Utilization of borogypsum as set retarder in Portland cement production

Boron ores are used in the production of various boron compounds such as boric acid, borax and boron oxide. Boric acid is produced by reacting colemanite(2CaO·3B₂O₃·5H₂O) with sulphuric acid and a large quantity of borogypsum is formed during this production. This waste causes various environmental problems when discharged directly to the environment. Portland cement is the most important material in the building industry. This material is produced by adding about 3-5% gypsum (CaSO₄·2H₂O) to clinker as a set retarder. The aim of this study was to stabilize borogypsum, and to produce cements by adding borogypsum instead of natural gypsum to clinker. Concrete using cement produced with borogypsum was tested to find the mechanical properties and the test values were compared with those of concrete from cement with natural gypsum. Compressive strength of concrete from cement produced with borogypsum was found to be higher than that of natural gypsum. Also, the setting time of cement with borogypsum was longer than that of the Portland cement.     

Differentiating seawater and groundwater sulfate attack in Portland cement mortars

The study reported in this article deals with understanding the physical, chemical and microstructural differences in sulfate attack from seawater and groundwater. Portland cement mortars were completely immersed in solutions of seawater and groundwater. Physical properties such as length, mass, and compressive strength were monitored periodically. Thermal analysis was used to study the relative amounts of phases such as ettringite, gypsum, and calcium hydroxide, and microstructural studies were conducted by scanning electron microscopy. Portland cement mortars performed better in seawater solution compared to groundwater solution. The difference in performance could be attributed to the reduction in the quantity of the expansive attack products (gypsum and ettringite). The high Cl concentration of seawater could have played an important role by binding the C₃A to form chloroaluminate compounds, such as Friedel's salt (detected in the microstructural studies), and also by lowering the expansive potential of ettringite. Furthermore, the thicker layer of brucite forming on the specimens in seawater could have afforded better protection against ingress of the solution than in groundwater.     

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

Influence of the aggregate on the electrical conductivity of Portland cement concretes

The electrical conductivity and compressive strength of several high-performance Portland concretes with different amounts of crushed aggregate and sand have been measured at early age in isothermal conditions (20 °C). The total aggregate volume fraction varied from 0 (plain paste) to 0.75 and a constant weight ratio (1.2) between crushed aggregate and sand was used. The w/c ratio was 0.37 and microsilica (in slurry form) and a superplasticizer in water solution were used.The time taken before the electrical conductivity began to drop correlated very well with the induction period. The drop of conductivity was slightly delayed by the aggregate. The analysis of the electrical data, by means of different numerical and analytical models [hard core soft shell model (HCSS), differential effective medium theory (DEMT), Lu-Torquato, Maxwell], allowed an estimate of the properties of the interfacial transition zone (ITZ). In particular, an ITZ thickness of about 9 μm and an ITZ to bulk conductivity ratio of ∼2.5 were found. The existence of a percolating pathway through the interfacial regions was found by both electrical measurement and modeling when the aggregate volume fractions exceeded 60%. Finally, a new relationship among electrical conductivity, compressive strength, and aggregate amount was derived.     

Properties of plain and latex modified Portland cement pastes and concretes with and without superplasticizer

This paper deals with the effects of latex concentration on the workability and strength characteristics of Portland cement pastes with and without superplasticizer. Durability assessments are made by immersing these pastes in 5% Na₂SO₄ and 2.5% NaCl solutions. From the results obtained, it is found that the superplasticizer and superplasticizer-latex combinations may improve the workability of the Portland cement pastes. The Portland cement pastes with superplasticizer have much higher strengths than the latex modified Portland cement pastes with and without superplasticizer. In general, curing in lime-saturated water adversely affect the strength of the pastes containing latex from about 28 days onwards. In the durability test, the resistance of latex modified Portland cement pastes with and without superplasticizer to NaCl is decreased. Degradation mechanism depends on the characteristics of the corrosive medium as well as the resistance of the material itself to the resulting chemical action. The character of strain-stress data of latex modified concretes becomes more prominent as the latex concentration increases. These data are anomalous when compared with the data normally observed for concretes without admixture. The proposed equations are found adequate to describe the stress-strain behaviour of latex modified concretes in compression. These equations can also be applied in calculating the initial modulus of elasticity and proportional limit in the case of polymer modified concretes, which exhibit non-linear behaviour at high stress.     

