Effect of curing regime on polymerization of C-S-H in hardened cement pastes

The effect of two different curing regimes on the polymerization degree of C-S-H in hardened cement pastes within 28 d were investigated by measuring the chemical environments of ²⁹Si with magic angle spinning (MAS) nuclear magnetic resonance (NMR) and by analyzing the ²⁹Si NMR spectra with deconvolution technique. The experimental results indicate that, at curing regime of constant temperature of 20 °C, the polymerization of C-S-H increases and then decreases with curing age, and the Al/Si ratio increases gradually with curing age, furthermore, the two non-bridging oxygen bonds of bridging silicate tetrahedra in C-S-H structure mainly bond to H⁺. At curing regime of variable temperature, the polymerization of C-S-H firstly increases then changes slightly and subsequently decreases with the temperature from low to high and then to low, and the Al/Si ratio firstly increases then keeps invariant and subsequently slightly decreases. Moreover, the temperature decreasing is advantageous for the Ca²⁺ to be bonded to the bridging silicate tetrahedra and entering into the interlayer of C-S-H structure. The polymerization of C-S-H at curing regime of variable temperature is higher than that cured at constant temperature, but the curing regime of constant temperature is more beneficial to the substitution of Al³ for Si⁴⁺ than that of variable temperature.     

Effect of Curing Regime on Degree of Al3＋ Substituting for Si4＋ in C-S-H Gels of Hardened Portland Cement Pastes

The effect of curing regime on degree of Al3+ substituting for Si4+(Al/Si ratio) in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning(MAS) nuclear magnetic resonance(NMR) with deconvolution technique. The curing regimes included the constant temperature(20, 40, 60 and 80 ℃) and variable temperature(simulated internal temperature of mass concrete with 60 ℃ peak). The results indicate that constant temperature of 20 ℃ is benefi cial to substitution of Al3+ for Si4+, and Al/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 ℃ is less than that at 20 ℃ for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 ℃ curing stage from 28 to 180 d at variable temperature regime, is benefi cial to the increase of Al/Si ratio which is still lower than that at constant temperature regime of 20 ℃ for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length(MCL) of C-S-H gels, furthermore, the amount of Al3+ which can occupy the bridging tetrahedra sites in C-S-H structure is insuffi cient in hardened Portland cement pastes.     

养护制度对粉煤灰复合水泥浆体干燥收缩及孔径分布的影响

将水泥试件在水中养护6天和13天后放入干燥养护箱中,测定其质量损失和干燥收缩。采用氮吸附方法测定不同龄期时的孔径分布。结果表明:粉煤灰复合水泥浆体,干燥收缩早期变化较为明显,随着龄期延长,变化逐渐变缓;水养时间延长,干燥收缩减小;不同水养时间养护后,粉煤灰复合水泥浆体4-40nm孔径体积含量减小,相应干燥收缩减小。     

变温下 Ba2＋对水泥浆体C－S－H微结构的影响

为从分子尺度优化C－S－H微结构提供理论依据,采用模拟大体积混凝土内部变温历程的养护制度,运用29 Si魔角旋转核磁共振（29 Si MAS NMR）结合去卷积技术,研究了变温条件下Ba（OH）2掺量为1.0％时对水泥浆体C－S－H微结构的影响规律。结果表明：在变温条件下掺加Ba（OH）2提高了水泥浆体中硅酸盐矿物水化程度,尤其在水化早期（3 d）时硅酸盐矿物水化程度增幅较大,进而使C－S－H结构中硅氧四面体二聚体数量增加,导致其C－S－H平均分子链长（MCL）显著低于纯水泥浆体,避免了纯水泥浆体在降温阶段出现C－S－H的MCL降低的现象。同时,掺加Ba（OH）2进一步降低了水泥浆体在变温过程下C－S－H中Al3＋取代Si4＋的程度。     

Influence of curing duration on thaumasite formation of Portland-limestone cement pastes

