The effect of calcium hydroxide on the mechanical and microstructural characteristics of geopolymer produced from alkaline activation of aluminosilicate wastes produced from demolition works were investigated. Demolition wastes are coarse aggregates, including waste concrete and demolished walls with cementing binder, both passing a sieve of 90 µm. A trial was carried out by making a mix containing demolished walls/waste concrete in the ratio of $60/40$ % by weight and other mixes having a hydrated lime content of 0–25% of the dry weight as a partial replacement for demolished walls. Half of the waste concrete was substituted with metakaolin, produced by firing kaolin at 800°C for 3 h. The alkaline activators used were sodium hydroxide and sodium silicate in the ratio of $3:3\%$ by weight of the dry weight. Curing of the geopolymer mixes was performed at ambient temperature (23°C) in tap water and under a mild temperature of 40°C [100% relative humidity (RH)]. Drying treatment at 80°C for 24 h was carried out on each specimen mix before subjecting it to compressive strength so as to improve the mechanical properties, and the resulting properties of dried and saturated samples were compared. Addition of the calcium compound improved the mechanical and microstructural properties of the specimens with the increase of hydrated lime up to 10 % by weight for mix specimens that were water-cured under ambient temperature, whereas the properties slightly decreased for those specimens cured in 100% RH at 40°C.

The influence of silica fume (SF) addition on properties of geopolymer materials produced from alkaline activation of alumino-silicates metakaolin and waste concrete produced from demolition works has been studied through the measurement of compressive strength, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy (SEM) analysis. Alumino-silicate materials are coarse aggregate included waste concrete and fired kaolin (metakaolin) at 800°C for 3 h, both passing a sieve of 90 μm. Mix specimens containing silica fume were prepared at water/binder ratios in a range of 0.30 under water curing. The used activators are an equal mix of sodium hydroxide and silicate in the ratio of 3:3 wt.%. The control geopolymer mix is composed of metakaolin and waste concrete in an equal mix (50:50, wt.%). Waste concrete was partially replaced by silica fume by 1 to 10 wt.%. The results indicated that compressive strengths of geopolymer mixes incorporating SF increased up to 7% substitution and then decreased up to 10% but still higher than that of the control mix. Results indicated that compressive strengths of geopolymer mixes incorporating SF increases up to 7% substitution and then decreases up to 10% but still higher than the control mix, where 7% SF-digested calcium hydroxide (CH) crystals, decreased the orientation of CH crystals, reduced the crystal size of CH gathered at the interface, and improved the interface more effectively.

Influence of Nano-SiO₂ (NS) addition on properties of Geopolymer materials has been studied through measurement of physicomechanical and morphological characteristics of the hardened specimens. Alumino–silicate materials are water-cooled slag, albite, kaolin, and metakaoline. Materials were prepared at water/binder ratios in a range of 0.244:0.320 for water-cooled slag based materials, while it increased to 0.46 for albite-based mixes, whereas the used activator sodium hydroxide is 10 wt.%. The control geopolymer mix has been composed of water-cooled slag, metakaoline, and kaolin in the ratio of (4:2:1). Albite used for comparison with slag to demonstrates the possibility of producing lightweight geopolymer binder. Nano-silica was added in the range from 0:3% from the total weight with 0.5% increment. Results indicated that, compressive strengths of geopolymer mixes incorporating NS were obviously higher than those of control one, specially at early ages as well as up on using 2.5% NS with the lowest percentage of water absorption; also uses of albite instead of slag results in formation of light binder with promising mechanical characteristics.

Nowadays kaolin raw material is usually used to produce nano-kaolin for geopolymer enhancement by using firing method. In the present study, kaolin used was taken from the Naqus Formation (Cambro-Ordovician age), west of Gabal El Gunna, Sinai, Egypt. Nano-kaolin material is an ultrafine material and was prepared from the taken kaolin by the firing process at 800 °C for 2 h with a heating rate of 5 °C/min. Six mixes were prepared and their laboratory specimens were made and cured up to 90 days. Water cooled slag was used as starting material, and sodium hydroxide and sodium silicate were used in the study as activators for the used kaolin. The formed geopolymer mixes with different ratios (1%, 1.5%, 3%, 5%, and 7%) of nano-kaolin as a partial replacement for the raw kaolin were investigated. Gelenium Ace super plasticizer was added in the ratio of 4% from the dry weight to ensure good dispersing of the used nano clay. Results showed that increasing the percentage of nano-kaolin up to 3% results in an enhancement in the mechanical properties as compared with the control mix up to 90 days of curing, while higher ratios are not preferable where they lead to agglomeration of the added nano materials and matrix dilution.

This paper presents a study on thermal stability as well as acid resistance of geopolymer materials prepared from ground granulated blast furnace slag (GGBFS), air-cooled slag (ACS), silica fume (SF) and cement kiln dust (CKD) using 6% of equal mix from alkaline sodium hydroxide and sodium silicate activators. The addition of ACS, SF and CKD as partially replaced GGBFS is investigated so as to improve the mechanical and microstructural properties of the geopolymer mix. Compressive strength tests and scanning electron microscopy were utilised in these studies. Materials were prepared using water/binder of 0·30 at 38°C and 100% relative humidity (RH). Results showed that geopolymer materials prepared using alkali-activated slag exhibit large changes in compressive strength with increasing fired temperature from 300 to 1000°C and possess an enhancement in thermal stability as compared to concrete specimens. Replacement of GGBFS by 15% ACS resists thermal deterioration up to 1000°C. It was suggested to be suitably applied in refractory insulation applications as well as for production of concrete nuclear reactors. On the other hand, geopolymer mixes exhibit low stability when subjected to different concentrations of a mix of nitric and hydrochloric acid in equal parts (1:1). Current studies of geopolymer microstructure are focused on morphology as well as its relationship with compositions and mechanical properties.