Unlike Portland cement, water is not incorporated directly into the geopolymer gel product. Only a small percentage of the mixing water remains as interstitial water in the geopolymer gel. This means there is a large excess of unbound or free water, which can evaporate from the hardened paste under low relative humidity conditions at ambient temperature. Despite the lack of chemically bound water, it still plays an important role in structural stability and if excess water is not removed properly, extensive shrinkage cracking of specimens can occur.
The water lost before drying shrinkage occurs is arbitrarily referred to as “free water”. Since this water is not chemically bound, it can simply be removed from the open macro-pore network without creating any capillary pressure in the micropore network. However, removing more than the free water provokes shrinkage. This water has been defined as "structural water" in the literature.
The question I want to ask is, "If free water does not play an active role in drying shrinkage if drying shrinkage occurs with the removal of structural water from the structure, can these conditions be achieved at room temperature? That is, can the chemically bound water be removed from the geopolymer at room temperature?"
1. No Chemical bound water cannot be removed at room temperature.
2. To stop hydration process or to remove chemical bound water.
3. The samples should be dip to isopropanol or acteone solution then oven dry at 50 to 60 degree Celsius for 24 hours or microwave oven methods has also been used.
4. This method ensures that water is not available for chemical reaction.
Dear Muralidhar Kamath ,
Thank you very much for taking the time to answer my question. But water is not chemically bound to its geopolymer structure, is it? It is said in the literature that there is no drying shrinkage with free water removal. If the structural water does not go away at room temperature, then what is the reason for drying shrinkage?
Dear Cansu,
geopolymerization isn’t hydraulic process as you correctly wrote, so there isn’t crystalline water inside, after condensation and complete evaporation. Liquid reagent is water based but this water isn’t free, but charged on Sodium or potassium salts. You can verify checking fire resistant testing. If some hydrated minerals were in geopolymer matrix, after 300°C, this water becoming gas should exit from material destroying the surface. This is spallation effect well known for ordinary Portland mortars and concretes. Instead Geopolymer materials can resist more than 1200°C and working on the GP binder also more than 1500°C.
So to answer at your question if at room temperature you have so high shrinkage could depend on several reasons:
- the free water ,usually is called extra water and added to the GP mortar or concrete to increase workability. If it’s too much you will have shrinkage even at room temperature , if you don’t protect the surface (just covering with plastic film)
- wrong dosage of liquid reagent (sodium or potassium based silicates for alkaline Geopolymers or the right acid used for the Geopolymers obtained in acidic medium ) that doesn’t balance Aluminum silicate amorphous used as precursor. Too much silicate bring inside too much salts and water too, so shrinkage and efflorescence are promoted.
- if your material is AAM (Alkali Activated Materials) because AAM aren’t Geopolymers ,but still hydrated materials, without polymer like organization. Water participates to hydration and due to not organized reaction can evaporate faster, promoting shrinkage specially at room temperature.
- when aggregate curve is not properly made , maybe too fine and lightweight , during condensation, can’t stabilize the system.
From my experience that follows Prof. Davidovits rules to create stable genuine Geopolymer systems, you could solve the issue about shrinkage, checking your raw materials in terms on : mineralogy, reactivity , size and chemistry, balancing with good quality of aggregate , selected at best granulometry (there are minerals more suitable than others of course).
Have a nice work
Best
Alex ReggianI
Hi
I think that the injected polymers can mainly apply viscose force to the residing fluids similar to a drag force which its physical nature is considerably weaker than the chemical bonds. Thus, No.
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