废弃混凝土再生新技术探索【摘要】本文对目前废弃混凝土再生技术的研究做了论述,并指出了其中存在的一些问题。
结合混凝土各组成部分的结构特点,提出了通过低温煅烧对废弃混凝土综合利用的新方法。
在750℃温度条件下煅烧1h,可以实现水泥浆与骨料的分离。
脱水后的水泥浆可以重新获得水化活性。
得到的混凝土骨料可以满足使用要求。
【关键词】废弃混凝土;再生技术;煅烧;水化活性;压碎指标【中图分类号】TU352·8【文献标识码】A【文章编号】1001-6864(2009)09-0004-02国家“十五计划”纲要指出:“坚持资源开发与节约并举,把节约放在首位,法保护和合理使用资源,提高资源利用率,实现永续利用。
推进资源综合利用技术研究开发,加强废旧物资回收利用,加快废弃物处理的产业化,促进废弃物转化为可用资源。
”保护环境、节约能源、减少废料、以持续的方式使用可再生资源是可持续发展战略的重要内容。
建材工业是典型的基础原料工业,在国民经济发展中具有重要作用。
建材工业又是典型的资源、能源消耗型工业,在其快速发展的同时,面临着资源、能源的过度消耗和环境的严重污染。
建筑和建材行业的根本出路就是走可持续发展的道路[1]。
起初,我国对混凝土的利用仅是简单的破碎充当再生粗骨料,这种生产的再生骨料性能与天然粗骨料的性能存在一定差异,主要表现在密度低、吸水率高、压碎指标大,表明再生骨料的空隙率高,强度低,这主要是由于其表面附着有大量水泥砂浆及在破碎过程中引入一定量的微裂纹的缘故[2],生成的混凝土性能低,耐久性、抗冻融、抗腐蚀能力差。
研究者根据再生骨料再利用过程中存在的问题,对再生骨料进行了物理、化学改性以及整形改性。
如朱崇绩等通过整形除去再生骨料表面的砂浆,使颗粒变得光滑,需水量降低,使所配制的混凝土收缩降低,但仍高于天然骨料混凝土[3]。
没有解决再生骨料中微裂纹带来的弊端。
目前有研究者对废弃混凝土进行了综合利用研究,通过筛分获得砌筑砂浆或进步筛分生产具有水化活性的再生水泥。
如孙荣光等[4]对旧水泥浆高温处理后的再水化胶凝特性研究,得出再生水泥具有再水化的能力,同时生成C-S-H凝胶、Aft和CH等物质,说明水化产物结构相同,但由于大量脱水相的存在使水化速度快。
余睿等[5]通过对水泥浆的研究得出石膏和粉煤灰组成改性剂能延长活化水泥浆的初凝时间,增强其抗压强度,但不能减少活化水泥浆的标准稠度需水量。
由于易水化的水泥石脱水需要时间,所以煅烧时间和脱水温度对再生水泥性能不容忽视。
1．废弃混凝土裂解温度确定混凝土是由水泥、粗细集料、矿物掺合料等加水拌合,经水化硬化而形成的一种微观不均匀,宏观均匀的人造石。
废弃混凝土在低温煅烧时的温度由水泥脱水温度与石灰石分解温度共同决定。
1·1水泥水化产物脱水温度文献认为,含水矿物中普通吸附水的脱水温度一般为100~110℃,存在于层状硅酸盐结构中的层间水或胶体矿物中的胶体水多数要在200~300℃以内脱水,个别要在400℃以内脱水;架状结构的硅酸盐结构水则要在400℃左右才大量脱出。
结晶水在不同结构中的矿物中结合程度不同,其脱水温度也不同。
结构水是矿物中结合最牢的水,脱水温度较高,一般要在450℃以上才脱水[5]。
为了确定废弃混凝土的低温煅烧温度,保证在该温度条件下骨料和水泥浆能够充分分离,水泥石完全脱水,本课题对水泥浆进行了热重分析(TGA),热失重曲线及(TG)及差热曲线(DTG)如图1所示。
从图1可以看出,水泥浆在100℃前后分别有一个明显的失重过程,理论上应当是失去自由水和弱吸附水所形成的过程。
在500℃以前又有一个明显的失重过程,该失重过程温度范围较窄,但失重速度较快,从水泥石的组成看应当是Ca(OH)2失水形成的。
另一个水泥石脱水过程发生在682~775℃之间,脱水的峰值速率发生在740℃。
这一过程脱水过程剧烈,持续时间较长,这一失水过程使C-S-H凝胶分解,重新生成为具有潜在水化活性的胶凝材料。
温度继续升高,直至1000℃再无明显的吸热峰出现。
另外,温度再渡升高可能影响混凝土骨料的结构。
因此,混凝土分解温度确定为750℃。
经过高温处理的水泥浆因脱水而重新获得水化活性,这一性能已在实验中得到证实。
1·2碎石分解温度粗骨料(碎石)的成分主要由CaCO3和在少量的MgCO3组成,其中MgCO3的分解温度较低,600℃就开始剧烈反应,CaCO3剧烈分解温度则更高达到900℃左右,但其开始分解温度较低,在650℃就开始分解。
所以在550~900℃范围内煅烧混凝土时,随着温度的升高,骨料发生分解的量增大,使骨料的强度降低。
过渡区是混凝土的薄弱环节,一方面搅拌时在骨料表面被水膜覆盖,妨碍了水泥粒子与骨料表面接触,骨料周边水灰比变大,另一方面,水化反应溶出的迁移性大的Ca2+、Al3+、SO4-3离子为使浓度平衡向骨料表面扩散。
在骨料表面附近,六角板状结晶的氢氧化钙和针状结晶的钙钒石的析出比较显著,形成大且连续的空隙构造[8]。
使过渡区的水分也几乎以层间水及自由水的形式存在,所以在较低的煅烧温度就可实现骨料与水泥砂浆的分离,理论上在300℃以上即可实现分离。
为了探究煅烧温度对骨料强度的影响,本试验分别对600、700、800℃煅烧温度时所得到的粗骨料压碎值进行了研究,并与天然碎石和直接破碎的混凝土进行了比较,实验结果如图所示。
从图2可知在燃烧温度600℃时,碎石的压碎指标略高于天然碎石,而随燃烧温度的提高,压碎指标明显增大,标志着骨料强度降低。
但在800℃所得到的骨料强度仍高于混凝土直接破碎得到的骨料强度,所以在混凝土煅烧温度不超过800℃时是可行的。
2结语根据混凝土各组分的结构特点,采取煅烧的方法把骨料和水泥浆分离是可行的。
经分解后的粗骨料可以在新拌混凝土中应用,其强度高于混凝土直接破碎得到的再生骨料。
经分离得到的水泥浆重新获得了再次水化的活性。
为废弃混凝土的回收以及骨料和水泥浆的再生和利用开辟了一条新的途径。
参考文献[1]水中和,曹蓓蓓,万惠文.废弃混凝土再生利用技术及其发展前景[J]·资源节约与综合利用,2006,(3):8-11·[2]王耀新.混合应用天然与再生骨料混凝土的基本性能[J]·混凝土,2005,(8):49-53·[3]朱崇绩,李秋义,李云霞.颗粒整形对再生骨料混凝土耐久性的影响[J]·水泥与混凝土,2007,(3):6-10·[4]孙荣光,余睿,玄东兴,水中和.旧水泥浆高温处理后的再水化胶凝特性研究[J].武汉理工大学学报,2007,(9):115-118·[5]余睿,水中和,玄东兴.再生水泥浆的改性研究[J]·2009,(4):104-107·[6]SidneyMindess·混凝土(原著第二版)[M].吴科如,等译·北京:化学工业出版社,2005·1·[7]王培铭,许乾慰.材料研究方法[M].北京:科学出版社,2005,236-249·[8]徐新生,徐卫生.