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Cementitious materials obtained from different sources, although evaluated and classified by the same methods and criteria, inevitably produce concrete with compressive strength variance despite other inputs and constituents remaining constant. This paper investigates the effect of cement by type and source on strength as a function of supplementary cementitious material (SCM) replacement and aggregate type. This was accomplished by compiling and analysing compressive strength data from previous research initiatives; it was concluded that a cement source change can produce coefficients of variation, range and range divided by average compressive strength values up to 18%, 2700 psi (18·6 MPa) and 0·40, respectively, at 14 to 56 d time frames. Concrete comprising SCMs displayed a statistically significant increase in compressive strength variation and a general decrease in compressive strength predictability against mixtures without SCMs when a cement source was changed, and specific aggregate/cementitious material combinations influenced compressive strength variability due to a change in the cement source.

Proficiency with instrumental analysis via GC–MS is an important skill for chemistry students. The application of analytical techniques and fundamental theoretical principles to real-world problems can be valuable learning exercises for undergraduates which can also improve their analytical thinking skills. Xylitol is generally considered safe for human consumption and is frequently used in sugar-free gum; however, it is extremely toxic to dogs. In this laboratory experiment, upper-level undergraduate chemistry students extract xylitol from both fresh and chewed gum sticks followed by direct aqueous injection GC–MS analysis. Students learn the proper steps and techniques required for sample extraction and preparation and GC–MS analysis, and they determine concentrations of xylitol present in gum samples. Identification and quantification of the chemical components in gum extract occurs via GC–MS analysis; however, other GC detectors could be used. Students also compare external and internal standard calibration methods for xylitol quantification. Upon quantification of xylitol in chewed and unchewed gum samples, students are able to calculate the level of hazard for dogs upon ingestion.