Modeling the degradation of Portland cement pastes by biogenic organic acids

Reactive transport models can be used to assess the long-term performance of cement-based materials subjected to biodegradation. A bioleaching test (with Aspergillus niger fungi) applied to ordinary Portland cement pastes during 15 months is modeled with HYTEC. Modeling indicates that the biogenic organic acids (acetic, butyric, lactic and oxalic) strongly accelerate hydrate dissolution by acidic hydrolysis whilst their complexation of aluminum has an effect on the secondary gel stability only. The deepest degradation front corresponds to portlandite dissolution and decalcification of calcium silicate hydrates. A complex pattern of sulfate phases dissolution and precipitation takes place in an intermediate zone. The outermost degraded zone consists of alumina and silica gels. The modeling accurateness of calcium leaching, pH evolution and degradation thickness is consistently enhanced whilst considering increase of diffusivity in the degraded zones. Precipitation of calcium oxalate is predicted by modeling but was hindered in the bioleaching reactor.     

A new test method to assess the bacterial deterioration of cementitious materials

The biodegradation of cement-based matrices by agro-industrial effluents is a very complex phenomenon. In this work, a test was developed – the Build-Mat Bio-test (BMB test) – to examine the degradation mechanisms caused by microbial activity on any type of building material. The main objective of this device was to analyze and distinguish between the effects caused by the bacteria and those caused by their metabolites in the deterioration. In this study, the BMB test was used to evaluate the impact on cementitious matrices of the bacterium Escherichia coli, found in liquid manure. The mechanisms and kinetics of deterioration resulting from exposure to the bacterial culture or to the metabolites were compared with those obtained with synthetic organic acids alone. It was notably observed that the bacterial suspension caused more intense deterioration and higher alteration kinetics as compared to the medium without microorganisms and to the synthetic acid solution.     

Poroplastic properties of calcium-leached cement-based materials

Asymptotic calcium leaching of cement-based materials produces a new material composed of C-S-H with low C/S ratio on the order of 1 and a high porosity generated by the dissolution of Portlandite (CH) crystals, which creates a new pore-size family in the micrometer range. This paper investigates the role of these two phenomena in the multiaxial inelastic and hardening deformation behavior, in compression, of calcium-leached cement pastes and mortars. From triaxial tests and SEM microscopy, it is shown that the low C/S C-S-H matrix is highly plastically deformable, which is consistent with the high degree of polymerization and the effect of the C/S ratio on the intrinsic cohesion of C-S-H. The validity of the effective stress concept is experimentally proven for calcium-leached cement paste and mortar and provides evidence that the low C/S C-S-H solid phase of the cement paste is a pure cohesive incompressible material. In turn, the large pores created by the CH dissolution provides expansion space for the incompressible solid during compressive loading. Once this porosity is filled, the volume deformability is exhausted, and the material dilates to failure. In a similar way, the early tendency of mortars to dilate is found to be a consequence of a competition between plastic material behavior of the matrix (plastic hardening) and porosity-controlled structural deformation (geometrical hardening) triggered by frictional dilation mechanisms in the Interfacial Transition Zone (ITZ).     

Toughening cement-based materials through the control of interfacial bonding

Mortars are made from inherently brittle components: sand grains and hardened cement paste. Under normal circumstances, cracks will propagate rapidly through the cement matrix, bypassing the strong sand grains but fracturing some of the weakest. The approach of the work described in this paper was to modify the mortar in order to alter this process. These modifications produced tensile residual stresses between the matrix and the aggregate, which when released by an additional applied tensile stress produced microcracking, debonding of matrix from aggregate, a small expansion and increased toughness. This work demonstrates toughening in sand/Portland cement mortars modified with different expansive admixtures: sodium sulphate or dead-burnt lime. Additionally, mortars of sand/ASTM Type K cement were tested. In order to give additional insight into the toughening mechanism, spherical and angular aggregate have been used to ascertain the consequences of microcracking and aggregate-bridging. The role of aggregate-bridging, especially when related to fracture paths, is also discussed and suggests that the bond between the aggregate and the matrix has been found in some cases to control not only the crack path but consequently the apparent toughness.     

