Hydration of high-volume fly ash cement pastes

The hydration processes of high-volume fly ash cement paste were investigated by examining the non-evaporable water content, the CH content, the pH of pore solution and the fraction of reacted fly ash, curing at either 20°C or elevated temperatures after an initial curing at 20°C. The replacement percentage levels of fly ash were 40%, 50% and 60% by weight, respectively. The results revealed that the non-evaporable water content in high-volume fly ash cement pastes does not develop as plain cement pastes does, so it may be improper to apply the non-evaporable water content to evaluate the hydration process in high-volume fly ash cement matrix. The reduction in CH content increases with the progressing of hydration process and varies linearly with the logarithm of curing age. The addition of 3.0% of Na₂SO₄ could accelerate the pozzolanic reaction of fly ash at early ages. At 20°C, the pH of pore solution of high-volume fly ash cement paste was reduced to a great extent at early ages and it continued to decline at later ages due to the inclusion of large amount of fly ashes. At elevated temperatures, however, this trend was not found. The fraction of reacted fly ash directly reflects the pozzolanic reactivity of fly ash both at normal and elevated temperatures. There is some inherent correlation between the reduction in CH content, the pH of pore solution and the fraction of reacted fly ash. For specified matrix, the consumption of CH and the pH of pore solutions change linearly with the increase of the fraction of reacted fly ash.     

Microconcrete with partial replacement of Portland cement by fly ash and hydrated lime addition

The reduction in Portland cement consumption means lower CO₂ emissions. Partial replacement of Portland cement by pozzolans such as fly ash has its limitations due to the quantity of calcium hydroxide generated in the mix. In this work we have studied the contribution of the addition of hydrated lime to Portland cement + fly ash systems. We have also studied several levels of cement replacement, ranging from 15% to 75%.The best mechanical results were obtained replacing 50% of Portland cement by the same amount of fly ash plus the addition of hydrated lime (20% respect to the amount of fly ash). In these systems, an acid-base self-neutralization of the matrix has occurred through a pozzolanic reaction of fly ash with portlandite liberated in the hydration of Portland cement and the added hydrated lime. It has been identified for these mixtures a significant amount of hydrated gehlenite, typical reaction product from rich-alumina pozzolans.     

Effects of colloidal nanoSiO2 on fly ash hydration

The influences of colloidal nanoSiO₂ (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO₂ had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO₂ on cement/FA hydration at later ages were proposed.     

Effects of colloidal nanoSiO2 on fly ash hydration

The influences of colloidal nanoSiO₂ (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO₂ had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO₂ on cement/FA hydration at later ages were proposed.     

The interfacial zone and bond strength between aggregates and cement pastes incorporating high volumes of fly ash

The weak transition zone between aggregate and cement paste controls many important properties of concrete. A number of studies dealing with interfacial zone are available in the literature for normal concrete and concrete containing silica fume. High-volume fly ash concrete for structural applications was developed at CANMET in the 1980s, but to date there has been no information available for interfacial zone in high-volume fly ash concrete.In this paper, the orientation index and mean size of Ca(OH)₂ crystals in the aggregate-paste interfacial zone were determined by the X-ray diffractometer. The bond strength between the aggregate and paste was also investigated. It was found that, at the age of 28 days, there was no obvious transition zone between the aggregate and cement paste incorporating high volumes of fly ash. The higher the paste strength, the higher is the bond strength.     

Preparation of alinite cement from municipal solid waste incineration fly ash

The feasibility of partially substituting ordinary raw materials with municipal solid waste incineration (MSWI) fly ash in alinite cement production was investigated by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF) and scanning electron microscopy (SEM). The physical properties and leaching behavior of the produced cement were also evaluated. Experimental results show that good quality clinkers can be obtained by firing the raw mixes, in which the replacement of MSWI fly ash reaches to 30%, at 1200 °C for 2 h. Alinite cements have higher early strengths at all gypsum additions, while the best result having acceptable early and 28-day strengths is obtained at 5% of gypsum addition. Results also show that the leaching toxicity of heavy metals is far lower than that of the regulatory limit at all testing ages. Based on this study, MSWI fly ash is viable as an effective, alternative raw material in alinite cement production.     

