An Application of Computer‐Aided Molecular Design (CAMD) Using the Signature Molecular Descriptor–Part 2. Evaluating Newly Identified Surface Tension‐Reducing Substances for Potential Use as Shrinkage‐Reducing Admixtures

In this study, the use of computer‐aided molecular design (CAMD) is validated as a tool for enabling the discovery of new shrinkage‐reducing compounds for possible use in portland cement composites and is framed as one of many multiscale modeling tools in a broad hierarchy of possibilities. Twelve additives were tested for their ability to inhibit shrinkage in Type I ordinary portland cement under both autogenous and drying conditions. The 12 additives included two commercial shrinkage‐reducing admixtures (SRAs), two active ingredients of a commercial admixture [one of which was used to establish the quantitative structure–property relationships (QSPR)], two additional classified as potential SRA compounds based on the patent literature, four newly identified compounds predicted by using CAMD and an inverse quantitative structure–property relationship (I‐QSPR), and two other compounds use to establish the QSPR relationship. The newly identified I‐QSPR compounds were targeted for their ability to reduce the surface tension of water, a primary consideration for shrinkage‐reducing activity. Results for both drying shrinkage and autogenous shrinkage indicate that the designed compounds perform similar to commercial admixtures, yet have different chemical functionalities. Hydration data and set measurements were also considered since selection of new SRAs is a multiparameter problem with many constraints. Thus, these newly identified shrinkage‐reducing compounds can potentially provide additional options for use in portland cement concrete applications.     

Influence of shrinkage-reducing admixtures on the reduction of plastic shrinkage cracking in concrete

Fresh concrete exposed to high evaporation rates is prone to plastic shrinkage cracking, especially in structures with large surface area/volume ratios. The present work shows that the reduction of the surface tension of the mixing water is an effective way for decreasing such cracking. In this study, conventional and high strength concretes with superplasticizers and shrinkage reducing admixtures (SRAs) were exposed to drying in the plastic state. Continuous monitoring of the surface displacement facilitated the identification of the different stages of plastic shrinkage cracking. Measurements of capillary pressure, settlement, internal temperature and evaporation rate were also made. The results show the effectiveness of SRAs in reducing plastic shrinkage cracking, even in high strength concrete. This is attributed to the reduction in the evaporation rate, delay of the peak capillary pressure due to the development of menisci in the pores and lower settlement.     

Effect of Chloride‐Based Accelerator in the Presence of Water‐Reducing and Retarding Admixture on Autogenous Shrinkage

Rapid repair concrete mixtures commonly used for full‐depth concrete pavement repair sections can use large dosages of accelerating admixtures to increase strength gain rates and decrease the time to traffic opening. Most often, these mixtures also contain water‐reducing and retarding admixtures (WRRAs) to allow for the use of a low water–cementitious material ratio in order to meet strength requirements. The use of large dosages of accelerating admixtures in combination with retarding admixtures could have significant side effects on concrete. Autogenous shrinkage of low water–cementitious concrete can contribute to high tensile stresses and cracking problems. The effect of calcium chloride‐based accelerating admixture dosage, when used with WRRAs, on autogenous shrinkage was measured. It was found that the inclusion of calcium chloride‐containing accelerating admixtures has a nonlinear effect on the pore size distribution and consequently a nonlinear increase on the autogenous shrinkage.     

Behavior of organic compounds in the pore solution of cement—experimental and molecular dynamics study

This study summarizes molecular dynamics (MD) simulations and experiments using four ester‐based cement shrinkage reducing agents in pure water and simulated cement pore solution. The esters were found to completely dissociate in the alkaline pore solution to form acid and alcohol analogue compounds. The alcohol analogues were further found to be entirely responsible for the surface tension reduction capacity of the parent compounds. In addition, the surface tension reduction capacity of the alcohol analogues are not affected by relevant counterions present in the pore solution. The MD‐based analysis show that water molecules form stable hydrogen bonds with head groups of the alcohol analogues and that relevant dissolved ions, K⁺ and Na⁺, have virtually no effect on hydrogen bond stability. Furthermore, the higher the self‐diffusion coefficient of water molecules in the neighborhood of the head groups the lower the resulting surface tension at the air‐water interface.     

