First, a 'cement' mixer is an incorrect term used by people who are ignorant about concrete. They also tend to say things like 'cement countertops' and 'cement driveway'. If I feel snarky, I ask them if they make their sandwiches with two pieces of flour.
A drum mixer is for traditional, gravel-based concrete. For concrete to mix, cement particles must be subjected to high shear forces, and gravel in a rotating drum does that. But a drum mixer is pretty much useless for a sand mix, it will give you the same quality of concrete as mixing it in a wheelbarrow with a shovel.
Mortar is concrete made with sand only, no gravel. A mortar mixer must provide shear forces some other way, so the drum is stationary, and there are rotating paddles on a shaft that do the mixing. Usually, the paddles have rubber tips that scrape the walls of the mixer. If you put gravel in a mortar mixer, say goodbye to the drum. You can have horizontal shaft mortar mixers (e.g. Stone) or vertical shaft mortar mixers (e.g. Imer 360), and it's really a matter of preference. The size of the motor or engine makes all the difference. People who have vertical shaft mortar mixers will disagree with me on this one, they swear by their Imers for GFRC mixes.
Personally, I find a good handheld mixer with an egg beater paddle is very effective for sand mixes, and it's about 1/10th of the cost.
Mixer Options for Decorative Concrete
High shear, barrel, drum, even the terms used can be hard to figure out.
There is no magical process occurring when mixing concrete, just good material dispersion which leads to more complete hydration. You are not “chopping up” the cement, just dispersing it (polycarboxalate supers help with Portland based mixes). I’ve mixed and tested a few million yards in everything from wheel barrows to 10 yard high shear mixers, it comes down to dispersion over a given mixing time; the less efficient the mixer, the longer the required mix time.
Simple barrel mixers, the type where the barrel rotates, are used to batch everything from jobsite mortar to highway pavement. Barrel mixers rely on the material folding through itself to mix properly. Barrel mixers mix best at certain batch sizes for a given mixer: too small and the mix rolls around without folding, too big and the mixing action becomes inefficient. Examples of barrel mixers would be ready mix trucks, highway pug mills, or any small rotating barrel portable mixer.
In high shear mixers the barrel remains stationary while the mixer paddles work through the material, this results in a more efficient mixing action which leads to faster mixing cycles (better dispersion faster). You can achieve similar results in a barrel mixer with a carefully controlled charging and mixing procedure but it would take longer in most cases. Examples of high shear mixers would be a drill and paddle used for bucket mixing or a vertical shaft Imer mixer. High shear mixers tend to mix high cement content mixes much faster than barrel type mixers.
Going with what you’ve got.
The optimum mixing method for GFRC is a high-shear mixer. A small high-shear mixer example would be a heavy-duty drill and paddle. Drill and paddle mixers work well for small face-coat batches with no fiber. Much larger-capacity specially made high-shear mixers are available to mix larger quantities of back coat at a time.
Another option is to mix your back coat in a barrel mixer or a vertical shaft mixer. I can mix 250 pounds of back coat at a time in my small barrel mixer and about 800 pounds of back coat at a time in my Imer vertical shaft mixer. Mixing without getting cement lumps is a little tricky.
Add all of the sand, some of the water and one fourth of the cement, all of the System 1 GFRC Admix and mix it to a wet slurry. http://www.trinic.us/System-1-GFRC-Admix-5-lbs-System-1-GFRC-Admix-...
Gradually add the rest of the cement and water.
Once this is fully mixed, add the fiber. Do not over mix the fiber, because you will break it up, rendering it useless. I would say that with my mixers, 1 minute of mixing time after the fiber is added gets it mixed in.
With a little care and some knowledge everyone has the ability to produce high performance decorative concrete.
Thank you Alla and Mark.. now i am enlightened! :)
Thanks to Alla and Mark for the explanations. I'm a little more qualified today. I've picked up a tidbit from both of you.
Alla, I'm afraid the terminology is an uphill battle for you. Around here anyway, even the masons call a rotating drum a cement mixer or cement truck.
When Alla gets snarky you better watch out.