Preparation And Performance Study Of Hydrophobically Modified Cementitious Materials By Internal Doping Method

With the increase of various damage problems caused by the environmental complexity of concrete so that its service life is far from the expected,concrete durability issues are getting more and more attention.Traditional methods to improve durability by reducing the watercement ratio,etc.have limited effect,and hydrophobic modification of concrete can also improve the durability of concrete.The basic principle of hydrophobic modification is that by covering the concrete surface particles with hydrophobic substances,thus forming a barrier that repels water,it can largely reduce the possibility of concrete being attacked by water,salt and chemicals,thus extending the service life of concrete.In addition,with the depletion of natural sand and gravel resources,the increase in ecological protection requirements and the continuous increase in demand for construction projects,the use of manufactured sand instead of natural sand for concrete is an inevitable trend in the future development of concrete applications,and the research on manufactured sand concrete not only responds to the national policy,but also meets the future development needs.Therefore,this paper investigates the effects of stearic acid content,water-cement ratio and cement-sand ratio on the wettability,mechanical properties and durability of mortar by using stearic acid as hydrophobic material,ethanol and tuff powder as dispersant and carrier of stearic acid,respectively,and tuff mechanism sand as fine aggregate,and investigates the effects of stearic acid on cement hydration as well as pore structure by means of microscopic analysis.Based on the low cost and low toxicity of stearic acid,this paper uses stearic acid as a hydrophobic material.Since stearic acid is insoluble in water,ethanol and tuff stone powder were used as dispersant and carrier of stearic acid respectively,and tuff mechanism sand was used as fine aggregate.In this paper,the effects of stearic acid content,water-cement ratio,cement-sand ratio,stone powder addition ratio and addition method on mortar wettability(contact angle and water absorption rate),mechanical properties(compressive strength and flexural strength),durability(chloride ion penetration resistance and frost resistance)of the mortar were investigated,and the effects of stearic acid on the hydration and pore structure of the cement were investigated by microscopic analysis to support the analysis.The main conclusions from the study are as follows:(1)The XRD analysis showed that stearic acid does not affect the hydration products of cement.SEM results show that stearic acid affects cement hydration.The pore structure analysis shows that stearic acid increases the porosity of mortar,but the modified stone powder formed by mixing and grinding with stone powder can reduce the porosity of mortar.(2)Stearic acid can be grafted into the mortar by reacting with the hydroxyl groups in the mortar,and its coverage on the pore surface will form a waterproof lining and increase the contact angle,thus weakening the capillary effect.This has a significant effect on the impermeability of the mortar,and the improved impermeability is beneficial to the durability of the cementitious material.(3)In the method of stearic acid-ethanol mixed system,when the water-cement ratio was0.4,the addition of stearic acid was 0.9% of cement mass,and the cement-sand ratio was 1:1,the mortar flux decreased by 56.1% compared with the control group,the frost resistance level was improved from F40 to F90,and the water absorption rate decreased by about 41.6%.All of which were the maximum values among the test groups of this method,which indicated that the cement-sand ratio had a significant effect on the action efficiency of stearic acid.(4)Based on the gray correlation analysis,the degree of influence of each method on the durability of mortar was considered,and the water absorption rate and compressive strength were used as references.It was found that when the water-cement ratio was 0.5,adding the modified stone powder directly into the mortar had a better effect on the durability,and the compressive strength of mortar was higher than that of the control group.