Effect of fly ash–gypsum blend on porosity and pore size distribution of cement pastes

Abstract

Abstract

This paper reports results on the porosity and pore size distribution (PSD) of cement paste containing simulated desulphurised waste (SDW). The SDW was chosen due to the variability in chemical composition of real desulphurised waste. The SDW is a combination of fly ash and gypsum. The content of fly ash in the SDW changed from 0 to 100% by weight. The water to binder ratio was 0·5. The binder consists of cement and SDW. Cement in the pastes was partially replaced with 25 wt-% SDW. The porosity and PSD of cement pastes at 28 days of curing is reported. Increasing amount of gypsum does not seem to greatly change the pore volume; however, there is tendency of obtaining coarser pore structure in the presence of gypsum. The compressive strength increases with increasing amounts of gypsum. Correlation between strength and PSD is conducted.     

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%.     

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C₃S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C-S-H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C-S-H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C-S-H at Ca/Si ∼ 1.2 and eventually to the decomposition of C-S-H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C-S-H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.     

Effect of magnesium and sulfate ions on durability of silica fume blended mixes exposed to the seawater tidal zone

The effect of silica fume on deterioration resistance to sulfate attack in seawater within tidal zone and simulated wetting-drying condition has been studied in Portland cement concretes and pastes containing silica fume (SF) with/without ground granulated blast furnace slag (GGBS). Changes in the compressive strength and capillary water absorption of specimens as a function of SF content have been investigated combined with phases determination by means of scanning electron microscopy and X-ray energy dispersion analysis. The strength change factors (SCFs) of specimens with SF (the more SF content, the higher strength loss) were greater than that of the mixes without SF or cured under tap water. Mg²⁺ ion originated attack found to be the dominating deterioration mechanism as confirmed by X-ray and chemical analyses.Further, the incorporation of GGBS with SF mixes in different exposure conditions led to the worst performance in all of the test environments. Lower cement content and hydration rate accompanied with particular chemical composition of GGBS made concrete and paste specimens to be more susceptible to deleterious seawater environment.     

硫酸盐侵蚀下硬化水泥浆体微结构演变及膨胀过程的数值模拟

根据硫酸盐侵蚀机理,利用改进的CEMHYD3D水化模型和随机概率方法,建立了硫酸盐侵蚀下硬化水泥浆体的微结构演变模型。在微观层次上,模拟了浆体孔溶液中硫酸根离子的自由扩散、随机碰撞和转化反应,分析了膨胀性侵蚀产物生长导致的微结构损伤和体积膨胀,计算了侵蚀过程中石膏和钙矾石的生成量及浆体的膨胀应变,并与已有试验结果对比分析验证了模型的合理性。在此基础上,数值模拟了硫酸盐侵蚀下不同水灰比水泥浆体的微结构演变及膨胀过程。结果表明：同一硫酸盐浓度下,硬化水泥浆体中氢氧化钙和含铝物相与孔隙的接触面积越小,浆体的膨胀应变越低;水灰比为0.25、0.30和0.35的硬化水泥浆体的孔隙填充程度分别达到9.09%、9.27%和9.41%时,浆体膨胀应变开始快速增大;硫酸盐侵蚀溶液浓度增大,浆体体积快速膨胀的时间提前。     

Evaluation of the pozzolanic activity of fluid catalytic cracking catalyst residue (FC3R). Thermogravimetric analysis studies on FC3R-Portland cement pastes

Spent fluid catalytic cracking catalyst (FC3R) from a petrol refinery has shown a great pozzolanic activity in lime pastes as have been demonstrated in previous studies. Based on these results, the pozzolanic activity of the FC3R in Portland cement pastes has been investigated. This evaluation has been carried out by means of thermogravimetry (TG) of cured FC3R-Portland cement pastes. The influence of water/binder ratio and the replacement percentage of FC3R on the pozzolanic reaction were investigated. Due to the chemical composition of FC3R that is similar to metakaolin (MK), and knowing that MK has a high pozzolanic activity, the latter was used as a material of comparison in the study of the water/binder ratio influence. The scope of this study is the determination of pozzolanic activity of FC3R when incorporated to Portland cement, and the evaluation on amount and nature of pozzolanic products. FC3R has shown a similar reactivity to MK, yielding similar pozzolanic products: CSH, CAH and CASH. The optimum replacing percentage in Portland cement pastes was in the 15-20% range.     

The influence of water removal techniques on the composition and microstructure of hardened cement pastes

The removal of water from hardened cement paste for analysis or to arrest ongoing hydration has been reported to affect the composition of hydrated phases and microstructure. The effect that arresting the hydration of hardened cement paste by replacing the pore water with acetone before drying, and by removing the water by freeze, vacuum and oven drying has on the hardened cement paste has been investigated. Two pastes were studied, a cemented iron hydroxide floc where a high proportion of ordinary Portland cement (OPC) had been replaced by pulverised fuel ash, and a pure hydrated OPC. The results showed that none of the water removal techniques caused any major deterioration in the composition and microstructure of the hardened cement pastes studied, but the pores appeared better preserved after arresting hydration using acetone quenching. Freeze drying appeared to cause more cracking of the microstructure than the other water removal techniques.     

