Influence of ultra-fine fly ash on hydration shrinkage of cement paste

Hydration shrinkage generated by cement hydration is the cause of autogenous shrinkage of high strength concrete. It may result in the volume change and even cracking of mortar and concrete. According to the data analysis in a series of experimental studies, the influence of ultra-fine fly ash on the hydration shrinkage of composite cementitious materials was investigated. It is found that ultra-fine fly ash can reduce the hydration shrinkage of cement paste effectively, and the more the ultra-fine fly ash, the less the hydration shrinkage. Compared with cement paste without the ultra-fine fly ash, the shrinkage ratio of cement paste reduces from 23.4% to 39.7% when the ultra-fine fly ash replaces cement from 20% to 50%. Moreover, the microscopic mechanism of the ultra-fine fly ash restraining the hydration shrinkage was also studied by scanning electron microscopy, X-ray diffraction and hydrated equations. The results show that the hydration shrinkage can be restrained to a certain degree because the ultra-fine fly ash does not participate in the hydration at the early stage and the secondary hydration products are different at the later stage.

Adsorption and Desorption Characteristics of K^+ and Na^+ Ions in Fly Ash Blended Cement Pastes

Cement pastes containing 0%, 15%, 25% and 35% fly ash were prepared. After being cured for 90 days, all fly ash blended cement pastes were crushed and ground into powders with a particle size less than 80 μm and then the powders were immersed in alkali solutions. Adsorption characteristics of K^+ and Na^+ ions in the pastes were investigated. Meawhile, the desorption characteristics of the adsorbed alklai ions and the inherent K^+ and Na^+ ions in the pastes were also investigated. Results showed that the contents of K^+ and Na^+ ions adsorbed by the pastes increased with increasing the substitution levels of fly ash and/or the concentrations of alkali solutions. Each paste was characterized by having the same adsorption capacity for K^+ or Na^+ that was essentially independent of alkali concentration. Adsorption mechanism of K^+ and Na^+ ions by the pastes is believed to be an effect of charge compensation of the C-S-H gel. Adsorption-desorption of the adsorbed K^+ and Na^+ ions in the pastes is reversible. The inherent K^+ and Na^+ ions in the pastes entered rapidly into the de-ionized water during the first 120 minutes, and then they were released at a relatively slow rate. A steady-state alkali partition was reached at about 720 minutes. Some K^+ and Na^+ ions which were originally "bound" by the hydration products were considered to be released into de-ionized water. Leaching tests showed that there was no significant effect of fly ash on the retaining of available alkalis in the pastes. A part of the released alkali ions exists in the pore solutions and the other part may be physically adsorbed by the hydration products.

Effects of water content, water type and temperature on the rheological behaviour of slag-cement and fly ash-cement paste backfill

The pumping ability and placement performance of fresh cemented paste backfill(CPB) in underground mined cavities depend on its rheological properties. Hence, it is crucial to understand the rheology of fresh CPB slurry, which is related to CPB mixture design and the temperature underground. This paper presented an experimental study investigating the effects of binder type, content, water chemical properties and content, and temperature, on the rheological properties of CPB material prepared using the tailings of a copper mine in South Australia. Portland cement(PC), a newly released commercially manufactured cement called Minecem(MC) and fly ash(FA) were used as the binders added to the mine tailing materials. Various amounts of two different water types were added to the mixtures in the preparation of backfill material slurry. Six different temperatures ranging from 5 to 60 °C were to investigate the effect of temperature on CPB rheology. Overall, the increasing water content and decreasing temperature lead to lower yield stress. Based on the results obtained from the rheological properties of CPB slurry, it was found that at room temperature(25 °C), with regards to the unconfined compressive strength(UCS) performance, the replacement of 4% PC mixed CPB(28 days UCS 425 k Pa) to 3% MC mixed CPB(28 days UCS 519 k Pa), reduced the slurry yield stress from 210.7 to 178.5 Pa. The results also showed that the chemical composition of water affects the yield stress of CPB slurry and that MC mitigates the negative effect of mine-processed water(MW) and thus lead to improve the rheological properties of the slurry. However, the results suggested that the rheological properties of a mixture using MC is very sensitive to the water volume and temperature change. Therefore, using MC in backfill requires better quality control in slump mixing.