混凝土过渡区对混凝土物质渗透性影响[J]·山东建材,1999,(3):10-13·EXPLORATION ON RECYCLING TECHNOLOGY OF WASTE CONCRETEMA Xin-wei1,ZHANG Dong-qing2,YINWei-feng2,LI Ning2 (1·Department of Civil Engineering, Harbin Institute ofTechnology at Weihai, Shandong Weihai 264209, China;2·School ofMaterials Science and Engineering, Harbin Institute ofTechnology, Harbin 150001, China)Abstract:The current research situation on the recycling technology ofwaste concrete and existing problems in applicationwas summarized·Consideringthe structural characteristics of cement paste and aggregate, a new recycling technology through calcination is presented·The cement paste and aggregates could be separated at 750℃for one hour·The dehydrated cement paste can have reactivity again, and the aggregates can meet the requirement of application·Key words:waste concrete;recyclyingte;chnology;calcination;reactivity;crushing indexNational "tenth five-year plan" outline pointed out: "insists resource development and save the save itself, in the first place, protect and rationally use method of resources, and improve the resource utilization rate, to achieve sustainable use. Push technology research and development resources comprehensive utilization of waste materials, strengthen recycling, accelerate the industrialization, promote waste disposal of waste into usable resources." Protect the environment and save energy, reducing waste, with continuous way, using renewable resource is the important content of strategy of sustainable development. The building materials industry is typical of basic raw materials industry in the national economy development, plays an important role. The building materials industry is typical resources, energy consumption industries in its rapid development, at the same time, facing the excessive consumption of resources, energy and environmental pollution. Construction and building materials industry is the fundamental way to walk the path of sustainable development [1].At first, the use of concrete in the crushing only a simple as recycled coarse aggregate, the production of natural regeneration aggregate performance and the performance of coarse aggregate, main show there is some difference in density low, bibulous rate is high, crush index is big, show the gap regeneration aggregate rate high, low intensity, and it is mainly because the surface adhesion has large cement mortar and crushing process in certain micro cracks of introducing the sake [2] and generated concrete performance low, durability, freezing-thawing resisting, corrosion resistance is poor. The researchers based on renewable aggregate reuse the problems existing in the process of recycled aggregate, carried on the physical and chemical modification and plastic modification. As such ZhuChong performance by plastic toremove the mortar, renewable aggregate surface water that particles, smooth, prepared by reducing shrinkage of concrete lower, but still higher than the natural aggregate concrete [3]. Not solve the regeneration aggregate micro cracks in the malpractices. Currently have researchers of the waste comprehensive utilization of concrete research, through the screening get bonding mortar or progress screening production of renewable cement hydration activity with. If SunRongGuang etc [4] to the old cement. after high temperature treatment of slurry rehydration characteristics study gel, it is concluded that renewable cement with a rehydration ability, while generating C - S - H gel, Aft and CH that hydration products such material, but same structure dehydrated phases because of the existence of hydration speed to. YuRui etc [5] for water mud through the research that fly ash and composition modifier gypsum slurry can extend the initial setting activation time and strengthen the compressive strength, however