Interacción entre cementos de diferente composición y aditivos superplastificantes

H Y Ghorab,Z G Abdel All,I M Kenawi

doi:10.3989/mc.2012.63610

Abstract

The slump behavior of ordinary Portland-, pozzolanic (red brick powder)-, sulfate resistant-, and limestone cement pastes caused by ≤ 1% additions of polycondensates and polycarboxylates superplasticizers are monitored for up to 90 minutes. With the plolycondensates, Portland- and pozzolanic cements gain fluidity at higher dosages than sulfate resistant and limestone cements. Limestone cement shows the best slump retention. The aluminate and sulfate phases play a major role in the fluidity. With the polycarboxylates, all cements gain fluidity with dosages of ≤ 0.3%. A polycarboxylate with no resonance of methyl methylene proton in the main chain identified in the NMR spectra creates good slump retention. This is explained by a low mobility of the structure and the predominance of the steric effect. The polycarboxylate shows also strong ether bands relative to the ester groups in the IR spectra and a low polydispersity observed in the elution of few low molecular weight species in the HPLC chromatogram.

Highlights

The high-range water reducers superplasticizers, first known in the sixties (1) are organic admixtures, which cause gain in strength of cementious systems by allowing less water to be used while improving the workability
The mechanisms explaining the dispersion of cement in the presence of organic admixtures are (3): 1) Electrostatic force by the adsorption of sulfonated types such as polynaphthalene sulfonate (PNS) and polymelamine sulfonate (PMS). 2) Steric hindrance effect by grafted chains of the adsorbed polycarboxylates
The X-ray diffractogram of the red brick powder is shown in Figure 1 and proves the presence of quartz (Q) as main crystalline component, small amounts of anhydrite (An), feldspar (F), hematite (He), traces of illite (I) and traces of calcite (Cc)

Summary

Introduction

The high-range water reducers superplasticizers, first known in the sixties (1) are organic admixtures, which cause gain in strength of cementious systems by allowing less water to be used while improving the workability. The polycarboxylates admixtures are dispersants invented in Japan in the eighties (2). By modifying their chemical structures, various properties such as dispersion performance of the cements after mixing, slump retention and setting can be controlled. The mechanisms explaining the dispersion of cement in the presence of organic admixtures are (3): 1) Electrostatic force by the adsorption of sulfonated types such as PNS and PMS. 2) Steric hindrance effect by grafted chains of the adsorbed polycarboxylates. The key parameters which control the adsorption of the polycarboxylates are their ionic charge density, the side chain density and the strength of the electrostatic and steric stabilization (4). The flexibility of the backbone structure may play a significant role in the adsorption process of some adsorbents (5)

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