Generic mathematical programming formulation and solution for computer-aided molecular design

This short communication presents a generic mathematical programming formulation for computer-aided molecular design (CAMD). A given CAMD problem, based on target properties, is formulated as a mixed integer linear/non-linear program (MILP/MINLP). The mathematical programming model presented here, which is formulated as an MILP/MINLP problem, considers first-order and second-order molecular groups for molecular structure representation and property estimation. It is shown that various CAMD problems can be formulated and solved through this model.     

减缩型聚羧酸减水剂提高混凝土早期抗裂性的作用研究

减缩型聚羧酸减水剂(SRPC)是一类兼具减缩与减水双重作用的新型混凝土外加剂,它对混凝土早期抗裂性的作用将影响其在工程中的应用。本文利用平板抗裂试验对比测试了掺SRPC、聚羧酸减水剂(PCE)及小分子减缩剂(SRA)的混凝土早期抗裂性,并通过对表面张力、收缩率、孔结构、水化热等参数的分析,讨论了SRPC影响混凝土抗裂性的作用机理。试验结果表明,尽管SRPC在掺量0.15%(质量分数)下的减缩效果弱于掺量1.5%(质量分数)的SRA,但SRPC表现出更佳的早期抗裂性能。机理分析表明,SRPC除了与SRA都是通过降低孔溶液表面张力及改变毛细孔结构从而减少混凝土的收缩外,还通过延缓水泥水化速率、降低早期水化放热量、抑制混凝土中水分蒸发等作用,改善混凝土的早期抗裂性。     

基团贡献法分子设计研究的进展

利用基团贡献法可预测化合物的性质,还能用于化合物的计算机辅助分子设计(CAMD).本文论述了基于基团贡献法CAMD的基本原理,以及在溶剂和聚合物等领域分子设计的应用,对分子设计的计算方法也作了简单的介绍.随着绿色溶剂和新型聚合物材料需求的增加,基团贡献法CAMD将大有应用前景.     

Interactions between shrinkage reducing admixtures (SRA) and cement paste's pore solution

Shrinkage reducing admixtures (SRA) have been developed to combat shrinkage cracking in concrete elements. While SRA has been shown to have significant benefits in reducing the magnitude of drying and autogenous shrinkage, it has been reported that SRA may cause a negative side effect as it reduces the rate of cement hydration and strength development in concrete. To examine the influence of SRA on cement hydration, this study explores the interactions between SRA and cement paste's pore solution. It is described that SRA is mainly composed of amphiphilic (i.e., surfactant) molecules that when added to an aqueous solution, accumulate at the solution-air interface and can significantly reduce the interfacial tension. However, these surfactants can also self-aggregate in the bulk solution (i.e., micellation) and this may limit the surface tension reduction capacity of SRA. In synthetic pore solutions, SRA is observed to form an oil-water-surfactant emulsion that may or may not be stable. Specifically, at concentrations above a critical threshold, the mixture of SRA and pore fluid is unstable and can separate into two distinct phases (an SRA-rich phase and an SRA-dilute phase). Further, chemical analysis of extracted pore solutions shows that addition of SRA to the mixing water depresses the dissolution of alkalis in the pore fluid. This results in a pore fluid with lower alkalinity which causes a reduction in the rate of cement hydration. This may explain why concrete containing SRA shows a delayed setting and a slower strength development.     

粉煤灰-偏高岭土-石粉对混凝土微结构及早期收缩的影响研究

以偏高岭土、粉煤灰和石粉作为复合掺和料,结合混凝土孔结构、界面过渡区(ITZ)及水化热等表征研究多元复合掺合料对混凝土抗压强度及早期收缩性能的影响.研究结果表明:粉煤灰-偏高岭土-石灰石粉多元复合掺和料对混凝土抗压强度有促进作用,其28 d龄期强度增长10%以上,降低孔隙率,减少混凝土试样200 h收缩率13%~23%,且收缩率随着石粉含量增加和偏高岭土含量增加而降低.偏高岭土复合掺和料加速了水泥水化反应过程,促进了CSH凝胶的快速形成,密实了界面过渡区.     

