Research status and analysis of cement and geopolymer hydrophobic composites
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摘要: 混凝土的长期耐久性问题是其面临的主要问题之一,造成耐久性破坏的主要原因是水在混凝土的多孔结构中的迁移,使有害离子更容易进入基材内部。采用超疏水材料对水泥及地聚物进行改性复合处理,赋予其超疏水特性,避免水分在其孔隙中的传输,从而防止有害离子的迁移及侵蚀,增强混凝土的耐久性。本文总结了目前研究当中对于水泥及地聚物胶凝材料超疏水改性方法,包括整体改性和表面改性两种;归纳了整体改性方式中超疏水改性剂加入到水泥及地聚物混凝土中的改性机制,以及与无机物基体中的连接键合方式;概括了目前研究当中表面改性常用的改性方法,包括喷涂法、浸渍法、模板法等,并分析了表面改性机制。与表面改性所得到的超疏水复合涂层相比,整体改性的水泥及地聚物基复合材料在实际应用场景当中具有更大的优势。此外,分析了疏水改性后对复合材料润湿性、防水性、抗压性能以及防腐性能的影响规律,发现其抗压强度降低了约20%~60%。最后,阐述了水泥及地聚物复合材料的疏水改性研究中存在的一些问题并对未来的研究方向进行了展望,建议从体积型超疏水、提高抗压强度、成本控制及疏水外加剂在材料内部实现均匀化分散等方面进行研究。Abstract: The long-term durability of concrete is one of the main problems it faces, and the main cause of durability damage is the migration of water in the porous structure of concrete, which makes it easier for harmful ions to enter the interior of the substrate. Modifying the cement and geopolymer with superhydrophobic materials is a valid method to avoid the transmission of water, thereby preventing the migration of harmful ions and increased its durability of concrete. This review summarized the superhydrophobic modification methods of cement and geopolymer cementitious materials and categorized into superhydrophobic surface and bulk modification; the modification mechanism of superhydrophobic modifier added to cement and geopolymer and their bonding mode with inorganic matrix in the internal modification method. Besides, the modification methods commonly used for surface modification in the present research are summarized, divided into external coating, maceration, template method, etc., and the surface modification mechanism is analyzed. Compared with the superhydrophobic composite coatings, the monolithically modified cement and geopolymer matrix composites have greater advantages in practical application scenarios. In addition, the effects of hydrophobic modification on the wettability, waterproofing, compressive properties and anti-corrosion properties of composites are concluded, and their compressive strength was reduced by about 20%~60%. Finally, some problems in the research of hydrophobic modification of cement and geopolymer composites are described and the future research direction is prospected, and it is suggested to carry out research on volumetric super-hydrophobicity, improvement of compressive strength, cost control, and homogeneous dispersion of hydrophobic admixture inside the material.
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传统的水泥混凝土是以水泥、砂、骨料等为主要材料通过掺水拌合而成;地聚物作为新型胶凝材料,是一种绿色的铝硅酸盐建筑材料,常应用于建筑、土木工程、交通运输等领域。水泥基混凝土由于其天然的亲水性,会产生由酸雨侵蚀、冻融损伤、钢筋腐蚀、收缩开裂等与水侵入有关的问题[1-2],同时水泥基材料中多余的水分的蒸发会产生孔隙,导致外部水和侵蚀离子在静水压力下通过毛细吸收或流经毛细孔穿透材料[3],并且在生产的过程中也会释放大量CO2[4],对环境造成极大的负担;地聚物表面同样存在未缩合的羟基易于吸附水分[5],有害离子通过水的运输侵入建筑材料内部,导致钢铁腐蚀和碱金属离子溶解[6-7],从而破坏其应用耐久性。
为了提高混凝土的应用耐久性,受“荷叶效应”的启发[8],研究者们通过在水泥及地聚物复合材料的内外表面构建微纳结构或覆盖低表面自由能物质的方式对复合材料进行疏水改性[9]。研究表明,通过添加疏水外加剂如硅烷[10-12]、脂肪酸[13]等可以大大降低吸水率,疏水添加剂可以在材料表面形成疏水膜,从而阻止水的渗透[1-4],并且表面超疏水或整体超疏水特性可以赋予混凝土自清洁[15-16]、防腐[17]、防结冰[18]等特性。
受到超疏水材料微观结构的启发,许多研究者试图通过在水泥及地聚物材料内外表面构建微纳结构或覆盖低表面自由能物质的方式对复合材料进行疏水改性[19]。疏水改性后的水泥及地聚物材料的内表面或外表面由亲水性转变为疏水性或超疏水性。本文根据目前研究当中的两种主要超疏水改性方式,表面改性和整体改性两个方面入手,对比了水泥和地聚物复合材料疏水改性机制的区别,归纳了改性后复合材料的性能变化规律,总结了超疏水水泥及地聚物复合材料目前研究中存在的问题,并对未来的研究方向进行了展望。
1. 表面改性方法及机制
通常将水接触角(WCA)大于150°,滚动角(SA)小于10°的表面定义为超疏水表面,水或其他液体投射或滴落在基材表面能够迅速滚落或弹起。其表面形态通常具有多层次的凹凸结构或者具有纳米级的特殊纹理,减小与水的接触面积,降低二者粘附力,并且具有低表面能,通常小于水的表面能,使表面具有较低的表面能和较高的接触角。常见的超疏水改性剂包括聚二甲基硅氧烷(PDMS)、聚甲基氢硅氧烷(PMHS)等有机硅改性剂和氟碳聚合物、氟烷基硅氧烷等氟碳改性剂,以及纳米SiO2颗粒、纳米Al2O3颗粒等纳米材料改性剂。材料是否超疏水是由材料表面的粗糙度和材料的化学组分共同决定的[20]。以下从水泥基复合材料和地聚物复合材料表面改性中分别归纳了两种复合材料表面改性的方法及超疏水改性机制。
1.1 水泥基表面超疏水改性方法及机制
超疏水涂层是通过构建微纳多层次结构复合低表面自由能改性获得的[21]。通常制备超疏水涂层的方法有模板法[22]、喷涂法[23-24]、浸渍法、蚀刻法[25]、电沉积法[26]等,以下对表面超疏水涂层制备方法及改性机制进行总结。