Simulated microstructure and transport properties of ultra-high performance cement-based materials

Ultra-high performance cement-based materials expected to be used in nuclear waste containers were submitted to a leaching test in order to evaluate their long-term durability. Reactive powder concretes (RPC) were attacked by de-ionized water. Previous studies revealed a superficial degradation after leaching with a sound zone underneath an altered porous zone in which anhydrous silicates C₃S and C₂S were dissolved. To predict the long-term durability of RPC, the hydration rate of cement minerals, pozzolanic reactivity of silica fume, pore structure, and mechanisms of chemical reactions were needed. So first, the microstructure of RPC matrix was simulated using the NIST microstructural model. Then the transfer of Ca ions through percolating water was estimated using DIFFU-Ca, a model based on the local chemical equilibrium. This double modeling validates the damage process related to an instantaneous dissolution of anhydrous cement silicates at the degradation front which results in a higher connected pore space, and is in good agreement with experimental results. The long-term behavior is expressed as the depth of the altered zone, the leaching kinetics and the evolution of Ca concentration in the material.     

Degradation of recycled PET fibers in Portland cement-based materials

In order to investigate the durability of recycled PET fibers embedded in cement-based materials, fiber-reinforced mortar specimens were tested until 164 days after mixing. Compressive, tensile, and flexural strengths, elasticity modulus, and toughness of the specimens were determined. The mortars were also analyzed by SEM. The results have shown that PET fibers have no significant influence on mortars strengths and elasticity modulus. However, the toughness indexes I₅, I₁₀, and I₂₀ decreased with time due to the degradation of PET fibers by alkaline hydrolysis when embedded in the cement matrix. Fourier transform infrared spectroscopy (FT-IR) and SEM analysis of PET fibers immersed and kept for 150 days in alkaline solutions supported the conclusions.     

Cement pastes alteration by liquid manure organic acids: chemical and mineralogical characterization

Liquid manure, stored in silos often made of concrete, contains volatile fatty acids (VFAs) that are chemically very aggressive for the cementitious matrix. Among common cements, blast-furnace slag cements are classically resistant to aggressive environments and particularly to acidic media. However, some standards impose the use of low C₃A content cements when constructing the liquid manure silos. Previous studies showed the poor performance of low-C₃A ordinary Portland cement (OPC). This article aims at clarifying this ambiguity by analyzing mechanisms of organic acid attack on cementitious materials and identifying the cement composition parameters influencing the durability of agricultural concrete. This study concentrated on three types of hardened cement pastes made with OPC, low-C₃A OPC and slag cement, which were immersed in a mixture of several organic acids simulating liquid manure. The chemical and mineralogical modifications were analyzed by electronic microprobe, XRD and BSE mode SEM observations. The attack by the organic acids on liquid manure may be compared with that of strong acids. The alteration translates into a lixiviation, and the organic acid anions have no specific effect since the calcium salts produced are soluble in water. The results show the better durability of slag cement paste and the necessity to limit the amount of CaO, to increase the amount of SiO₂ (i.e., reduction of the Ca/Si ratio of C-S-H is not sufficient) and to favor the presence of secondary elements in cement.     

Physicochemical equilibria of cement-based materials in aggressive environments—experiment and modeling

Assessment of the integrity of concrete structures during their service life begins by considering the durability of the material in its environment. Experiments have clearly improved the understanding of the degradation mechanisms of concrete, mortars, and cement pastes under various aggressive environments. As far as radioactive waste containers are concerned, leaching by water has to be considered. Leaching experiments of cement pastes by aggressive solutions are shown to result in degradations with different kinetics. Three cement pastes with variable water-to-cement (w/c) ratio (0.25, 0.4, and 0.5) in two solutions (pure water and mineralized water) were investigated by TG/DTA, SEM-EDS, and by application of the NIST (National Institute of Standards and Technology) microstructure models. Leaching kinetics, evolution of the solid skeleton, and pore solution were experimentally studied and successfully modeled, using a reactive-transport approach. The discrepancies between modeling and experimental results highlight the understanding of complex degradation mechanisms. New results on the interactions on the aggressive solution and the cementitious material, through the pore solution, are presented.     

Quasi-static compressive strength of cement-based materials

Wet cementitious materials show a noticeable dependence on the rate of quasi-static loading. While dry cementitious materials are almost independent of loading rate in the quasi-static region, the mechanical strength of wet materials increases with increasing rate of loading. Therefore, the Abrams' formula for the static mechanical strength cannot provide reliable values with wet materials at higher rates and should be corrected. Some possibilities for its improvement have been discussed.     

