Study On A New Water-repellent And Preventing Cracks Admixture

The calcined kaolin, aluminium powders and aluminum sulfate were used as components of water-repellent agent, which effects on cement hydration and concrete impermeability were discussed in this paper. Their effects on the chemical shrinkage, self-shrinkage, hole structure of cement stone and on the shrinkage of mortar and concrete were also studied thoroughly, and a concrete admixture with high performance of crack-resistant and water-repellent was prepared.The research results revealed that activated alumina content of calcined kaolin of kaolin had a strong correlation with the calcined temperatures and time. The activated alumina content of calcined kaolin increased with the calcined temperatures increasing from 750 C to 950 C, and increased with the calcined time prolonging at 750 C, 800 C, 850 C, 900 C and 950 C. However, the elevated speed of the activated alumina content decreased at 950 C for 4h. The activated alumina content had a reduce with time prolonging from 2h to 5h at calcined temperature of 1000 C. The research results also revealed that products obtained by sudden cooling from high temperature had a higher content of activated alumina than that by cooling naturally. Considering energy conservation and effects of calcined kaolin on the strength of cement, it was optimum method to produce calcined kaolin by calcining it at 980 C for 0.5h and then cooling it suddenly. The analysis results of reaction between calcined kaolin and saturation Ca(OH)2 solution and XRD results of reaction products of calcined kaolin and cement indicated that calcined kaolin has a high volcanic ash active. The experiment was done that the cured cement stone with addition of calcined kaolin were dipped into NaCl solution of 2.5wt%, and the experiment results showed that addition of calcined kaolin made cement stone pore fine and cement mortar displayed high property of impermeability. However, the addition of calcined kaolin to cement caused setting time of cement to prolong and water requirement of cement to reduce.The addition of calcined kaolin to cement increased the chemical shrinkage of cement stone, but it had no distinct impact on the self-shrinkage of cement stone. The addition of aluminum sulfate caused the chemical shrinkage of cement mortar to reduce a little, but induced net plasma self-shrinkage to grow into expansion. That reason was that ettringite was produced by reaction between aluminum sulfate and hydration products. The addition of aluminium powders induced cement mortar to produce a mount of pore, the existing of pore could block path of through pore, so increased cement mortar impermeability. The experiments results revealed that appropriate addition amount of aluminium powders in cement mortar was 0.004-0.005 wt%.The relation between addition content of calcined kaolin, aluminium powders and aluminum sulfate in concrete and compressive strength, slump constant, 30min slump and permeability altitude were studied, and according to the experiment results, the component percent contents of crack-resistant and water-repellent admixture in cement were determined as following: calcined kaolin 6%, aluminum sulfate 4% and aluminium powders 0.005%. The experiment, results revealed that crack-resistant and water-repellent concrete prepared on those proportion had a good restricted expansion coefficient and high impermeability.According to standard JC474-1999 "water-repellent admixture for mortar and concrete", properties of the admixture were measured and they all satisfied with the specified standard of first quality product.