喷涂法是采用表面喷涂制备微纳米的粗糙结构以获得超疏水性。Song[27]等选用DC-30(主要含有辛烷-硅烷和硅氧烷)作为改性剂,将彩色超疏水粉末通过孔径小于100μm的筛网进行筛分,按一定比例混合制成彩色超疏水涂层,采用喷涂法开发了一种彩色超疏水混凝土涂料,改性剂中有三个以上的硅羟基,阻碍了C—O基团的形成,加入水泥和沙子的混合物中形成 Si—O基团,使得C—O基团的比例更低,表面能更低,赋予其水泥混凝土表面超疏水性,基本原理为硅烷可以与大气中的水分或混凝土表面孔隙中的水发生反应,通过三个烷氧基的水解形成硅烷中的含硅烷醇的基团,然后缩聚成低聚物,通过硅烷和混凝土之间的羟基键合,与材料表面形成共价连接[28]。其疏水机制如下图1(a)所示。Kong[29]等以一种新型的无机胶凝材料作为粘结剂,即采用矿渣和碱激发剂混合制备的地聚合物涂层作为粘结层,将微硅藻土与纳米Al2O3以及无水乙醇按比例为2∶1.1∶100构建微纳结构,将其加入到比例为1∶50的硬脂酸与乙醇溶液中制备出超疏水溶液从而创造出一种持久耐用的S-混凝土涂层。S-混凝土水化产物之间的Si—O结合进一步增强了硅藻土与基体的结合,增强了其表面粗糙度;微硅藻土和纳米Al2O3与Ca(OH)2在混凝土中发生反应,形成了大量的C—A—S—H 凝胶,硬脂酸上的疏水基团—CH3与C—A—S—H 凝胶交联,降低了混凝土的表面能,使其具有良好的疏水性能。She[30]等通过将有机粘结剂复合纳米SiO2颗粒直接喷涂或涂刷在混凝土表面来建造坚固的超疏水混凝土。通过正丙基三乙氧基硅烷的水解缩聚制备的疏水溶液与纳米SiO2运用水热法制备的5wt%的胶态SiO2纳米颗粒溶液混合,其形成的超疏水溶液直接喷涂于混凝土表面,硅官能团在水中水解生成硅醇,与纳米SiO2颗粒和混凝土的—OH基团反应,形成自组装膜,在混凝土表面形成稳定的超疏水网络结构。Wang[31]等以白色硅酸盐水泥为基体,硫化硅橡胶为疏水剂,TiO2为太阳反射填料,采用简单的一步涂刷法制备了水泥基涂料,在催化剂的作用下,四乙氧基硅烷水解成硅醇,将107胶与硅醇缩聚得到交联结构的三维聚合物网络,端羟基缩聚的中间产物可以与TiO2表面的羟基发生反应,并通过Si—O键与颗粒表面化学键合,使表面表现出超疏水性。Yang[32]等通过在水泥砂浆中加入比例为1∶2的纳米TiO2和黑色“cool cold”红外热反射颜料(黑色“冷冷”红外反射颜料,主要成分为CrO3和Fe3O4),干燥后用低表面能的1H,1H,2H,2H-全氟癸基三乙氧基硅烷(PFDTS,其中1H,1H,2H,2H表示在该全氟代烷基链上,第一个和第二个碳原子上存在未被氟取代的氢原子。)溶胶修饰,制备了热反射和超疏水水泥混凝土表面。
浸渍法是使用特殊的试剂通过浸渍渗透到材料的内部结构并填充孔隙,疏水浸渍不影响孔隙,只起到防水剂的作用[42]。Wang[33]等以月桂酸钠作为改性剂,普通硅酸盐水泥浆体表面在室温月桂酸钠水溶液中浸泡5 s后,其反应产物月桂酸钙的长链烷基有十二个碳,使月桂酸钙具有较低的表面能。月桂酸钙形成不规则的层状结构,覆盖硬化的水泥浆体表面,获得表面超疏水性。
模板法是通过化学或物理方法将相关材料沉积到具有特定结构的模板上,之后去除模板制备超疏水涂层的方法[34]。Gao[35]等人以不同粒度砂纸为正模板,采用纳米铸造法制备PDMS负极模板,将砂纸的粗糙表面分别复制到硅酸盐水泥和氯氧镁水泥上,最终获得 WCA>140°的疏水性表面,其疏水改性机制如下图1(c)所示。表1罗列了使用不同的方法对水泥及地聚物材料表面改性的优劣对比。其水接触角为涂层磨耗前的接触角,表中对其耐久性的测试方法主要为砂纸磨耗和附着力测试两种,砂纸磨损是测试其涂层在一定压力下所能承受的磨损次数或磨损总长度,可以直接观察到涂层的水接触角的变化,而表1中的附着力测试是ASTM C3359-17[39]中的压敏胶带剥离附着力测试,相较对砂纸磨损来说能够更加直观的观察到涂层与基材之前的粘附力。同时通过表1看到用甲基三乙氧基硅烷和二乙氧基二甲基硅烷对纳米Al2O3颗粒进行改性[37],通过简单热压处理的方法具有较好的耐久性,同时固体树脂表面还有大量的活性基团(羟基甲基),在热压过程中可与碱性交联剂形成三维结构,并且在
5000 磨粒的砂纸磨损后,材料仍具有良好的疏水性。Cementitious types Coating preparation Method Test method WCA/(°) Surface coating of cementitious materials DC-30 (contains mainly octane-silane and siloxane)[27] External coating Abrasion of 200 grit sandpaper for 20 m under a load of 2.5 kpa 158 Aqueous solution of sodium laurate[33] Maceration — 150 Sandpaper and polydimethylsiloxane [35] Template — 142 Triethoxyoctysilane and diatomaceous earth low
surface materials [36]External coating Sandpaper for 18.00 m under 24.50 kpa load 158 Organosilicon functionalized Al2O3 + solid resins materials [37] External coating 400 cm abrasion of sandpaper under 200 g load 165 Surface coating of geopolymer materials Polymethylhydrosiloxane [40] External coating — 161 Polydimethylsiloxane solution containing
polytetrafuoroethylene /
stearic acid and fly ash [39]Maceration Polydimethylsiloxane -only coatings have higher adhesion than polydimethylsiloxane coatings containing fly ash 159 Notes: WCA—water contact angle 1.2 超疏水基的地聚物表面超疏水改性方法及机制
地聚物因其表面存在的活性羟基基团使其具有亲水性,致使有害离子随水分渗透到基材中,导致破坏其耐久性。通常对地聚物表面进行超疏水处理的方法有喷涂法[38]和浸渍法[39]。
Cui[40]等以偏高岭土前驱体、石英砂和液化水玻璃为原料,在玻璃基材上制备了地聚物涂层,通过涂覆含氢硅油,成功制备了表面具有密实纤维束的地聚物基超疏水复合涂层。在制备过程中,高碱浓度作为活化剂打破Si—O键和Al—O键,释放单体,少数金属阳离子(Na+)负责构造地聚合物结构,少量的PMHS与尖端孔道中的水发生水解反应生成硅醇,随着水解产物的增加,硅醇将与活性羟基(—OH)重新聚合和沉淀,获得表面原位生长的硅纳米丝构建了表面多层次结构。Wang[41]等利用废粉煤灰颗粒和商用胶粘剂成功地制备了一种坚固的多功能超疏水表面,即对粉煤灰进行球磨处理,提高粉煤灰颗粒的微米/纳米级分级粗糙度,接着用全氟癸基三乙氧基硅烷包覆粉煤灰颗粒,采用商用粘合剂通过喷雾沉积或筛沉积的方法粘接在各种基材上。
浸渍法与水泥基材料的浸渍法相同,不影响其孔隙,只起到防水剂的作用。Pachana[39]等采用浸渍法在地聚合物表面制备了含有聚四氟乙烯或硬脂酸钙微粒与粉煤灰混合的PDMS复合涂层,获得超疏水表面。
水泥或地聚物复合材料的超疏水表面改性的基本原理是提高材料表面粗糙度及降低表面自由能。提高表面粗糙度可以通过覆盖铜网、砂纸或是加入纳米颗粒来获得,本文所阐述的降低材料表面的自由能主要是通过加入有机硅化合物来实现的。硅烷基疏水涂料与传统涂料的不同之处在于,大多数硅烷都具有小分子结构,更容易进入毛细孔,从而在表面覆盖一层疏水层,减少水进入材料基体的可能性[43]。下图1分别为材料表面硅烷的疏水改性机制、喷涂法、模板法和浸渍法的示意图。
喷涂法由于其简易性和可扩展性在三种制备方法中最具优势,同时结合表1可以看出,制备出超疏水溶液直接喷涂在材料表面,其具有较好的机械强度,并且水接触角(WCA)均超过150°,并且相较于模板法和浸渍法来说,喷涂法的重复利用率更高,成本更低,更适合现代工程中的应用。
2. 水泥及地聚物整体改性方法及机制
2.1 水泥整体超疏水改性方法及机制
与表面改性不同的是,整体超疏水混凝土的制备是在混凝土搅拌中加入疏水剂,与水泥共混,并粘附在水泥水化产物上。
