Dear Zainab,

shrinkage in Geopolymer materials is lower than Portland cement based ones because :

1. Geopolymer technology isn’t promoted by water like for Portland cement , so there aren’t hydrated minerals with crystalline water inside. Water amount is slower in Geopolymer systems. Extra water, that isn’t already in the silicate but could be added as rheology modifier, like plastificant must be accounted with care, because this is free so can promote locally little shrinkage phenomena (fortunately almost ever just superficial and with only little aesthetic issue) .

2. Silicates called reagent or hardeners are water based because water is the best and the cheapest solvent so alkaline salts can be distributed better to form full reacted geopolymer binder when mixed with precursor that could be fly ash (only type F) or metakaolin or blend of each with blast furnace slag (never alone, it hasn’t the correct mineralogy). This water inside silicate is charged of salts, isn’t free water so will evaporate slowly when geopolymerization process happened. Of course changing precursor will changes also silicate demand that depends on size of particles, mineralogy, shape of particles and amounts. So mix design of GP concrete can be managed to obtain very low shrinkage without any additives or fibers.

3. If evaporation rate is regular, the superficial porosity of material allows that no defects can be visible. There isn’t strong change of volume and density between fresh material and after condensation so hardening. While for Portland cement you must use admixtures (expansive or SRA) that reduce volume changing due to hydration process of cement.

Best ,

Alex Reggiani

The major difference is the setting chemistry.

For Portland cements, the set cement forms a semi-crystalline structure which is held together with a lot of water. There is chemical shrinkage which occurs as the hydrated cement occupies less space than the components of the cement.

As well, the amount of water held in the concrete depends on the humidity of the surrounding environment. If the environment is drier than the concrete, then shrinkage will will occur (this is the normal case). If the environment is wetter than the concrete, such as when the concrete is submerged, the concrete will expand.

Geopolymers condense as a polymer, and are not significantly affected by humidity.

You could look at the diagram in this wiki article https://en.wikipedia.org/wiki/Geopolymer_cement.

Zainab A.Mohammed geopolymer or pozzolan concrete has a perfect microstructure than normal concrete due to the mixture containing geopolymer or pozzolan will cause :

- Reduce the number of Ca(OH)2 crystals

- Acting as a filler due to its particle size (especially in case of fly ash, nano-silica (NS), or rice husk ash )so it can fill the spaces between particles of C–S–H gel.

- Increased resistance to chemical attack (including chloride permeability)

All previous parameters will make the concrete denser due to perfect particle packing which causes shrinkage less than normal concrete.

Dear, I have observed high shrinkages for geopolymeric concretes based on metakaolin, cured at room temperature: Article Self-compacting geopolymer mixture: Dosing based on statisti...

There may be some cases where geopolymer products exhibit low shrinkages.

However, the literature tells otherwise.

Please check this publication:

Li, Zhaoheng, Wei Zhang, Ruilan Wang, Fangzhu Chen, Xichun Jia, and Peitong Cong. "Effects of reactive MgO on the reaction process of geopolymer." Materials 12, no. 3 (2019): 526.

Thank you.