Extensive researches have been carried out on the conventional sulfate attack, while it has been found that the thaumasite form of sulfate attack(TSA), sulfate attack at low temperature, has just been discovered and its mechanism is not well understood so far. In this study, the sulfate attack of cement paste incorporating 30% mass of limestone powder was investigated. After 20 ℃ water cured for 7, 14, and 28 days,respectively, 20 mm cube specimens were exposed in a 5% magnesium sulfate solution at(6 ±1) ℃ for periods up to 240 days. Their appearance change, compressive strength development were examined at different storage time, and selected paste samples were examined by X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FTIR), scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS). The results indicate that all Portland-limstone cement pastes suffer from appearance deterioration to some extent. The compressive strength of cement paste initially increases and after 120 days decreases with increasing exposed period. In addition, the cement paste with short curing time is more susceptible to sulfate attack, which directly leads to the formation of non-binder thaumasite crystal accompanied by the formation of ettringite, gypsum and brucite, and becomes a white, mushy, and incohesive matrix. Additionally, the extent of sulfate attack is greater and the formation of thaumasite is observed earlier for shorter curing time.     

Effect of Al3+ on Tobermorite Crystallinity

The difference between the systems of CaO-SiO2,-H2O and CaO-SiO2-Al(OH)3,-H2O, was studied, especially the effect of Al3+on the crystallinity of tobermorite was focussed. As a result, in the system of CaO-SiO2-H2O, tobermorite formed early, however, withreaction time forward, tobermorite was replaced by xonotlite. So far as CaO-SiO2-Al(OH)3,-H2O was concemed, hydrothermal reactionwas cdried out under the same condition as CaO-SiO2-H2O to study the morphological changes of tobermorite crystals. It was found thatAl3+ accelefated the crystal growth of tobermorite to some extent and was in faVor ofplaty crystals. Moreovef, with the Al3+ content in-creasing in the starting material, the morphology of tobermorite did not change magnificently, but platy crystals became more and moreeminent. As soon as the Al2O3 content was over 15.6%, synthetic mineral greatly changed in smicthe and haled into hibschite differentfrom 1 .1 nm tobermorite. Obviously, xonotlite was not apt to form in the presence of Al3+.     

Effect of curing regime on bonding strength of EVA modified mortar to tile

Ethylene-Vinyl Acetate (EVA) redispersible powder and latex were used to modify mortar. Three kinds of curing regimes: standard curing, high temperature curing and freeze-thaw circle curing were adopted to cure the bonded samples. Bonding strength of EVA modified mortar was tested at 28 days. The development of bonding strengths under all three curing regimes were discussed and compared. The experimental results show that bonding strength increases with the increase of EVA content in mortar. The curing regime used within 28 days is critical according to the bonding strengths values under three curing regimes for different ages. The reasons of that the EVA can improve the bonding strength were analyzed.     

甲酸钙对水泥浆体抗压强度、水化及孔结构的影响

研究了甲酸钙对普通硅酸盐水泥浆体和硅酸盐水泥浆体抗压强度的影响,采用XRD、压汞法和灼烧法,测试了甲酸钙对水泥浆体水化产物组成、孔结构及化学结合水的影响。结果表明：甲酸钙能加速水泥的水化,促进Ca（OH）2的生成,提高水泥浆体的水化程度;甲酸钙能明显改善水泥浆体的孔结构,减少孔隙率和细化孔径;甲酸钙对P·O42．5普通硅酸盐水泥浆体早期强度的提升要比对P·152．5硅酸盐水泥的效果好。     

初凝后水泥浆体的弹性模量研究

在复合材料力学理论与流变学开尔文模型基础上建立了一个用以描述水泥浆体水化早期弹性模量成长的模型.该模型从微观角度出发,在假设水泥颗粒与水化产物为圆球状且水泥浆体形成的孔结构为圆孔形的基础上,应用线弹性理论与有孔介质力学理论研究随着时间的增加、水化度不断增大的情况下,硬化水泥浆体的有效弹性模量.将该有效弹性模量与流变学开尔文模型理论结合起来得出初凝后水泥浆体弹性模量时变方程.研究结果表明:该模型可用于描述水泥浆体初凝后的弹性模量变化,且其变化规律与精度和实验数据符合良好.     