Wang[47]等选用了PDMS作为疏水外加剂对水泥砂浆进行疏水改性,PDMS分子与硅酸盐水泥水化产物水化硅酸钙(C—S—H)和Ca(OH)2表面化学键和,固化剂(TEOS)在碱性条件下水解成硅醇,PDMS的两个羟基和硅醇与水化产物表面的羟基发生反应,通过Si—O键在水泥表面形成稳定的Si—O—Si化学键。Zhang[48]等通过覆盖金属网,加入疏水端羟基聚二甲基硅氧烷(HPDMS),制备了具有良好机械稳定性和可回收性的超疏水性氯氧镁水泥基复合材料。HPDMS的固化剂(四乙氧基硅烷)在二月桂酸二丁基的催化作用下水解成硅醇,HPDMS和硅醇都能与氯氧镁水泥颗粒表面的羟基发生反应,通过稳定的Si—O—Si键键合在氯氧镁水泥颗粒表面,同时表面具有亚毫米级网格结构和纳米级针状相形成的双层分形结构使表面变得更加粗糙,—CH3和—CH2基团可以有效降低表面能。反应机理图如下图2(a)。Wang[49]
等利用硬脂酸对水泥浆体进行改性,制备出了疏水性水泥浆体(MCP)。水泥颗粒表面富含羟基,硬脂酸与羟基的酯化反应将硬脂酸的疏水长尾链接枝到水泥浆体上,疏水基团—CH3和—CH2提供了低表面能,同时花状硬脂酸钙晶体大大增加了水泥浆体的表面粗糙度。除了直接加入有机硅烷改性剂对其整体进行改性外,同时还可以通过对集料表面进行改性处理后加入水泥砂浆中,形成多层次的超疏水微纳结构,使其获得整体的超疏水特性。Xiang[50]以SiO2为集料,以甲基三甲氧基硅烷(MTMS)为疏水改性剂,设计开发了自制固体超疏水粉体,可直接与水泥、砂混合制成整体超疏水混凝土(SC)。MTMS首先进行水解和缩合反应生成硅醇和甲醇,SiO2表面上的—OH与硅醇反应通过氢键结合,使得硅醇附着在SiO2表面,而MTMS的另一端在SiO2颗粒之间形成空间位阻防止其团聚,改性的SiO2制备而成的S-粉与水泥、砂混合制成整体超疏水混凝土。其反应机理图如下图2(b)。Romero[51]等也通过添加大脂肪族分子基团(正十二硅烷)共价连接SiO2,通过SiO2表面的硅烷醇基团与硅烷的乙氧基基团的偶联反应对SiO2进行改性,添加到水泥混凝土中,制备出了具有整体疏水性的水泥混凝土。此外,除了上述直接添加外加剂或是对疏水颗粒进行改性来制备整体超疏水性混凝土之外,Sun[52]等提出了一种采用牺牲模板的方法,制备出了具有力学坚固性的超疏水混凝土。将溶解在酒精中的硬脂酸均匀包裹在碎石表面,与水泥和水混合,形成砂浆混凝土,将制备好的混凝土块浸入表面处理过的纳米SiO2颗粒和PDMS均匀分散的环己烷混合溶液中,溶解硬脂酸模板,PDMS作为粘合剂,使超疏水SiO2紧密地粘结在混凝土的内基体和外表面,在硬脂酸的帮助下,SiO2渗透到基体内部进行疏水改性,最大限度的保留了抗压强度,并且具有优异坚固性和自清洁性能,为制备整体超疏水性混凝土提供了新的思路。
2.2 超疏水基的地聚物整体改性机制
地聚物进行整体疏水改性时通常是引入疏水性填料或是加入有机改性剂,同时还可以通过调整混凝土的配合比如水灰比、砂石粒径分布等,改变混凝土的孔隙结构,以提高疏水性能。
通常用到的有机硅改性剂有PDMS、PMHS和异辛基三乙氧基硅烷等。Shao[53]等开发了一种新型的“二合一”策略,通过引入PMHS作为双功能剂同时实现了粉煤灰基地聚合物发泡和疏水。样品PMHS分解成硅醇和H2引起发泡。硅醇将进一步与活性羟基(—OH)重新聚合,疏水基团在 N—A—S—H 接枝在凝胶表面,赋予地聚物材料优异的疏水性能。Zhu[54]等以端羟基聚二甲基硅氧烷(HPDMS)为添加剂,改善粉煤灰-矿渣地质聚合物的疏水性。羟基PDMS和N—A—S—H凝胶在碱性环境下发生脱水缩合反应,PDMS可与N—A—S—H凝胶形成Si—O—Si键连接,由于—CH3是疏水基团,PDMS与N—A—S—H凝胶交联后,N—A—S—H凝胶被疏水基团包围,在样品表面形成疏水膜。反应机理如下图3(a)。Yan[55]等研究了PDMS和疏水颗粒双掺入对偏高岭土基地聚物的影响,在地聚合过程中,聚合体PDMS分子会吸附和沉积在凝胶中,通过Si—O—T(T代表Si/Al)键接枝到凝胶上改变了地聚物表面的化学成分,导致疏水性增强。Chen[56]等对硅烷(异辛基三乙氧基硅烷)改性粉煤灰/矿渣(FSBG)浆料的疏水性和反应产物进行了研究。硅烷在高碱性环境下迅速水解,羟基取代烷氧基,当硅烷水解完成后,由于偶联效应,聚合形成的硅醇单体和多聚体倾向于接近前驱体颗粒并被吸附在硅烷表面,随着水化产物的大量形成,水解后的硅烷首先被氢键吸附在其表面,形成Si—O—T稳定键合。反应机理如下图3(b)。Yang[57]等以PDMS为外加剂对矿渣砂浆(AAS)进行整体改性,含有大量Si—O和Si—CH3的PDMS分子与AAS的水化产物形成了化学键合, PDMS通过共价键与C—S—H结合起桥接作用,内部具有多级的微观结构,赋予矿渣砂浆疏水性能。
此外,除了上述硅烷有机物作为外添加剂对地聚物进行改性以外,Pang[58]等采用一种新颖的方法构建了脱氮粉煤灰的拓扑结构。将粉煤灰中所含颗粒的Si—O/Al—O键在高碱环境中发生断裂并重新排列,形成无定形硅酸/铝酸凝胶,碳酸化反应形成连续的N—A—S—H在液桥的影响下相互粘附形成桥接凝胶,并形成其多级粗糙度,PMHS又覆盖于多级粗糙结构表面得到疏水内表面。但该方法需要严格控制蚀刻介质的温度和碱度,同时也为批量利用废粉煤灰提供了新的思路。
综上所述,水泥材料的疏水改性主要是水泥与水接触后发生水化反应,超疏水有机改性剂当中的硅烷有机物可以与水泥中的硅酸盐矿物发生反应通过Si—O—Si键合形成连续的有机膜,将水化产物结合成一个整体网络。而地聚物复合材料是Si—O和Al—O在碱性环境中下断裂后再重组缩聚,在碱性激发剂的作用下,原料中的硅酸根和铝酸根发生解聚释放出硅酸根离子和铝酸根离子,随着反应的进行,水化硅酸根和铝酸根离子开始相互聚合,形成N—A—S—H凝胶,有机硅化合物与N—A—S—H通过Si—O—Si或Si—O—Al键合,N—A—S—H被疏水基团包围,使复合材料具有整体疏水性。
下表2罗列了整体改性对超疏水水泥及地聚物材料的水化/聚合作用的影响。疏水改性后对水泥及地聚物的水化和聚合反应都产生了一定的滞后作用,地聚物复合材料大都使用硅烷对其进行疏水改性。其中将PMHS作为发泡剂和防水剂[53]制备的地聚物材料具有较好的疏水性能,但PMHS的剂量多少会改变材料的孔隙结构,进而影响抗压强度;而Pang[58]等利用PMHS制备出的地聚物材料也表现出优异的超疏水特性,但需要严格控制蚀刻介质的温度和碱度。
Cementitious types Coating preparation Test method Effects on hydration/polymerization WCA/(°) ordinary silicate cement[62] non-toxic lauric acid and covering metal mesh XRD Integral superhydrophobic concrete has fewer hydration products than ordinary silicate concrete. 153 magnesium oxychloride based cement [48] hydroxyl-terminated polydimethylsil-oxane XRD、SEM Nanoscale needle-like phases are covered by hydrophobic silicone rubber. >150 ordinary silicate cement[50] functionalization of SiO2 with fluorine-free silanes XRD、FTIR Silanes in superhydrophobic powders react with cement hydration products to slow down the cement hydration rate. 153.8 ordinary silicate cement[61] Stearic acid modified fly ash TGA Fly ash can increase the water-cement ratio and facilitate cement hydration, providing more nucleation sites for cement hydration. 93.2 ordinary silicate cement[64] nano-silica and isobutyl-triethoxysilane isothermal calorimeter Silane can mitigate the loss of flowability caused by nano-silica to some extent, while nano-silica can completely compensate for the delay of silane in the early hydration process. 153.5 fly ash based polymer materials [53] polymethylhydrosiloxane Hot Plate Method Thermal conductivity decreases with increasing amount of polymethylhydrosiloxane, and the higher the porosity, the lower the bulk density. 