Microstructural characterization of leaching effects in cement pastes due to neutralisation of their alkaline nature: Part I: Portland cement pastes

When concrete is exposed to the elements, its underlying microstructure can be attacked by a variety of aggressive agents; for example, rainwater and groundwater. The knowledge of concrete resistance to long term water aggression is necessary for predictions of their performance in different environments. This study aims to analyse the effects of leaching on the microstructure of Portland cement binders. Leaching of cement pastes was performed by an accelerated extraction leaching test that produces significant degradation and helps to achieve equilibrium or near-equilibrium conditions between the leachant medium and cement paste. FTIR spectroscopy, TG-DTA thermal analysis, low temperature nitrogen gas sorption, and geochemical modelling were used to characterize the microstructural changes produced in cement pastes at different equilibrium pHs reached during the leaching process.     

Accelerated tests of hardened cement pastes alteration by organic acids: analysis of the pH effect

Effluents, such as liquid manure and silage effluents, stored in silos often made of concrete, contain organic acids that are chemically very aggressive for the cement-based matrix. The pH of liquid manure is comprised between 6 and 8, and the pH of silage effluent is about 4.There has already been much research done on manure's effect on concrete using aggressive solutions with a pH of or inferior to 4, in order to accelerate alteration kinetics. These studies aimed at simulating liquid manure and silage effluent, equally.The goal of this article is to validate the use of solutions with a pH of 4 to implement accelerated studies on alterations occurring to structures exposed to the acidic part of liquid manure.In this study, the alteration mechanisms of the cement-based matrix produced by two solutions of organic acids with pH of 4 and 6 were compared.At the end of the experiment, carried out on ordinary Portland cement and slag cement pastes, the kinetics of alteration of the cement pastes immersed in the solution with a pH of 4 was ninefold higher than in the solution with a pH of 6.The chemical and mineralogical modifications of the paste were analyzed by electron microprobe, XRD and BSE mode observations.It was shown that the alteration mechanisms of the paste are sensibly identical for both solutions: almost complete decalcification, the disappearance of the crystallized or amorphous hydrated phases and the probable formation of a silica gel containing aluminum and iron, mainly. The differences in alteration mechanisms between the two solutions are minor and mainly concern the stability of the anhydrous phases: C₄AF and slag grains.     

The influence of potassium-sodium ratio in cement on concrete expansion due to alkali-aggregate reaction

In concrete containing potentially reactive aggregates, deleterious alkali-aggregate-reaction (AAR) can be prevented by the use of suitable mineral admixtures or by limiting cement content and alkalis (Na₂O-equivalent) of the cement. However, the Na₂O-equivalent of cement may not always accurately define the potential of cement to cause AAR. In this study, the potential reactivity of concrete produced with cements having similar Na₂O-equivalents but different K/Na-ratios has been measured and the composition of gel has been analyzed. Additionally, pastes and mortars have been produced to study the development of pore solution composition.The expansion of the concrete mixtures shows significant differences depending on the cement used. The different K/Na-ratio present in the cements is reflected in the pore solution of pastes and mortars and in the gel present in aggregates of the concrete mixtures. As the hydroxide concentration in the pore solutions of pastes and mortars produced with the different cements is nearly identical, the difference in K/Na-ratio has to be the reason for the observed differences in concrete expansion.     

Development of low-pH cementitious materials based on CAC for HLW repositories: Long-term hydration and resistance against groundwater aggression

One of the most accepted engineering construction concepts of underground repositories for high level waste considers the use of low pH cementitious materials. This research is focused on the development of those based on Calcium Aluminate Cements (CAC) with high mineral admixture contents that significantly modify their microstructural properties. Once their short-term hydration is known, this paper deals with the modifications generated in the pore solutions and in the solid phases of low-pH cement pastes based on CAC after 2 years of hydration, observing a high stability of the solid phases formed in the short-term. This paper also deals with the resistance of these materials to long term groundwater aggression using two types of aggressive agents: deionised water and groundwater from the real site of Äspö. Leaching tests show a good resistance of low-pH concretes against groundwater aggression but dependent on the leaching agent and on the concrete composition.     

Binding of chloride and alkalis in Portland cement systems

A thermodynamic model for describing the binding of chloride and alkalis in hydrated Portland cement pastes has been developed. The model is based on the phase rule, which for cement pastes in aggressive marine environment predicts multivariant conditions, even at constant temperature and pressure. The effect of the chloride and alkalis has been quantified by experiments on cement pastes prepared from white Portland cements containing 4% and 12% C₃A, and a grey Portland cement containing 7% C₃A. One weight percent calcite was added to all cements. The pastes prepared at w/s ratio of 0.70 were stored in solutions of different Cl (CaCl₂) and Na (NaOH) concentrations. When equilibrium was reached, the mineralogy of the pastes was investigated by EDS analysis on the SEM. A well-defined distribution of chloride was found between the pore solution, the C-S-H phase, and an AFm solid solution phase consisting of Friedel's salt and monocarbonate. Partition coefficients varied as a function of iron and alkali contents. The lower content of alkalis in WPC results in higher chloride contents in the C-S-H phase. High alkali contents result in higher chloride concentrations in the pore solution.     

