Operator-Induced Variability Caused by Hand Mixing of Cement Paste—Effects on Fresh and Hardened Properties

Laboratories typically adapt ASTM C305-14, Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency, ASTM C1738-19, Standard Practice for High-Shear Mixing of Hydraulic Cement Pastes, and other standardized mixing methods to prepare cement paste and mortar specimens for testing. Hand mixing is adapted when small amounts of material are available (for example, pure phases) or necessary for testing (for example, isothermal calorimetry testing). Although a majority of past research has focused on studying the dependence of cement paste rheology on mixing, relatively fewer studies have focused on hydration and hardened properties of cement pastes. In this study, fresh and hardened cement paste properties were tested at two water-to-cement ratios (w/c): 0.36 and 0.45. The following tests were performed: flow test, set time, isothermal calorimetry, thermogravimetric analysis (TGA), apparent density, and entrapped air. Results from hand mixing were compared to those from mechanical mixing (ASTM C305-14). Operator-induced variability that was due to hand mixing was assessed by comparing results obtained by three operators. Data were quantitatively evaluated using the coefficient of variation (CV) values for hand-mixing results, and the percentage difference (PD) between the mechanical mixing result and the average hand-mixing result. For most results from the tests conducted, CV and PD were below 5 %, and hence, they may be considered insignificant. A few measured parameters (certain flow diameters, set times, peak heat time) showed CV and PD values ranging from 5 to 10 % and could be potentially significant. The highest values of CV and PD were found for entrapped air, likely because values of entrapped air were low. These results suggest that for carrying out certain tests on cement pastes in the lab such as isothermal calorimetry and TGA, hand mixing could potentially be used instead of ASTM C305-14 without altering results.