The effect of liquid push out of the material capillaries under sulfate ion diffusion in cement composites

The paper treats complex phenomena that accompany the diffusion of sulfate ions into cement paste or systems. The ion distribution within the material was studied by designing a specific diffusion model. The model accounts for two phenomena: capillary filling with products of the chemical reactions and the subsequent liquid push out of the capillary. The approach allows to quantify the concentration of free ions having penetrated the cement stone and that of chemically reacted ions, and to assess the liquid push out. Experimental data are also presented.     

Sulfate ingress in Portland cement

The interaction of mortar with sulfate solutions leads to a reaction front within the porous material and to expansion. Thermodynamic modelling coupled with transport codes was used to predict sulfate ingress. Alternatively, “pure” thermodynamic models - without consideration of transport - were used as a fast alternative to coupled models: they are more flexible and allow easy parameter variations but the results relate neither to distance nor to time. Both transport and pure thermodynamic modelling gave comparable results and were able to reproduce the changes observed in experiments. The calculated total volume of the solids did not exceed the initial volume of the paste indicating that not the overall volume restriction leads to the observed expansion but rather the formation of ettringite within the matrix and the development of crystallisation pressure in small pores. The calculations indicate that periodic changing of the Na₂SO₄ solution results in more intense degradation.     

Numerical simulation of 3D sulfate ion diffusion and liquid push out of the material capillaries in cement composites

This study treats 3D diffusion of sulfate ions into cement composite and simultaneous effects, such as microcapillary filling and subsequent liquid push out of them. A numerical algorithm provides account for various conditions specified at the interface, as well as possibility to model separate volume sections as inert fillers. Cases discussed illustrate the capabilities of the proposed model and those of the algorithm designed to study diffusion and liquid push out for specimen complex configuration. Results found allow for the assessment of the effects of convective and diffusion ion transfer. Comparison with experimental observations of corrosion processes that have more or less one-dimensional character prove the necessity to use a 3D mathematical model for a fuller clarification of sulfate corrosion.     

Long-term performance of cement paste during combined calcium leaching-sulfate attack: kinetics and size effect

This paper presents experimental results obtained on cement paste samples (water/cement ratio of 0.4) subjected to a low-concentration (15 mmol/l) external sulfate attack during several weeks. Chemical and microstructural analyses include the continuous monitoring of calcium loss and sulfate consumption within the cement paste, periodic layer by layer X-ray diffraction (XRD)/energy-dispersive spectrometer (EDS) analyses of the solid constituents of the cement matrix (ettringite, portlandite, gypsum) within the calcium-depleted part of the samples. Scanning electron microscopy (SEM) and visual observations are used to follow the crack pattern evolution during the external sulfate attack. The relation between the size of the specimen and crack initiation/development is investigated experimentally by performing tests on samples with different thickness/diameter ratios.     

Modeling the effects of solution temperature and concentration during sulfate attack on cement mortars

Simple chemistry-based empirical models have been developed to assess the role of temperature and concentration of the sulfate solution in the process of expansion of cement mortars that are subjected to external sulfate attack. ASTM Type I PC mortars, prepared according to ASTM C-109, were immersed in sodium and magnesium sulfate solutions at five different concentrations and four different temperatures. For both solutions, the trends in the measured expansion suggested the use of a simple rate law to analyze the effect of concentration. For the effect of temperature, an Arrhenius relationship was developed to determine the activation energy required to initiate expansion in sodium sulfate solution. Regression-based statistical models were found to be sufficient to explain the effect of temperature of magnesium sulfate solution on the expansion. Implications of using these models for developing potential test methods, as well as to enable interpretation of data from nonstandard test methods, are discussed.     

Impact of admixtures on the hydration kinetics of Portland cement

Most concrete produced today includes either chemical additions to the cement, chemical admixtures in the concrete, or both. These chemicals alter a number of properties of cementitious systems, including hydration behavior, and it has been long understood by practitioners that these systems can differ widely in response to such chemicals.In this paper the impact on hydration of several classes of chemicals is reviewed with an emphasis on the current understanding of interactions with cement chemistry. These include setting retarders, accelerators, and water reducing dispersants. The ability of the chemicals to alter the aluminate–sulfate balance of cementitious systems is discussed with a focus on the impact on silicate hydration. As a key example of this complex interaction, unusual behavior sometimes observed in systems containing high calcium fly ash is highlighted.     

