Effect of Sand Content on Strength and Pore Structure of Cement Mortar

The effects of four sand contents on the compressive, flexural and splitting-tensile strength of cement mortars were evaluated. Moreover, we experimentally investigated the pore structure of cement mortar brought about by changing the sand content and water/cement ratio. The changes in the pore structure were quantified by measuring the porosity and pore size distribution obtained by using mercury intrusion porosimetry（MIP） technique. The test results show that the strengths of cement mortar increase with increasing sand content. It is also suggested that the traditional water/cement ratio law can be applied to cement mortar with different sand contents, provided that a slight modification is introduced. Sand content is an important parameter influencing the pore structure of cement mortar. Moreover, there is a good relationship between the pore structure and strength of cement mortar.

Pozzolanic Activity of Burned Coal Gangue and Its Effects on Structure of Cement Mortar

The pozzolanic activity of coal gangue burned at different burning temperatures was investigated. The burned coal gangue was mixed with portland cement in different proportions （ 20% - 60% ）. The pozzolanic activity of coal gangue burned and the hydration products were examined, the compressive strengths of the pastes of the mixtures were tested, and the mechanism of the reaction was discussed. The experimental results slum, that the coal gangue burned at 750 ℃ has the optimum pozzolanic activity, and the burned coal ganguc with SiO2 and Al2 O3 is in an active form. When the coal gangue burned at 750℃ is mixed into portland cement, the content of calcium hydroxide in paste is significantly reduced, while the contents of hydrated calcium silk.ate and hydrated calcium aluminate are increased accordingly, hence resulting in the improvement of the microstructure of mortar. The compressive strength of cement paste decreases with increasing the content of burncd coal gangue. The decease in strength is small in the range of 20% - 30% coal gangue substitution and significant in 30%- 60% substitution.

Effect of strain rate and water-to-cement ratio on compressive mechanical behavior of cement mortar

Effects of strain rate and water-to-cement ratio on the dynamic compressive mechanical behavior of cement mortar are investigated by split Hopkinson pressure bar(SHPB) tests. 124 specimens are subjected to dynamic uniaxial compressive loadings.Strain rate sensitivity of the materials is measured in terms of failure modes, stress-strain curves, compressive strength, dynamic increase factor(DIF) and critical strain at peak stress. A significant change in the stress-strain response of the materials with each order of magnitude increase in strain rate is clearly seen from test results. The slope of the stress-strain curve after peak value for low water-to-cement ratio is steeper than that of high water-to-cement ratio mortar. The compressive strength increases with increasing strain rate. With increase in strain rate, the dynamic increase factor(DIF) increases. However, this increase in DIF with increase in strain rate does not appear to be a function of the water-to-cement ratio. The critical compressive strain increases with the strain rate.

Mechanical properties of cement mortar in sodium sulfate and sodium chloride solutions

To investigate the mechanical properties of cement mortar in sodium sulfate and sodium chloride solutions, uniaxial compression test and ultrasonic test were performed. Test results show that the relative dynamic elastic modulus, the mass variation,and the compressive strength of cement mortar increase first, and then decrease with increasing erosion time in sodium sulfate and sodium chloride solutions. The relative dynamic elastic moduli and the compressive strengths of cement mortars with water/cement ratios of 0.55 and 0.65 in sodium sulfate solution are lower than those in sodium chloride solution with the same concentration at the420 th day of immersion. The compressive strength of cement mortar with water/cement ratio of 0.65 is more sensitive to strain rate than that with water/cement ratio of 0.55. In addition, the strain-rate sensitivity of compressive strength of cement mortar will increase under attacks of sodium sulfate or sodium chloride solution.