161 fly ash based polymer materials [58] polymethylhydrosiloxane TEM Grafting of poly(methylhydrosiloxane) is not exactly proportional to the amount of geopolymer produced. 152 fly ash-slag base polymer materials [56] isooctyltriethoxysilane BSE、EDX Silanes slow down the hydration kinetics, while the increase in the modulus of the alkali activator inhibits the formation of hydration products. 118.1 slag mortar [57] polydimethylsiloxane MIP、SEM、EDS Polydimethylsiloxane increases internal defects in the body. 128 3. 超疏水基的水泥及地聚物的性能
3.1 润湿性
材料表面的润湿性用接触角来表示,接触角是在固、液、气相处于平衡状态时,液滴与液-气界面和固-液界面的夹角[59]。具体来说,超疏水表面具有两个关键的润湿特性:高接触角(WCA)和低滚动角(SA)。其高接触角是指水滴在涂层表面静止时与表面形成的角度,超疏水涂层的接触角通常大于150°;低滚动角是指水滴在涂层表面滚动时需要克服的最大角度,通常滑动角小于10°,这就意味着水滴可以轻易地在涂层表面滚动,而不会因为表面与水滴之间的黏附而受到阻碍。
材料的润湿性主要由表面形貌和化学成分两个因素决定[60]。根据Cassie-Baxter模型:
cosθr=f1(cosθ1+1)−1 其中θr和θ1分别为粗糙表面和平坦表面上的WCA,f1为固体界面上水的面积比。
超疏水涂层是因为有空气层的存在增加了空气-水界面的接触面积,从而使涂层具有较高的接触角,表现出良好的疏水性。表3罗列了超疏水基的水泥混凝土的接触角和滑动角,表4罗列了超疏水基的地聚物混凝土的接触角和滑动角,包括表面改性和整体改性,部分含有所使用的改性剂的用量,为将来超疏水复合材料的改性剂的选择和剂量提供了可靠的依据。
Cementitious types Hydrophobic Modifiers Modification type WCA/(°) SA/(°) ordinary silicate cement [62] 0.8 wt% non-toxic lauric acid Integral 153 10 magnesium oxychloride based cement [48] 6 wt% hydroxyl-terminated polydimethylsiloxane Integral >150 <10 high belite sulphoaluminate cement [65] lauric acid Integral 153.2 — ordinary silicate cement [66] 1H, 1H, 1H, 2H-perfluorodecyl-triethoxysilane Surface 163.3 — ordinary silicate cement [67] contains mainly octane-silane and siloxane Surface 160±1 6.5±0.5 ordinary silicate cement[68] hydrophobic silica nanoparticles Surface 160 1.7 Notes: SA—sliding angle Geopolymer types Hydrophobic Modifiers Modification type WCA/(°) SA/(°) calcined clay and slag [69] 5 wt% polydimethylsiloxane Integral 120 — fly ash [70] 5 wt% stearic acid Integral 96.67 — metakaolin[71] 5 wt% polydimethylsiloxane Integral 127.5 — metakaolin [40] polymethylhydrosiloxane Surface 161 2 dust/silicate cement [72] polydimethylsiloxane Surface 154.1 6.1 通过分析表3和表4看出对水泥基复合材料进行超疏水改性后,不管是表面改性还是整体改性,其水接触角均大于150°,滑动角均小于10°,说明改性后的水泥基复合材料具备超疏水性,此外,Lu[63]等利用PMHS通过原位发泡和低表面能特性,制备出三维超疏水泡沫混凝土,发现当水灰比为0.5,PMHS为3 wt%时,水接触角高达164°;表面改性后的地聚物复合材料水接触角大于150°,滑动角小于10°,而表中整体改性后的地聚物复合材料的水接触角仅大于120°,虽未达到超疏水,但其在一定程度上提高了材料的耐蚀性,可以获得良好的抗污染、抗磨损和抗腐蚀性能,从而延长复合材料的使用寿命。通过控制和改变接触角和润湿面积,以此影响水流轨迹和液体流动状态,这在工业应用中具有重要意义。
3.2 防水性能
孔隙率和吸水率是决定复合材料耐久性和使用寿命的重要参数。吸水率和吸湿率是量化和评估材料湿敏感性和耐久性的另一个关键因素[73]。
3.2.1 孔隙率
参考水泥基材料的孔隙分类,也可以分析地聚合物的孔隙结构,一般可分为4类:凝胶孔(直径<0.01 μm)、过渡孔(直径0.01~0.1 μm)、毛细管孔(直径0.1~1 μm)和大孔(直径大于1 μm)[54]。Sun[52]等利用硬脂酸、纳米SiO2颗粒和PDMS采用模板法制备出的整体超疏水混凝土(SH),其孔隙率为15.03%,与普通混凝土孔隙率14.71%相比,略微增大。Lee[66]等利用PFDTS烷制备出的超疏水混凝土的孔隙率略高于普通混凝土,并经过200次冻融循环后发现普通混凝土孔隙率高于改性后混凝土孔隙率,说明改性后的混凝土能有效阻断水分子的运输。Romero[51]等利用正十二烷基三乙氧基硅烷(DTES)对纳米SiO2颗粒功能化后,加入到水泥砂浆中制备的超疏水混凝土比普通混凝土的总孔隙率低11.52%,由于官能化SiO2颗粒的平均尺寸约为178 nm,因此这些颗粒的孔隙填充主要发生在大孔隙中。Wang[74]等使用一种低成本的水性硬脂酸乳液制备出了超疏水水泥混凝土,发现改性剂的加入其阈值孔径304 mm明显大于原始阈值孔径59 mm,并增加了其孔隙连通率;同时其吸水性仅为
0.0392 g/cm3,下降了大约86%。疏水改性后的地聚物材料的孔隙率变化与改性后水泥基材料相似,大都呈现出一种增长的趋势。Zhu[54]等用以HPDMS改性的粉煤灰-矿渣地聚物(GPFS),其孔隙率相较于未改性的地聚物复合材料略微增大,仅增加了0.18%。Zhang[75]等通过在偏高岭土基中加入甲基硅酸钠制备出的疏水性地聚物混凝土(GS),其总孔隙率增加了1%。Yan[80] 等通过在偏高岭土基地聚物中加入PDMS发现,随PDMS的加入,其阈值直径和峰值增大,总体上凝胶孔的孔隙度降低,其他类型孔隙(直径>100 nm)的孔隙度增加。Yan[81]等同时将PDMS和聚丙烯纤维加入到偏高岭土基地聚物中(P-PPF),发现与无任何外加剂的地聚物相比,其孔隙率从0.22%增加到5.236%。下图4为水泥/地聚物改性后孔隙率变化图。
改性后的水泥及地聚物的孔隙率都有不同程度的增加,但对纳米SiO2颗粒功能化后加入到水泥基材料中[51],其孔隙率得到了下降,Xiang[50]等用无氟硅烷对纳米SiO2颗粒进行改性后加入到水泥基材料中也制备出超疏水混凝土,但并未对孔隙率进行详细的介绍。
3.2.2 吸水率
Hou[76]等通过仿生矿化和低表面能硅烷改性在混凝土表面制备超疏水表面,发现比普通混凝土砂浆的吸水率下降了86%。She[30]等通过将有机粘结剂复合纳米SiO2直接喷涂或涂刷在混凝土表面制得了超疏水涂层,使其表面孔隙率下降,并且累积吸水率下降了90%。Xu[77]等通过PDMS改性降低表面能,用硬脂酸钙和羟丙基甲基纤维素优化孔隙结构,同时制造表面粗糙度,制备出超疏水泡沫混凝土,其吸水率降低了约86%。Mao[78]等加入PDMS、硬脂酸钙、纳米SiO2,并用200目金属丝网覆盖试件表面制备出的整体超疏水泡沫混凝土,其吸水率下降率高达90%。这也为普通混凝土和泡沫混凝土如何降低其吸水率和提高其使用寿命提供了思路。
