M. Isabel Tejedor

Detection of Aggregate Clay Coatings and Impacts on Concrete

This article will discuss how deleterious clay minerals often enter concrete as coatings on aggregates. The impacts of the certain clays introduced by this mechanism is the subject of this study. The effects of different clay properties, including cation exchange capacity (CEC) and the nature of the exchangeable cations on concrete, were measured. The material selected for this research was an igneous clean coarse aggregate which, for purposes of repeatability and experimental control, was lab-coated with four different clay suspensions (kaolin, illite, sodium montmorillonite, and calcium montmorillonite). The coated aggregate was then used to make concrete test specimens. The results clearly show that the impact of the clays is not only a function of the CEC of the clay but also of the nature of the exchangeable cation.

Nanotechnology to Manipulate the Aggregate-Cement Paste Bond Effects on Mortar Performance

It is well recognized that the area of contact between cement paste and aggregates, known as the interfacial transition zone (ITZ), is one of the most vulnerable areas of concrete. The microstructure, chemistry, porosity, and mineralogy of this interface affect the adhesion between the aggregate and the cement paste and therefore, under certain conditions, dictate the performance of concrete. The traditional strategy for improving the ITZ has been the addition of pozzolanic materials having a close relation between surface area and particle size. However, the beneficial effects of these additions to the ITZ are diluted because the additives largely end up in the bulk of the cement paste. Research was done to show the benefit and practicality of depositing pozzolanic materials as thin films on the surface of aggregates to improve the ITZ. Preliminary results show that a small dose of these materials can significantly modify adhesion between aggregate and cement paste in the ITZ. The consequence is an overall improvement of the principal mechanical properties and a change in the microstructure of mortar. In particular, mortar made with a 0.0032 silica oxide–cement ratio deposited as a surface coating of just one-third of the total fine aggregates showed an average 35% improvement in compressive, flexural, and tensile strength at early ages along with decreased chloride penetrability. These results clearly indicate that the addition of silica oxides into concrete as thin films on aggregate surfaces has a high potential for improving the overall performance of concrete.

PHYSICAL RESPONSES TO THE INTERACTION OF DEICER AND NATURAL MICROFINES

The interactions of natural microfines and potassium acetate deicer were investigated in laboratory specimens. Microfines (material smaller than 75 µm [0.003 in.]) from five aggregate sources from locations across the western United States were characterized and subsequently studied when subjected to deicer solution while in concrete. Expansion and freezing-and-thawing durability test specimens were monitored and evaluated for damage. Based on the incurred failures and mineralogy type, the microfines were assessed. Among the microfines tested, those incorporating phylosilicates showed the greatest degradation of concrete when subjected to potassium acetate deicer over the duration of the study.

Influence of Curing Conditions on the Impact of Natural Aggregate Coatings on Concrete Performance

Traditionally the effect of microfines in concrete has been evaluated in specimens subjected to a moist curing regimen. However, this type of curing is not representative of real pavement construction conditions in the field where evaporation of water from new pavement surfaces can occur before curing compounds are applied. This study evaluated the impacts of natural microfines from both aggregate fractions, fine and coarse, under ASTM C33–specified requirements, in the performance of concrete cured under a wet (~90% relative humidity) and a dry (30% to 60% relative humidity) environment. The examined concrete batches were fabricated with seven different aggregates, and their associated microfines fell into two major categories: siliceous stone and dolomitic limestone. Results showed that current ASTM C33 thresholds of microfine content were reliable only when specimens were moist cured. The damage induced by dry curing was exacerbated by the presence of these microfines.