Investigation of the Effect of Using Different Fly Ash on the Mechanical Properties in Cemented Paste Backfill

In the cemented paste backfill(CPB)method,which can also be used for fortification purposes in mines,different additive materials with pozzolanic properties can be employed as substitutes instead of cement that is the main binder.One of the most popular pozzolanic materials that can be employed instead of cement is fly ash,which is thermal power plant tailings.But the compositions of fly ash and tailings used in high amounts in the CPB method,as well as the chemical structures that these materials form by interacting with the cement binder,affect the mechanical properties of the material depending on time.In this study,fly ash with 4 different chemical compositions(TFA,SFA,YFA,and CFA)was used as a cement substitute in CPB.By substituting fly ash with different chemical compositions in different proportions,CPB samples were created and their strength was elucidated according to 28,56,and 90-day curing times.The results of the study revealed that TFA with the highest CaO/SiO_(2) and SO_(3) ratios remained stable at the strength values of 6 MPa(total 9% binder)and 10 MPa(total 11% binder)in the long term.However,CFA with the lowest CaO/SiO_(2),SO_(3),and the highest SiO_(2)+Al_(2)O_(3)+Fe_(2)O_(3) ratios resulted in the greatest strength increase at a 20%substitution rate(11% of the total binder).Nevertheless,it was found that the SFA,which is in Class F,increased its strength in the early period based on the CaO rate.

Effect of Fly Ash on Rheological Properties of Cement Paste

This paper is aimed to study the factors influencing rheological properties of cement paste with fly ash,including the quantity of cement replaced by fly ash,types of the fly ash,degree of cement hydration and the microstructure of cement paste. By changing the water-cement ratio,mixing different amounts of fly ash,adding different types fly ash,the rheological parameters of cement paste are measured with a rheometer,and the needle penetration are tested by Vical needle. A ultrasound velocity test was aslo employed to monitor hydration of paste at early ages. Combinations of different factors on the rheological properties of cement paste are studied. The objective is to reveal the effect mechanism of material nature and proportion of each component on the rheological properties of cement paste with fly ash.

Utilization of Low-Alkalinity Cementitious Materials in Cemented Paste Backfill of Gold Mine Tailings

The purpose of this paper was to explore the possility of using low alkalinity cementitious materials as binders,in which ground blast furnace slag and fly ash acted as a partial replacement of ordinary Portland cement,and CaSO_(4),Na_(2)SO_(4),and CaO were used as a sulfate activator and alkali activated additives,to solidify gold mine tail-ings for preparation of a green,inexpensive cemented paste backill(CPB).For this target,the effects of cement/tailings ratio,superplasticizer dosage,solid content,tailings fineness on the mechanical properties of the CPB were inves tigated.Additionally,the hydration mechanism of the CPB was analyzed based on X-ray diffraction and scanning electron microscopy results.The results showed that the fuidity of the CPB slurry could be improved by adding polycarboxylic acid superplasticizer.The unconfined compressive strength(UCS)of the CPB specimens was increased with the increase of cement/tailings ratio and solid content.Under the same experi-mental conditions,the 28 d UCS of the CPB specimens was 3.8-4.9 times higher than that of ordinary Portland cement.The softening coefficient of the CPB specimens was increased with the increasing cement/tailings ratio,ranging from 0.83 to 0.92.The shrinkage rate of the CPB specimens was decreased from 0.70%to 0.54%with the increase of cement/tailings ratio from 1:12 to 1:4 The UCS of the full tailings CPB was the highest,followed by the fine tailings CPB specimens,and the UCS of the coarse tailings CPB specimens was the lowest.The low alka-linity binder was proved to be a promising material to improve the engineering performances of the CPB.The optimal mixing ratio is 1:6 cement/tailings ratio,0.15 wt% superplastizer dosage,and 70 wt%solid content.Pre-pared by this mixing ratio,the UCS values of the CPB after 3,7,and 28 d curing ages reached 1.85,5.87,and 9.16 MPa,respectively,which were suitable as CPB for the Zhaoyuan gold mine in terms of strength requirements.

