Ambient temperature drying shrinkage in metakaolin‐based geopolymer pastes exposed to low relative humidity environments has been investigated. The effect of varying the geopolymer composition (water content, Si : Al ratio, Na : Al ratio, and Na ⁺ or K ⁺ cations) on the sensitivity to ambient temperature drying shrinkage is reported. The definition of “structural” water as being the minimum water content required that prevents contractions in the gel structure, and thus drying shrinkage from occurring, is introduced. From the results presented, it is clear that the ionic charge density of cations, the total quantity of cations, and the relative quantities and stabilities of cation: AlO ₄ ⁻ pairs in the paste are major factors affecting the sensitivity of pastes to ambient temperature drying shrinkage.

The recovery of phosphorus from sewage sludge ash samples obtained from 7 operating sludge incinerators in the UK using a sulfuric acid washing procedure to produce a technical grade phosphoric acid product has been investigated. The influences of reaction time, sulfuric acid concentration, liquid to solid ratio and source of ISSA on P recovery have been examined. The optimised conditions were the minimum stoichiometric acid requirement, a reaction time of 120 min and a liquid to solid ratio of 20. Under these conditions, average recoveries of between 72% and 91% of total phosphorus were obtained. Product filtrate was purified by passing through a cation exchange column, concentrated to 80% H(3)PO(4) and compared with technical grade H(3)PO(4) specifications. The economics of phosphate recovery by this method are briefly discussed.