Experimental Comparative and Numerical Predictive Stu-dies on Strength Evaluation of Cement Types: Effect of Specimen Shape and Type of Sand

Quality of cement is evaluated via group of tests. The most important, and close to understanding, is the compressive strength test. Recently, Egyptian standards adopted the European standards EN-196 and EN-197 for specifying and evaluating quality of cements. This was motivated by the large European investments in the local production of cement. The current study represents a comparative investigation, experimental and numerical, of the effect of different parameters on evaluation of compressive strength. Main parameters are shape of specimens and type of sand used for producing tested mortars. Three sets of specimens were made for ten types of cements. First set were 70.6 mm cubes molded according to old standards using single sized sand. Second group were prisms molded from standard sand (CEN sand) according to the recent standards. Third group were prisms molded from local sand sieved and regenerated to simulate same grading of CEN sand. All specimens were cured according to relevant standards and tested at different ages (2,3,7,10 and 28 days). Results show that CEM-I Type of cement does not fulfill, in all of its grades, the strength requirements of Ordinary Portland cement OPC specified in old standards. Also, the use of simulated CEN sand from local source gives strengths lower than those obtained using standard certified CEN sand. A limited number of tests were made on concrete specimens from two most common CEM-I types to investigate effect on concrete strength and results were also reported. Numerical investigation of the effect of specimen shape and type of sand on evaluation of compressive strength of mortar specimens, presented in the current study, applies one of the artificial intelligence techniques to simulate and predict the strength behavior at different ages. The Artificial Neural Network (ANN) technique is introduced in the current study to simulate the strength behavior using the available experimental data and predict the strength value at any age in the range of the experiments or in the future. The results of the numerical study showed that the ANN method with less effort was very efficiently capable of simulating the effect of specimen shape and type of sand on the strength behavior of tested mortar with different cement types.

An Experimental Study of the Physicochemical Properties of a Cement Matrix Containing Dredged Materials

Recently, the amount of dredged soil material (DM) has been rapidly increasing in Korea due to four major river maintenance projects and new harbor construction. DM waste is mostly dumped into the ocean, while only a small part of it has been utilized for coastal reclaiming, or as filling and backfilling material. This study carried out physical and chemical tests to map out a specific plan for utilizing DM in a mortar mixture. The compressive strength tests and microstructure analysis using XRD and SEM of cement mortar contained DM were performed as a replacement for fine aggregate or as a filler material of mortar matrix. The study measured the impact of contaminants contained in DM and how silt and clay influenced the compressive strength of the mortar.

硫酸镁侵蚀作用下高流态水泥砂浆的早期强度劣化特征

高流态早强(HFES)水泥砂浆被广泛应用于土木工程的各个领域。在复杂的工程环境下,HFES水泥砂浆的硫酸盐侵蚀不容忽视。尽管硫酸盐对水泥基材料性能的影响已有一些研究,但HFES水泥砂浆与传统水泥基材料相比有明显不同,其在硫酸盐侵蚀情况下的劣化机制尚不明确。本文研究了不同硫酸镁溶液浓度和侵蚀时间下HFES水泥砂浆的早期抗折和抗压强度,通过扫描电镜(SEM)、X射线衍射(XRD)和差示扫描量热仪(DSC)测试,分析了硫酸镁溶液浓度和侵蚀次数对HFES水泥砂浆微观结构和水化产物的影响,揭示了硫酸镁对HFES水泥砂浆早期强度形成的影响机制。

钢纤维-海泡石纤维优化铁尾矿砂-石英砂混合砂浆性能试验研究

为了在保证经济性的同时改善铁尾矿砂-石英砂混合砂浆的性能,选用钢纤维、海泡石纤维优化混合砂浆,研究了钢纤维、海泡石纤维单掺和以不同比例混掺对混合砂浆工作性能、力学性能和自然干燥收缩率的影响。结果表明,在单掺情况下,海泡石纤维对流动性的负面影响高于钢纤维,两种纤维分别以合适掺量掺入混合砂浆中,钢纤维对抗折强度和28 d自然干燥收缩率的改善效果优于海泡石纤维,而海泡石纤维对抗压强度的改善效果更佳;钢纤维-海泡石纤维以0.50%-1.0%和0.75%-1.0%组合混掺对矿浆力学性能和干燥收缩改善效果最佳。从性能改善效果和经济性角度综合考虑,确定了最优纤维掺量组合,即钢纤维、海泡石纤维掺量分别为0.50%、1.0%。在此掺量组合下,混合砂浆3 d、7 d和28 d抗折强度分别提升了15.8%、13.0%和12.5%,混合砂浆3 d、7 d和28 d抗压强度分别提升了16.2%、14.7%和13.9%,28 d自然干燥收缩率降低了32.8%,且经济性良好,每吨混合砂浆仅增加28.8元左右的成本。

