Magnesia-phosphate bond for cold-setting cordierite-based refractories

A cold-setting refractory material was developed using the magnesia-phosphate reaction. A cement paste based on alumina, silica fume, magnesia and orthophosphoric acid or monoaluminum phosphate was designed to form cordierite-mullite during heating. This cement paste set at room temperature and MgHPO₄·3H₂O phase (newberyite) was observed, but amorphous phases were predominant. Two exothermic effects were detected during the setting process corresponding to the acid-base reaction of magnesia with phosphates and to the formation of bonding hydrates. At 1100 °C, C-AlPO₄ was formed by reaction of alumina with orthophosphoric acid or monoaluminum phosphate. At 1350 °C, the principal crystalline phases were cordierite and mullite. A refractory concrete with the obtained cement paste and a cordierite-mullite aggregate (scrap refractory material) was prepared. At 1350 °C, this concrete had a thermal expansion coefficient of 1.0×10⁻⁶ °C⁻¹ and a flexural strength of 10 MPa.     

Synthesis and evaluation of Mg-Al hydrotalcite formation and its influence on the microstructural evolution of spinel-forming refractory castables under intermediate temperatures

The mechanical properties of in-situ spinel (MgAl₂O₄)-forming alumina-based castables under intermediate temperatures are of critical importance before the refractory lining system reaches normal operating conditions. The objective of this study is to elucidate the role of the hydrotalcite formed within a fine-grained castables matrix, in which no strength loss of the MgO-bonded alumina-based castables without silica fume was observed. Numerous fundamental studies were conducted to examine the factors influencing hydrotalcite formation within the blended pastes composed of MgO and Al₂O₃ nanopowders; dead burned or fused magnesia and Al₂O₃ nanopowder; dead burned magnesia and water-dispersed sol of fumed alumina by using: XRD and DSC-TG-EGA(MS). The XRD, FTIR and ²⁷Al MAS NMR analysis of the hydrotalcite calcination products revealed that the spinel begins to form at temperatures as low as 700 °C. Finally, the physical properties, phase composition and microstructure of the refractory castables bonded with the hydrotalcite decomposition-routed nanostructured spinel were evaluated.     

Study on the high‐temperature behavior and rehydration characteristics of hardened cement paste

The high‐temperature behavior and rehydration characteristics of the hardened cement paste and their mechanisms have been studied in this paper. X‐ray diffraction and thermogravimetry are used to establish the effect of elevated temperatures on the mineralogical changes that occurred in the hardened cement paste. The change of microstructure was characterized by scanning electron microscopy. The results showed that with the temperature increased, the compressive strength of hardened cement paste first increased and then decreased. According to micromeasurements, at 400°C, the porosity and average pore diameter of hardened cement paste increased slightly, while at 800°C, the porosity and average pore diameter of hardened cement paste increased sharply. When hardened cement paste was cured after exposing to 400°C, its pore structure and phase composition had no change, while when hardened cement paste was cured after exposing to 800°C, there are new hydration products, and its pore structure may be finer, but it cannot fully recover to the original state. Copyright © 2014 John Wiley & Sons, Ltd.     

Soluble phosphate salts as setting aids for premixed calcium phosphate bone cement pastes

Recently, premixed calcium phosphate cement pastes have been proposed as biomaterials for bone tissue repair and regeneration. Use of premixed pastes saves the time and removes an extra step during a medical operation. α-Tricalcium phosphate (α-TCP) based cements set to form calcium deficient hydroxyapatite which has a moderate bioresorbtion speed. α-TCP cements require a setting aid, usually a sodium or potassium phosphate salt, to speed up the setting process. Within the current research we investigated which setting aid has significant advantage, if α-TCP is used in form of non-aqueous premixed paste. This approach offers the application of simple ingredients to produce a premixed calcium phosphate cement. The following properties of cement formulations were evaluated: cohesion, phase composition, microstructure, pH value of the liquid surrounding the cement, and compressive strength.Compositions using mixture of basic and acidic potassium phosphate salts (KH₂PO₄ and K₂HPO₄) in sufficient amounts give the best overall results (adequate cohesion and pH of the surrounding liquid, hydrolysis of starting materials within 48 h, and compressive strength of 12 ± 3 MPa). Cement prepared with basic sodium phosphate salt (Na₂HPO₄) as setting aid had considerably higher compressive strength 22 ± 1 MPa, but the pH of the surrounding liquid was basic (9.0).     

Hydration products of alkali-activated slag-red mud cementitious material

A new kind of alkali-slag-red mud cementitious material, abbreviated as ASRC, with both high early and ultimate strength and excellent resistance against chemical attacks has been developed by the application of composite solid alkali activator into slag-red mud mixture system. The hydration products of this cement at ambient temperature have been investigated by means of XRD, IR, TG-DTA, TEM, EDXA, etc. The results showed that the hardened cement paste was mostly consisted of C-S-H gel, being very low in Ca/Si ratio, very fine in size and extremely irregular in its shape. Neither Ca(OH)₂ and AFt, which are usually present in the hardened Portland cement paste, nor zeolite-like products have been detected. These characteristics are considered to be the chemical reasons for the high early and ultimate strength and good resistance against chemical attacks of the hardened ASRC cement paste.     

