Dry mixing of soil cement

[Andrew Alcorn]

 Johe Saunders  wrote:

>I thought the subject of soil cement in this forum 
>might spark some thoughts amoung those who are 
>interested in stabilizing a mixture of earth which
>may need "help".
>
>Of course dry mixing Portland with earth requires
>work and a machine is in order.
>
>If I were to try soil cement I would use my morter
>mixer, the soil would have to be sifted first and be
>of low moisture content.  I guess I would use a 10%
>by volume proportion for starters.

In NZ cement is the most common stabiliser used in earth construction. It is
used in rammed earth, pressed block, and 'adobe' where a stabiliser is
necessary. Rammed earth and pressed block require a 'dry' mix. As you
surmise the soil needs to be seived. A standars mortar mixer will work,
although the speed needs to be reduced and the angle of the drum reduced.
Sometimes chains are added to stop 'balling' of the mix. Cement is mixed
with the moist soil, and water added last.

With a wet mix, say for paving or adobe, water is put in the mixer first,
then cement, then the soil.

With a good soil it may be possible to omit a stabiliser. An even mix of
sand/silt/clay is the best to aim for. 

In all cases shrinkage, compression and modulus of rupture (bending) tests
need to be done on the soil you have. This should be done on the
unstabilised soil first before experimenting with the minimum amount of
stabiliser to achieve the desired result. 7% is a common ratio for general
wall construction. No hard and fast amounts can be stated though because of
the variability of soils and requirements. It all depends.


Andrew Alcorn 
Designer/Consultant, Heliotrope Design/Research,
Secretary, Earth Building Association of New Zealand,
Energy Researcher, Centre for Building Performance Research, School of
Architecture, Victoria University of Wellington.

PO Box 600, Wellington,New Zealand, 6002
Tel:  + 64 4 802 6233,          + 64 4 3838 439,        Fax: + 64 4 802 6204