The AFm phase in Portland cement

The AFm phase of Portland cements refers to a family of hydrated calcium aluminates based on the hydrocalumite-like structure of 4CaO·Al₂O₃·13-19 H₂O. However OH⁻ may be replaced by SO₄²⁻ and CO₃²⁻. Except for limited replacement (50 mol%, maximum) of sulfate by hydroxide, these compositions do not form solid solutions and, from the mineralogical standpoint, behave as separate phases. Therefore many hydrated cements will contain mixtures of AFm phases. AFm phases have been made from precursors and experimentally-determined phase relationships are depicted at 25 °C. Solubility data are reported and thermodynamic data are derived. The 25 °C stability of AFm phases is much affected by the nature of the anion: carbonate stabilises AFm and displaces OH and SO₄ at species activities commonly encountered in cement systems. However in the presence of portlandite, and as carbonate displaces sulfate in AFm, the reaction results in changes in the amount of both portlandite and ettringite: specimen calculations are presented to quantify these changes. The scheme of phase balances enables calculation of the mineralogical balances of a hydrated cement paste with greater accuracy than hitherto practicable.     

X-ray microtomography (microCT) of the progression of sulfate attack of cement paste

High-resolution X-ray computed tomography (i.e., microCT or microtomography) was used to study the sulfate attack of cylinders of Type I cement paste cast with water-cement (w/c) ratios of 0.45, 0.50 and 0.60. Damage levels in samples exposed to a Na₂SO₄ solution with 10,000 ppm sulfate ion concentration were qualitatively rated from 0 (no damage) to 4 (extreme damage) based upon visual examination of the samples' exteriors and microtomography of the samples' interiors. The greater the w/c ratio, the more rapid the onset of sulfate damage. The corners of the cylinders appeared to be particularly susceptible to spalling, and damage may have continued into the cement paste by formation of subsurface cracks.     

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.     

The thaumasite form of sulfate attack in concrete of Yongan Dam

According to microanalytical investigations, it is shown that the concrete of Yongan Dam is deteriorated due to the thaumasite form of sulfate attack (TSA). Analysis results of scanning electron microscopy (SEM), Energy Disperse X-ray (EDX) and X-ray Diffraction (XRD) are supported by the analysis of the concrete composition and the geographical conditions of the dam.     

Predicting the durability of Portland cement systems in aggressive environments—Laboratory validation

Portland cement systems are often exposed to severe environments, and their long-term performance is of concern. The main results of a comprehensive investigation of deterioration processes that may affect the behavior of Portland cement systems exposed to chemically aggressive environments is presented. As part of this investigation, well-cured cement paste discs were fully characterized and exposed to deionized water and sodium sulfate solutions. Degradation experiments were conducted under saturated and unsaturated conditions. At the end of the exposure period, microstructural alterations were investigated by microprobe analyses, scanning electron microscope observations and energy-dispersive X-ray analyses. Test results provide information on the basic aspects of various degradation phenomena, such as decalcification and external sulfate attack. Experimental results were also compared with results obtained by a numerical model. The analysis reveals that the intricate microstructural features of the degraded samples could be accurately reproduced by the model.     

Utilization of weathered phosphogypsum as set retarder in Portland cement

In this study, usability of weathered phosphogypsum (PG) from residue areas as set retarder in Portland cement was investigated. The effects on the setting and mechanical properties of PG added in ratios 1, 3, 5, 7, 10, and 12.5 wt.% to Portland cements were studied and compared with a Portland cement containing natural gypsum (NG). It was found that PG can be used in place of NG for Portland cement according to Turkish standards. The highest 28-day compressive strength was found in the sample with 3 wt.% PG.     

The permeability of Portland limestone cement concrete

The effect of limestone addition on the air permeability, water permeability, sorptivity, and porosity of limestone cement concrete has been investigated. Six Portland limestone cements (PLCs) with different limestone content (10-35% w/w) were produced by intergrinding clinker, gypsum, and limestone. A water-to-cement ratio (w/c) of 0.70-0.62—depending on the cement strength class—was used to prepare concrete of the compressive strength class C20/25 of EN 206-1. A modified commercial triaxial cell for 100-mm-diameter samples was used for the determination of the gas (N₂) and the water permeability of concretes. In addition, the sorptivity and porosity of the samples were measured, while thin sections of the concrete specimens were examined by means of optical microscopy. It is concluded that the PLC concrete indicates competitive properties with the ordinary Portland cement (OPC) concrete. Furthermore, the limestone addition has a positive effect on the water permeability and the sorptivity of concrete.