疏水改性后,水泥基复合材料的吸水率明显降低,疏水改性后的地聚物也表现出了相似的现象。Wang[79]等以超疏水铁矿尾矿(S-IOT)为原料,以硬脂酸为改性剂,制备了具有良好憎水耐蚀性能的疏水砂浆,当S-IOT≤30%时,其吸水率下降了43%。Yan[55]等将疏水改性偏高岭土与石英颗粒以及PDMS助剂混合,制备了具有优异防水性能的地聚物复合材料,发现其大孔和毛细孔的孔隙率增大,并且当疏水颗粒和PDMS的双掺入时,低吸湿阶段会长达150 h。Yan[81]等通过添加疏水丙烯纤维(PPF)和PDMS制备出了疏水地聚合物复合材料,发现PPF和PDMS可以增加微米级孔隙的体积分数和尺寸,初级吸水率降低了约70%且延长吸附性低诱导时间可达60 h。下表5罗列了超疏水水泥复合材料和地聚物复合材料的吸水率降低的程度和原因。
categories Materials and Dosages Modification type water absorption/% reason ordinary silicate cement 1 wt% stearic acid[74] Integral −86% Surface water is rejected by the surface. 1.4 wt%
cetyltrimethoxysilane[76]Integral −86% The internal inorganic mineralized layer further prevents water intrusion through a relatively dense micro/nano-scale two-layer structure. 5 wt% SiO2 silica solution[30] Surface −90% Hydration products and some unhydrated nanoparticles can clog capillaries, thus blocking water transfer paths. geopolymer materials 60 wt% iron ore tailings +1.5 wt% stearic acid[79] Integral −43% The particle size of superhydrophobic iron ore tailing is much smaller than that of sand, and the fine superhydrophobic iron ore tailing can easily fill up the pores of the mortar, making the mortar more dense and leading to a decrease in water absorption. 10 wt% hydrophobic metakaolin + polydimethylsiloxane [55] Integral −26%~−27.6% Weakening of the capillary's ability to absorb and hold water. polydimethylsiloxane + polypropylene fiber [81] Integral −70% Polypropylene fiber easily adsorbs polydimethylsiloxane but does not easily trap water vapor, blocking the water vapor diffusion channel. 通过表5分析可知,不管是水泥或是地聚物胶凝材料进行超疏水改性后,其吸水率都有不同程度的降低,水泥基材料的吸水率大多降低了80%~90%,可以达到良好的疏水效果;地聚物复合材料疏水改性后,其吸水率也有所降低,但吸水率下降程度不如水泥基混凝土。
3.3 抗压强度
3.3.1 超疏水水泥基复合材料的抗压强度
疏水改性方法不同对复合材料的强度影响也有明显的差异。通常来说,对混凝土表面进行疏水改性通常不会影响基体材料的强度,疏水改性剂以外加剂的方式掺入到混凝土中会对复合材料的强度造成不同程度的下降。而硅烷改性通常会对水泥基材料的强度造成负面影响。Xiang[82]等通过添加防水剂(WA,含硅烷和硅氧烷)和铜网覆盖层,制备了超疏水混凝土(SC),其抗压强度虽随维护天数增加而增加,养护28 d的抗压强度比普通混凝土降低了20%。Xiang[50]等以SiO2为基材,以无氟硅烷(MTMS)为疏水改性剂制备成的整体超疏水混凝土(SC),由于制成的固体超疏水粉体(s-粉)中的硅烷与水泥水化产物发生反应,减缓水泥的水化速率,抑制混凝土强度的发展,相比之下其28 d的抗压强度降低了18.06%。徐怡[83]等将有机异辛基三乙氧基硅烷与无机纳米SiO2复合,通过“二元协同”仿生方法,制备了整体超疏水水工混凝土,发现其抗压强度在标准养护条件下比在自然养护条件下高。Lee[66]等在浇筑过程中添加PFDTS和合成的纳米TiO2和SiO2制备了混合水泥砂浆(ACM),由于疏水材料阻碍了水泥与水之间的火山灰水化反应,其抗压强度为12.1 Mpa,比普通砂浆降低了约60%。Wang[47]等用PDMS作为一种水泥外加剂疏水改性水泥砂浆(M-HCM),28 d抗压强度为22.43 Mpa,降低了约48%。
除了上述硅烷改性剂外,Sun[52]等采用模板法制备出的超疏水混凝土(SH)抗压强度为37.2 MPa,仅下降了7.9%。Zhang[48]等通过在氯化镁水泥基材料中,加入疏水HPDMS并覆盖金属网,制备了具有良好机械稳定性和可回收性的超疏水性氯化镁水泥基复合材料,随着HPDMS含量的增加,抗压强度从105.48 MPa下降到71.25 MPa。Moon[84]等考察了疏水SiO2和硅灰作为补充胶凝材料,对普通硅酸盐水泥(OPC)力学性能和水化行为的影响,发现含5 wt%和10 wt%硅灰的砂浆强度分别提高了21.5%和36.9%,而含5 wt%和10 wt%疏水SiO2的砂浆强度在28天内分别提高了32.4%和46.8%,得出纳米SiO2的水泥浆体比硅灰的水泥浆体表现出更好的抗压强度。此外,下图5为不同超疏水水泥基复合材料的28 d抗压强度降低程度对比,得出利用纳米SiO2和PDMS运用模板法制备的水泥基混凝土其强度下降最少。
在提高水泥基混凝土强度方面,Wang[85]等通过添加适宜量磷石膏和磨碎的高炉矿渣对再生水泥改性后,发现不仅可以延长其凝固时间,且其28 d和90 d的抗压强度分别提高了21.1%和39.9%。此外,Wang[86]等还发现在水泥砂浆中添加C—S—H纳米颗粒,其7 d的抗压强度提高了11%,通过观察含有C—S—H纳米颗粒水泥样品,可以明显发现纤维状C—S—H,如下图6(a);而加入硅烷疏水改性后的水泥样品[82]抑制了水泥水化,同时水化过程中产生的气泡也阻止了针状生长和孔隙的形成,导致了抗压强度的降低,如下图6(b)。Wei[64]等发现将异丁基三乙氧基硅烷(IBTEO)和纳米SiO2一起添加到水泥中, 纳米SiO2可以补偿IBTEO造成的大部分抗压强度损失,尤其是早期抗压强度(前3 d),而IBTEO可以部分补偿抗弯强度损失。同时对水泥浆体预碳化处理后可以使其在高温下的结构更加稳定[87],这也为水泥基混凝土进行疏水改性后如何提高其抗压强度提供了新的思路。下图6为未疏水改性和疏水改性的水泥样品的SEM对比图。
3.3.2 疏水地聚物复合材料的抗压强度
砂浆的抗压强度关系到其能否用于建筑施工的一项重要力学指标。Wang[79]等用无污染的硬脂酸对超疏水铁尾矿进行改性,制备了超疏水铁尾矿(S-IOTs),发现当质量分数为1.5%硬脂酸复合30%的铁尾矿时,28 d的抗压强度比普通砂浆提高了12%;而硬脂酸不变的情况下,铁尾矿含量增加到60%时,28 d抗压强度比普通砂浆降低了11%。Yang[57]等研究了PDMS改性碱活化矿渣(AAS)砂浆的吸水特性和力学性能,当PDMS体积分数为8%时,复合材料具备超疏水性能,但其在28 d的抗压强度为42.2 MPa,降低了22%左右,分析是由于PDMS增加了基体内部缺陷。Pasupathy[88]等疏水气相SiO2和硅烷乳液(一种含有硅烷和硅氧烷混合物的奶油状浸渍剂)两种疏水剂对不同用量的砂粒进行包覆制备了疏水复合材料GPC-Sil和GPC-FS,发现当硅烷乳液含量为20 wt%的地聚物复合材料(GPC-Sil),28 d的抗压强度下降了32.9%;而疏水气相SiO2剂量为2 wt%时得到的复合材料(GPC-FS),28 d的抗压强度反而提高了20.1%。Zhang[89]等介绍了一类具有可调体积超疏水性能的新型杂化有机地聚合物,可以调节三官能团和四官能团硅氧烷单元的比值(即T/Q),发现当T/Q为
