Formulation development of metakaolin geopolymer with good workability for strength improvement and shrinkage reduction

This study aims to optimize formulation and preparation of metakaolin based geopolymer for good workability, high compressive strength, and reduced drying shrinkage by considering alkaline activator composition, liquid to solid mass ratio, curing condition, and admixtures. The optimized geopolymer paste was further modified by granulated blast furnace steel slag (GBFS) and ordinary Portland cement (OPC) as calcium sources and carbon fibers as reinforcing additive for further strength improvement and shrinkage alleviation, in which the improvement mechanisms were manifested through mechanical strengths and microstructure observations. An optimized combination of modifiers and curing condition was obtained by Orthogonal Array Testing method. Experimental results indicate that sodium silicate is essential in the synthesis of alkali activator with nominal composition of Na2O·1.3SiO2·10.8H2O for strength development and cracking prevention. Good workability and high compressive strength of paste specimens could be attained by liquid to solid mass ratio of 1.21 between alkaline activator and metakaolin with ambient curing. The modified geopolymer pastes presented further improved compressive strength and reduced drying shrinkage. The microstructure images showed that the formation of cement hydration products, ettringite crystals, and more condensed matrix structure by using OPC as additive in geopolymer. The optimized geopolymer paste was made with 2% carbon fiber, 33% Portland cement and 10% GBFS and cured at ambient temperature achieved ultimate compressive strength of 73 MPa with reduced drying shrinkage. This ultimate compressive strength was 55% and 65% higher than non-modified geopolymer and ordinary Portland cement pastes, respectively, and was higher than 80% values in related studies.