Heat evolution of high-volume fly ash concrete

In this paper, the results of a laboratory investigation conducted with heat evolution of high-volume fly ash (HVFA) concrete are presented. Heat evolution of concrete was studied by measuring the temperature increase in concrete under adiabatic curing condition. Characteristic of heat evolution of fly ash concrete was found to be strongly dependent on the replacement level of fly ash and dosage of superplasticizer used to maintain workability. It was also found that using fly ash as cement replacement resulted in a reduction on the maximum temperature rise. Increasing the replacement level of fly ash caused lower temperature rise in concrete. Superplasticizer caused a delay in peak temperature rise time; this is taken as an indicator that high-dosage superplasticizer used in concrete caused retardation in hydration of cement. Concretes having similar ingredients showed similar peak temperature rise whether they are superplasticized or not.     

New insights into tricalcium silicate hydration in paste

The knowledge of the aqueous phase composition during the hydration of tricalcium silicate (C₃S) is a key issue for the understanding of cement hydration. A new in situ method of computing calcium ion concentration from the measurement of the electrical conductivity on paste was coupled to isothermal calorimetry and BET measurements to get new insights on the early hydration of C₃S. Ion concentrations of the aqueous phase are mainly dependent on the degree of hydration and the water to C₃S ratio. In the case of C₃S paste, the calcium and silicon concentrations determined at low degrees of hydration can be related to the equilibrium curve of C-S-H having C/S = 1.27 and named C_1.27SH_y. It is expected that C_1.27SH_y thermodynamically controls the aqueous phase composition at this early stage. Indeed, the formation of C_1.27SH_y is quasi-immediate when C₃S is in contact with water inducing a very rapid increase of the specific surface area that remains constant during the induction period. At higher degrees of hydration, the aqueous phase composition departs from the C_1.27SH_y equilibrium curve. C_1.27SH_y appears to be a metastable C-S-H that could be related to an intermediate phase previously reported. The quasi-immediate precipitation of C_1.27SH_y on C₃S surface explains why calcium and silicon concentrations remain low during early hydration even though C₃S is strongly undersaturated. This also agrees with the control of the end of the induction period by the nucleation and growth of more stable C-S-H.     

Effect of calcium to silica ratio on the synthesis of calcium silicate hydrate in high alkaline desilication solution

High alkaline desilication solution (DSS), a high volume byproduct from the pretreatment of high-alumina fly ash, was used as low-cost mother liquor for the synthesis of calcium silicate hydrate (C-S-H). Through the combined analysis of X-ray diffraction, thermogravimetric analysis, X-ray fluorescence, ²⁹Si MAS NMR, and Brunauer-Emmett-Teller, the relationship between chemical composition and structure of C-S-H synthesized under Ca/Si of 0.83:1 to 2.0:1 was investigated. Silicon conversion and yield of product have a positive correlation with Ca/Si. Sodium uptake in C-S-H is inhibited as Ca/Si increases. The formation of sodium in C-S-H transfers from “bound Na” to “mobile Na” and aluminum from tetrahedrally coordinated Al (IV) to octahedrally coordinated Al (VI). The increase of Ca/Si leads to shortening of silicate chain and formation of more dimers, which causes more water bound in C-S-H. The mechanism of calcium addition on silicate chain obtained from DFT calculation primarily results from more interlayer calcium occurrence to affect bridging tetrahedron and cationic bounding states reorganization. Reasonable control for Ca/Si momentously contributes to the adjustment for composition and structure of C-S-H synthesized in DSS.     

粉煤灰掺量对天然盐渍土强度和变形的影响

选取天然盐渍土在不同粉煤灰配合比条件下进行三轴试验、压缩试验和溶陷试验,研究粉煤灰对天然盐渍土的改良特性规律。试验结果表明:同种类盐渍土在一定条件下,盐渍土的抗剪强度随着粉煤灰配合比的增加逐渐增加;粉煤灰可改良盐渍土的压缩特性,压缩系数变化较大的垂直压力在400kPa之前,粉煤灰掺量越多,改良效果越好。氯渍土的压缩系数较硫酸盐渍土小,相应的压缩模量较大;粉煤灰对天然盐渍土溶陷特性的改良存在峰值压力为400kPa,对于粉煤灰的配合比也存在一界限值,氯盐渍土的粉煤灰最优配合比为15%,亚硫酸盐渍土的粉煤灰最优配合比为20%。     

Pair distribution function analysis of nanostructural deformation of calcium silicate hydrate under compressive stress