0.0625 时,复合材料具有超疏水性,但其抗压强度从9.8 MPa降低至0.5 MPa。Zhu[73]等研究了稻壳灰对粉煤灰基地质聚合物(F8R2)防水性能和微观结构性能的影响,发现稻壳灰剂量为20 wt%时,其28 d抗压强度下降了约8.8%,虽其憎水性能有所增加,但并未达到疏水性。下图7为不同超疏水地聚物复合材料28 d抗压强度降低值。Zaoui[90]等运用分子动力学和密度泛函理论对石墨烯纳米涂层地聚合物表面进行了研究,发现其耐久性提高,但接触角并未超过90°。此外,在未疏水改性的前提下,Xie[91]等以粉煤灰和矿渣为胶凝材料,以橡胶粉为细骨料制备了橡胶化地聚物混凝土,发现当橡胶粉含量为10%时,其抗压强度提高了19.1%;且研究了矿渣和粉煤灰对再生粗骨料制备的地聚物混凝土的硬化性能的影响[92],发现相较于水胶比,矿渣掺量对其抗压强度的影响更为显著,当矿渣掺量为50 wt%和75 wt%的地聚物混凝土的抗压强度分别比普通硅酸盐混凝土提高50%和180%。地质聚合物粘结剂和再生粗骨料的结合可以表现出优异的抗压性[93]。在目前研究的基础上,疏水外加剂的加入会使复合材料的抗压强度降低,如何在保留复合材料抗压强度的基础上,保证其疏水性能是将来探索和需要解决的问题之一。
3.4 防腐性能
水泥及地聚物混凝土作为一种碱性材料,具有天然的亲水性,容易受到高温膨胀、酸性环境、盐水侵蚀以及微生物的侵蚀,从而使混凝土的内部结构被破坏,缩短其使用寿命。对腐蚀损坏后的混凝土进行维护或重修往往需要消耗更多的水泥,且其在建造过程中会加剧CO2的排放,进而对环境造成影响[94]。故在混凝土表面上制备超疏水涂层以及对混凝土进行整体疏水改性,可以有效减缓腐蚀离子的渗透,且由于疏水改性赋予了低表面能抑制了水分在其表面及内部的扩散。因此,提高混凝土表面防腐对提升工程建设基础设施的服役寿命及环境保护也具有重要意义[95]。
腐蚀实验是在3.5 wt%NaCl电解液中使用电化学工作站进行的。得到不同方法制备的超疏水复合材料的腐蚀电位(Ecror)和腐蚀电流密度(Icorr)。腐蚀电位越高,腐蚀电流密度越小,超疏水复合材料的耐腐蚀性能越好。Wang[47]等利用PDMS改性制备的超疏水复合材料测得的Ecror和Icorr分别为−72.04 mV和4.10×10−8A∙cm−2,且测试了不同循环次数对水泥基复合材料腐蚀电位的影响,且腐蚀电位在不同测试周期都保持较高的值。Kong[28]等制备出的超疏水混凝土(S-混凝土),测得的腐蚀电位为−
0.28225 V,由于涂层中的三维硅酸铝网络的形成,构成了一层均匀分布的防渗硅酸铝凝胶,有效防止氯离子渗透。Xiang[82]等利用制备的超疏水混凝土(SC),在16 V电压下电化学腐蚀20 min得到的Icorr为7.702×10−6 A·cm−2,Icorr值远低于普通水泥混凝土,具有良好的防腐性能。Wang[62]等用无毒月桂酸改性的超疏水混凝土,得出Ecror和Icorr分别为−0.477 V和1.26×10−5 A·cm−2,S-混凝土中的分层结构能有效防止腐蚀离子的进入。Sun[52]等制备的超疏水混凝土(SH),测得的腐蚀电位为−0.15 V,且3.5wt% NaCl溶液中浸泡一个月后,SH中包裹的钉子仍没有生锈,表现出良好的腐蚀性能。Wang[96]等利用硬脂酸改性的S型混凝土,测得Ecror和Icorr分别为−0.173 V和5.921×10−7 A·cm−2。Wang[79]等通过1.5 wt%硬脂酸改性得到的超疏水地聚物复合材料S-IOTs,Ecror和Icorr分别为−0.321 V和1.451×10−6 A·cm−2,当含有60%的铁尾矿时具有良好的防腐性能。下表6罗列了上述几种超疏水复合材料的腐蚀电位和腐蚀电流密度。Concrete material name Hydrophobic Modifiers Ecror /V Icorr/(A·cm−2) hardened cement mortar [47] polydimethylsiloxane − 0.07204 4.10×10−8 superhydrophobic surface for concrete [29] stearic acid − 0.28225 — superhydrophobic concrete [82] containing silane and siloxane — 7.702×10−6 superhydrophobic concrete [52] lauric acid −0.477 1.26×10−5 superhydrophobic concrete [96] stearic acid −0.173 5.921×10−7 superhydrophobic iron ore tailings [79] stearic acid −0.321 1.451×10−6 Notes:Ecror is a mixed electrode potential determined by the cathode and anode reactions on the corroded surface; Icorr is the amount of electricity per unit area per unit time of cathodic protection on a metal electrode. 分析表6可以看出利用PDMS改性制备的M-HCM复合材料能的Icorr最小,因此其能有效防止氯离子的侵蚀。王元战[97]等基于贻贝仿生原理,开发了一种由γ-氨丙基三乙氧基硅烷表面处理层、单宁酸(TA)/ SiO2微纳米层和十六烷基三甲氧基硅烷低表面能修饰层构成的新型单宁酸(TA)/ SiO2疏水涂层,能够至少减少75.31%的离子渗透进入混凝土。超疏水表面对于混凝土的防腐也有着显著的效果。Zhang[98]等分别以水性环氧树脂(WR),三种硅烷偶联剂(SCA)为1%,合成了有机改性偏高岭土基地质聚合物(MG)复合材料涂层,通过线性极化电极(LPR)测试测定了环氧树脂和硅烷偶联剂改性的地聚物复合材料,发现环氧树脂改性的复合材料极化电阻(Rp)越高,氯离子渗透能力越低,但文中并未提及两种复合材料的润湿性。此外,Wang[99]等制备了含有矿渣和粉煤灰基地质聚合物的再生骨料混凝土,发现其在未疏水改性条件下,抗硫酸盐性会随着矿渣含量的增加而增加。这也为地聚物复合材料在海洋中的应用打开了新的思路,但还需要进一步研究复合材料更实际的应用性能。
4. 结 论
本文通过对水泥和地聚物复合材料的疏水改性的研究现状进行总结,归纳了目前超疏水改性水泥及地聚物胶凝材料的改性方法及机制,并对改性后的复合材料的性能包括润湿性、防水性、抗压性能和防腐性能进行了分析评价。经对比发现,不同方法制备的超疏水复合材料可以使防水性得到较大提升,但也会导致其他性能的下降,目前发现研究当中存在的不足如下:
(1)表面改性不会影响到基体的力学性能,但不会渗透到基体内部,一旦涂层被破坏其疏水性能丧失。
(2)整体改性的水泥基复合材料由于掺入的疏水物质的疏水性会抑制水泥的水化反应,从而导致约20%~60%抗压强度的降低。相较于整体疏水改性的水泥基复合材料,地聚物复合材料的降低相对较小,但抗压强度仍会降低。
(3)常用的疏水改性剂大多为有机硅化合物,如聚二甲基硅氧烷(PDMS)、聚甲基氢硅氧烷(PMHS),但此类材料不仅价格昂贵而且不环保。
(4)纳米材料如疏水SiO2颗粒等通常通过内掺的方式对复合材料进行整体改性,但纳米材料的团聚使其难以均匀分散到基体内部,从而使整体改性的复合材料各个部位的疏水性能出现参差。
综上所述,将水泥及地聚物复合材料的疏水改性今后研究方向总结为以下几点:
(1)探究超疏水材料与玻璃纤维或碳纤维等材料的结合,提高复合材料的力学性能,并探究其复合材料在实际环境中的应用。
(2)提高疏水集料在水泥和地聚物复合材料中的均匀化分散,使其具有整体性超疏水。
(3)可以结合纳米材料的尺寸效应和硅烷的偶联效应,进一步提高水泥及地聚物材料的疏水性能。
(4)应该进一步探讨有机硅化合物与地聚物之间的键合方式(Si—O—Si/ Si—O—Al)是哪种优先或是更加趋于哪种键合,以及其键和方式对材料改性后的性能影响如何,这对于理解有机硅化合物对地聚物材料的疏水改性有着重要意义。
(5)当前超疏水材料的合成方法存在成本高、产量低、操作复杂等问题,开发新的合成策略,寻求降低成本、操作简便、环境友好的方法。
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表 1 不同改性方法对水泥及地聚物材料表面改性的优劣对比
Table 1 Comparison of advantages and disadvantages of different modification methods for surface modification of cement and geopolymer materials
