Solar Concrete

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Concept

OSE's interest is local cement production. Currently, the cement industry is highly concentrated, and only 34 of the 50 states have cement production [1]. OSE envisions increasingly localized and ecological production that does not contribute to greenhouse gas emissions. Whereas cement releases large amounts of CO2 into the atmosphere, lime will reabsorb the CO2 created during its manufacture to make it essentially carbon neutral. [2]


Since we are far from mainstream industry taking the lead in solar concrete, OSE intends to lead an industry transformation where abundant solar electric energy is used instead of fossil fuels. With modern technology, it is feasible to distribute cement production.

Use PV to make cement from limestone. PV heats limestone, emits CO2 and produces cement. Cement is used in construction, and it absorbs C02 to make rock-hard structure. Overall process is carbon neutral, and thus solves the issue of vast CO2 emissions from industry standard cement production. OSE calls for distributed production as such, to be deployed in resilient communities worldwide at low cost.

Assumptions and Test Hypothesis

Limestone is hard - anywhere from 2000 - 14000 PSI [3]. When burned, it turns to lime. When water is added, and lime reacts back with carbon dioxide - a rock of the same original strength is formed.

This link [4] shows that 100% lime is only 750 PSI strong. That is still acceptable for a wide range of construction purposes. The disadvantage is a week [5] as opposed to a day curing time for sufficient cure to resist the elements. This means that construction schedules must do concrete work and then possibly wait.

OSE wants to test whether it is feasible to convert limestone into building material - concrete - and whether such concrete is structurally sound and easy to make. How strong is it? What is its practical range of applications? How can we make the quicklime [6] return to as much strength as it turns back into limestone? And, what else can we add to make it stronger or faster to set?

At stake here we have the reinvention of the cement industry - to local production of lime concrete [7] anywhere that limestone can be found.

Disadvantages

  1. Hydraulic lime usually gains strength in time greater than the time in which cement concrete gains same value of strength.
  2. Lime cement takes a long time to cure, and while the ancient world had lots of time, today time is money.
  3. Lime cement does not harden in water but stays soft. So there are situations where it cannot be used.


Uses and precautions

  1. Lime concrete is very widely used for foundation bases of load bearing walls, columns, and under layers of floors.
  2. Due to its flexibility it adjusts very well with the underneath base ground and upper construction of cement base.
  3. For better quality of lime concrete it is important to compact & cure concrete properly. Lime causes rashes on human skin so the persons which are dealing lime concrete should be provided with suitable rubber gloves.
  4. Persons should use oil on their skin to avoid rashes and cracking of their skin due to reaction of lime.
  5. To achieve good quality lime concrete, certain admixtures, fibers etc can be used

Numbers

  • Embodied energy of portland cement - 5.6 MJ/kg. Note that concrete blocks are about 1.5 MJ/kG
    • But for lime - it is 3.3 MJ/kg.
  • 1 MJ = 0.3 kWhr (1 kWhr = 3.6 MJ)
  • Thus, a 10 kW solar panel array, affordable by any University Solar Concrete Project - pr donated for conscience - has 60kW hrs of energy per day
  • Concrete block is only 10-15% cement
  • 1 kWhr gets us .6 kg of cement
  • 60 kWhr gets us 36 kg of portland cement or 61 kg of lime cement
  • 8x8x16" cinder blocks [8] are 30 lb or 15 kg
  • Concrete block - [9]
  • 1:3:6 cement for concrete blocks - [10]
  • We can make 36 kg of cement per day with a 10 kW array, or 360 kg of block - or 24 cinder block per day. Or 72 full sized CEB Blocks.
    • Standard concrete blocks are $1.40 each - [11]
    • The value of electricity for 60kWhr is $6. The value of cement is $9 or 50% more. Value of block is $34 or 600% more.
  • The key here is using excess electricity from inexpensive PV, which otherwise may be left unused.
  • Negotiating power sales to the grid can be attractive pending ability to disconnect from grid if company terms are not favorable, so there is a strong case for the negotiating power of small producers. But this depends on the ability of producers to have storage, for night time use, if disconnected from the grid.
  • $25*300= $7500 - a good value per year for an experimental club in school which stockpiles cement or special use block such as pervious pavement blocks.
  • Notes - calculations above are for portland cement, so lime cement should be 30-40% less energy intensive?

Links

  • Energy to calcine limestone - 755 Mcal per ton = 0.755 Mcal / kg. [12]
  • 1 Mcal = 4.18 MJ [13]
  • Thus, 1 kg of lime takes 3.3 MJ/kg
  • 40 lb bag is 0.01 cu yd [14]
  • Thus - one day of lime cement gives 0.03 cu yd of cement - or 0.3 cu yd of concrete. So 3 days of solar cement for a Microhouse foundation which is 6" tall and 14 feet on each side - which is about a cubic yard.