Despite enormous interest in calcium silicate hydrate (C–S–H), its detailed atomic structure and intrinsic deformation under an external load are lacking. This study demonstrates the nanostructural deformation process of C–S–H in tricalcium silicate (C₃S) paste as a function of applied stress by interpreting atomic pair distribution function (PDF) based on in situ X‐ray scattering. Three different strains in C₃S paste under compression were compared using a strain gauge, Bragg peak shift, and the real space PDF. PDF refinement revealed that the C–S–H phase mostly contributed to PDF from 0 to 20 Å whereas crystalline phases dominated that beyond 20 Å. The short‐range atomic strains exhibited two regions for C–S–H: I) plastic deformation (0‐10 MPa) and II) linear elastic deformation (>10 MPa), whereas the long‐range deformation beyond 20 Å was similar to that of Ca(OH)₂. Below 10 MPa, the short‐range strain was caused by the densification of C–S–H induced by the removal of interlayer or gel‐pore water. The strain is likely to be recovered when the removed water returns to C–S–H.     

粉煤灰掺量及颗粒群分布与水泥浆体流变性的关系研究

研究了原状粉煤灰与磨细粉煤灰不同掺量和不同颗粒群分布与水泥浆流变性能的相互关系。结果发现:原状粉煤灰加入水泥后具有降低水泥浆的粘度与屈服值的作用,且随粉煤灰细度及掺量的增加而更明显;磨细粉煤灰加入水泥后具有提高水泥浆的粘度与屈服值的效应,且随粉煤灰细度及掺量的增加而更突出。用灰色关联分析原理证实了原状灰<10μm的颗粒含量与水泥浆的流变性呈负关联,即对改善水泥的流动性是有益的;而磨细灰<10μm的颗粒含量与水泥浆的流变性呈正关联,即对提高水泥的流动性是不利的。     

分选与磨细粉煤灰对水泥胶砂性能的影响

研究了分选与磨细粉煤灰的颗粒分布与形貌的差异及对水泥胶砂性能的影响。研究结果表明:当勃氏比表面积相近,磨细粉煤灰的中位粒径大于分选细粉煤灰,其圆珠状颗粒较少,表面较为粗糙。在相同水胶比的条件下,掺分选粗粉煤灰的水泥胶砂流动度及强度均低;分选粗粉煤灰磨细后,不仅减少了颗粒的粘连,增加了比表面积,而且提高了粉煤灰的反应活性和水泥胶砂流动度及强度,虽其水泥胶砂流动度仍小于掺分选细粉煤灰的水泥,3d水泥胶砂强度也略低,但其28d水泥胶砂强度略高于掺分选细粉煤灰的水泥;在相同水泥胶砂流动度的条件下,掺磨细粉煤灰配制的水泥胶砂3d强度低于掺分选细粉煤灰的水泥,但随着水化龄期的增长,其差距逐步缩小,至60d时可超过后者。     

煤粉炉粉煤灰与循环流化床粉煤灰矿物学性质比较

为了提高粉煤灰的利用率,通过化学成分分析、扫描电子显微镜(SEM)分析、X射线衍射光谱(XRD)分析和核磁共振分析,对煤粉炉和流化床2种粉煤灰的形貌、物相组成和活性进行了表征,研究了2种粉煤灰矿物学性质的差别。试验结果表明:2种粉煤灰在形貌和物相上存在较大的区别。形貌上,煤粉炉粉煤灰中存在大量的玻璃微珠,而流化床粉煤灰由于成灰温度低不存在玻璃微珠;物相上,煤粉炉粉煤灰中存在较大量的结晶类矿物,而流化床粉煤灰多为非晶玻璃态物质。通过核磁共振分析发现煤粉炉粉煤灰中硅氧结构和铝氧结构的聚合度较高,不利于活性组分溶出。     

不同粒径改性粉煤灰对磷酸根吸附性能的影响

废水排放过量的磷导致水体污染日益严重,将粉煤灰通过化学改性制备成了水化硅酸钙吸附剂,研究了改性吸附剂对磷酸根的吸附效果。利用XRD, SEM及BET比表面积等手段对粒度分级前后的吸附剂进行表征,研究不同粒级吸附剂对磷酸根的吸附性能,并考察其吸附机理。结果表明,不同粒级的吸附剂其化学成分出现了明显的偏析现象,孔隙结构也差异显著。相比其他粒径下的吸附剂颗粒,颗粒粒径在50?75 μm时,吸附剂中钙和硅含量较多,铝、铁和镁含量较低,水化硅酸钙组分含量最高,且伴有含铝的托贝莫来石晶体出现,钙离子的增加使其可以与更多的磷酸根结合形成沉淀。同时此粒径下具有较高的比表面积及孔隙度,疏松多孔的结构为钙离子提供更多活性位点。当使用粒径在50?75 μm的吸附剂吸附磷酸根时,磷的饱和吸附量可达到17.1 mg/g,比未分级的吸附剂高19.58%。     