Wavelet-based analysis of ultrasonic longitudinal and transverse pulses in cement-based materials

Ultrasonic pulse velocity has been used for decades to detect localized damage and to estimate concrete properties. More recent applications aim at diffuse damage characterization, such as environmental and mechanical damage. In most applications the methodology to calculate pulse speed is a very important issue. This work, applying continuous wavelet transform (CWT) to construct time-frequency signal representations, calculates frequency-dependent velocity of longitudinal and transverse ultrasonic pulses using wavelet scales. The method is applied to a total of 14 specimens of 5 different mixes and frequency-dependent velocities are calculated using four wavelet families. The CWT capability to decompose the inquiring pulse spectrum and analyze phase velocities is discussed with regard to wavelet, pulse type, and mixture. Frequency-dependent velocity of longitudinal pulses at lower frequencies (from 100 kHz up to 250 kHz) was proven to be much more sensitive to mix proportions than transverse pulses.     

Characterization of pore structure in cement-based materials using pressurization-depressurization cycling mercury intrusion porosimetry (PDC-MIP)

Numerous mercury intrusion porosimetry (MIP) studies have been carried out to investigate the pore structure in cement-based materials. However, the standard MIP often results in an underestimation of large pores and an overestimation of small pores because of its intrinsic limitation. In this paper, an innovative MIP method is developed in order to provide a more accurate estimation of pore size distribution. The new MIP measurements are conducted following a unique mercury intrusion procedure, in which the applied pressure is increased from the minimum to the maximum by repeating pressurization-depressurization cycles instead of a continuous pressurization followed by a continuous depressurization. Accordingly, this method is called pressurization-depressurization cycling MIP (PDC-MIP). By following the PDC-MIP testing sequence, the volumes of the throat pores and the corresponding ink-bottle pores can be determined at every pore size. These values are used to calculate pore size distribution by using the newly developed analysis method. This paper presents an application of PDC-MIP on the investigation of the pore size distribution in cement-based materials. The experimental results of PDC-MIP are compared with those measured by standard MIP. The PDC-MIP is further validated with the other experimental methods and numerical tool, including nitrogen sorption, backscanning electron (BSE) image analysis, Wood's metal intrusion porosimetry (WMIP) and the numerical simulation by the cement hydration model HYMOSTRUC3D.     

Effects of calcination condition on expansion property of MgO-type expansive agent used in cement-based materials

Previous research indicated that the expansion property of MgO-type expansive agent (MEA) depended strongly on the calcining conditions, i.e. kiln temperature and residence time. However, the intrinsic effect of calcination condition on the expansion property of MEA has not been clearly demonstrated. In the present work, the effects of calcination condition on the microstructure, hydration activity, and expansion property of MEA have been investigated, and their correlations are also studied. Results indicate that the microstructure of MEA is the intrinsic factor that controlling its expansion property, which is influenced by the calcination condition. MEA produced under higher temperature and longer residence time has less interior pores, larger crystal size of MgO, and smaller specific surface area, thus resulting in lower hydration activity and slower expansion at early age, but larger “ultimate” expansion at late age. While, a new expansion model of MEA is proposed to explain these results.     

Degradation of alumina refractory bricks by sintering Mn low-alloy steels

The economic importance of the corrosion and wear of refractory materials is indisputable because these processes determine the viability of any high-temperature liner used in metallurgical processes. The degradation mechanism of lining materials (refractory bricks) in contact with corrosive gases can be studied by examining the penetration rate or the chemical corrosion that results from the circulation of the atmosphere over the refractory material (by diffusional and convective transport). During the sintering of steel containing Mn, the high vapour pressure of Mn enables its sublimation during thermal cycling; therefore, Mn is incorporated into the sintering atmosphere. Although the diffusion of Mn in steel samples is beneficial, the presence of Mn in a sintering atmosphere can modify the composition of refractory components. As a result of atmosphere-refractory interactions, a new phase is formed. In this study, the changes in refractory materials as a function of exposure time to atmospheres containing Mn_(g) at the most common sintering temperature, 1120 °C, were investigated. The microstructural changes in the refractory materials and the consequences of the presence of Mn_(g) were analysed using optical microscopy, electron microscopy with X-ray (EDS) microanalysis, X-ray diffraction, and X-ray fluorescence (XRF).     

Water-tolerant Ru-Starbon® materials for the hydrogenation of organic acids in aqueous ethanol

Highly active and stable ruthenium-supported nanoparticles show excellent activities and selectivities to target products in the aqueous hydrogenation of organic acids including succinic, fumaric, itaconic, levulinic and pyruvic acids. The materials were also highly reusable and comparably more active than related supported Ru materials.     