Corrosion Resistance and Physical-Mechanical Properties of Reinforced Mortars with and without Carbon Nanotubes

Following the evolution of currently enforced Performance Based Design standards of reinforced concrete (RC) structures for durability, the designer, rather than complying with given prescriptive limits, may instead specify a cementitious mix design that is proven to exhibit a code prescribed resistance level (class) to a given exposure environment. Such compliance will lead to the protection of the steel reinforcement from corrosion and the cementitious mortar from degradation, during the design lifespan of the structure, under aggressive environmental exposure conditions such as, marine or deicing salts and carbonation. In this context, the enhancement of the physical and durability properties of common cement-based mortars under chloride exposure are experimentally investigated herein. In particular, the experimental program reported herein aims to evaluate the influence of incorporating multi-walled carbon nanotubes on the physical and mechanical properties of reinforced mortars against chloride ions. Furthermore, the anticorrosion protection of cementitious composites prepared with nanomaterials at 0.2% w/w is further investigated, by comparing all test results against reference specimens prepared without any additive. Electrochemical (Half-cell potential, corrosion current) and mass loss of reinforcement steel measurements were performed, while the porosity, capillary absorption and flexural strength were measured to evaluate the mechanical and durability characteristics of the mortars, following a period of exposure of eleven months;SEM images coupled with EDX analysis were further recorded and used for microstructure observation. The test results indicate that the inclusion of the nanomaterials in the mix improved the durability of the mortar specimens, while the nano-modified composites exhibited higher chloride penetration resistance and flexural strength than the corresponding values of the reference mortars. The test results and the comparison between nanomodified and reference mortars showed that the use of CNTs as addition led to protection of steel reinforcing bars against pitting corrosion and a significant improvement in flexural strength and porosity of the mortars.

粗糙度对磷酸钾镁水泥砂浆-混凝土界面剪切特性的影响

为研究粗糙度对磷酸钾镁水泥砂浆-混凝土黏结界面剪切强度与变形特性的影响,对磷酸钾镁水泥砂浆-混凝土试样开展了不同界面粗糙度和不同法向应力下的直剪试验,探讨了黏结界面的峰值抗剪强度、残余抗剪强度、剪切扩容、破坏模式以及强度参数等剪切力学特性。结果表明:粗糙度和法向应力决定了黏结界面的剪切变形特性和剪切破坏模式,当粗糙度大于0时,界面峰值抗剪强度与粗糙度呈二次关系,与法向应力呈线性相关;磷酸钾镁水泥砂浆-混凝土黏结界面剪切破坏体现为脆性破坏,当粗糙度大于0且法向应力大于1 MPa时,局部剪切破坏面发生在磷酸钾镁水泥砂浆内部,在随后的剪切过程中发生爬坡效应,并出现明显的剪胀现象。磷酸钾镁水泥砂浆-混凝土界面的黏聚力为6.5~7.5 MPa,内摩擦角为44.9°~49.0°,且随着粗糙度的增大,黏聚力和内摩擦角均呈先增大后减小的趋势。研究结果可为损伤混凝土的快速修补加固提供理论基础和技术支撑。

玄武岩纤维水泥胶砂力学性能试验研究

为了研究长度为12 mm的玄武岩纤维对改善水泥胶砂力学性能效果,通过室内试验和微观结构扫描相结合的方法开展不同期龄玄武岩纤维水泥胶砂试件的抗折强度、抗压强度以及微观结构测试研究,结果表明:当玄武岩纤维掺量低于2%时,拌和物的和易性和泌水性较好,试件收面容易;当玄武岩纤维掺量为0.31%时,试件密度平均值最大,此时砂浆和纤维连接最为紧密;掺少量(0.31%)的玄武岩纤维在一定程度上有助于提高水泥胶砂试件的早期抗折强度,掺量过多会使试件整体结构松散,导致抗折强度和抗压强度有所降低。