复掺高性能矿物掺合料对高强机制砂混凝土性能的影响

为定量研究S105矿粉与其他矿物掺合料共同作用对C80高强机制砂混凝土的和易性、抗压强度和干燥收缩性能的影响规律,通过试验得到不同龄期(3 d、7 d、28 d、60 d)下,S105矿粉单掺,以及掺S105矿粉的同时以不同含量的微珠、超细矿粉、硅灰分别取代水泥时,高强机制砂混凝土的坍落度、扩展度、抗压强度和干燥收缩率,并利用图表分析及拌合物实际状态对比等对其性能的变化趋势进行分析。结果表明:在一定掺量范围内复掺多种矿物掺合料,有利于提高高强机制砂混凝土的和易性和抗压强度,并显著减小其干燥收缩。在保证混凝土和易性良好的条件下,相比于单掺S105矿粉,S105矿粉与不同矿物掺合料双掺对提高混凝土的综合性能有更显著的作用。综合考虑对和易性、抗压强度和干燥收缩性能的影响,当超细矿粉取代水泥的质量分数为3%时,即水泥与S105矿粉和超细矿粉的质量比为33:11:1时,高强机制砂混凝土的性能处于较好的水平,其粘聚性和流动性都有显著改善,其3 d和60 d抗压强度分别增长3.1%和5.1%,其干燥收缩率则减小了4.0%。     

Thermal behavior of cement matrix with high-volume mineral admixtures at early hydration age

Thermal cracks that usually occur in mass concrete are closely related to the thermal behavior of cement matrix, such as heat liberation, temperature rise and thermal shrinkage. Cement pastes added with large-volume mineral admixtures that are usually used for thermal controlling were cast into well-sealed plastic cylinder and covered by heat insulation materials to simulate the pseudo-adiabatic condition of mass concrete. The deformation and temperature rise of cement specimens under the heat insulation condition have been examined at early hydration age. Results show that with addition of fly ash, coal gangue and blast furnace slag the heat liberation and peak temperature of cement paste decrease, while its total shrinkage increases.There is no shrinkage but expansion of the pastes during the temperature rise process, which may be ascribed to the complete compensation of the shrinkage by thermal dilation of the pastes. The thermal dilation coefficient (TDC) of cement paste changes drastically with the hydration duration, and it is also related to the addition of mineral admixtures.     

煤结构的计算机辅助分子设计研究

本文简要评述了煤结构的计算机辅助分子设计技术的最新进展，详细讨论了煤分子结构构象的建立，能量的极小化，密度及其表面积和分形维数等内容，最后还介绍了以ＳＩＮＡＴＵＲＥ软件为代表的计算机辅助结构解析方法的具体应用步骤。     

Correlation between water permeability of latex-modified concrete (LMC) and water diffusion coefficient of latex film

Cement-based materials are generally known as weak materials in flexure and tension in comparison with compression. Polymers are used in cement-based materials to improve their flexural and tensile behaviors. The composite is called as polymer-modified concrete/mortar. Furthermore, polymers decrease permeability of water into cementitious matrices. Polymers are usually used as admixtures in concretes in form of latexes. Latexes are water-based polymers, which are consistent with water-based concrete matrices. On this basis, these kinds of products are called latex-modified concretes (LMCs). However, it has been found that chemical composition, particle size distribution, molecular weight, physical/mechanical properties of latexes affect performance of modified concretes. In this investigation, six latexes in three categories (acrylic, SBR and polyvinyl acetate) were used as concrete admixtures. They were characterized for chemical composition (by FTIR analysis), minimum film formation temperature, pH, glass transition temperature (T _g), particle size and particle size distribution to evaluate the effect of each property on LMC performance. Due to the formation of latex film in the microcracks and pores of concrete microstructure, it was suggested that diffusion of water into films controls permeability of whole concrete structures. On this basis, the diffusion coefficient of the latex films subjected to water was measured using a new method (continuous FTIR analysis). Capillary water absorption test was performed on concrete specimens to verify validity of the suggestion. It was found that there is a correlation between capillary water absorption of LMCs and water diffusion coefficient of latex films.     

