1、 中文 4300 字 Combined effect of mineral admixtures with superplasticizers on the fluidity of the blended cement paste Abstract： The new concrete often incorporates several organic and mineral admixtures which interact with the various constituents of the cements and cause some problems of hardness and
2、 workability. In the present study, limestone cement (C1) and pozzolanic cement (C2) were used to make cement paste with two types of superplasticizer； SP1 based on polynaphthalene sulphonate (PNS)； and SP2 based on resins melamines (PRM). Marsh cone test was adopted to check the combined effects of
3、 the following factors on the fluidity namely the type of cement, the type and the dosage of the superplasticizer, the type and the replacement rate of the mineral admixture and the watercement ratio (W/C). The results of this work show that limestone cement presents a high fluidity with low loss af
4、ter 1 h relatively to the pozzolanic cement within the saturation proportioning. Superplasticizer SP1 constitutes an incompatibility case when it is mixed with cement containing high C3A or alkali content such as C2 cement. Also, limestone powder is found to be the best mineral admixture when it rep
5、laces a part of cement, where more fluidity is exhibited caused by the dilution effect. Keywords： Cement， Limestone powder， Natural pozzolan， Superplasticizer， Fluidity 1. Introduction Currently, the essential novelty appeared in cement industry is actually the increase use of the mineral admixtures
6、, substituting a part of cement to reduce the carbonic gas emission, to minimize the cement cost and to improve some technical performances. High performances concretes made with low W/C ratio require the use of suitable and compatible superplasticizers with the new cements which can transform a con
7、crete with high consistency into a concrete with high workability. During the use of superplasticizers in concrete, certain cements can sometimes present some problems of incompatibility of cementsuperplasticizer； irregularity of slump and rapid workability loss. The principal approach provided to c
8、ombat against this difficulty is to select the most efficient couple cementsuperplasticizer, enabling to obtain a maximum water reduction, a better workability and an acceptable rheology during the placement and the finishing concrete. The incorporation of some mineral admixtures such as blast furna
9、ce slag, fly ash, silica fume or natural pozzolan can make the interaction between the cementitious materials and superplasticizers more complex, and therefore the selection of the compatible couple requires further consideration. The adsorption of superplasticizer molecules on hydrated phases creat
10、es an electrostatically charged germ which participates to the electrostatic repulsion and avoids flocculation. Also, the cement paste characterized by the long needle of ettringite formed at early age usually decreases the paste fluidity . There exists an optimum soluble alkali content with respect
11、 to the fluidity and fluidity loss, which was found to be 0.40.5% Na2O equivalent. At this optimum alkali content, the initial fluidity is maximum and fluidity loss is minimum. Neubauer et al.4noted that superplasticizer causes the zeta potential of the cement pastes to become increasingly negative；
12、 it suggests that this superplasticizer begins to disperse the cement particles. When new superplasticizers are developed, an interaction problem must be anticipated, cement and superplasticizer will be able to cause sharp variation in fluidity and produce stiffness, depending upon the combination o
13、f cement and superplasticizerSwamy et al. works concluded that it is possible to reduce the content of superplasticizer by incorporating slag in the cement； the replacement of the cement by 70% slag reduces 10% of the amount of superplasticizer necessary to get the same workability. The results cond
14、ucted by Duval and Kadri confirmed that the superplasticizer adsorption depends both on the amount of C3A and the presence of soluble alkali sulphates in the cement. It was proved that the incorporation of fly ash in concrete reduces the need of superplasticizer necessary to obtain a similar slump f
15、low compared with the concrete containing only cement as binder. On the other hand, Sone et al. observed a total loss of fluidity when Portland cement was replaced by blended cement, where superplasticizer content changes from 0.5% to 3%. When the superplasticizer presents a compatibility with a cer
16、tain mixture composition, it will lose it as soon as the mineral admixture is substituted. Similarly, Bensebti and Houarifound that the fluidity of the cement paste decreases with the introduction of the fillers, this reduction is proportional to their replacement level and type. The investigation o
17、f cementsuperplasticizer (CSP) compatibility can be realized by measuring flow time of grout as proposed by several researchers1113. The cement paste fluidity results usually are represented by a curve indicating the flow time CSP system according to superplasticizer dosage at 5 and 60 min age. The
18、type of curve obtained presents three essential points which control the rheological behavior of the cementsuperplasticizer studied and are expressed as follows: Saturation superplasticizer dosage corresponding to a break in the curve when superplasticizer is added over the saturation point； it does
19、 not improve any more the fluidity of CSP but only increases the risk of sedimentation and delays the cement setting time. Fluidity level reached for this saturated dosage (flow time)； which will be small as much as the paste is fluid. In this text, the fluidity term is the opposite of viscosity. Fl
20、uidity loss related to the two curves at 5 and 60 min； it can be expressed by the difference between these two times, which must be very low for compatible couples of cementsuperplasticizer. The objective of this work is to study the interaction cementsuperplasticizer by measuring the fluidity of the cement paste in order to select the compatible superplasticizer with the given cement.