Evaluating the use of supplementary cementitious materials to mitigate damage in cementitious materials exposed to calcium chloride deicing salt

This paper discusses the role of supplementary cementitious materials (SCM) in reducing damage caused by calcium oxychloride formation. Calcium oxychloride is a destructive product of a reaction between calcium hydroxide (CH) that exists in a cementitious matrix and CaCl₂ that can enter the pores of the matrix when it is used as a deicing salt. Paste samples were prepared where a percentage of ordinary portland cement was replaced with various types of SCM (including fly ash, slag, and silica fume). This paper examined the amount of calcium oxychloride that formed using low-temperature differential scanning calorimetry, and damage development detected using acoustic emission. Thermogravimetric analysis was also performed to determine the relationship between the amount of CH in cementitious materials and the amount of calcium oxychloride formation. The results show that the use of SCM is effective in reducing the calcium oxychloride formation and resulting damage when cementitious materials are exposed to various compositions of solution containing CaCl₂. The explanation of the benefit of using SCM is that it can reduce the calcium oxychloride formation due to a reduction in the amount of CH in the cementitious materials through pozzolanic reaction and dilution of cement. As a result, cementitious materials with SCM exposed to CaCl₂ may experience less damage and have a longer service life.     

Morrocan composite cement with minor addition of fly ash

In order to lower the consumption of natural raw materials, to save fuel energy for clinker production and to make appreciable reduction of CO₂ emission, the Morrocan plants cements become to produce some composite cements types by adding some components. This paper describes the production of composite cements by intergrinding clinker, gypsum and limestone with a minor addition of fly ash up to 10%. Physical and mechanical properties are discussed, and the main result is that the addition of fly ash with low quantity acts as grinding agent by reducing the required time to obtain the same percentage of particles retained on an 80 μm sieve compared to the cement without addition of fly ash. From 28 days to 90 days, the compressive strength increases rapidly in the case of cement with a minor addition of fly ash.     

硫酸盐环境下灌浆材料长龄期力学性能及微观结构分析

本文将抗硫酸盐水泥和中热水泥掺粉煤灰制备的水泥浆体浸泡在5%Na2SO4溶液至1110d,研究长龄期硫酸盐侵蚀下各试件的力学性能和微观结构。结果表明:限制空间中形成的细小钙矾石是引起基体开裂的主要原因,石膏的形成会引起水泥基材料剥落,抗硫酸盐水泥不能有效防止石膏型硫酸盐侵蚀;大掺量粉煤灰的二次水化反应能够消耗大量氢氧化钙,从而降低侵蚀过程中石膏相的形成且能有效改善浆体微结构;水电工程中采用中热水泥+50%粉煤灰制备的水泥基材料能够有效抑制钙矾石和石膏的形成,其抗硫酸盐侵蚀性能和经济性明显优于抗硫酸盐水泥制备的水泥基材料。     

Influences of fly ash on magnesium oxychloride mortar

The application of magnesia-based construction materials draws much research interests nowadays due to the ever increasing awareness of environmental protection. By incorporation of fly ash into magnesium oxychloride (MOC) cement, an energy efficient and environmentally friendly repair material can be formed for successful industrial applications. In the current research, an appropriate formulation of the MOC matrix with a suitable combination of the molar ratios MgO/MgCl₂ and H₂O/MgCl₂ has been characterized by using phase diagram, X-ray diffractograms and scanning electronic microscope. Subsequently the influences of fly ash on the properties of both MOC cement and mortar are investigated. It is found that the incorporation of fly ash can enhance the workability or fluidity, retard the setting time, and improve the water resistance of the MOC mortars. With the enhanced performance and a slightly expansive nature, the MOC mortars incorporated with fly ash has a good potential to be used as a repairing material.     

The influence of an Austrian fly ash on the reaction processes in the clinker phases of Portland cements

The aim of the paper is to establish the influence of fly ash on the hydration process of the cement and fly ash mixtures. Particular attention was paid to the influence on the main clinker phases, C₃S, C₂S and C₃A being investigated by X-ray methods at various points during the reaction period. The reaction partners used were two normal Austrian Portland cements plus an Austrian brown-coal ash. In addition to the pure cements, a mixture with 30% fly ash and 70% of the respective cements was also investigated. For purposes of comparison, it was also necessary to analyse in each case a cement mixture with an inert substance in the same ratio.     

