resulting corrosion, and a value of 1.5 was found to be an optimum modulus for the activator for improving the chloride transport and the corrosion resistance.

A Multi‐Ionic Continuum‐Based Model for C3S Hydration

A computationally efficient strategy for modeling tricalcium silicate hydration based on through‐solution‐phase kinetics is demonstrated. This study extends a recently introduced advanced continuum‐based single particle model by including rigorous multi‐ionic transport, nonlinear reversible reaction kinetics and portlandite precipitation. Model parameters were either fixed based on known values, estimated using experimental measures or extracted by model fitting to benchmark experimental datasets. The model is now able to generate calorimetric hydration and evolution of pore solution chemistry responses that are in good agreement with available experimental results and predictions of other multiphysical modeling platforms. Oncecalibrated, the model was tested to see if it could predict the effect of water to cement ratio (w/c) and particle size on hydration outcomes. The findings support the need for a mechanism that limits the volume into which product can form.

Cofired biomass fly ashes in mortar: Reduction of Alkali Silica Reaction (ASR) expansion, pore solution chemistry and the effects on compressive strength

This document comprehensively studies reduction of ASR expansion by cofired biomass fly ashes in mortar and compares it to pure woody and coal ashes. An experimental matrix by ASTM C 227 at ambient temperature includes (1) six fly ashes (three cofired, one pure woody and two from coal), (2) three fly ash/cement (mass) ratios of 15/85, 25/75 and 35/65 and (3) testing periods up to 600days. The cement samples (no fly ash), no opal and opal, the most reactive aggregate in ASR expansion, serve as low and high expansion control. Three cofired biomass fly ashes coming from herbaceous/coal and one woody combustion, despite of their more than doubled available alkali of Class C, can reduce ASR expansion comparable to or much better than Class C. Pore solution chemistry with high pressure mold squeezing has unveiled that alkali dilution, alkali absorption and alkali ions transfer reduction, or combinations of the above three factors, play roles in reduction of ASR expansion. Alkali leaching from the curing containers by ASTM C490 is about 10%. This investigation provides significant motivation to revisit the provisions of ASTM C618, to include biomass fly ash in concrete rather than exclusion just by their “non coal” origin.