Ⅰ级粉煤灰对水泥净浆开裂的影响

采用环约束法,试验研究了Ⅰ级粉煤灰对不同水胶比下水泥净浆开裂的影响。试验研究发现：Ⅰ级粉煤灰在低水胶比（0.24）情况下对净浆开裂几乎没有影响,当水胶比为0.32和0.40时抗裂作用明显;较高水胶比（0.32、0.40）情况下,Ⅰ级粉煤灰对净浆的抗裂性能有较好的改善作用,在20%～65%掺量范围内,随着掺量的增大,净浆的开裂龄期延长,抗裂性能提高。     

Corrosion rates of steel embedded in cement pastes

Dynamic polarization techniques were used to estimate the corrosion rates of steel embedded in cement paste made with different binder systems. A corrosion model is proposed. The corrosion rates were determined by employing the Evans diagrams constructed from cathodic polarization curve of steel embedded in chloride free cement paste and anodic polarization curve of steel embedded in cement pastes containing chloride. Within the limitations of the adopted experimental conditions and sample configuration, the preliminary results indicate that corrosion rates of steel embedded in blended cement pastes generally fall within the range shown by steel embedded in portland cement pastes.     

Detection of Failures around Main Cracks and Their Relation to the Fracture Toughness of Cement Pastes

Fluorescence microscopy examinations revealed the characteristics of crack path and the presence of local failures around the main cracks in hardened cement pastes. Reduction in the water: cement ratio and the addition of silica fume led to a decrease in locally damaged regions around the main cracks. However, the R - curve of the silica fume-free cement paste with the water: cement ratio of 0.55 was much different from that of the silica fume-bearing paste. The silica fume-bearing cement paste exhibited a relatively flat R -curve, whereas reduction in the water: cement ratio in the silica fume-free paste led to a rising R-curve. These results were related to the toughening mechanisms of cement pastes derived from the characteristics of the crack path and the areas of local failures around main cracks that were revealed by fluoresence microscopy. Presumably, these differences in fracture toughness resulted from the nature of hydration products and/or their inherent microstructure.     

Processing of electron microprobe data from the analysis of altered cementitious materials

The purpose of this paper is to present a method for processing electron microprobe data coming from the analysis of degraded cementitious materials. The application domain is that of cement matrices exposed to attack by aggressive media inducing the leaching of one or more chemical elements from the cement paste — typically attacked by neutral or acid aqueous environments. As raw data obtained from electron probe microanalysis (EPMA) come in the form of relative quantities of elements, the aim of the processing is to obtain the absolute variation of the specimen's chemical composition in the degraded zone and consequently to allow a better understanding of the degradation mechanisms by the aggressive environment under consideration. This method was designed for degraded specimens in which the porosity was not filled with resin before the EPMA. The method uses the titanium content of the cement matrix as a control, this element being stable during attack by neutral or acidic media. The method is described in detail using the example of the attack of cement pastes by a mix of several organic acids (notably acetic acid) at pH 4. The use of EPMA for sound and altered cement pastes is also validated. The application domain of the correction method is discussed.     

Application of Powers’ model to modern portland and portland limestone cement pastes

Powers’ model is a simple approach for estimating the relative volumes of hydration products, porosity, and chemical shrinkage present in portland cement paste as a function of its starting water‐to‐cement ratio (w/c) and current degree of hydration. It forms an important link between cement composition, microstructure, and performance, necessary for modeling cement‐based systems. Previous researchers have adapted Powers’ model for inert fillers to illustrate their effects on the hydration, porosity, and chemical shrinkage of blended cements; however, it is well‐documented that limestone is not, in fact, an inert filler, but rather participates in cement hydration through both chemical and physical processes. This research experimentally investigates the applicability of Powers’ model to modern portland cements containing up to 15% by mass finely divided limestone. The results demonstrate that the modified Powers’ model is insufficient for predicting the influence of finely divided limestone additions on the chemical shrinkage of both ordinary portland cement pastes and portland limestone cement pastes. Possible explanations for the discrepancy are discussed and a plausible source is proposed.     

Microstructure of Mature Alite Pastes

Tricalcium silicate and Fe-bearing alite pastes hydrated 4 years and portland cement paste hydrated 33 days were studied by SEM, TG, and XRD. SEM studies of the effect of water, NaCl, and MgSO₄ solutions on the microstructure of alite are reported. The characteristics of the attack were clarified by examining both sides of the fracture plane. The Ca(OH)₂ phase present in the fresh fracture surface is very unstable toward water, NaCl, and MgSO₄ solutions. When exposed to MgSO₄ solution the Ca(OH)₂ reacts, producing Mg(OH)₂ and CaSO₄·2H₂O. The C-S-H gel phase is more resistant to the various reactants. The presence of calcite, aragonite, and Ca(OH)₂ and their proportion in the surface layer depend on the characteristics of the environment.     

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.     