Calcium aluminate cements in fly ash/calcium aluminate blend phosphate cement systems: Their role in inhibiting carbonation and acid corrosion at a low hydrothermal temperature of 90°C

Study was focused upon formulating sodium polyphosphate-modified fly ash/calcium aluminate blend (SFCB) geothermal well cements with advanced anti-carbonation and anti-acid corrosive properties. At a low hydrothermal temperature of 90°C, to improve these properties, we investigated the effectiveness of various calcium aluminate cement (CAC) reactants in minimizing the rate of carbonation and in abating the attack of H₂SO₄ (pH 1.6). We found that the most effective CAC had two major phases, monocalcium aluminate (CA) and calcium bialuminate (CA₂), and a moderate CaO/Al₂O₃ ratio of 0.4. The reaction between sodium polyphosphate (NaP) and CA or CA₂ at room temperature led to the formation of amorphous dibasic calcium phosphate hydrate and anionic aluminum hydroxide caused by the decalcification of CA and CA₂. When SFCB cement made with this CAC was exposed to 4% NaHCO₃-laden water at 90°C, some carbonation of the cement occurred, forming calcite that was susceptible to the reaction with H₂SO₄. This reaction resulted in the deposition of gypsum gel scales as the acid corrosion product on the cement surfaces. The scale layer clinging to the cement protected it from further corrosion. Under such protection, the amorphous dibasic calcium phosphate hydrate crystal hydroxyapatite and anionic aluminum hydroxide crystal boehmite phase transitions were completed in acid solution. Meanwhile, the further chemical and hydration reactions of NaP with fly ash led to the formation of additional crystalline Na-P type zeolite phases. Thus, we propose that passivation of the surface of the cement by deposition of gypsum, following the formation of these reaction products, which are relatively inert to acid, are the acid corrosion-inhibiting mechanisms of the SFCB cements.     

Utilization of municipal solid waste incineration ash in Portland cement clinker

Municipal solid waste incineration (MSWI) ash is used in part as raw materials for cement clinker production by taking advantage of the high contents of SiO₂, Al₂O₃, and CaO. It is necessary for environmental reasons to establish a material utilization system for the incineration waste ash residue instead of disposing these ashes into landfill. The aim of this paper is to study the feasibility of replacing clinker raw materials by waste ash residue for cement clinker production. MSWI bottom ash and MSWI fly ash are the main types of ashes being evaluated. The ashes were mixed into raw mixture with different portions of ash residue to produce cement clinker in a laboratory furnace at approximately 1400°C. X-ray diffraction and X-ray florescence techniques were used to analyze the phase chemistry and chemical composition of clinkers in order to compare these ash-based clinkers with commercial Portland cement clinker.     

Influence of fly ash blending on hydration and physical behavior of belite–alite–ye’elimite cements

A cement powder, composed of belite, alite and ye’elimite, was blended with 0, 15 and 30 wt% of fly ash and the resulting blended cements were further characterized. During hydration, the presence of fly ash caused the partial inhibition of both AFt degradation and belite reactivity, even after 180 days. The compressive strength of the corresponding mortars increased by increasing the fly ash content (68, 73 and 82 MPa for mortars with 0, 15 and 30 wt% of fly ash, respectively, at 180 curing days), mainly due to the diminishing porosity and pore size values. Although pozzolanic reaction has not been directly proved there are indirect evidences.     

On the interaction of Class C fly ash with Portland cement–calcium sulfoaluminate cement binder

Shrinkage cracking in concrete is a widespread problem, especially in concrete structures with high surface-to-volume ratio such as bridge decks. Expansive cements based on calcium sulfoaluminate phase were developed to mitigate the shrinkage cracking of concrete. The compressive stress induced due to restrained expansion of concrete has been shown to counteract the tensile stress generated during drying shrinkage. This research attempts to address the differential behavior of fly ash type (i.e., Class C vs. Class F) on early-age expansion and hydration characteristics of ordinary Portland cement (OPC)–calcium sulfoaluminate (CSA) cement blend. It was observed earlier that the presence of Class C fly ash (CFA), unlike Class F fly ash, shortened the expansion duration of OPC–CSA cement blend, which was hypothesized to be correlated to early depletion of gypsum. This paper presents a detailed verification of the hypothesis. Addition of external gypsum to OPC–CSA–CFA blend led to simultaneous increase in expansion and disappearance of a shoulder peak in the calorimetric curve. Thermodynamic calculations using a geochemical modeling program (GEMS-PSI) revealed higher saturation levels of ettringite in presence of external gypsum, which led to higher crystallization stress, and thereby increased expansion.     

