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Question:

How much cement would I need to make a 13 diameter stone?

13 inch diameter solid concrete stone - yes it will be heavy = approximately 90-95lbs.

Answer:

If you are using only Portland cement and water in the 1:4 ratio for the reaction to complete properly, you will need 80% cement by volume. Very few applications use pure Portland Cement however. V(sphere) = (4/3)(pi)(r^3) r = d/2 = 6.5in V = (4/3)(3.14)(6.5^3) = 1150in^3 1150in^3(0.80) = 920in^3 of Portland cement One sack of Portland cement (94 pounds) is approximately 1 cubic foot which is 1728in^3 920/1728 = 0.53 or about 1/2 sack or 47 pounds of Portland cement. If you are using the traditional 1 : 2 : 3 mix of cement:sand:gravel, you use 1/6 cement, or 47/6 ~ 8 pounds. At 80% due to 1:4 water mix = 8(0.80) = 6.4 pounds. At 133 lbs/ft^3 or 0.077 lb/in^3, 1150in^3 weighs: 1150(0.077) ~ 89 lbs Your answer is then either 47 pounds of pure Portland Cement or 6.4 pounds as part of a 1:2:3 dry mix combined with water at a rate of 1:4 (the latter is the practical answer). 6.4/94 ~ 1/15 sack. Remember that these figures are based on general rules of thumb and do not take into account any particular service requirement. - .--
hoe high is the stone

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