Experimental Study On Early Shrinkage And Cracking Of Sea Sand Mortar

With the sustained and rapid development of China's construction industry,in response to the growing shortage of river sand resources,the use of sea sand has been gradually increasing in recent years because of our vast sea area and rich sea sand resources.The large amount of chloride ions contained in sea sand have a great influence on the hydration of cement in concrete,thus affecting the mechanical properties and volume stability of concrete.However,there is no systematic research on early shrinkage and cracking of sea sand concrete.The mechanical properties,shrinkage,crack resistance and microscopic pore structure of the sea sand mortar were determined in this thesis.It will provide certain theory for wide use of sea sand.In this thesis,compressive and flexural strength of sea-sand mortar under different water-cement ratio,chloride ion concentration and mineral admixture were studied.The results showed that when the mineral admixture was not blended,flexural and compressive strength at 3d increased with the increase of chloride ion concentration,flexural and compressive strength at 28 d decreased with the increase of chloride ion concentration;After adding mineral admixture,flexural and compressive strength at 3d decreased with the increase of mineral admixture dosage,and the flexural and compressive strength increased slightly at 28 d.Based on different water-cement ratio,chloride ion concentration and mineral admixture,the autogenous shrinkage and drying shrinkage of sea sand mortar were studied.When there was no mineral admixture and the water-cement ratio was 0.26 or 0.34,the results showed that the autogenous shrinkage and the drying shrinkage increased with the increase of chlorine ion concentration.However,when the water-cement ratio was 0.42,the drying shrinkage decreased.When the chloride ion concentration was 0.12%,with increasing water-cement ratio,autogenous shrinkage decreased and drying shrinkage increased.When the chloride ion concentration was 0.24% or 0.36% and the water-cement ration was 0.34,the drying shrinkage was largest in all the groups.At a fixed chloride ion concentration,with decreasing water-cement ratio,autogenous shrinkage increased.After slag content was added,autogenous shrinkage incereased and drying shrinkage decesased with the increase of slag content.After adding slag and fly ash together,with the content of mineral admixture increasing,autogenous shrinkage and drying shrinkage decreased at the same time.The cracking test was conducted by comparing the normal mortar and the mortar containing mineral admixture,which have the largest and smallest ratio of the total shrinkage.The results showed that the mortar with the highest total shrinkage had shortest cracking time and largest crack width,while the mortar with the least total shrinkage had longest cracking time and smallest crack width.Based on studying the mechanical properties and volume stability of mortar,the pore structure of the mortar was charactered using the mercury intrusion porosimetry.For a fixed water-cement ratio,with increasing chloride ion concentration,the characteristic parameters of the pores decreased and the pore structure was refined.After mineral admixtures were added,the proportion of harmless pores in the mortar increased,the proportion of harmful pores decreased,The pore structure of the mortar blended with 30% slag and 30% fly ash was the best.