Influence of chloride ions and pH level on the durability of high performance cement pastes

The resistance to chemical attack of low water to binder ratio pastes containing silica fume was studied by soaking small paste disks in three different pH controlled solutions, with or without sodium chloride, for periods of up to three months. The pastes were made using water to binder ratios of 0,25 and 0,38. The three solutions in which the paste disks were soaked were the following: 3% NaCl (by weight) at a pH level of 8,5,0% NaCl at 8,5, and 0% NaCl at 4,5. After three months of exposure, the results show that the pH level of the aggressive solution is the most important factor controlling the durability of cement pastes subjected to chemical attack. The total porosity and the depth of decalcification was found to increase with the decrease of the pH level. It was also found that the3water to binder ratio does not significantly affect the deterioration processes, but only influences the kinetics of these processes. The decrease of the water to binder ratio reduces significantly the rate of deterioration. Chloroaluminate crystals were observed only in the cement pastes having a water to binder ratio of 0,38.     

Pore solution composition and reinforcement corrosion characteristics of microsilica blended cement concrete

Plain and microsilica blended cement pastes with water-cement ratio of 0.6 were prepared using a 14% C₃A cement. Two levels of chloride from NaCl corresponding to 0.6% and 1.2% by weight of cement were added through mix water. The pastes were allowed to hydrate in sealed containers for 180 days and then subjected to pore solution expression. The expressed pore fluids were analyzed for chloride and hydroxyl ion concentrations. The results show that the OH⁻ ion concentration in the pore solutions of both chloride-free and chloride-bearing pastes drop steeply with increasing cement replacement by microsilica. For 10% microsilica cement pastes the pH for both 0.6% and 1.2% chloride addition was found to be around 13.30. However, the pH drops to a level below that of saturated Ca(OH)₂ solution when cement replacement by microsilica is increased from 10% to 20%. This is ascribable to the consumption of Ca(OH)₂ by microsilica as shown by the DTA/TGA results. 10% and 20% microsilica blending more than doubles the free chloride ion concentration in the pore solutions of the chloride-bearing pastes. 10% microsilica replacement raises the Cl⁻/OH⁻ ratio 4 to 5 fold, whereas for 20% microsilica replacement, the Cl⁻/OH⁻ ratio is increased to 77 and 39 folds over the corresponding values for the plain cement pastes for 0.6% and 1.2% chloride additions respectively. Accelerated corrosion monitoring tests carried out on steel bars embedded in plain and microsilica blended cement concretes exposed to 5% NaCl solution show a 3 fold superior performance of microsilica blended cement concretes in terms of corrosion initiation time. This corrosion behaviour is contrary to the prediction from the increased aggressivity of pore solution composition in terms of highly elevated Cl⁻/OH⁻ ratios. This is attributable to the densification of cement matrix by the pozzolanic reaction between microsilica and calcium hydroxide. No discernable advantage in terms of corrosion initiation time is evident by increasing microsilica blending from 10% to 20%.     

Effect of Curing Conditions on the Capillary Porosity of Hardened Portland Cement Pastes

The capillary pore structure of hardened portland cement pastes cured by high-pressure steam, chemical acceleration, high-pressure steam with reactive SiO₂, water immersion, water immersion and high-pressure steam, and hot-pressing was measured using mercury porosimetry to 50,000 psi. Differences of > 2 orders of magnitude exist in the average capillary pore diameters of the cement pastes studied. The largest pores (∼1 to 3 μm in diameter) are associated with high-pressure steam-cured pastes. The smallest average capillaries observed were 0.02 μm for pastes steam-cured with reactive SiO₂. Hot-pressed pastes had essentially no porosity accessible to mercury. The application of pore size control to problems of polymer-impregnated concrete is discussed.     

Fate of Calcium Chloride Dissolved in Concrete Mix Water

The extent to which chloride ion incorporated in portland cement concrete as calcium chloride accelerator at the usual treatment levels remains dissolved in the pore solution was investigated. This was examined by direct analysis of pore solutions expressed from cement pastes. The chloride ion concentration of the pore solution remains high during the first day of hydration and only gradually declines. It appears that appreciable concentrations of chloride ion likely remain in solution indefinitely .     