Cementitious types Coating preparation Method Test method WCA/(°) Surface coating of cementitious materials DC-30 (contains mainly octane-silane and siloxane)[27] External coating Abrasion of 200 grit sandpaper for 20 m under a load of 2.5 kpa 158 Aqueous solution of sodium laurate[33] Maceration — 150 Sandpaper and polydimethylsiloxane [35] Template — 142 Triethoxyoctysilane and diatomaceous earth low
surface materials [36]External coating Sandpaper for 18.00 m under 24.50 kpa load 158 Organosilicon functionalized Al2O3 + solid resins materials [37] External coating 400 cm abrasion of sandpaper under 200 g load 165 Surface coating of geopolymer materials Polymethylhydrosiloxane [40] External coating — 161 Polydimethylsiloxane solution containing
polytetrafuoroethylene /
stearic acid and fly ash [39]Maceration Polydimethylsiloxane -only coatings have higher adhesion than polydimethylsiloxane coatings containing fly ash 159 Notes: WCA—water contact angle 表 2 整体改性对超疏水水泥及地聚物材料的水化/聚合作用的影响
Table 2 Effect of Integral Modification on the Hydration/Polymerization of Superhydrophobic Cementitious and Geopolymer Materials
Cementitious types Coating preparation Test method Effects on hydration/polymerization WCA/(°) ordinary silicate cement[62] non-toxic lauric acid and covering metal mesh XRD Integral superhydrophobic concrete has fewer hydration products than ordinary silicate concrete. 153 magnesium oxychloride based cement [48] hydroxyl-terminated polydimethylsil-oxane XRD、SEM Nanoscale needle-like phases are covered by hydrophobic silicone rubber. >150 ordinary silicate cement[50] functionalization of SiO2 with fluorine-free silanes XRD、FTIR Silanes in superhydrophobic powders react with cement hydration products to slow down the cement hydration rate. 153.8 ordinary silicate cement[61] Stearic acid modified fly ash TGA Fly ash can increase the water-cement ratio and facilitate cement hydration, providing more nucleation sites for cement hydration. 93.2 ordinary silicate cement[64] nano-silica and isobutyl-triethoxysilane isothermal calorimeter Silane can mitigate the loss of flowability caused by nano-silica to some extent, while nano-silica can completely compensate for the delay of silane in the early hydration process. 153.5 fly ash based polymer materials [53] polymethylhydrosiloxane Hot Plate Method Thermal conductivity decreases with increasing amount of polymethylhydrosiloxane, and the higher the porosity, the lower the bulk density. 161 fly ash based polymer materials [58] polymethylhydrosiloxane TEM Grafting of poly(methylhydrosiloxane) is not exactly proportional to the amount of geopolymer produced. 152 fly ash-slag base polymer materials [56] isooctyltriethoxysilane BSE、EDX Silanes slow down the hydration kinetics, while the increase in the modulus of the alkali activator inhibits the formation of hydration products. 118.1 slag mortar [57] polydimethylsiloxane MIP、SEM、EDS Polydimethylsiloxane increases internal defects in the body. 128 表 3 不同改性剂的超疏水水泥基材料的接触角和滑动角
Table 3 WCA and SA of superhydrophobic cementitious materials with different modifiers
Cementitious types Hydrophobic Modifiers Modification type WCA/(°) SA/(°) ordinary silicate cement [62] 0.8 wt% non-toxic lauric acid Integral 153 10 magnesium oxychloride based cement [48] 6 wt% hydroxyl-terminated polydimethylsiloxane Integral >150 <10 high belite sulphoaluminate cement [65] lauric acid Integral 153.2 — ordinary silicate cement [66] 1H, 1H, 1H, 2H-perfluorodecyl-triethoxysilane Surface 163.3 — ordinary silicate cement [67] contains mainly octane-silane and siloxane Surface 160±1 6.5±0.5 ordinary silicate cement[68] hydrophobic silica nanoparticles Surface 160 1.7 Notes: SA—sliding angle 表 4 不同改性剂的超疏水地聚物材料的接触角和滑动角
Table 4 WCA and SA of superhydrophobic geopolymer materials with different modifiers
