Effect of silica fume on the thaumasite form of sulfate attack on cement-based materials

Thaumasite form of sulfate attack(TSA) can be observed in cement containing limestone under sulfate condition at low temperature. Mixing with suitable mineral admixture could be a good choice to improve the TSA resistance performance of cement-based materials. We investigated the durability performance of limestone-cement mortars reinforced with silica fume(SF) in 5% MgSO_4 solution at 5 ℃. The mortars, which were immersed in aggressive condition, were prepared with 2%, 4%, 6%, 8%, and 10% cement replacement by SF at a fixed water-to-binder ratio. Appearance, compressive strength, change of length and mass and corrosion products were investigated to evaluate the TSA resistance performance of SF based specimens. The results showed that specimens in the absence of SF almost disintegrated. Increasing SF dosage can reduce the degree of deterioration of SF mortars in TSA environment. Mortar mixtures with more than 6% SF merely show slight degeneration in relation to macroscopic and microscopic tests and characterizations.     

外加剂对混凝土TSA腐蚀的抑制作用

研究了掺加化学外加剂的水泥净浆试件在（5±2）℃、质量分数5%的硫酸镁溶液中浸泡300d的外观变化、强度发展及侵蚀产物,采用X射线衍射（XRD）、傅立叶转换红外光谱（FTIR）等方法分析了侵蚀产物的组成。结果表明：甲酸钙能延缓水泥石发生TSA腐蚀,但不能阻止TSA发生,在0.3%-0.7%范围内,随着甲酸钙掺量的增加,水泥石抗TSA腐蚀的能力增强。硝酸钡能有效抑制水泥石中碳硫硅钙石的生成,原因是硝酸钡能结合生成碳硫硅钙石所需的SO42-,从而降低了水泥石发生TSA腐蚀的程度。硝酸钡掺量在1%左右时,水泥石抗TSA性能较好。     

Effect of limestone powder and fly ash on magnesium sulfate resistance of mortar

The effect of limestone powder and fly ash on magnesium sulfate resistance of mortar was studied by testing on the strength, expansion and hydration products of the specimens stored in MgSO₄ solution at certain periods. The experimental results show that the strength of mortar stored in MgSO₄ solution increases a little before 28 d, but decreases fast subsequently. The more the contents of limestone powder and fly ash, the less the strength losses. Mortar swells in the MgSO₄ solution with the soaking time. And the more the contents of limestone powder and fly ash, the less the expansion rate is. The expansion or strength loss of mortars results from the expansion of gypsum, as well as the loss of Ca(OH)₂ and other hydration products of cement. The magnesium sulfate resistance of the mortars containing limestone powder and fly ash is high.     

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.     

Effects of Thaumasite Formation on the Performance of Portland-limestone Concrete Stored in Magnesium Sulfate Solution

The influence of theaumasite formation on the performance of Portland-limestone cement concrete stored in magnesium sulfate solution was studied.The experimental results show that the deterioration of Portlandlimestone cement concrete is higher than that of Portland cement concrete.The more the content of limestone,the more serious the deterioration of concrete,and also the lower the temperature,the earlier the deterioration of concrete.Thaumasite was detected to form in the Portland-limestone pastes when stored in 10wt% MgSO4 solution at 3-10℃ and is was easy to form at lower temperatures.     

早期养护条件对水泥石碳硫硅钙石型硫酸盐腐蚀的影响

研究了早期高湿度空气养护、水中标准养护及密封养护对水泥石碳硫硅钙石型硫酸盐腐蚀的影响。采用X射线衍射（XRD）、傅立叶转换红外光谱（FTIR）和扫描电子显微镜（SEM）等方法分析了水泥石经180 d硫酸盐侵蚀后的腐蚀产物。结果表明,早期高湿度空气养护,水泥石碳化生成的CaCO3填充于孔隙中,使表层结构更加致密,阻碍了SO4^2-等有害离子侵入,延缓了水泥石碳硫硅钙石型硫酸盐腐蚀过程。比较而言,早期密封养护,水泥水化不充分,水泥石内部缺陷相对较多,SO4^2-等有害离子容易侵入,水泥石碳硫硅钙石型硫酸盐腐蚀最为严重。     

碱激发胶凝材料抗硫酸盐侵蚀机理的探讨

对碱-矿渣、碱-粉煤灰、碱-偏高岭土和普通硅酸盐水泥的抗硫酸盐侵蚀机理进行了探讨,其中用普通硅酸盐水泥作为对比样.采用实验室加速试验法(即干湿循环法),并对侵蚀后的碱激发胶凝材料的产物进行X射线衍射(XRD)分析.结果表明:在5%的硫酸钠溶液中,碱激发胶凝材料砂浆主要是由于环境中硫酸钠溶液浸入试块孔隙中,使盐结晶产生体积膨胀而破坏,而普通硅酸盐水泥砂浆主要是由于生成膨胀性产物石膏和硫酸钠盐结晶而破坏.     

