Pore solution composition and alkali diffusion in inorganic polymer cement

Extraction of pore solutions from hardened inorganic polymer cement (“geopolymer”) paste samples shows that the pore network of these materials is rich in alkali cations and has pH > 13, with a relatively low dissolved Si concentration. However, there is little soluble Ca available in these materials to play a buffering role similar to Ca(OH)2 or high-Ca C–S–H in hydrated Portland cements, meaning that preventing alkali loss is essential in ensuring the protection of reinforcing steel. It has been seen previously that calcium in an inorganic polymer cement binder is important in the formation of a low-permeability pore system; alkali diffusion measurements confirm these observations and highlight the role of Ca in reducing effective alkali diffusion coefficients by up to an order of magnitude. This is crucial for the durability of inorganic polymer concretes containing steel reinforcement, as it appears that the use of calcium-containing raw materials will be highly preferable.