矿渣微粉-粉煤灰-高效减水剂与水泥净浆的流动度

通过矿渣微粉单掺、矿渣微粉与不同的高效减水剂双掺、矿渣微粉与粉煤灰双掺以及矿渣微粉、粉煤灰与高效减水剂三掺来研究水泥浆体的流动度与矿渣微粉之间的关系.结果表明:矿渣微粉的减水作用只有在和高效减水剂同时使用时才能得到充分发挥.     

用粉煤灰制备聚硅酸氯化铝铁絮凝剂的研究

对聚硅酸氯化铝铁(PSAFC)絮凝剂的制备及其方法优化进行了实验研究.考察了粉煤灰中有效成分的最佳溶出方案,通过实验确定了制备PSAFC的最佳摩尔比(n(Al+Fe)/n(Si))、最佳的活化pH值以及活化时间.在实验中发现,硅酸活化问题是复合絮凝剂制备的关键.结果表明:复合絮凝剂PSiFAC的絮凝效果好,矾花大且沉降...     

Effect of the densification of C–S–H on hydration kinetics of tricalcium silicate

Effect of water to cement (w/c) ratio and temperature profiles on the densification of C–S–H (calcium silicate hydrate gel) and hydration kinetics of triclinic tricalcium silicate (C₃S) is studied beyond the first day of hydration. Calorimetry and quantitative X‐ray diffraction/Rietveld analysis show that degree of hydration is unaffected by w/c up to 7 days and marginally thereafter. Coupling the degree of hydration with the portlandite content measured from thermal analysis indicate that C/S ratio of C–S–H decreases with increasing w/c. There is a clear increase in the portlandite content with increasing w/c, even though the degree of hydration is unchanged, due to the variations in C/S ratio of C–S–H. On the other hand, when C₃S is initially cured at a lower temperature (20°C) and then at a higher temperature (40°C), there is a significant increase in the reactivity even until 28 days and vice versa. These experimental results were explained using the densified volumetric growth hypothesis, which assumes that hydration kinetics are dependent on the internal surface area of C–S–H.     

粉煤灰制备P型分子筛工艺研究

通过对粉煤灰成分的系统分析,论述了利用粉煤灰制备P型分子筛的可能性,并对其制备条件和产品性能进行了实验研究。结果表明,只要条件控制得当,就可以制得性能较好的P型分子筛,为粉煤灰的综合利用开辟了一条新的途径。     

Cl-对高钙粉煤灰硬化水泥浆体的影响

研究了高钙粉煤灰硬化水泥浆体中Cl^-的扩散和渗透规律，以及Cl^-对高钙粉煤灰硬化水泥浆体性能的影响，并探讨了氯盐对高钙粉煤灰水泥浆体产生破坏的机理。试验结果表明，高钙粉煤灰的掺量以及水灰比和水化时间对Cl^-的扩散都有影响。     

粉煤灰水泥基材料的水化产物

用热重仪-差示扫描量热仪、扫描电子显微镜、透射电子显微镜、X射线能量散射、高分辩电子显微镜、压汞仪,测定了粉煤灰水泥基材料水化产物的形貌特征、微观结构、化学组成及水泥石的孔结构,讨论了水化产物性质及水泥石孔结构随粉煤灰掺量的变化规律。结果表明：粉煤灰的大量掺加,可以改善凝胶的化学组成。水化后期,粉煤灰与Ca(OH)2及由熟料水化生成的高n(Ca)／n(Si)的水化硅酸钙(C—S—H)凝胶发生二次水化反应,生成低n(Ca)／n(Si)的C—S—H凝胶,此种凝胶的固碱能力强,可减少碱-集料反应的危害性。同时,二次水化产物能够填充那些对水泥石强度和耐久性极为不利的孔隙空洞,使水泥石的结构更加致密,优化水泥石的孔结构,对提高水泥基材料的耐久性作用极大,为进一步提高工业废渣利用的技术水平奠定了基础。     

固硫灰渣和粉煤灰的特性对比分析

介绍了固硫灰渣的形成过程,并将固硫灰渣与粉煤灰的物化特性进行对比,分析表明:固硫灰渣与粉煤灰的化学成分类似,矿物组成差异较大;固硫灰渣与粉煤灰一样,具有较高的火山灰活性,用于建筑材料的生产完全可行.     