Damage in cement pastes exposed to MgCl<Subscript>2</Subscript> solutions

Magnesium chloride (MgCl₂) reacts with cement pastes resulting in calcium leaching and the formation of calcium oxychloride, which can cause damage. This paper examines the damage in different cement pastes exposed to MgCl₂ solutions. Volume change measurement and low temperature differential scanning calorimetry are used to characterize the formation of calcium oxychloride. Thermogravimetric analysis and X-ray fluorescence are used to quantify calcium leaching from Ca(OH)₂ and C-S-H. The ball-on-three-balls test is used to quantify the flexural strength reduction. Calcium oxychloride can form in cement pastes exposed to MgCl₂ solutions with a (Ca(OH)₂/MgCl₂) molar ratio larger than 1. As the MgCl₂ concentration increases, two-stages of flexural strength reduction are observed in the plain cement pastes, with the initial reduction primarily due to calcium leaching from Ca(OH)₂ and the additional reduction due to the calcium leaching from C-S-H (at MgCl₂ concentrations above 17.5 wt%). For the cement pastes containing fly ash, there is a smaller reduction in flexural strength as less Ca(OH)₂ is leached, while no additional reduction is observed at high MgCl₂ concentrations due to the greater stability of C-S-H with a lower Ca/Si ratio. The addition of fly ash can mitigate damage in the presence of MgCl₂ solutions.     

Recommendation of RILEM TC 238-SCM: determination of the degree of reaction of siliceous fly ash and slag in hydrated cement paste by the selective dissolution method

The performance of slag and fly ash in hydrated cementitious materials depends on the degree of reaction developed at the evaluated age. Several methods for the determination of the reaction degree of supplementary cementitious materials are available, among which the selective dissolution method is one of methods developed the earliest. This is a direct method that aims to quantify the amount of unreacted slag or fly ash in the sample by applying a selective acid attack. The degree of reaction is obtained from the comparison between the remaining unreacted SCM, which should not dissolve, and the total amount initially included in the mix. This recommendation indicates suitable procedures for computing the degree of reaction by selective dissolution of cement pastes containing slag and fly ash. Specific considerations are indicated for necessary corrections due to the imperfect selective dissolution when the procedure is applied to hydrated cement paste.     

Synthesis of low-temperature calcium sulfoaluminate-belite cements from industrial wastes and their hydration: Comparative studies between lignite fly ash and bottom ash

The aim of this research was to study the production of calcium sulfoaluminate-belite (CŜAB) cement from industrial waste materials via hydrothermal-calcination process. Lignite fly ash and bottom ash were used as starting materials for comparison. Other waste materials viz., Al-rich sludge and flue gas desulfurization gypsum were also key players in raw mixes for the synthesis of CŜAB cement. For lignite fly ash as a starting material, mixed phases between ye'elimite and larnite were obtained, whereas for lignite bottom ash as starting material, only ye'elimite was obtained The hydration reaction was studied in terms of heat evolution, setting time, compressive strength and hydration product formation with various gypsum contents. The results showed a rapid formation of ettringite as a main hydration product mixed with calcium silicate hydrate, monosulfate and strätlingite phases as minority, with a fast final setting time of 24–26 min and high early compressive strength of 16.0 and 18.0 MPa in 1 day for CŜAB cements made of fly ash and bottom ash, respectively.     

纳米SiO2与粉煤灰协同改性水泥基材料性能研究

通过调整纳米SiO_2与粉煤灰的比例,研究了两者协同作用对水泥基材料性能的影响。结果表明,纳米SiO_2(NS)和粉煤灰协同作用效果优于NS单一掺加,3%(质量分数,下同)纳米SiO_2和不大于30%的粉煤灰同时掺加可以补偿粉煤灰引起的早期强度降低,且砂浆28d抗压强度不降低。随着NS掺量增加水泥基材料的干燥收缩增大,粉煤灰可以改善纳米SiO_2对干燥收缩的不利影响。随着NS掺量的增加,试件的抗冻性和抗氯离子渗透性能均得到提升,掺加3%NS与30%粉煤灰使水泥基材料达到最佳耐久性能。NS可以缩短水泥水化诱导期,加速水泥水化进程,且使胶凝体系总放热量增加。在水泥粉煤灰体系中掺入NS后,非蒸发水含量在早期明显增多,但在后期增长缓慢。     

水泥-粉煤灰-硅灰基超高性能混凝土水化过程微观结构的演变规律

超高性能混凝土(UHPC)具有卓越的力学性能和耐久性能,应用前景广阔。采用扫描电镜背散射电子图像、热重法和氮气吸附法系统研究了水泥-粉煤灰-硅灰基UHPC浆体水化过程中微观结构的演变过程。结果表明:UHPC浆体在早期水泥水化较快,但7d后水化变得较为缓慢,粉煤灰在UHPC浆体中反应较为缓慢,28d时反应程度仅为7%;UHPC浆体中Ca(OH)2含量早期上升快速,由于硅灰和粉煤灰的火山灰反应逐渐消耗,3d后含量开始下降,但28d时浆体中仍存在部分Ca(OH)2;此外,在水化过程中,UHPC浆体的比表面积不断降低,孔隙率逐渐下降,水化产物变得更为致密。