Use of nanocomposites as permeability reducing admixtures

Permeability is one of the fundamental properties of concrete structures as it is strictly related to durability. Mitigation of the degradation processes induced by aggressive solutions can be achieved by controlling water penetration through the pore network. In this study, we test the potential use of nanocomposites as waterproofing agents in concrete. Macroscopic measurements show that the addition of a small amount of nanoparticles effectively reduces the extent of the water permeation front. A combination of experiments, based on X‐ray tomography, mercury intrusion porosimetry, and BET nitrogen adsorption, and of numerical simulations, are used to interpret the macroscopic observations. These investigations show that C–S–H precipitation away from cement surfaces, induced by the presence of nanoparticles, leads to a refinement of the pore network. Such a microstructural change in the cement matrix results in a net reduction in the overall concrete permeability.     

Effect of temperature on pore solution composition in plain cements

Cement pastes with a water-cement ratio of 0.6 were prepared using three ordinary portland cements with C₃A contents of 2.43, 7.59 and 14%. Three levels of chlorides 0.3, 0.6 and 1.2% by weight of cement, derived from sodium chloride, were added through mix water. The pastes were allowed to cure in sealed containers at 20 and 70°C for 180 days and then subjected to pore solution extraction. The expressed pore solutions were analyzed for chloride and hydroxyl ion concentrations. Results show that increase in temperature from 20 to 70°C increased unbound chlorides and decreased hydroxyl ion concentration of pore solutions for all the three cements. The simultaneous increase in unbound chlorides and decrease in hydroxyl ion concentration drastically increased Cl⁻/OH⁻ ratio of the pore solution, thereby indicating an increase in corrosion risk. This adverse effect of increase in the Cl⁻/OH⁻ ratio of the pore solution with increase in temperature is higher in the high 14% C₃A cement than in the low C₃A cements, and is also higher for the low 0.3% chloride treatment level than the higher chloride inductions. Increase in temperature is also expected to cause an increase in ionic diffusion to steel embedded in concrete as well as in the rate of corrosion reaction. All these factors tend to increase corrosion risk of steel reinforcement in concrete with an increase in temperature.     

The combining of sodium chloride and calcium chloride by a number of different hardened cement pastes

A non-destructive method is presented to study the combining properties of a number of hardened cement pastes for NaCl and CaCl₂. Cements hardened with solutions of NaCl and of CaCl₂ are equilibrated in solutions of these electrolytes in a special way. To interpret the results, the equilibrium between the pore solution and the hardened cement is taken as a general solid/solution equilibrium. The equilibrium between the pore solution and the equilibrating solution is considered to be a liquid/liquid equilibrium. That means that the equilibrium concentrations in the equilibrating solution and the pore solution are equaL. Thus it is possible to calculate the dependence of the amount of chloride combined by the cement on the total amount added to the cement.     

Measurements of Chloride Activity Coefficients in Real Portland Cement Paste Pore Solutions

The study of chemical and electrochemical equilibria between concrete or embedded steel and ionic species in concrete's pore solution requires the knowledge of ion activities (free concentrations in solution). Recently, a method has been developed for measuring chloride-ion (Cl⁻) activities in simplified synthetic concrete pore solutions using calibrated Cl⁻ selective electrodes. Cl⁻ activities also have been calculated using Pitzer's specific ionic-interaction model. In this work, we have applied both of these methods, experimental and theoretical, for obtaining Cl⁻ activities in solutions obtained using the technique of "pore pressing," which has been applied to cement pastes with varied admixed Cl⁻ concentrations. A reasonably good agreement has been found between experimental and theoretical results, but the presence of hydroxide ions in these highly alkaline electrolytes interferes with the measurements at low Cl⁻ concentrations.     

Methods of obtaining pore solution from cement pastes and mortars for chloride analysis

Two techniques for the recovery of pore solution from cement mortars are examined: pore solution expression and miscible displacement using a high pressure permeameter. In the former, the pore solution is expressed from the mortar by crushing; in the latter, it is eluted from the mortar over 30 min by miscible displacement with water. Experimental results are presented for a range of cement pastes and mortars into which known amounts of chloride ion have been incorporated by using sodium chloride solution as the mix water. The results show that both eluted and expressed solutions exhibit a decrease in chloride ion concentration as the cement matrix ages, with the elution method showing a greater sensitivity to mix composition. Both methods show a decrease in chloride concentration as the water: cement ratio of the mix is increased. Overall, the high pressure elution method is capable of recovering a significantly higher proportion of the incorporated chloride. The application of these techniques to pore solution analysis is discussed.     