Degradation of Chlorocarbons Driven by Hydrodynamic Cavitation

To provide an efficient lab‐scale device for the investigation of the degradation of organic pollutants driven by hydrodynamic cavitation, the degradation kinetics of chloroform and carbon tetrachloride and the increase of conductivity in aqueous solutions were measured. These are values which were not previously available. Under hydrodynamic cavitation conditions, the degradation kinetics for chlorocarbons was found to be pseudo first‐order. Meanwhile, C‐H and C‐Cl bonds are broken, and Cl₂, Cl^., Cl^– and other ions released can increase the conductivity and enhance the oxidation of KI in aqueous solutions. The upstream pressures of the orifice plate, the cavitation number, and the solution temperature have substantial effects on the degradation kinetics. A decreased cavitation number can result in more cavitation events and enhances the degradation of chlorocarbons and/or the oxidation of KI. A decrease in temperature is generally favorable to the cavitation chemistry. Organic products from the degradation of carbon tetrachloride and chloroform have demonstrated the formation and recombination of free radicals, e.g., CCl₄, C₂Cl₄, and C₂Cl₆ are produced from the degradation of CHCl₃. CHCl₃ and C₂Cl₆ are produced from the degradation of CCl₄. Both the chemical mechanism and the reaction kinetics of the degradation of chlorocarbons induced by hydrodynamic cavitation are consistent with those obtained from the acoustic cavitation. Therefore, the technology of hydrodynamic cavitation should be a good candidate for the removal of organic pollutants from water.     

Microstructure and texture of hydrated cement-based materials: A proton field cycling relaxometry approach

We show how the measurement of proton nuclear magnetic spin-lattice relaxation as a function of magnetic field strength (and hence nuclear Larmor frequency) can provide reliable information on the microstructure (specific surface area and pore size distribution) throughout the progressive hydration of cement-based materials. We present in details the experimental and theoretical characteristic features of the relaxation dispersion to support an interpretation in terms of coupled solid-liquid relaxation at pore interfaces, surface diffusion, and nuclear paramagnetic relaxation. The measurement does not require any drying temperature modification and is sufficiently fast to be applied continuously during the progressive hydration of the material. Coupling this method with the standard proton nuclear spin relaxation and high resolution NMR allows us to follow the development of micro-scale texture within the material.     

Recent advances in the field of cement hydration and microstructure analysis

This paper is a bibliographic tool reviewing experimental and theoretical studies related to cement hydration and microstructure development that have been published within the four years of the interim period between the 12th and 13th International Congress on the Chemistry of Cement.     

Role of moisture in the Seebeck effect in cement-based materials

Moisture in the form of liquid water contributes little, if any, to the Seebeck effect in cement-based materials. Moisture loss has no effect on the absolute thermoelectric power, but increases the electrical resistivity.     

Material and environmental parameter effects on the leaching of cement pastes: Experiments and modelling

Results already published on the leaching of cement pastes have shown that the kinetics depends sensitively on the material and environment. However and because of the variability of the tested materials and leaching protocols, it is difficult to compare these data and quantify the effect of each parameter. In this paper, a large experimental database on the leaching kinetics of cement pastes is built. Four parameters are investigated: type of cement (portland cement, silica fume cement, slag cement, ternary cement with slag and fly ash); water-to-cement ratio (0.5; 0.4; 0.25), temperature (26 °C; 72 °C; 85 °C) and chemical composition of the leaching solution (pure water, mineralised water, ammonium nitrate solution). Firstly, the database is used to calculate the leaching kinetics of the cementitious materials. Secondly, a simplified model predicting the one-dimensional leaching kinetics for other water-to-cement ratios and temperature up to 85 °C is presented.     

Influence of limestone addition on calcium leaching mechanisms in cement-based materials

In order to investigate the mechanisms of calcium leaching and their implications on the long-term durability of cement-based materials, four different systems (C₃S, C₃S+C₃A+CaSO₄·2H₂O, C₃S+C₄AF+CaSO₄·2H₂O, and Portland cement) were prepared, mixed with water, and cured for several weeks. Four percentages of calcium carbonate addition (0%, 5%, 10%, and 20%) were used in the preparation of these systems. In all cases, the water/solid ratio was fixed at 0.5. At the end of the curing period, 1-mm thick samples were cut and immersed in distilled water. The kinetics of degradation was assessed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) analysis after 10, 20, 40, and 90 days of immersion. The volumetric stability was also followed by length-change measurements. Test results clearly indicate that the mechanisms of leaching are directly affected by the mineralogical composition of the system, in particular by the calcium carbonate content.