A case history: Effect of moisture on the setting behaviour of a Portland cement reacting with an alkali-free accelerator
A case history: Effect of moisture on the setting behaviour of a Portland cement reacting with an alkali-free accelerator
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DOI：
10.1016/j.cemconres.2007.02.020
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发表时间：
2007-06-01
影响因子：
11.4
通讯作者：
Salvioni, D.
中科院分区：
文献类型：
--
作者：
Maltese, C.;Pistolesi, C.;Salvioni, D.
闪凝促进剂是隧道开挖过程中常用于喷射混凝土的一类外加剂。它们可以立即凝固混凝土，该混凝土可以粘合到基材上，而不会发生危险的倒塌。这种涂层可以防止截面收敛，即其挤压的趋势。喷射混凝土的早期强度发展（直到1小时）可以与掺有促进剂的水泥浆的最终凝固时间相关。终凝时间越短，早期强度就应该越高。喷射混凝土主要使用两种方法：湿法和干法。如今，出于安全原因并获得更均匀的混凝土层，湿法变得更加普及。该技术需要使用液体闪凝加速器，将其泵送到喷嘴并立即投射到岩壁上。在欧洲市场，主要有两种促凝剂：富碱和无碱。近年来，由于多种原因，无碱促进剂的需求增长非常迅速。它们是总碱金属含量（以Na2O当量表示）低于1%的硫酸铝水溶液。为了开发新的、性能更好的促进剂，正在进行一些研究来阐明其作用机制和影响加速混凝土凝结的因素。在对几种水泥浆体与无碱促进剂反应的凝结行为的实验研究中，我们发现随着时间的推移，水泥表现出持续的凝结时间减少。我们发现这种影响与水泥暴露在湿气中有关。对这种特殊水泥进行的矿物学研究表明，存在半水合物作为凝结调节剂。为了研究无碱促进剂与湿老化水泥之间的相互作用，对水泥浆样品进行了一些形态（ESEM-FEG）、晶体化学（XRD）、物理化学（水化温度曲线）和化学（ICP）分析。这项研究表明，当水泥浆样品由含有先前暴露于湿气的β-半水合物的波特兰水泥组成时，与无碱促进剂混合的水泥浆样品的凝结时间显着缩短。这种效果似乎与β-半水合物溶解速率的降低有关。 (c) 2007 年，爱思唯尔有限公司出版。
Flash setting accelerators are a class of admixtures commonly used for sprayed concrete during tunnel excavation. They allow an immediate set of concrete which can bind to the substrate without dangerous collapses. Such a coating layer prevents the section convergence that is its tendency to squeeze. The early strength development (till 1 h) of the sprayed concrete can be connected to the final set time of cement pastes admixed with the accelerator. The lower is the final set time, the higher should be the early strength. Two main methods are used to project concrete: wet and dry. Today, for safety reasons and to obtain more homogeneous concrete layers, the wet method is becoming more diffused. This technique requires the use of liquid flash setting accelerators which are pumped to a nozzle and immediately projected onto the rock wall. In the European market, two kinds of setting accelerators are mainly available: alkali-rich and alkali-free. Recently, for several reasons, the demand of alkali-free accelerators is growing very rapidly. They are water solutions of aluminium sulphate with a total alkali metal content (expressed as equivalent of Na2O) lower than 1%. In order to develop new and more performing accelerators, several studies are in progress to elucidate their action mechanism and the factors affecting the setting of accelerated concretes.During an experimental study on the setting behaviour of several cement pastes reacting with an alkali-free accelerator, we found a cement showing, as time proceeded, a persistent setting time reduction. We discovered that the effect was connected to the cement exposure to the moisture. A mineralogical investigation performed on this particular cement revealed the presence of hemihydrate as setting regulator. In order to study the interaction between the alkali-free accelerator and this moist aged cement, some morphological (ESEM-FEG), crystal-chemical (XRD), physical-chemical (hydration temperature profile) and chemical (ICP) analyses on cement paste samples were carried out. This study showed a significant setting time reduction of cement paste samples admixed with an alkali-free accelerator when they are composed of a Portland cement containing beta-hemihydrate that was previously exposed to moisture. Such an effect seems to be related to the reduction of the beta-hemihydrate dissolution rate. (c) 2007 Published by Elsevier Ltd.