Application of image analysis based on SEM and chemical mapping on PC mortars under sulfate attack

The degradation mechanisms of cementitious materials exposed to sulfate solutions have been controversial, despite considerable research. In this paper, two methodologies of image analysis based on scanning electron microscope and chemical mapping are used to analyse Portland cement mortars exposed to sodium sulfate solution. The effects of sulfate concentration in solution and water to cement ratio of mortar, which are considered as the most sensitive factors to sulfate attack, are investigated respectively by comparing the macro expansion with microstructure analysis. It is found that the sulfate concentration in pore solution, expressed as sulfate content in C-S-H, plays a critical role on the supersaturation with respect to ettringite and so on the expansion force generated.     

Properties and Acceleration Mechanism of Cement Mortar Added with Low Alkaline Liquid State Setting Accelerator

Low alkaline liquid state setting accelerator(LSA) for Portland cement was prepared in laboratory from aqueous solution of several inorganic sulfate salts and some organic chemical substances. Properties of cement with addition of LSA relating to its setting time and strength development as well as its resistance to sulfate attack for short and long term exposure were experimentally examined. The experimental results showed that 5%-7% addition of LSA significantly accelerated the initial and final setting of Portland cement in the presence or absence of the blending of mineral admixtures, the initial and final setting time being less than 3 min and 6 min respectively. Meanwhile, the early 1 day curing age compressive strength increased remarkably by 20%, while the late 28 th day curing age compressive strength remained almost unchanged as compared with that of the reference accelerator free cement mortar specimen. Furthermore, mortar specimens of cement added with LSA and exposed to 5% Na2SO4 solution showed their excellent resistance to sulfate attack, with their short and long term curing age resistance coefficient to sulfate attack being around 1.04 to 1.17, all larger than 1.0. XRD analysis on hardened cement paste specimens at very early curing ages of several minutes disclosed the existence of more ettringite in specimens added with LSA than that of the reference specimens, meanwhile SEM observation also revealed the existence of well crystallized ettringite at very early hydration stage, suggesting that the accelerated setting of Portland cement can be attributed to the early and rapid formation of ettringite over the whole cement paste matrix due to the introduction of LSA. MIP measurement revealed that hardened cement paste specimens with the addition of LSA presented less medium diameter pores, more proportion of small pores and less proportion of large capillary pores, which is in a very good coincidence with the improvement of strength development of cement mortars added with LSA.     

Deterioration mechanism of sulfate attack on concrete under freeze-thaw cycles

The experiments of concrete attacked by sulfate solution under freeze-thaw cycles were investigated. The sulfate solution includes two types of 5% Na2SO4 and 5% MgSO4. Through the experiment, microstructural analyses such as SEM, XRD and TGA measurements were performed on the selected samples after freeze-thaw cycles. The corrosion products of the concrete were distinguished and quantitatively compared by the thermal analysis. Besides, the damage mechanism considering the dynamic modulus of elastically of concrete under the coupling effect was also investigated. The experimental results show that, under the action of freeze-thaw cycles and sulfate attack, the main attack products in concrete are ettringite and gypsum. The corrosion products exposed to MgSO4 solution are more than those to Na2SO4 solution. Furthermore, the content of gypsum in concrete is less than that of ettringite in test, and some of gypsum can be observed only after a certain corrosion extent. It is also shown that MgSO4 solution has a promoting effect to the damage of concrete under freeze-thaw cycles. Whereas for Na：SO4 solution, the damage of concrete has restrained before 300 freeze-thaw cycles, but the sulfate attack accelerates the deterioration process in its further test period.     

碱激发矿渣陶粒混凝土空心砌块砌体抗剪试验

为研究碱激发矿渣陶粒混凝土空心砌块砌体的受剪性能,对108个用Mb25~Mb130碱激发矿渣净浆和用Mb25~Mb80碱激发矿渣陶砂砂浆砌筑的碱激发矿渣陶粒混凝土空心砌块砌体进行抗剪试验.试验结果表明:碱激发矿渣净浆作砌筑浆体的砌体抗剪强度低于碱激发矿渣陶砂砂浆作砌筑浆体的砌体.碱激发矿渣陶砂砂浆作砌筑浆体的砌体抗剪强度低于用水泥砂浆和混合砂浆作砌筑浆体的砌体.砌体的抗剪强度随砌筑浆体抗压强度的提高而增大,Na2O含量、砂灰比对砌体抗剪强度的影响不容忽视.基于试验结果,分别建立了用碱激发矿渣净浆和碱激发矿渣陶砂砂浆作砌筑浆体时的碱激发矿渣陶粒混凝土空心砌块砌体抗剪强度计算公式.     

