Quantitative analysis of interactions between portland cements and superplasticizers
Abstract
A new approach to studying interactions between chemical admixtures and cements has been developed in this research. Features include repeated analyses at short intervals of physico-chemical parameters important in admixture effects including the sulfate-bearing phases of the cement (gypsum, hemihydrate, and insoluble anhydrite), ettringite, and concentrations of the admixture and of inorganic ions in the paste solution. The gypsum, hemihydrate, and ettringite analyses are made by DSC; anhydrite is analyzed by QXRD; admixture concentrations are measured by UV spectroscopy; and inorganic ion concentrations are measured by conventional methods. The physical behavior of the paste is also monitored. Studies have been carried out using both naphthalene sulfonate and melamine sulfonate superplasticizer admixtures. The methods developed can be applied for any chemical admixture. Two different conventional portland cements and a special white cement have been used. The white cement has a very low alkali content, and anhydrite is the single form of calcium sulfate present. The effectiveness of the superplasticizer has been found to depend on maintaining a reasonable concentration of it in solution. Cement mixes that do not were found to stiffen and set prematurely. This response was found to be associated with a low concentration of sulfate in the paste solution. Adding sulfate reduces the early uptake of superplasticizer and allows the admixture to function properly. In all cases it has been found that the presence of the superplasticizer modifies the early pattern of ettringite development. Na$\sp+$ ions balancing the negative sulfonate sites in the superplasticizers are immediately detected in the paste solution. As the dissolved polymer is incorporated into the hydrating cement, the sulfonate sites are replaced by OH$\sp-$ ions, leading to a permanent increase in the OH$\sp-$ ion concentration. The early absorption of superplasticizer by the hydrating cement was found to be reversible under some circumstances. When excessive amounts of alkali hydroxides were added to superplasticized white cement, premature stiffening of a variant kind was developed.
Degree
Ph.D.
Advisors
Diamond, Purdue University.
Subject Area
Civil engineering
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