Susan,

A home made ferro-cement tank is a lot of physical work and takes plenty of planning, but is possible.

Ferro-cement vessels are not new. During WWII some liberty ships were made of ferro-cement. As you probably know, cement theoretically never finishes drying. This factor was made clear when those liberty ships kept settling lower and lower in the water. The last I heard, which was along time ago, those ships were still being used as a breakwater somewhere along the southwest coast. The problem was the cement used in those ships had a soluble chemical that was released as the cement dried. Over time that chemical was displaced with water. Making the ship heavier.

Back to the subject. Ferro-cement is a combination of ferric metal, usually rebar, and cement. The strength of a ferro-cement structure is the metal and the glue and filler is the cement. Before there was precast structures, I designed structures used underground in all the various soil types. As you stated, for standard cement, to much water weakens the mixture. But with less water, it is harder to work. The actual design to strength of the structures I specified were fairly thin, but it was physically very difficult to make the thick cement mixture flow to the bottom of the form around and through all that rebar. If we over used vibration techniques to get the cement mixture to flow to the bottom of the form, we would have the heavier aggregate settle below the fines which weaken the structure. Inadequate flow resulted in voids. To combat this problem, I changed the design to make the forms wider. This increased cost and weight but it worked.

You might try the ferro-cement boat building technique of forming the tank with rebar. The bottom, sides, and top all at once. Don’t forget the input and output fixtures as well as the access lid and provision for a ladder. There are many books on ferro-cement boat building techniques. Now for the really hard part. Get a number burley men and on both the inside and outside of the tank have them force the thick fairly dry cement mixture into the rebar tank structure between them, similar to a riveter and his helper. It is best if this task can be completed in one day for the bottom and sides. The top can be cemented later. Just don’t be the one on bottom of the top pushing up. It is very tiring and messy. Remember, neatness counts. There are chemical adhesives that let you continue between successive days if you can’t finish in one day but it is second choice. You won’t have a problem with different expansion rates between rebar and cement with such a small structure. Remember for every foot height of water in the tank the pressure per square inch increases by 2.31 pounds. So, a seven foot high water tank will have a force of 16.17 pounds per square inch at the bottom of the side walls.

There are a number of coatings you can use to line the tank. Some have been used for thousands of years. Ceramic tile is one of them. Don’t think that bacteria, mold, and algae will not grow without sun light. Every pore in the cement will harbor nasties and will be difficult to clean. All concrete pipes now used for water distribution are ceramic coated on the inside to avoid this problem.

Also familiar yourself with confined space safety procedures. I have a good video on the subject but it is for commercial applications. The best procedures can be condensed into three steps. Purge, test, and ventilate. Rent a ducted ventilator and an explosive gas detector. Once you determine the cubic volume, there are tables that tell how long you should vent the tank. Then test for explosive gases. Once your inside the tank, keep the ventilator on full time. You don’t want to fall victim to inert gas narcosis. Don’t throw in a lit match to test instead of using an explosive gas detector. I saw an idiot do that in down town Salem, Oregon during rush hour one morning and watched as successive manhole covers blew off for blocks in four directions.

If I can help further, if you call this help, let me know.