Fracture mechanics studies on concrete compounds

An experimental and analytical investigation was conducted to determine fracture mechanics characteristics of hardened cement paste, aggregates and aggregate-cement paste interfaces. For this the fracture toughness K_Ic was determined on wedge loaded CT-specimens. It was found that hardened cement paste, aggregates and interfaces exhibit unique K_Ic values which are independent of the initial crack length. In additional test series the ductility of various model concretes tested in flexure was determined. The ductility depends primarily on the fracture toughness toughness of the aggregate-hardened cement paste interfaces and is less affected by the fracture toughness of the hardened cement paste.     

硫化物对水泥基材料中锌的控制研究

研究了硝酸锌对水泥基材料物理性能的影响以及外加控制剂硫化物对其在水泥基材料中的反应情况和硫化物本身对水泥基材料物理性能的影响。并通过比较硝酸锌和硫化物在2种不同的添加方式下所测得的水泥基材料性能,探讨了硫离子对锌离子的控制作用。研究结果表明,硫化物本身对硬化水泥浆体的宏观性能影响很小,其作为外加控制剂使用能很好地钝化锌离子在硬化水泥浆体中的化学活性。     

磷酸钾镁水泥净浆抗盐冻性能研究

为了研究磷酸钾镁水泥抗盐冻性能,通过测定四种不同冻融介质中磷酸钾镁水泥净浆试件的质量损失、强度、体积变形和表观破坏形态,并与相同龄期的长期浸泡环境下的磷酸钾镁水泥净浆试件比较,借助微观手段研究磷酸钾镁水泥硬化体的物相组成和微结构.结果表明:磷酸钾镁水泥经过400次冻融循环时仍保持质量损失不超过5％、强度损失不超过25％、体积膨胀变形最大为0.275％,具有较好的抗盐冻性.四种冻融介质中,以5％硫酸钠溶液为冻融介质时破坏最严重,可作为评价磷酸钾镁水泥抗盐冻性能好坏的指标.     

Phase transformation on bone cement: Monocalcium phosphate monohydrate into calcium-deficient hydroxyapatite during setting

This study evaluated the phase transformation of calcium phosphate cement (CPC) using a mixture of monocalcium phosphate monohydrate (MCPM) and CaCO₃ as the solid phase and either water or a sodium phosphate buffer (SPB) solution (pH=7.0) as the liquid phase. The synthetic CPC was characterized by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). The setting reaction in the SPB solution involved three phase transformations. Firstly, MCPM and CaCO₃ reacted with sodium phosphate immediately to form dicalcium phosphate dehydrate (DCPD) which continued to dissolve. Secondly, meanwhile, an intermediate amorphous calcium phosphate (ACP) was formed. Finally, ACP transformed into calcium-deficient hydroxyapatite (CDHA). In contrast, the reaction stopped at the first stage in water. Consequently, the SPB solution not only caused the dissolution of DCPD but also provided the buffering capacity to induce the conversion of the starting materials to CDHA.     

Magnesium monophosphate cements derived from diammonium phosphate solutions

Rapid-setting magnesium monophosphate cementitious materials were prepared by mixing calcined magnesium oxide (MgO) powder with an aqueous solution of diammonium phosphate (ADP) at 24°C. The activation energy for the curing reaction of the cement paste was determined to be 30.29 kcal/mole, and at age 1 hr the compressive strength was ≈900 psi (6.2 MPa). X-ray diffraction studies of the cured cement indicated that the major reaction product was magnesium orthophosphate tetrahydrate [Mg₃(PO₄)₂·4H₂O]. Magnesium ammonium phosphate hexahydrate [MgNH₄PO₄·6H₂O] and Mg(OH)₂ were also detected. Subsequent heating of the cement to 1300°C resulted in the conversion of the three compounds to a single phase of anhydrous magnesium orthophosphate [Mg₃(PO₄)₂]. The resultant product had a compressive strength of 7000 psi (48.23 MPa) and was thermally stable in air at temperatures >1000°C.     

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。     

高温条件下G级油井水泥原浆及加砂水泥的水化和硬化

利用X射线衍射仪和扫描电子显微镜研究了80～240℃温度范围内温度、硅砂对G级油井水泥水化硬化的影响,检测和分析了硬化体的水化产物、微观结构和强度,揭示了水化产物组成、微观结构及硬化体抗压强度的变化特点.结果表明:当养护温度超过110℃时,不添加硅砂的水泥原浆的主要水化产物由CSH(Ⅱ),C2SH2,C3S2H3转变为C2SH,硬化体微观结构由三维网络状结构转变为板快状或团块状结构,原浆水泥石抗压强度随温度升高而降低;在相对较高的温度条件下,添加硅砂的水泥主要水化产物则分别转变为C5S6H5,C6S6H(>150℃),C5S5A0.5H5.5,C3.2S2H0.8及其他类型的水化硅酸钙晶体,硬化体的微观结构相应地变为纤维网状、粗框架、短平行针状及团块状,在温度为100～150℃范围时,添加硅砂的水泥硬化体抗压强度随温度升高而增加,而在温度为150～240℃范围时.抗压强度随温度升高而降低.对于温度超过120℃的深井,合理的硅砂加量为30%～40%.     