川藏铁路石灰石粉–水泥基材料抗低温硫酸盐侵蚀研究

为研究川藏铁路掺石灰石粉混凝土抗低温硫酸盐侵蚀性能,通过对低温硫酸盐溶液浸泡侵蚀和电脉冲低温硫酸盐侵蚀作用下掺不同石灰石粉的水泥基材料的外观侵蚀变化等级、质量评价参数、抗压强度评价参数、孔隙特征以及侵蚀产物的测试,研究了低温环境下石灰石粉掺量对水泥基材料硫酸盐侵蚀行为的影响规律,并建立非单调Wiener随机过程模型对低温环境下石灰石粉-水泥基材料硫酸盐侵蚀服役寿命进行了预测。结果表明:（1）低温环境下硫酸盐浸泡侵蚀反应缓慢,侵蚀周期内,石灰石粉-水泥基试件外观未出现可见剥落、质量持续增加;强度先提高后降低,在侵蚀120d时出现转折;孔隙先细化后劣化,在侵蚀90d时出现转折;在侵蚀180d时有少量碳硫硅钙石生成。（2）电脉冲加速了低温环境下石灰石粉-水泥基硫酸盐侵蚀反应,电脉冲低温硫酸盐侵蚀60d时,试件表面开始剥落,质量和强度下降,内部孔隙劣化。石灰石粉掺量越多,碳硫硅钙石侵蚀破坏就越严重,石灰石粉掺量为40%的试件在侵蚀60d时就有碳硫硅钙石生成,而石灰石粉掺量为10%、20%的试件在侵蚀90d才生成碳硫硅钙石。（3）基于Wiener随机过程的可靠性寿命退化分析表明,电脉冲低温硫酸盐侵蚀作用下掺10%、20%和40%石灰石粉-水泥基材料室内加速侵蚀寿命分别为265d、130d及105d。     

Diffusion characteristic of sulfate anion in surface region of cement mortar at early stage

The SO_4~(2-)concentration distributions in surface region of cement mortar immersed in sulfate solution at early stage were measured by layered sampling method combined with chemical analysis, and the diffusion coefficients of SO_4~(2-)anions in surface region of mortar into internal area were calculated by means of instantaneous plane diffusion theory. The experimental results showed that the SO_4~(2-)concentration gradually reduced when the diffusion depth increased in the surface region of mortar. Diffusion coefficient(D) was relevant with the concentration and kind of environmental sulfate solution, which reduced with immersion time at the beginning, and then rose slowly after a period of time. The calculation of initial diffusion coefficient(D_0) and starting time of deterioration(t_∞) caused by sulfate attack was further attempted based on the data of diffusion coefficient, and it was found that D_0 and t∞ were all relevant with concentrations of sulfate and different kind of sulfate as well.     

Influences of polypropylene fiber and SBR polymer latex on abrasion resistance of cement mortar

Influences of polypropylene (PP) fiber and styrene-butadiene rubber (SBR) polymer latex on the strength performance, abrasion resistance of cement mortar were studied. The experimental results show that the flexural strength, brittleness index (σ_F/σ_C) and abrasion resistance can be improved significantly by the addition of PP fiber and SBR polymer latex. The relationship between the flexural strength and abrasion resistance was analyzed, showing a good linear relationship between them. The reinforced mechanism of PP fiber and SBR polymer latex on cement mortar was analyzed by some microscopic tests. The test results show that the addition of SBR polymer latex has no significant influence on the cement hydration after 7 d curing. Adding SBR polymer latex into cement mortar can form a polymer transition layer in the interfaces of PP fiber and cement hydrates, which improves the bonding properties between the PP fiber and cement mortar matrix effectively.     

Synthesis and Application of Sodium-carboxymethylcellulose Type Superplasticizer in Cement Mortars

The polymeric admixture, the sodium-carboxymethylcellulose(CMC)/poly sodium p-styrene sulfonate(PSS)/poly vinyl acetate(PVAc) was synthesized and applied in cement mortars. The polymer was tested by FTIR and SEM, and the results indicate that the ideal molecular structure is synthesized. The effect of addition amount of polymeric admixture and water-to-cement ratio on mechanical properties of cement mortars was studied. The polymer-modified mortars under the optimum water cement ratio and optimum polymer cement ratio, the flexural strength of polymer-modified mortars are 1.45, 1.21, and 1.17 times higher than the plain cement mortar at age of 3, 7, and 28 d, respectively. The compressive strength of polymer-modified mortars at age of 3, 7, and 28 d are 1.55, 1.40, and 1.2941 times higher than that of the plain cement mortar,respectively. Scanning electron microscope(SEM), FTIR and TG were used to analyze the effect of polymer emulsion on cement hydration reaction. The results show that the polymer emulsion can promote the hydration reaction of cement.     

