RHA acts as a pozzolan, so you need to have a certain amount of cement in the mixture which hydrates to produce calcium hydroxide. As RHA is almost pure silica, the optimum amount might be close to what has been used for silica fume - close to the 20% by mass of cement you are using now. Moreover, since RHA absrorbs water from the mix due to its fine pore structure, the optimum amount may also depend on the rehology of the mixture and on how much superplasticizer you want to add.

Hi Arunachalam,

I have not heard of 100% of cement with rice husk ash so far. In my humble opinion, I dont think its possible without a binder. Cement acts as a binder giving out CH that reacts favorably with the ash.

Unless you have an activating agent, it believe it is not possible. On a side not, I read in literature that the optimum replacement is 20-25%.

thank u sir @samuel chuah @ P.L.Ng @ Gaochuang cai  for u r suggestions! i will work on it!

Dear sir it is observed that The optimum strength is obtained at the level of 5 % of OPC replaced by RHA for M20 grade concrete. As the rice husk ash is a waste material, it reduces the cost of construction.As the replacement of cement by RHA in concrete increases, the workability of concrete decreases.

http://www.ijste.org/articles/IJSTEV2I11190.pdf

From the moment that there is an optimum by substituting the cement by RHA, generally the substitution rate is between 15 and 25%. On this, one can not completely substitute the cement by the RHA. In other hand,  RHA acts as a pozzolan, consequently, you need to have a certain amount of cement in the mixture to produce calcium hydroxide.

Hello,

it is not possible.

This is not possible since it is only a pozzolanic material, it needs the Cement to contribute to the properties of the Concrete or Mortar, or you can obtain to activate it with alkaline solutions = Alkali Cements, Activated Cements ...

Recently I made an edit 15 % sustitution of cement .

if we replace 100% cement by RHA i think we will get diffuculty to applied it, i have been apllied to used RHA as additive material for concrete and 5% RHA give optimum result

Dear: Arunachalam Ryder

I think it doesn't work. Actually, RHA is using as the substitution material in cement, whether reduce the cement content or as the optimization technique. 

The important thing is you should find the optimum percentage of the RHA  substitution in cement.

Thank you

Depends on material's finenesss and the burning conditions (temperture vs time)..but 20% is sufficient enough

Dear: Arunachalam Ryder

As per my idea you can use RHA by replacement of 10%,20%,30%(max)  of cement.

RHA can't not be completely substituted for cement in concrete. Adding RHA to mix helps to improve the characteristics property of the concrete.

Thanks.

hi, i think that the RHA's silica should find some cement's calcium hydroxide to react with.

RHA acts as a pozzolan, so you need to have a certain amount of cement in the mixture which hydrates to produce calcium hydroxide. As RHA is almost pure silica, the optimum amount might be close to what has been used for silica fume - close to the 20% by mass of cement you are using now. Moreover, since RHA absrorbs water from the mix due to its fine pore structure, the optimum amount may also depend on the rehology of the mixture and on how much superplasticizer you want to add.

cements works as the substrate for strength of concrete while rice husk role is that of nutrient. even if replaced 100% the strength of concrete will be reduced plus the gain of strength of concrete will be so slow that it will not be suitable for practical use. so if you can come up with any solution to meet these two shortcomings than only it will be feasible to use 100% Rice Husk ash in place of cement. one of the viable option can be rice husk ash with addition of lime instead of cement and you may have your results.

RHA may be used as partial substitute of cement, its +ve impact may stay up to 30% but not to go for 100%. 

Hi Arunachalam,

Without activation it seems unlikely to me! As you know most of pozzolans achieve their binding properties after being activated. This activation may occur in the presence of Ca(OH)2 or such alkali conditions. Geopolymer concrete is a great example of what you are looking for. In geopolymers indeed 100% of cement is replaced by pozzolans like RHA, but usage of chemical activator is inevitable.

Dear Ryder,

RHA only contains silica that doesn't have any binding capacity alone. It only reacts with the hydration product Ca(OH)2  to form secondary CSH gel. If you want to replace 100% cement, then you need to consider the source of Ca(OH)2  in the system. 

Following 2 articles could be useful for you to understand the effect of RHA  as a pozzolanic material:

https://www.researchgate.net/publication/236619188_Pozzolanic_Contribution_of_Ricechusk_Ash_in_Cementitious_System

https://www.researchgate.net/publication/309384156_Physical_and_chemical_contributions_of_Rice_Husk_Ash_on_the_properties_of_mortar

Article Pozzolanic Contribution of Ricechusk Ash in Cementitious System

Article Physical and chemical contributions of Rice Husk Ash on the ...

You may also consider following article for the alternative source of  Ca(OH)2 :

http://www.sciencedirect.com/science/article/pii/S0261306912007261

As I know RHA does not have any cementing properties, it has more than 90 percent silica content, but very poor in cementing properties therefore when RHA replace 100% cement than some admixture must be added to it for enhancing its cementing property.