Modeling the effect of fineness and filler in early‐age hydration of tricalcium silicate

Alite (impure C₃S) being the major and most reactive phase in ordinary portland cement has been studied extensively. This paper focuses on the mathematical modeling of new hypothesis developed, densified volumetric growth, applied to the main hydration peak in alite hydration. This hypothesis assumes a time and particle size‐dependent growth rate and densification rate of C–S–H controlled by its internal surface area. To test this hypothesis, a new microstructural modeling platform (Cementitious Reactions Simulator) has been developed. The model was used to calibrate and predict the isothermal calorimetric results from the literature and two sets of original experimental results with alite replaced with different replacement fractions of quartz, fly ash, and slag of different fineness. The model is able to capture and predict the effect of fineness and fillers without the need of varying the parameters for the particular set of simulations. The values of the parameters of simulation reflect the current experimental evidence from the literature and thus provide validation of the hypothesis which suggests that the main hydration period including deceleration period in the alite hydration is induced by the densification and reducing outer growth rate of C–S–H.     

高硫灰与渣中硫的测定及影响因素研究

为了准确测定高硫灰及渣中硫,研究了库仑滴定法和高温管式炉燃烧红外热导法测定高硫灰及渣中硫含量的方法及其影响因素。结果表明,灰的粒径对库仑滴定法和红外热导法测定的准确性均有显著影响,需研细到0.04 mm以下时,才能消除粒径对高硫灰测定的影响。库仑滴定法测定高硫灰的时间较煤中全硫和低硫灰长200 s以上。三氧化钨的催化作用主要体现在促进灰中硫酸钙的分解。对硫含量高于现有灰标准物质的高硫含量灰,可以通过增大灰标准物质使用量的方法标定库仑测硫仪后准确测定,或通过使用煤标准物质标定库仑测硫仪后,在坩埚底部添加0.1 g活性炭法准确测定,添加活性炭能明显减少灰中硫的测定时间。     

High calcium cementless fly ash binder with low environmental footprint: Optimum Taguchi design

Despite the myriad of research efforts on exploiting fly ash as an alternative binder, its current role in industry is largely restricted to the supplementary use, which enables only partial replacement of conventional portland cement. Herein, we propose an unprecedented binder composite with the promising early-age strength, which is cost-effective and reduces the CO₂ footprint compared with portland cement. The major constituent is fly ash occupying 76.4%-80.3% by the total mass of the constituents, while calcium oxide, nanosilica, and the minimum amounts of sodium-based activators are added to induce the early-age strength development. Optimization of the composition via the Taguchi design of experiments produced the early (7-day) compressive strength of 16.18 MPa. This value is encouraging considering that it is comparable to that of conventional portland cement and that a cementless composition with the minimum amounts of sodium-based activators was employed. The extensive materials analysis demonstrates that the starting Ca/Na molar ratio and the amount of nanosilica play instrumental roles in strength development by influencing the formation of key reaction products, which include the sodium-substituted AFm phase (the U-phase), katoite and portlandite. Overall, the promising early-age strength coupled with the significantly decreased amount of sodium-based chemicals and the reduced CO₂ footprint will lay a foundation for development of low-cost, environmentally friendly binder in diverse industries.     

Effects of decalcification on the microstructure and surface area of cement and tricalcium silicate pastes

Thin coupons of white portland cement (WPC) and tricalcium silicate paste were decalcified by leaching in concentrated ammonium nitrate solutions, resulting in calcium-to-silicon molar ratios (C/S) ranging from 3.0 (control) down to 0.3. The microstructure and surface area were measured using both small-angle neutron scattering (SANS) and nitrogen gas sorption. The intensity in the SANS data regime corresponding to the volume fractal C-S-H gel phase increased significantly on leaching, and the total surface area per unit specimen volume measured by SANS doubled on leaching from C/S=3.0 to near C/S=1.0. The nitrogen BET surface area of the WPC pastes, expressed in the same units, increased on decalcification as well, although not as sharply. The primary cause of these changes is a transformation of the high-density “inner product” C-S-H gel, which normally has a low specific surface area as measured by SANS and nitrogen gas sorption, into a morphology with a high specific surface area. The volume fractal exponent corresponding to the C-S-H gel phase decreased with decalcification from 2.3 to 2.0, indicating that the equiaxed 5 nm C-S-H globule building blocks that form the volume fractal microstructure of normal, unleached cement paste are transformed by decalcification into sheetlike structures of increasing thickness.