热活化煤矸石对水泥基材料性能影响研究

将热活化煤矸石用作辅助胶凝材料,替代部分水泥,制备水泥砂浆,研究热活化煤矸石掺量对水泥砂浆抗压强度和抗折强度的影响,及热活化煤矸石增强水泥基材料机械强度的机理。结果表明,在利用热活化煤矸石替代部分水泥制备砂浆过程中,适量热活化煤矸石具有良好的火山灰效应和微集料效应,可促进砂浆基体中相互交联网状C—S—H凝胶的形成,增强砂浆基体中孔隙的填充效果,改善砂浆基体的微观结构,进而提升水泥基材料机械强度。当热活化煤矸石掺入量为20%时,砂浆试样水化28 d的抗压强度可达46.2 MPa,抗折强度达6.79 MPa,其机械强度与纯水泥砂浆相当,完全可以满足一般建筑行业使用要求。

环境荷载对CRTS Ⅰ型CA砂浆抗压强度与弹性模量定量关系的影响

抗压强度和弹性模量作为材料力学特性的表现,两者存在必然联系。为研究CRTSⅠ型水泥乳化沥青砂浆(CA砂浆)抗压强度与弹性模量内在关系及环境荷载的影响,利用电子万能试验机对CRTSⅠ型CA砂浆进行应力-应变压缩实验,建立CRTSⅠ型CA砂浆抗压强度和弹性模量的定量关系,分析龄期、浸水时间和干湿循环次数对CRTSⅠ型CA砂浆强度和弹性模量定量关系的影响。结果表明:单轴应力条件下CRTSⅠ型CA砂浆具有明显的速率效应,且随着加载速率的增大,CRTSⅠ型CA砂浆的弹性模量和抗压强度均增大,当加载速率从0.03 mm/min增加至30 mm/min时,CA砂浆的抗压强度和弹性模量分别增大1.96倍和1.47倍;CRTSⅠ型CA砂浆的力学性能受环境荷载影响较大,CRTSⅠ型CA砂浆的抗压强度和弹性模量随着浸水时间和干湿循环次数的增大而降低;CRTSⅠ型CA砂浆的弹性模量随抗压强度的增大而增大;不同工况下,CRTSⅠ型CA砂浆的弹性模量均随抗压强度的增大而增大,两者呈线性关系,拟合优度均在0.97以上;CRTSⅠ型CA砂浆的弹性模量与强度由其组成及微观特征决定,CRTSⅠ型CA砂浆的龄期、浸水时间和干湿循环次数对这种内在关系基本无影响。

不同类型聚合物对水泥砂浆粘接性能的影响研究

研究了丙烯酸酯聚合物(AC)、聚乙烯醇(PVA)、丁苯橡胶(SBR)和乙烯醋酸乙烯酯(EVA)对聚合物改性水泥砂浆(PCM)力学性能的影响。使用了一种聚合物剂量(3%),并将该剂量对相变材料(PCM)的影响与含有0%聚合物的对照砂浆混合物进行了比较,测量了抗压强度和粘接强度,分析了聚合物的添加对改善聚合物改性水泥砂浆粘接性能的影响。结果表明,含有EVA的相变材料表现出最好的性能,28 d后抗压强度提高了33%。相比之下,AC、PVA和SBR在28 d后粘接强度的提高较小。含有EVA的PCM在7 d时表现出最高的粘接强度;28 d后,粘接强度最高的是含有SBR的PCM,高出对照砂浆约29%。

周期性温变作用下水泥砂浆相对含冰量变化及其与抗压强度的相关性

为研究孔隙相对含冰量影响下的水泥砂浆力学性能演化规律,基于周期性温变作用下水泥砂浆的电阻率测试结果,计算并分析得到水泥砂浆相对含冰量时程变化规律,并利用PPR建模技术建立了不同相对含冰量影响下的相对抗压强度计算模型。结果表明:周期性温变作用下,水泥砂浆电阻率与相对含冰量呈现较强的相关性,建立的计算模型能较好地预测周期性温变作用下相对抗压强度随水泥砂浆的水胶比、饱和度、循环次数、最大相对含冰量的变化。

煤矸石与气化渣协同作为水泥掺合料的性能影响研究

煤矸石和气化渣是煤炭工业利用过程的副产品,堆存和填埋处置对环境已造成严重的污染和危害。为实现固体废物的资源化利用,基于对煤矸石碱溶液离子快速溶出评价、气化粗渣活性分析等样品分析,采用净浆流动性、胶砂试件的抗压强度与抗折强度、SEM图形分析等多种活性评价方法以确定煤矸石最佳活化温度,即采取胶砂强度试验对煤矸石及气化渣协同作为水泥掺合料对胶砂试件的性能影响规律进行研究。结果表明煤矸石的最佳活化温度为750℃,将活化后煤矸石与机械活化后的气化粗渣进行互配,以30%掺量替代P.O 42.5水泥,得出相较于活化煤矸石单一掺入水泥,煤矸石及气化粗渣协同作为水泥掺和料不但能提高复合胶凝材料的力学性能且对节能减排放起到良好作用。