Early-age acoustic emission measurements in hydrating cement paste: Evidence for cavitation during solidification due to self-desiccation

In this study, the acoustic emission activity of cement pastes was investigated during the first day of hydration. Deaired, fresh cement pastes were cast in sealed sample holders designed to minimize friction and restraint. The majority of acoustic emission events occurred in lower water to cement ratio pastes, while cement pastes with higher water to cement ratios showed significantly less acoustic activity. These acoustic events occurred around the time of setting. A layer of water on the surface of the cement pastes substantially reduced acoustic emission activity at the time of setting. According to these experimental results, the acoustic emission measured around setting time was attributed to cavitation events occurring in the pores of the cement paste due to self-desiccation. This paper shows how acoustic emission might be used to indicate the time when the fluid–solid transition occurs in a cement paste, often referred to as time-zero. Knowledge of time-zero is fundamental for determining when mechanical properties develop and in calculations of residual stresses.     

Modeling of frost salt scaling

This paper discusses the numerical modeling of deterioration in cement-based materials due to frost salt scaling (FSS). Several aspects of FSS are investigated such as carbonation, microstructure, mechanical properties and testing conditions. Mainly blast-furnace slag cement (henceforth slag cement) systems are of interest in this paper since several reports have been indicated that cementitious materials bearing slag-rich cement are critically vulnerable under combined attack of frost and de-icing salts.In the first part, the paper deals with the effect of carbonation on the micromechanical properties and FSS resistance of 1-year-old slag cement and ordinary Portland cement pastes with W/C 0.45. The micromechanical properties were evaluated by the nano-indentation technique and the results are used to evaluate the behavior of these pastes under frost salt attack. FSS damage on the paste samples is modeled according to the glue-spall theory with the aid of Delft Lattice Model. Additionally, the carbonated cement paste microstructures are characterized by ESEM/BSE.In the second part, parameters that are varied in the investigation are the salt concentration in the external water layer and ice-layer thickness on the surface. Again the lattice type model is used to simulate the mechanism in which the material structure is implemented using digital images of the real material. Both experiments and the simulation with the model show that the amount of scaling increases with increasing thickness of the ice layer on the surface. Furthermore it is shown that with the model the well known pessimum effect for salt concentration in the water (which causes maximum damage at 3% salt) can be reproduced.The outcome of the model indicates that glue-spall theory can successfully explain FSS.     

Performance of irradiated blended cement paste composites containing ceramic waste powder towards sulfates,chlorides, and seawater attack

Within the scope of this study, blended cement pastes were prepared by replacing different proportions of ordinary Portland cement with ceramic waste powder (CWP). The hardened blended cement pastes were cured under tap water for different periods of time up to 180 days. Physico‐mechanical properties of specimens were studied in terms of free lime content, chemically combined water, compressive strength, and particle size distribution. The results manifested that the optimum content of ceramic waste which gave a marked improvement in the mechanical properties was 10% as compared to the other specimens at the same curing age. Besides that, similar specimens of the hardened blended cement paste containing 10% CWP were impregnated with unsaturated polyester and exposed to different doses of gamma rays from 10 to 50 kGy. Both the impregnated specimens that irradiated at a dose of 30 kGy and the neat blended cement paste that contained 10% ceramic waste were soaked in 1, 3, and 5% magnesium sulfate, sodium chloride solutions, and in seawater for up to 180 days. The results indicated that the composite specimens became more resistant to aggressive solutions and their durability increased as compared to the neat blended cement paste prepared under the same previous conditions. J. VINYL ADDIT. TECHNOL., 26:24–34, 2020. © 2019 Society of Plastics Engineers     

Accelerated carbonation of Friedel's salt in calcium aluminate cement paste

The stability of Friedel's salt with respect to carbonation has been studied in calcium aluminate cement (CAC) pastes containing NaCl (3% of Cl⁻ by weight of cement). Carbonation was carried out on a powdered sample in flowing 5% CO₂ gas at 65% relative humidity to accelerate the process. At an intermediate carbonation step, a part of the sample was washed and dried up to 10 cycles to simulate a dynamic leaching attack. The two processes were followed by means of X-ray diffraction (XRD), pH and Cl⁻ analyses in the simulated pore solution.     

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.     

Physicochemical characteristics of acrylic-acid polymer-impregnated cement pastes

Various Portland cement pastes were made using water cement ratios of 0·20, 0·25, 0·35 or 0·40 and then cured for 1, 3, 7, 28, 90 or 180 days. These pastes were impregnated with acrylic acid monomer under vacuum and the monomer-impregnated samples were then treated at two different temperatures, 40 or 60°C, for the polymerization process, using benzoyl peroxide as initiator. Several physicochemical studies were carried out on each cement paste; these studies include compressive strength tests, bulk density, compressive strength versus gel/space ratio relationships, polymer load, X-ray diffraction analysis and differential thermal analysis. Results have indicated that compressive strength improvement in acrylic acid-polymer impregnated cement pastes is mainly dependent on initial water/cement ratio, curing time and gel/space ratio. The results of X-ray diffraction analysis and differential thermal analysis indicated that the intrusion of polymer into the cement paste matrix does not affect the phase composition of the Portland cement hydration products.