混合材料与水泥浆体间界面的形貌特征

用ＳＥＭ和ＥＤＳ研究了矿渣、钢渣、粉煤灰、石灰石、大理石和石英等混合材料和水泥浆体间所形成界面的形貌特征；讨论了水泥水化初期和后期混合材料颗粒界面状态的模型。

粉煤灰与水泥浆体间界面的形貌特征

用ＳＥＭ和ＥＤＳ研究了粉煤灰掺入水泥后与水泥浆体间所形成界面的形貌特征。结果表明，在水泥水化初期，粉煤灰表面呈三种状态：（１）被ＣＳＨ单层膜包裹；（２）被ＣＨＣＳＨ双层膜包裹；（３）嵌入块状ＣＨ晶体内。在水泥水化后期，粉煤灰颗粒表面层已完全与水泥水化产物发生反应，且形成若干层反应物，呈致密的粒状或环状凝胶体。

粉煤灰对水泥水化浆体微结构的影响

为探讨粉煤灰对水泥水化浆体微结构的影响，通过原子力显微镜以及能谱分析，对纳米尺度下基准及掺有粉煤灰水泥浆体中水化产物的形状、大小以及堆聚结构的形成机制进行了研究。试验结果显示，在基准水泥浆体中的水化产物大部分是粒级大约40～80mm的圆球形颗粒，而且试验还观察到在它们中还镶嵌着一些尺寸大于1μm的板状和薄片状水化产物。然而，圆形颗粒的尺寸随养护龄期的增长有逐渐减小的趋势，且圆球形颗粒有分散和团聚两种堆聚形式。在水化早期，水化浆体微结构呈较松散的堆聚结构；随着养护龄期增长，由于颗粒的重排列，形成更低尺度的颗粒呈紧密的堆聚结构。然而，粉煤灰的掺入会影响水化产物的形状和堆聚形式，研究结果表明，由于粉煤灰的掺入，降低了浆体Ca／Si，使水化浆体的微结构无论是早期还是后期，其堆聚结构均较基准水泥样松散，而且浆体中还观察到许多椭球状的水化产物，经分析认为这是致使浆体微结构呈疏松状态的主要原因。

大掺量矿渣复合胶凝材料的硬化浆体形貌特征

通过扫描电镜观察了大掺量矿渣复合胶凝材料在不同水化龄期的硬化浆体形貌,并与水泥硬化浆体形貌进行对比,研究了矿渣在复合胶凝材料中的水化性能与硬化浆体形貌特征的关系。结果显示:大掺量矿渣复合胶凝材料水化早期的浆体结构比水泥浆体结构疏松,28天的浆体结构比水泥浆体结构致密。水化早期,矿渣颗粒被水泥水化产物包裹,与水泥浆体界面薄弱;水化后期,矿渣颗粒的水化产物与水泥水化产物紧密结合,颗粒与水泥浆体的界面牢固。

粉煤灰颗粒-水泥浆体的界面特征的研究

本文通过研究原状灰、磨细灰、自然细灰与水泥浆体的界面特征,获得了分别掺加上述三种粉煤灰的水泥浆体宏观性能差异的原因.为有利于粉煤灰的利用,将粉煤灰颗粒-水泥浆体界面区划分为两种:光滑颗粒和粗糙颗粒形成的界面区.

软水环境下粉煤灰对水泥净浆溶蚀特性的影响

为了研究软水环境下掺入粉煤灰对水泥净浆溶蚀特性的影响,该文制备了不同粉煤灰掺量的水泥净浆薄片,进行去离子水中薄片试件浸泡溶蚀实验。通过饱水-干燥称重法、乙二胺四乙酸(EDTA)滴定法、扫描电子显微镜(SEM/EDS)等测试,分析了各试件孔隙率、钙溶出量、钙硅比和微观形貌等随粉煤灰掺量和溶蚀时间的变化规律。结果表明,掺入粉煤灰可增加水泥净浆的初始孔隙率,且试件的溶蚀深度随着粉煤灰掺量的增加而增大,进而导致薄片试件溶蚀进程加快;但掺入粉煤灰可降低水泥浆体中氢氧化钙含量及钙溶出量和溶出速度,且其火山灰反应所生成的致密水化硅酸钙(C-S-H)凝胶可有效填充浆体孔隙,延缓溶蚀过程中试件的孔隙率增长和密实度下降,使薄片试件的溶蚀程度降低。