超低温环境下双掺粉煤灰及硅灰对砂浆力学性能的影响

研究了粉煤灰、硅灰2种矿物掺合料双掺在不同掺量下对超低温冻融循环(-190~20℃)后水泥砂浆抗压、抗折强度和抗冻性的影响规律。结果表明:在超低温冻融循环后,对水泥砂浆性能的劣化程度远大于低温冻融循环(-20~20℃);当粉煤灰掺量15%、硅灰掺量10%时,水泥砂浆力学性能最优,10次超低温冻融循环后,抗折、抗压强度分别达到10.06、38.29 MPa;此时水泥砂浆强度的损失率最低,抗冻性最好;孔径达到最小为33.15 nm,孔隙率也是最小,为13.58%。

压滤后的混凝土废浆干粉对水泥砂浆性能的影响

研究了CSW干粉内外掺入对砂浆稠度、表观密度、保水率及3 d、7 d、28 d抗压强度的影响,并使用多元线性回归方法分析了各龄期抗压强度与CSW干粉掺量、表观密度之间的相关性。结果表明:CSW干粉内掺时砂浆稠度、表观密度及各龄期抗压强度与掺量呈反相关,保水率与掺量呈正相关;CSW干粉外掺时,除保水率与掺量呈正相关外,掺量对各指标的影响趋势不明显。通过多元线性回归分析得出CSW干粉内掺时抗压强度、掺量、表观密度之间具有良好的相关性;CSW外掺时3 d抗压强度、28 d抗压强度、掺量、表观密度之间具有良好的相关性,但7 d龄期的各指标之间的相关性较差。

纳米SiO_(2)-水泥复合胶凝材料早期水化动力学研究

采用纳米SiO_(2)(NS)部分取代水泥的方式,对比研究NS对于NS-水泥复合胶凝体系的初终凝时间、水泥胶砂抗压强度、抗折强度和水化热等的影响。使用水化热分析NS-水泥复合胶凝体系的水化特性,并基于Krstulović-Dabić水化动力学模型,分析NS-水泥复合胶凝体系的水化反应机理。结果表明:NS取代水泥掺入后,使水泥的初终凝时间提前,提高了水泥胶砂不同龄期的抗压强度,但会降低水泥胶砂的流动度;NS促进NS-水泥复合胶凝体系水化反应的诱导前期、诱导期、加速期和减速期的反应速率,并在减速期出现第三个放热峰,使得加速期和减速期提前结束;NS显著提高结晶成核和晶体生长(NG)、相边界反应(I)和扩散阶段(D)的反应程度,降低I阶段在整个水化进程的占比,加快I阶段向D阶段转变,使得浆体内部孔隙率和渗透率降低,水化反应阻力增加,稳定期提前。

稻壳灰煅烧温度对其改性水泥砂浆性能的影响

为探究不同煅烧温度下制备的稻壳灰对水泥砂浆性能的影响,借助X射线荧光光谱分析仪(XRF)与X射线衍射分析仪(XRD)测定了不同煅烧温度下稻壳灰中无定形SiO_(2)含量,通过稻壳灰的扫描电镜(SEM)图像分析其微观结构。将制得的稻壳灰代替水泥掺入到水泥砂浆中,通过水泥砂浆力学性能实验与抗渗性实验测定稻壳灰改性水泥砂浆的力学性能与抗渗性能;结合稻壳灰的微观测试结果,分析稻壳灰煅烧温度对水泥砂浆性能的影响机理。研究结果显示:当煅烧温度超过600℃时,稻壳灰中无定形SiO_(2)向结晶相转变;随着稻壳煅烧温度的提高,稻壳灰改性水泥砂浆早期强度与抗渗性能逐渐降低,但对其后期强度没有显著影响;稻壳灰的最佳煅烧温度为600℃。