PB-g-PS胶乳改性水泥砂浆的性能研究

PB-g-PS胶乳常用于改性水泥砂浆,聚丁二烯对苯乙烯的重量比的范围从70/30,50/50到30/70.保持水灰比为0.5的常数,在20°、相对湿度为90%的空气中养护24 h,接下来在20°的水中养护27 d.这项研究考察了聚丁二烯与苯乙烯的重量比和聚灰比对PB-g-PS胶乳改性水泥砂浆的流动度,抗压、抗折强度及吸...     

Effect of corrosive solutions on C-S-H microstructure in portland cement paste with fly ash

By means of ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) combined with deconvolution technique, X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as energy dispersive X-ray system(EDX), the effect of 5 wt% corrosive solutions (viz. 5 wt% Na₂SO₄, MgSO₄, Na2SO4+NaCl and Na₂SO₄+NaCl+Na₂CO₃) on C-S-H microstructure in Portland cement containing 30 wt% fly ash was investigated.The results show that, in MgSO₄ solution, Mg²⁺ promotes the decalcification of C-S-H by SO ₄ ^2- ,increasing silicate tetrahedra polymerization and mean chain length (MCL) of C-S-H. However, the substituting degree of Al³⁺ for Si⁴⁺ (Al[4]/Si) in the paste does not change evidently. Effect of Na²SO⁴ solution on C-S-H is not significantly influenced by NaCl solution, while the MCL and Al[4]/Si of C-S-H in fly ashcement paste slightly change. However, the decalcification of C-S-H by SO ₄ ^2- and CO ₃ ^2- attack, as well as the activation of fly ash by SO ₄ ^2- attack will increase the MCL and Al[4]/Si, which are both higher than that under Na₂SO₄ corrosion, MgSO₄ or Na₂SO₄ +NaCl coordination corrosion.     

Properties of cementitious rendering mortar prepared with recycled fine aggregates

The results of an experimental study on investigating the properties of cementitious rendering mortars prepared with a recycled fine aggregate (RA) were presented. The RA was obtained from a recycling plant in which mixed demolition wastes were processed by mechanical crushing, sieving and sorting operations. Two series of rendering mortar mixes were prepared with a constant water/cement and a constant aggregate/cement ratios of 0.55 and 3 respectively. River sand and natural crushed rock fine were originally used in the two series separately, and they were consistently replaced by 25%, 50%, 75% and 100% by the recycled aggregate. The experimental results showed that mechanical properties, such as compressive strength, flexural strength and modulus of elasticity of the mortars prepared with the RA were lower than the mortars made with the natural aggregates. Nevertheless, the bond strength at the interface between the mortar and masonry bricks determined by the Triplet test was found to be higher for the mortars prepared with the RA.     

碱—熟料—矿渣水泥混凝土的碱集料反应研究

采用砂浆测长法研究了碱－熟料－矿渣水泥砂浆的碱集料反应膨胀率;分析了碱组分种类及熟料掺量等因素对该类砂浆碱集料反应膨胀率的影响;综合砂浆测长结果及ＳＥＭ分析,探讨了碱－熟料矿渣水泥系统碱集料反应的特性。     

Effects of carbon nanotubes on mechanical and 2D-3D microstructure properties of cement mortar

To study the influence of multi-wall carbon nanotubes （MWCNTs） on the mechanical and microstructural properties of cementitious composites, 0.00, 0.02, 0.08, 0.10, and 0.20 wt% of multi-wall carbon nanotubes were added into cement mortar, in which the cement-sand ratio was 1：1.5. The flexural and compressive strengths of cement mortar at the age of 3, 7, 28 and 90 d and the fracture performance at the age of 28 d were determined, its 2D micrograph was tested by means of SEM, and the 3D defects distribution was firstly determined with or without CNTs by means of XCT （X-ray computerized tomography）. The results showed that 0.08 wt% of CNTs improved the compressive strength and flexural strength by 18% and 19~A, respectively, and a significant improvement of its fracture property was observed. Moreover lower addition of carbon nanotubes to cement mortars can improve its microstructure and decrease the defects significantly compared to the cement mortar without CNTs. With the increase of the content of CNTs, the mechanical properties of cement mortars presented to be declined largely due to the agglomeration of CNTs.