Role of Supplementary Cementitious Material Type in the Mitigation of Calcium Oxychloride Formation in Cementitious Pastes

Abstract

Premature deterioration has been observed at some joints in concrete pavements. This joint damage has been attributed, in part, to a deleterious chemical reaction between chloride-based deicing salts (e.g., calcium chloride) and calcium hydroxide from the cementitious matrix resulting in the formation of a compound referred to as calcium oxychloride. Calcium oxychloride formation can be mitigated in cementitious pastes through the replacement of cement with supplementary cementitious materials (SCMs) (e.g., fly ash or slag). Although various SCMs are beneficial in mitigating the formation of calcium oxychloride, little has been written to describe similarities or differences between the different types of SCMs relating to their ability to mitigate calcium oxychloride formation. This paper compares various SCM types. Cementitious pastes were prepared with between and 6% of the cement being replaced (by volume) with SCMs for water-to-cementitious materials ratios (w/cm) of .36 and .5, which are the lower and upper bounds of the w/cm typically used for pavements and sidewalks. Pastes were cured to develop a high degree of hydration, and subsequently ground to a fine powder for use in thermogravimetric analysis and low temperature differential scanning calorimetry testing to quantify the calcium hydroxide and calcium oxychloride amounts, respectively. The composition of the specific fly ash and slag used influences the calcium hydroxide and calcium oxychloride amounts. As the w/cm increases, calcium hydroxide and calcium oxychloride amounts increase; however, this amount was only between 4 and 3% for the calcium hydroxide and various SCMs. As the volume of supplementary cementitious materials replacing the cement increases, calcium hydroxide and calcium oxychloride amounts decrease. The supplementary cementitious materials reduce the calcium hydroxide and calcium oxychloride formed in the order calcined clay > silica fume ≫ fly ash > slag ≫ limestone. The contributions of the SCMs to reducing calcium hydroxide (CH) and calcium oxychloride (CAOXY) due to dilution and reaction are separated. The potential benefits of different SCMs in reducing calcium hydroxide amounts in paste can be predicted based on their pozzolanicity (determined from their response in pozzolanic tests based on isothermal calorimetry and thermogravimetric analysis).