外加剂对混凝土收缩与开裂性能影响的试验研究

针对传统混凝土中加入矿物外加剂会导致混凝土开裂的问题,提出在传统方法上加入粉煤灰和硅灰。为验证方法的可行性,采用不同实验配方比,并结合单掺和复掺方式,对上述方法进行验证。实验表明:复掺30%~40%的粉煤灰和矿粉可有效降低混凝土的开裂面积;硅灰与粉煤灰和矿粉在二元或三元复合下可降低砂浆自收缩和干燥收缩,且混凝土强度不会降低。由此得出混凝土收缩与开裂和砂浆收缩存在显著相关性,砂浆的自收缩和干燥收缩是影响开裂的主要因素。     

碱-矿渣水泥性能研究及应用

研究了碱-矿渣水泥水化产物组成、水泥石孔结构和孔液成分、力学性能、水化热、抗化学腐蚀性能和对钢筋的钝化作用等;并对混凝土的力学性能、干缩变形、耐久性和对外加剂的适应性等进行了研究。结果表明,碱-矿渣水泥具有早强、高強、低水化热和高耐久性等优良性能,但是干缩较大;选择适当的外加剂可以改善部分性能。介绍了利用该水泥的特性在多种工程中的应用情况。     

减缩抗裂型混凝土超塑化剂的性能及其作用机理

开发了一种新型的减缩抗裂型超塑化剂(shrinkage-reducing and anti-cracking polycarboxylate-based superplasticizer,SRPCA),掺量为水泥质量的0.2%时,减水率可达22.3%,且增强效果明显.和掺传统萘系减水剂的混凝土相比,掺SRPCA的混凝土90d的自收缩率和干燥收缩率分别降低了52.1%和44.9%.平板和圆环开裂实验证实:SRPCA大大提高了混凝土的抗裂能力.SRPCA的减缩抗裂效果基本接近减缩剂掺量为胶凝材料总质量2%的减缩抗裂效果,但SRPCA的掺量不到复掺萘系减水剂和减缩剂的1/10.     

低相对分子质量氨基甲酸酯丙烯酸酯的合成与研究

选用6种不同结构的小分子二元羟基化合物、异佛尔酮二异氰酸酯和丙烯酸羟乙酯或季戊四醇三丙烯酸酯为原料,合成了12种不同结构的低相对分子质量氨基甲酸酯丙烯酸酯(UA)与2种已商业化应用的不添加二元羟基化合物的无间隔结构UA对比样品。采用红外光谱进行结构表征,通过实时红外光谱研究了各样品的光聚合动力学,并进行了体积收缩率、力学性能与常规表面性能测试,研究了链长和链段结构类型对UA综合性能的影响。结果表明:带有小分子二元羟基化合物结构的低相对分子质量UA样品具有柔韧性好、硬度高、刚性高、体积收缩率低、双键转化率高等优点,并且样品对PET膜和ABS基材有较高的附着力。     

Effect of shrinkage-reducing admixtures on the properties of alkali-activated slag mortars and pastes

The effect of a shrinkage-reducing admixture (SRA) based on polypropylenglycol on the dimensional stability of waterglass-activated slag mortars was studied. The analysis also showed the effect of the admixture on pore structure of the mortars as well as on the mineralogical composition and microstructure of the alkali-activated slag pastes.The SRA reduced the shrinkage by up to 85 and 50% when the alkali-activated slag mortar specimens were cured at relative humidities of 99 and 50%, respectively. The mechanism primarily involved in shrinkage reduction is the decrease in the surface tension of pore water prompted by the admixture. The SRA also modified the pore structure - under both curing conditions - increasing the percentage of pores with diameters ranging from 1.0 to 0.1 μm. Capillary stress is much lower in these pores than in the smaller capillaries prevailing in mortars prepared without admixtures.Microstructurally, the SRA occasioned a slight increase in the proportion of Si units Q² in the CSH gel and a decrease in the percentage of Al replacing the Si in the gel structure. The admixture did not, however, modify the mineralogical composition of the pastes.Finally, the SRA admixture retarded the alkaline activation of the slag, more intensely at higher admixture dosages. While the admixture did not significantly alter the degree of reaction in pastes cured for 7 days at RH = 99%, the value of this parameter dropped by 7% in the presence of the admixture in pastes cured at 50% relative humidity.     

混凝土减缩剂的研究与发展

简单概述了混凝土的开裂问题和收缩机理,主要总结了混凝土减缩剂的国内外研究进展,其中对于减缩剂的化学结构、组成和掺量等作了详细描述。