Interaction of bentonite with supercritically carbonated concrete

Bentonite and concrete are essential components in construction of a geological high level nuclear waste (HLNW) repository. Conventional Ordinary Portland Cement (OPC) used for concretes gives a pore water leachate with a pH as high as 13.5 in contact with ground water. This alkaline plume of leaching waters might perturb the engineered barrier system, which might include bentonite buffer, backfill material or the near-field host rock. The accepted solution to maintain the bentonite stability, which is controlled by the pH, is to develop cementitious materials with pore water pH around 11. Four lixiviation experiments representative of long-term interaction of solids and pore fluids at the concrete/bentonite interface were performed with two types of cement paste, Portland and calcium aluminate cement, before and after being carbonated under supercritical conditions, with granite water at 80 °C. The evolution of the pH indicates that the supercritical carbonation reduced the alkalinity of the cement pastes and calcite likely controls the equilibrium of Ca at the end of the experiments. The bentonite helps to buffer the alkalinity of concrete leachates through several reactions such as dissolution of montmorillonite and precipitation of secondary products as trioctahedral smectite, zeolites (gismondine), and presumably Mg hydroxides and amorphous gels. Carbonation may reduce propagation of the alkaline plume and enhance the barrier performance.     

Studies on expansive cement II. Hydration kinetics,surface properties and microstructure

Hydration kinetics were followed by measuring non-evaporable water and free sulphate contents. Surface areas, total pore volumes and the microstructure of the hardened expansive cement pastes are discussed. Nitrogen and water vapour were used as adsorbates in the measurement of the surface areas and pore volumes and the results obtained are compared with each other. Scanning electron microscopy was employed to study the microstructures of the hardened pastes.     

The pore solution phase of carbonated cement pastes

Samples of hydrated cement pastes were exposed to atmospheres with various carbon dioxide concentrations at relative humidities controlled by different saturated salt solutions. When carbonated throughout their thickness, as indicated by the phenolphthalein test, they were resaturated with water and subjected to pore solution expression and analysis. The effects of the various carbonating environments on the pore solution composition and on aspects of the pore structure and mineralogy of the carbonated products are reported. Implications regarding the likely effects of different accelerated carbonation regimes on the corrosion behaviour of steel in concrete are discussed. In particular, it is shown that the use of saturated sodium nitrite solution to control the relative humidity of atmospheres with high concentrations of carbon dioxide may cause an evolution of gaseous oxides of nitrogen, which can result in the contamination of the pore solution with nitrite and nitrate salts.     

Effect of Fly Ashes with High Total Alkali Content on the Alkalinity of the Pore Solution of Hydrated Portland Cement Paste

The influence of two Spanish fly ashes (ASTM class F) with high total alkali content (equivalent to 2.0% and 2.6% Na₂O) on the alkalinity levels of the pore solutions expressed from hydrated portland cement pastes was studied during a period of 90 days from mixing. Mixtures with 0%, 15%, and 35% replacement of cement by fly ash were prepared with a water/mixture ratio of 0. 4. The effect of the fly ash on the pore solution depended mainly on the age and fineness of the fly ash.     

Effect of some superplasticizers on the mechanical and physicochemical properties of blended cement pastes

Blended cement pastes made of Portland cement and fine sand (known in Egypt as El-Karnak cement) were made using a water–cement ratio of 0.25 by weight. Three pastes containing admixture (water-soluble condensates) were also prepared using a water–cement ratio of 0.25 and condensate (superplasticizer) content of 0.25% by the weight of cement; the superplasticizers used are Na-phenol sulfonate formaldehyde, Na-polystyrene sulfonate, and Na-ß-naphthol sulfonate formaldehyde condensates. All pastes were cured for various time intervals within the range of 0.02–90 days. Compressive strength tests, hydration kinetics, X-ray diffraction analysis, thermal analysis, and surface properties were studied and related as much as possible to the pore structure of the hardened pastes. © 1995 John Wiley & Sons, Inc.     

Effects of two Danish flyashes on alkali contents of pore solutions of cement-flyash pastes

The influence of two Danish flyashes on alkali contents and other characteristics of cement paste pore solutions was investigated by preparing pastes with 30 percent replacement of cement with flyash, hydrating for up to six months, and expressing and analyzing the resulting pore solutions. Reference cement pastes (without flyash) were found to retain about 80 percent of the total potassium and about 60 percent of the total sodium of the cement in solution indefinitely, leading to solution concentrations of about 0.6M in combined alkalies. The alkali contents of the flyashes used were substantial (2.4 and 3.3 percent Na₂O equivalent), and some of this is “available” as measured by standard tests. Despite this, neither flyash was found to contribute to the alkali content of the pore solutions. One was essentially inert to alkalies, the other extracted a small proportion of alkalies from the pore solutions.