Effect of borax on hydration and hardening properties of magnesium and pottassium phosphate cement pastes

Magnesium and potassium phosphate cement (MKPC) sample were prepared by mixing dead burnt MgO powder, potassium phosphate and different dosages of retarder borax to investigate the effect of borax on its hydration and hardening characteristics. The pH value, fluidity, hydration temperature and strength development of MKPC paste were investigated, and the mineralogical composition and microstructural morphology of its hydration products were analyzed. The experimental results indicated that, within a certain dosage, borax caused an endothermal effect for MKPC paste, which decreased the early hydration rate of MKPC paste, increased the fluidity of MKPC paste. Thus, strength and micro-morphology of hardened MKPC are affected. It can be concluded that borax in MKPC paste retards the early hydration rate of MKPC paste by forming a film onto surface of MgO, decreasing the temperature and increasing the pH value of the system. As borax dosage varying, different factors may dorminate the effects.     

碳化作用下水泥浆内亚硝酸根离子的含量分布

通过电子探针微区分析(EPMA)和X射线衍射分析(XRD)技术探明了碳化对硬化水泥浆内亚硝酸根离子分布的影响,阐明了游离亚硝酸根离子的迁移和吸附规律,有利于精确评价亚硝酸根离子在混凝土中的阻锈作用。结果表明:含亚硝酸盐的水泥浆体水化后生成新的水化产物结晶相NO_2-AFm,并均匀分布于水泥浆内。NO_2-AFm在碳化过程中重新分解生成亚硝酸根离子并向未碳化区扩散,致使碳化区NO_2-含量减少,非碳化区NO_2-含量增加。由于碳化过程中C-S-H凝胶分解,吸附于凝胶表面的游离亚硝酸根离子重新转变成游离态,碳化区游离态亚硝酸根离子含量增多,同时硅元素从碳化区向未碳化区迁移,未碳化区C-S-H凝胶非晶态含量增加,提高了游离态亚硝酸根离子的固化率,使碳化区游离态亚硝酸根离子含量高于未碳化区。     

萘系减水剂/氧化石墨烯复合材料对水泥石微观结构和性能的影响

采用Hummers法和超声剥离制备了氧化石墨烯(GO)纳米片层分散液,再与萘系减水剂(NS)复合得到NS/GO复合材料.研究了NS/GO复合材料对水泥净浆流动性和水泥石抗压耐折强度的影响.研究结果表明,当NS/GO的掺量为4.8g/0.06g(每100g水泥)时,净浆流动度可以达到204mm,所得水泥石的抗压强度为65.4MPa,且耐折强度达到10.6MPa,分别比对照样品提高了29.3%和41.4%.SEM形貌结果表明,GO能够促使水泥水化反应形成致密的片层状结构.这些结果对于提高混凝土的抗压耐折强度,以及抗裂性和耐久性等具有重要的意义.     

Effect of Al in slag blended cement paste under sulfate attack

Unilateral sulfate attack of cementitious materials containing 40% slag with different water to binder ratios was investigated. The results showed that the degradation of slag blended cement pastes was nearly from the corners of paste surface with cracking and spallings, water-to-binder(w/b) ratio made a significant sense to the damage that low w/b ratio led to little weight loss, less cracking and spalling damage and vice versa. Microstructural experimental results demonstrated that in the three different stages of sulfate attack, degradation of pastes was primarily associated with the migration behavior and bonding configuration of aluminum, in the early ages Al was mostly present in C-(A)-S-H, and thus, the damage of pastes hardly appeared while at later ones Al had been largely transferred from C-(A)-S-H into AFt, leading to expansive damage.     

