Block mix design and cost calculation

Submitted by sbclark on Sun, 16/09/2012 - 06:54

Hello Geoffrey,

I know it has been awhile since hearing from us. Have been very busy and a little discouraged with the testing of the bricks and the different soils. I am sending three different soil samples that I have been working with to arrive to the perfect sample …if that is possible.

Sample 1 - has turned out to not be very favorable.
Samples 2&3 - I got from a local brick factory, upon their recommendation do a 60/40 mix and start from there. Have made several test all with good results. This is 60% one soil and 40% the other?
I made a brick for the factory which has new up to date equipment from Italy (impressive). With the 60/40 mix, no sand, no cement, to submit to their curing process. After several days in the dryer and a couple of hours under extreme heat, of which they were very impressed with the final product. We put it to the drop test and it broke. By the way they make a high quality industrial hollow brick and roof tiles with the 60/40 mix.
So I also made some test bricks and cut them in half to have more pieces to work with:
Sample 1- 90% dirt mix, 10% cement
Sample 2- 80% dirt mix, 10% river sand, 10% cement
Sample 3- 70% dirt mix, 20% river sand, 10% cement
Sample 4- 60% dirt mix, 30% river sand, 10% cement These sound like reasonable and interesting mixes.
·Curing process- next day after, 1 day in water, week kept wet covered in shade. Do you have a tarp so you can do solar curing? You do get some sun up in los cielos - no? For solar curing a week is enough for us. I think your temperatures are cooler and you should probably have a few more days. For shade curing I would expect you to cure for at least 21 days.
All were better bricks and stronger than the factory brick that was toasted in the oven. They didn´t break on the first drop (half bricks).
Sample 1 & 2 had about ½ cm of shrinkage, the others held their size. A 1/2 cm of shrinkage of a block is far too much. This shows you have far too much clay.
I was having problems getting them out of the mold without sticking to the bottom plate, even brushing water and diesel. This is another indication of too high of clay content.  So I made some more test that are curing at the moment. It look like the silica and water content was high and decided to make a dryer and more sandier mix.
Mix 1- 15% gravel, 40% river sand, 35% dirt mix, 10% cement
Mix 2- 15% gravel, 35% river sand, 40% dirt mix, 10% cement
Mix 3- 10% gravel, 30% river sand, 50% dirt mix, 10% cement
Mix 4- 10% gravel, 30% river sand, 50% dirt (60% red number 2, 40% number 1) 10% cement Why the gravel? How is your pulverizer working?
The reason I made a test with the number 1 dirt is I found a piece of brick that I threw away because it
broke coming out of the press and if looked impressive.
All of this to ask your advise, if you could look at the lab test and what I have done and come up with a ultimate mix.!?¿¿?. I would really appreciate tour imput on this matter. Steve
Steve, I want to think about this some more. Right now I would suggest trying these mixes:
soil sand
Try making without cement to see how they:
- mix
- press
- eject
- color
- texture
When you have a ratio you like, then try different cement ratios such as:
OK, I've got to go. Please write again.

Submitted by geoffrey on Sun, 16/09/2012 - 06:55



Dear Steve,
I'm starting to think about your work. In any new endeavor it is normal to have some problems. Normal. We can overcome the problems with knowledge, diligence and prayer. Hang in there.
I'm putting some comments in your post in blue.  

Submitted by sbclark on Sun, 16/09/2012 - 06:58

In reply to by geoffrey


Hello Geoffrey, Every step seems to present their own set of hurdles to overcome. I think I´ve finally found a good mix with the soil and other elements locally. The 1:4 mix seems to work the best with 10% cement (1 soil, 4 sand, 10% cement) and 21 days to cure. I had at one time one of your work sheets to help calculate costs, but have misplaced it. What I need to know is how many full size bricks can be produced per 1 square meter of mixed product? Steve

Dear Steve,
Your diligence and good thinking are admirable. I'm happy to hear you have a mix that you like and are confident with. The soil you have chosen apparently has a high clay content. So, you are using a high sand content mix with a very economical cement content. I believe you'd like to know how many blocks you can produce per a unit volume of sand, soil and unit weight of cement. In the appendix of one of my books there is a method for calculating the cost of a block. I have developed a new calculation method that I like better but the old one will also work. In any case, you need to start with your mix sheet which gives you amounts (weights and volumes) of materials in your mix. You also need to know the weight of your block; the charge weight is preferred (the weight of material that you put into the block press). The cured weight of a block might be slightly different.
The basic calculation method is: find the quantity of blocks that one mix can produce. Divide the amount of each individual material in the mix by that quantity of blocks. You'll have the amount of material per block. Then take your purchasing unit for each material and divide it by the amount of that material in one block. Your units must be consistent. For example, if one block weighs 8 kg and your total mix weight is 48.6 kg then you get 6.1 blocks per mix. If the mix contains 4.5 kg cement then the amount of cement in one block is 4.5kg / 6.1blocks. So, there is 0.738 kg cement/block If your cement comes in a 50 kg bag then take 50kg / bag divide by 0.738kg/block and you'll see that you can get 67.8 blocks per bag of cement. For the sand and soil you can calculate in the same manner only with units of volume (FYI, there are 1000 litres in a cubic meter).