does not reduce activation of standard slurry water viscosity. Due to easy hydration of cement stone dehydration need time, so calcining time and dehydration temperature of recycled cement performance cannot be ignored.1. Determine the decomposition temperature of waste concreteConcrete is made of cement, thick aggregate, such as mineral admixture, the water hydration sclerosis regarding mix-proportion formed a microscopic uneven, macro uniform man-made stone. Abandoned the concrete in low temperature calcination temperature by cement dehydration temperature decomposition temperatures decided together with limestone.1 · 1 cement hydration products dehydration temperatureLiterature in minerals that water cut the dehydration temperature ordinary adsorption water for 100 ~ 110 ℃commonly, exists in layer silicate structure between layers of the colloid water or colloid mineral water in 200 ~ 300 much ℃in within dehydration, individual ℃400 within dehydration; Frame shape structure of silicate structure in 400 ℃water is only around large emerge. The minerals in different structure gesso combination of different degree, the dehydration temperature is different. Structure combination of water is mineral water, the most jail dehydration temperature is higher, typically in 450 ℃above just dehydration [5]. In order to determine the low temperature calcining temperature waste concrete, guarantee the temperature conditions in slurry aggregates, and can be fully separation, cement stone completely dehydration, this topic to the slurry thermogravimetric analysis (TGA), thermo-gravimetric curve and (TG) and differential thermal curve (DTG) as shown in figure 1 below.Temperature（℃）Slurry thermogravimetric analysis curvesFrom figure 1 in 100 ℃, water slurry before an obvious weightlessness, respectively, in theory should be losing process of free water and weak adsorption water by the formation of the process. In 500 ℃ago and have an obvious of weightlessness process, this weightlessness process temperature range is narrower, but weightlessness faster, the composition of the cement paste.the watch from shall be the Ca (OH) 2 water loss formation. Another cement stone anhydration process occurs in 682 ~ 775 ℃between, dehydration rate occurred in 740 ℃peak. This process a severe dehydration process, continues for a long time, the water loss process makes C - S - H gel decomposition, born again into a potential hydration activity of cementitious material. Temperature continues to rise again until 1000 ℃, has no obvious heat-sink peak appearing. In addition, temperature rise again crossing may influence the structure of concrete aggregate. Therefore, concrete decomposition temperature determined for 750 ℃. The slurry high-temperature processing for dehydration and regain hydration activity, this performance has been confirmed in the experiment.1 ·2 gravel decomposition temperatureThe coarse aggregate (gravel) mainly by the ingredients in a small amount of CaCO3 and MgCO3 composition, which MgCO3 decomposition temperature is lower, 600 ℃began to react vigorously CaCO3 decomposition temperatures, severe ℃is high reached 900 around, but its start decomposing temperature is lower, at 650 ℃began to decompose. So in 550 ~ 900 ℃ scope when calcined concrete, with the rise of temperature, aggregate ratio increases