Dan
“While it shares some properties of other mineral admixtures, the key to RHA’s effectiveness is its particle surface. RHA’s average particle size is comparable to that of fly ash, and RHA’s silica content and degree of reactivity are comparable to silica fume’s.
“Silica in pozzolans reacts with portland cement paste’s weak and easily soluble calcium hydroxide to form stronger-bonding calcium silicate hydrates. In hardened concrete, more calcium silicate hydrates mean less capillary porosity and less permeability. This pozzolanic reaction also reduces the presence of soluble, alkaline calcium hydrates, which can leach out and weaken concrete.
“RHA used in concrete has an effect similar to other pozzolans. Research indicates a 10% replacement of the weight of cement by RHA produces higher compressive, tensile, and flexural strengths than ordinary concrete but slightly lower strengths than silica fume/cement concrete. “ ftp://66.151.101.167/woc/J970499.PDF
http://lejpt.academicdirect.org/A08/09_16.htm
Characteristics of Wood ASH/OPC Concrete
“Wood ash in this study is an admixture: a pozzolana. A pozzolana is a material rich in silica and alumina which in itself has little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties [3]. Wood ash is obtained from the combustion of wood. It can be related to fly ash since fly ash is obtained from coal, which is a fossilized wood [4]. Rice husk ash is also of plant origin. This implies that wood ash could be used as a pozzolana in concrete. Tarun, Rudolph and Rafat [5] reported the following elements in wood ash: carbon (5% to 30%), calcium (5% to 30%), carbon (7% to 33%), potassium (3% to 4%), magnesium (1% to 2%), phosphorus (0.3% to 1.4%) and sodium (0.2% to 0.5%). The following compound composition limits were also reported: SiO2 (4% to 60%), Al2O3 (5% to 20%), Fe2 O3 (10% to 90%), CaO (2% to 37%), MgO (0.7% to 5%), TiO2 (0% to 1.5%), K2O (0.4% to 14%), SO3 (0.1% to 15%), LOI (0.1% to 33%), moisture content (0.1% to 22%), and available alkalis (0.4% to 20%). The study revealed that all the major compounds present in wood ash are present in fly ash.”
Re: What is it that shrinks in papercrete?
Posted by: "Wayne" huon@intas.net.au casaway67
Fri Feb 25, 2011 5:28 pm (PST)
Seems a great idea Dan.
Can any sort of ash be used or does it have to be rice hull? Rice is not
grown here but I have access to other organic wastes such as trees.
Wayne
> Check out the following links, for example.
>
>
>
> It seems that rice hull *ash* is usually used as an additive to
> concrete, similar to and possibly better than pozzolan.
>
>
>
> Perhaps you could use both rice hulls and rice hull ash in your mix,
> getting twice the bang for your buck.
>
>
>
> Dan
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