Characteristics of two types of C-S-H gel in hardened complex binder pastes blended with slag

The characteristics of C-S-H gel in hardened Portland cement pastes and complex binder pastes with ground granulated blast furnace slag were investigated with nanoindentation. The composition of C-S-H gel was analysed with SEM-EDS. The obtained results showed that the volume fraction of LD C-S-H gradually reduced and the volume fraction of HD C-S-H increased with the prolongation of hydration age. Most of the C-S-H gel produced at later age was HD C-S-H. The volume fraction of HD C-S-H increased as the fraction of slag in complex binder pastes increased, suggesting that HD C-S-H was mainly in the hydration products of slag. The chemichal compositions of the two types of C-S-H gel were simlar, meaning that formation and transformation of the two types of C-S-H gel were not affected by their Ca/Si ratio.     

已硬化砼中未水化水泥安定性的检测及事故处理

在已硬化砼中水泥安定性不合格的主要原因是水泥熟料矿物中含有过多游离氧化钙或游离氧化镁。在砼结构中主要表现为局部酥松变形、砼开裂、构件崩溃等。在工程中实用的过烧游离氧化钙的检测方法有沸煮法、X射线衍射法和甘油精法。一旦发现施工现场水泥安定性不良，应按情况组织检查、退货、确定事故原因及影响范围，或打掉重浇，加强现场施工的监督和管理。     

膨胀型水泥硬化浆体变形的分形与粘弹性分析——含钙矾石和水镁石膨？…

应用分形学与粘弹性力学对含钙矾石和水镁石膨胀源的水泥硬化浆体的膨胀机理进行了研究与分析。得到在这种膨胀型水泥硬化浆体中的钙矾石和水镁石膨胀随ＳＯ３与ＭｇＯ质量分数和时间的变化规律;揭示了膨胀型水泥硬化浆体的分形行为,其膨胀曲线的分形元服从指数规律;解释了在钙矾石膨胀的基础上水镁石膨胀的非线性叠加现象;解释了来自钙矾石和水镁石的结晶膨胀能不可能全部用作膨胀功,而是部分消耗在变形过程中,部分贮存在硬化     

Influence of fly ash fineness and shape on the porosity and permeability of blended cement pastes

The effects of the fineness and shape of fly ash on the porosity and air permeability of cement pastes were investigated. Pulverized coal combustion (PCC) fly ash and fluidized bed coal combustion (FBC) fly ash classified into three different finenesses were used. River sand with particle size distribution similar to that of fly ash was also used for comparison. Portland cement was replaced with fly ash and ground sand at the dosages of 0, 20wt%, and 40wt%. A water-to-binder ratio (w/b) of 0.35 was used thr...     

Adsorption of superplasticizers in fly ash blended cement pastes and its rheological effects

The adsorption of superplasticizers in fly ash blended cement paste and its rheological effects were investigated.It is shown that the absorption of superplasticizer on portland cement particles is very different from that on fly ash particles.The fly ash particles have smooth surfaces and are negatively charged,so its adsorption capacity is weaker than the portland cement particles.The amount of adsorbed SP in the fly ash blended cement paste depends highly on the replacement proportion of portland cement with fly ash,and to a much less extent on the nature of the fly ash.However,the amount of adsorbed superplasticizer does not correspond well the ζ-potential of the solid particles,due the strong adsorbing capacities of the Portland cement particles.When fly ash replaces portland cement in the paste,the rheological behavior is radically changed,which is closely related to the fineness and density of the ash.The packing and agglomeration of the solid particles are the controlling factors on the rheological parameters of the fresh paste,instead of the amount and type of adsorbed superplasticizer.     

SEM-backscattered electron imaging and image processing for evaluation of unhydrated cement volume fraction in slag blended Portland cement pastes

Proper parameters for image taking and minimum field number for image processing were investigated to evaluate volume fraction of unhydrated cement(UHC) in both neat cement paste and slag blended cement paste. Our research suggested that magnification 250× was sufficient for the two pastes, and accelerating voltage should be set as 15 kV and 20 kV for BSE image taking of neat cement paste and slag blended cement paste respectively; the minimum field number increased while the total imaging area stayed the same as the magnification increased within certain statistical bias.