矿渣、粉煤灰和硅灰对硬化水泥浆体孔隙率的影响

本文用3种不同的试样制备方法和3种测孔方法分别测定了加矿渣、粉煤灰和硅灰等掺合料的硬化水泥浆体孔隙率。结果表明,试样制备方法和孔隙率测定方法一样,都会影响孔隙率的测定结果,掺加掺合料以后,这种影响更为显著。     

A reliable technique to determine the local mechanical properties at the nanoscale for cementitious materials

A Hysitron Triboindenter has been used to determine the mechanical properties of hardened cement paste and cement paste at the early age. This technique provides an in-situ scanning probe microscopy (SPM) imaging facility that allows pre and post-test observation of the sample. The same probe is used to indent and image eliminating the complicated situation of locating the same area with different instruments or coupling two different instruments, such as a SEM and a nanoindenter to work together. This paper presents preliminary results of experiments performed on hardened cement paste and cement paste at the early age.     

Preparation techniques and the microscopical examination of portland cement paste and C3S

The requirement for structural resolution in TEM is the preparation of electron-transparent sections, so that for hardened cement paste ion-beam milling is the most practicable technique. Polished thin sections of bulk cement paste and hydrated tricalcium silicate, C₃S, have been prepared and used for transmitted light microscopy, and to produce ion-beam machined thin-foils for examination in an analytical scanning transmission electron microscope. Scanning electron microscopy, used in conjunction with energy dispersive analysis, has proved particularly useful in providing a view of three-dimensional structural organisation and in helping to establish a connection with the images observed in TEM.     

硫酸腐蚀后油井水泥石的性能及微观结构

研究了硫酸对油井水泥石强度及微观结构的影响。结果表明：水泥石被硫酸腐蚀后,强度明显下降,硬化浆体中100 nm以上有害孔的数量显著增多,水化产物变得疏松多孔,硬化水泥浆体的物相组成发生变化,有新的腐蚀产物CaSO4·2H2O生成;水泥石抵抗酸性介质腐蚀的能力不仅与其致密程度有关,还与其硬化浆体的矿物组成密切相关;不同水化产物抵抗腐蚀的能力不同,Ca（OH）2比C-S-H凝胶更容易受到酸性介质的腐蚀;C-S-H凝胶被腐蚀后产生的孔隙主要是细小孔隙,而Ca（OH）2被腐蚀后产生的孔隙主要是100 nm以上有害孔,降低硬化浆体中Ca（OH）2的含量是提高水泥石抗腐蚀性能的关键。     

矿渣-粉煤灰混合材料水化产物、微观结构和性能

用X射线衍射仪和扫描电子显微镜等对矿渣、粉煤灰混合材料的水化产物、硬化体微观结构及强度进行了检测和分析,确定了水化产物的组成及微观结构特点,揭示了矿渣粉煤灰材料的水化作用特点及强度特征.结果表明:矿渣在激发剂作用下使玻璃体首先发生表面水解,产生水化反应,进而引发粉煤灰的火山灰作用;混合材料的水化产物组分以水化硅酸钙凝胶为主,硬化体具有与油井水泥相类似的网络状微观结构;随养护时间增长,混合材料后期强度持续增加.     

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.     

磷酸镁水泥性能试验研究

采用过烧镁砂与磷酸镁二氢钾为主要原料,按照一定配比制备出可工业化生产的磷酸镁水泥。研究了磷酸镁水泥的凝结硬化时间、不同龄期的强度、干缩率、耐腐蚀性及粉煤灰对其性能的改性,并采用X射线衍射分析和扫描电镜等手段对其水化产物进行了分析。研究结果表明,磷酸镁水泥凝结时间受水灰比、成型温度影响较大,早期强度发展迅速,干缩率较小,耐腐蚀性好,掺加适量的粉煤灰可有效地提高其后期强度。该体系水化产物主要是六水磷酸镁钾晶体和一些无定形物质。     

Petrographic evidence of calcium oxychloride formation in mortars exposed to magnesium chloride solution

Many researchers have reported chemical interactions between CaCl₂ and MgCl₂ solutions and hardened Portland cement paste. One potentially destructive phase reported in the literature is calcium oxychloride (3CaO·CaCl₂·15H₂O). In the past, limited numbers of researchers have reported identification of this phase by X-ray diffraction. In this work, petrographic evidence of oxychloride formation is presented based on optical microscopy, scanning electron microscopy and microanalysis. This evidence indicates that calcium oxychloride does form in mortars exposed to MgCl₂ solutions.