ASTM C305
ASTM C305: Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency
ASTM C305 standard outlines the method for mechanical mixing of hydraulic cement pastes and mortars to achieve plastic consistency. Adopted initially in the designated year and subsequently revised, this standard emphasizes the proper mechanical mixing techniques for testing hydraulic cements. It's essential to note that the standard doesn't address all safety concerns and requires users to establish appropriate safety and health practices, considering any regulatory limitations before use.
The standard references ASTM Standards, such as C 778 for Standard Sand, and emphasizes the importance of using appropriate materials and methods. The primary significance of this practice is to standardize the mechanical mixing of pastes and mortars for testing hydraulic cements, ensuring consistent and reliable results across different tests.
Equipment and Conditions
The mixer used should be an electrically driven mechanical mixer of the epicyclic type, capable of both planetary and revolving motion. It must have at least two speeds with specific rates for the paddle and planetary motion. The clearance between the paddle and the bowl is critically defined, and the apparatus includes an adjustment feature to maintain this clearance. The paddle, made of stainless steel, and the mixing bowl, typically with a 4.73 L capacity, should be designed according to specified dimensions and shapes. A nonabsorbing lid, a semirigid rubber scraper, and other supplementary apparatus for measuring and preparing materials are also required.
Temperature, Humidity, and Material Preparation
The room temperature for mixing should be maintained between 20 and 27.5 °C, with specific temperature ranges for dry materials and mixing water. The relative humidity in the laboratory should not be less than 50%. Materials used should conform to the requirements of the test for which the paste or mortar is being prepared.
Mixing Procedure for Pastes and Mortars
For pastes:
Place dry paddle and bowl in the mixer.
Introduce materials: first water, then cement.
Mix at slow speed for 30 seconds, followed by a 15-second pause for scraping.
Mix at medium speed for 60 seconds.
For mortars:
Same initial steps as for pastes.
After adding cement to water, mix at slow speed.
Gradually add sand over 30 seconds, continue mixing at slow speed.
Change to medium speed for 30 seconds, then pause for 90 seconds (first 15 seconds for scraping).
Finish with a 60-second mix at medium speed.
Both processes emphasize the importance of specific time intervals and speed settings for mixing to achieve the desired consistency. The standard also includes warnings about the clearances between the paddle and the bowl, especially when using coarser aggregates, to avoid damage to the mixer.
Conclusion
ASTM C305 standard provides detailed guidelines for the mechanical mixing of hydraulic cement pastes and mortars. This practice is crucial for ensuring consistent, repeatable, and reliable results in the testing of hydraulic cements, forming a fundamental part of quality control in cement manufacturing and construction. It's vital for users to follow the detailed specifications for equipment, mixing procedures, and environmental conditions to achieve accurate and dependable outcomes.