occurred decomposition, aggregate strength decreased. Transitional zone is the concrete and the weak link in aggregate, on one hand, mixing the surface is water film covering, hampered cement particles and aggregate surface contact, aggregate peripheral water-cement ratio change, on the other hand, the transference of hydration reaction dissloution Ca2 +, big Al3 +, SO4-3 ion to make to the aggregate balance of surface concentration of diffuse. In aggregate, hexagonal near the surface plate crystallization of calcium hydroxide and needle crystal stone precipitation of calcium vanadium, forming a notable large andcontinuous space structure [8]. Make the moisture in the transition between layers also almost free water and water exists in the form of lower, so in the calcining temperature can achieve aggregate and cement mortar in the separation of more than 300 ℃, the theory can be realized separation. In order to explore calcining temperature effect on the strength of aggregate respectively, and the test of 600, 700, 800 ℃ calcining temperature get the coarse aggregate crush values are studied, and directly with natural gravel and broken concrete were compared, and the experiment results as shown.600 700 800Natural aggregate broken concreteFrom graph 2 600 ℃in combustion temperature that when the crush index, gravel, and slightly higher than the natural macadam with combustion temperature rise, crush index increased, marked the aggregate strength decreased. But in 800 ℃obtained aggregate strength is still higher than the aggregate concrete crushing get directly in the concrete intensity, so no more than 800 ℃calcination temperature is feasible.2 .EpilogueAccording to the structure characteristics of various components of the concrete methods of, take the aggregate and pellets slurry separation is feasible. After the decomposition of coarse aggregate in concrete in new mix, its strength of concrete application of crushing get directly than renewable aggregate. The slurry isolated from getting the hydration activity again. For the collection and aggregate concrete waste water and mud regeneration and use open a new way.References[1] water and CaoBeiBei, WanHuiWen. Abandoned, recycled technology and its development of concrete [J] prospect, conservation of resources and comprehensive utilization, 2006, (3) : 8-11 ·[2] WangYaoXin. Mix with application of natural regeneration aggregate concrete, basic properties of concrete [J], 2005, (8) : 49-53 ·[3] ZhuChong performance LiQiuYi, LiYunXia. Particles, of recycled aggregate concrete durability plastic [J], the influence of cement and concrete, 2007, (3) : 6-10 ·[4] SunRongGuang, YuRui, XuanDongXing, the water and the mud. Old water high temperature treatment rehydration gelling properties research [J]. Journal of wuhan university of technology, 2007, (9) : 115-118 ·[5] YuRui, the water and the mud. XuanDongXing renewable water, modified study [J] DE 2009, (4) : 104-107 ·[6] SidneyMindess ·concrete (original second edition) [M]. WuKeRu, etc, Beijing: chemical industry translation press, 2005 · 1 ·[7] WangPeiMing, research methods XuQian comfort. Materials [M]. Beijing: science press, 2005,236-249 ·[8] XuXinSheng, XuWeiSheng transition region. Concrete of concrete material [J], osmotic influence of shandong building materials, 1999, (3) : 10-13 ·。