Geopolymer types Hydrophobic Modifiers Modification type WCA/(°) SA/(°) calcined clay and slag [69] 5 wt% polydimethylsiloxane Integral 120 — fly ash [70] 5 wt% stearic acid Integral 96.67 — metakaolin[71] 5 wt% polydimethylsiloxane Integral 127.5 — metakaolin [40] polymethylhydrosiloxane Surface 161 2 dust/silicate cement [72] polydimethylsiloxane Surface 154.1 6.1 表 5 超疏水水泥及地聚物复合材料的吸水率降低程度和原因
Table 5 Extent and causes of water absorption reduction in superhydrophobic cement and geopolymer composites
categories Materials and Dosages Modification type water absorption/% reason ordinary silicate cement 1 wt% stearic acid[74] Integral −86% Surface water is rejected by the surface. 1.4 wt%
cetyltrimethoxysilane[76]Integral −86% The internal inorganic mineralized layer further prevents water intrusion through a relatively dense micro/nano-scale two-layer structure. 5 wt% SiO2 silica solution[30] Surface −90% Hydration products and some unhydrated nanoparticles can clog capillaries, thus blocking water transfer paths. geopolymer materials 60 wt% iron ore tailings +1.5 wt% stearic acid[79] Integral −43% The particle size of superhydrophobic iron ore tailing is much smaller than that of sand, and the fine superhydrophobic iron ore tailing can easily fill up the pores of the mortar, making the mortar more dense and leading to a decrease in water absorption. 10 wt% hydrophobic metakaolin + polydimethylsiloxane [55] Integral −26%~−27.6% Weakening of the capillary's ability to absorb and hold water. polydimethylsiloxane + polypropylene fiber [81] Integral −70% Polypropylene fiber easily adsorbs polydimethylsiloxane but does not easily trap water vapor, blocking the water vapor diffusion channel. 表 6 超疏水水泥及地聚物材料的腐蚀电位和腐蚀电流密度
Table 6 Corrosion potential and corrosion current density of superhydrophobic cement and geopolymer materials
Concrete material name Hydrophobic Modifiers Ecror /V Icorr/(A·cm−2) hardened cement mortar [47] polydimethylsiloxane − 0.07204 4.10×10−8 superhydrophobic surface for concrete [29] stearic acid − 0.28225 — superhydrophobic concrete [82] containing silane and siloxane — 7.702×10−6 superhydrophobic concrete [52] lauric acid −0.477 1.26×10−5 superhydrophobic concrete [96] stearic acid −0.173 5.921×10−7 superhydrophobic iron ore tailings [79] stearic acid −0.321 1.451×10−6 Notes:Ecror is a mixed electrode potential determined by the cathode and anode reactions on the corroded surface; Icorr is the amount of electricity per unit area per unit time of cathodic protection on a metal electrode. -
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目的
混凝土的长期耐久性问题是其面临的主要问题之一,造成耐久性破坏的主要原因是水在混凝土的多孔结构中的迁移,使有害离子更容易进入基材内部。为了提高混凝土的应用耐久性,受“荷叶效应”的启发,通过在水泥及地聚物复合材料的内外表面构建微纳结构或覆盖低表面自由能物质的方式对复合材料进行疏水改性,疏水添加剂可以在材料表面形成疏水膜,从而阻止水的渗透。
方法本文根据目前研究当中的两种主要超疏水改性方式,表面改性和整体改性两个方面入手,介绍了以硅烷改性剂为主的水泥及地聚物复合材料的疏水改性机制。水泥及地聚物材料常用的表面改性方法包括喷涂法、浸渍法、模板法;而整体改性是引入疏水性填料或是加入有机改性剂,同时还可以通过调整混凝土的配合比如水灰比、砂石粒径分布等,改变混凝土的孔隙结构,以提高疏水性能。并总结分析了疏水改性后对复合材料润湿性、防水性、抗压性能以及防腐性能的影响规律。
结果水泥或地聚物复合材料的超疏水表面改性的基本原理是提高材料表面粗糙度及降低表面自由能。提高表面粗糙度可以通过覆盖铜网、砂纸或是加入纳米颗粒来获得,本文所阐述的降低材料表面的自由能主要是通过加入有机硅化合物来实现的。其硅烷改性的基本原理为硅烷可以与大气中的水分或混凝土表面孔隙中的水发生反应,通过三个烷氧基的水解形成硅烷中的含硅烷醇的基团,然后缩聚成低聚物,通过硅烷和混凝土之间的羟基键合,与材料表面形成共价连接。而水泥材料的整体疏水改性主要是水泥与水接触后发生水化反应,超疏水有机改性剂当中的硅烷有机物可以与水泥中的硅酸盐矿物发生反应通过Si—O—Si键合形成连续的有机膜,将水化产物结合成一个整体网络;地聚物复合材料是Si—O和Al—O在碱性环境中下断裂后再重组缩聚,在碱性激发剂的作用下,原料中的硅酸根和铝酸根发生解聚释放出硅酸根离子和铝酸根离子,随着反应的进行,水化硅酸根和铝酸根离子开始相互聚合,形成N—A—S—H凝胶,有机硅化合物与N—A—S—H通过Si—O—Si或Si—O—Al键合,N—A—S—H被疏水基团包围,使复合材料具有整体疏水性。通过对水泥基材料和地聚物材料进行表面改性或整体改性后其水接触角均表现出不同程度的增加;其孔隙率在对复合材料进行疏水改性后也出现了不同程度的增加,但也出现了孔隙率降低的情况;值得注意的是改性后材料的吸水率出现了大幅度降低的情况,尤其是水泥基材料的吸水率最高降低了约90%,阻隔了水分子进入到基体中,提高了材料的耐久性;并且通过对整体改性后的水泥或地聚物复合材料在3.5 wt%NaCl电解液中进行腐蚀试验,发现改性后的复合材料的防腐性能也有所增加;但对两种复合材料进行整体疏水改性后,材料的抗压强度表现出降低的趋势,其抗压强度降低了约20%~60%,这也是目前整体疏水改性所面临的问题之一。
结论通过对水泥和地聚物复合材料的疏水改性的研究现状进行总结,归纳了目前超疏水改性水泥及地聚物胶凝材料的改性方法及机制,并对改性后的复合材料的性能包括润湿性、防水性、抗压性能和防腐性能进行了分析评价。经对比发现,不同方法制备的超疏水复合材料可以使防水性得到较大提升,但也会导致其他性能的下降,例如抗压强度等。可以从体积型超疏水、提高抗压强度、成本控制及疏水外加剂在材料内部实现均匀化分散等方面进行研究。