Thano Drimalas

Characterization of Fly Ashes for Sulfate Resistance

The sulfate resistance of mixtures containing six fly ashes was studied with reference to their chemical and mineralogical compositions. Quantitative analysis of the mineralogical composition of fly ashes was carried out through X-ray diffraction (XRD) using the Rietveld method of analysis. Bulk and glass composition of the fly ash particles was also determined by using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDXA). The sulfate resistance of fly ash mortars was evaluated using the ASTM C1012/C1012M test. The performance of each fly ash in the sulfate resistance test was related to the chemical and mineralogical composition of the fly ash and its position on the ternary (CaO-SiO2-Al2O3) diagram. The study showed that the nature of glass is an important factor in determining the sulfate resistance of fly ash mixtures in a sulfate environment.

Five-Hour Autoclave Test for Determining Potential Alkali-Silica Reactivity of Concrete Aggregates: A Multi-Laboratory Study

Alkali-silica reaction (ASR) is a deleterious chemical reaction that can lead to expansion and cracking of concrete structures. The reaction occurs between alkali hydroxides in the pore solution and reactive forms of silica present in some aggregates. To successfully mitigate ASR in new structures, the potential alkali reactivity of the aggregates must be known. Standardized test methods for this purpose are sometimes too time-consuming for rapid military construction operations. To assess the viability of a five-hour autoclaved mortar bar test method for ultra-rapid identification of aggregate reactivity, researchers in five laboratories participated in a multi-laboratory study led by the U.S. Army Engineer Research and Development Center (ERDC). The test method uses fine aggregate in mortar bars like those used in ASTM C1260, Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method), but the equivalent alkali content of the mortar is boosted to 3.5 % by mass of cement. Mortar bars are cured for 48 hours prior to autoclaving at 130°C for five hours at peak temperature. This paper details the test procedure, expansions, comparisons with the accelerated mortar bar test and the concrete prism test, within- and multi-laboratory precisions, and investigation into the amount of alkali leaching during the test. For 85 % of the 20 aggregates tested, there was agreement between the autoclave test and ASTM C1260 in determining whether or not an aggregate was reactive to some degree. The agreement between the autoclave test and ASTM C1293, Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction, was 100 % for ten of the aggregates for which ASTM C1293 data were available. The average within- and multi-laboratory coefficients of variation were 5.9 and 20.0 %, respectively. Results showed alkali leaching to be between 6 and 9 % per mortar bar. This study demonstrated the viability of the five-hour autoclave test for rapidly assessing aggregate reactivity in military construction operations.