Bentonite Modification And Its Effect On The Permeability Of Cement-Based Materials

Nowadays,many kinds of waterproof technologies were applied in engineering and it can be mainly divided into internal waterproofing technology and external waterproofing technology according to different site construction methods.However,the waterproofing agent used in the internal waterproofing technology generally contains Na⁺,Cl^-,SO₄^2-etc.,which are harmful to concrete.Meanwhile,Organic polymer materials were usually used as the main waterproof material in the external waterproofing technology,and there are problems of aging and poor adhesion to the substrate.Therefore,how to further improve the performance of waterproof materials have aroused the attention of a much researchers.In recent years,bentonite has been used in waterproof engineering due to its water swelling properties.On the basis of the above,we proposed to modify bentonite to prepare magnesium-based bentonite.The microstructure and impermeability of cement-based materials which incorporating bentonite were investigated to provide new materials and technical support for concrete waterproof technology.Firstly,the effects of variety bentonite raw materials,heating temperature,different modifier types and concentration on the modification of bentonite were studied.The results showd that the higher the temperature,the higher the degree of magnesiumation when modified with calcium-based bentonite.However,the magnesium-based bentonite could be obtained under normal temperature conditions when the sodium-based bentonite was used in modification.In general,the molar concentration of Mg²⁺in the solution at 0.5 mol/L can satisfy the demand for magnesiumation,and more magnesium-based bentonite can be formed when magnesium sulfate was used as a modifier.The calcium-based bentonite and the magnesium-based bentonite had large ionic attraction,which can adsorb more water molecules,make the micro-layered structure denser and the water absorption capacity be weaker.However,the ionic charge attraction between the bentonite layers was low,the number of adsorbed water molecules was less,the microscopic layered structure was loose and the water absorption capacity was strong for sodium bentonite.Secondly,the effects of calcium bentonite,sodium bentonite and modified magnesium bentonite on the workability,mechanical properties and impermeability of mortar were studied.Mechanism of bentonite improving the impermeability of mortar was investigated by the use of SEM/EDS and MIP analysis.The workability of the fresh mortar was reduced due to the water absorption of bentonite,and the effect of sodium bentonite was most obvious.The surface of Na-bentonite generated more hydration products to reduce the porosity of the mortar,and the needle-like hydration products formed in the edge could be overlapped with the substrate,which all provided positive effect on the mechanical strength.Ca-bentonite had a weaker influence on mechanical strength because less hydration products were found on the surface of Ca-bentonite.The strength of mortar with Mg-bentonite was close to Ca-bentonite in the early age,however,the mortar strength would increase dramatically due to the Mg?OH?₂crystals that were formed by the exchange of Mg²⁺in the pore solution.The incorporation of bentonite can significantly improve the impermeability of the mortar.Mortar with Mg-bentonite had the best excellent waterproof performance,the improvement of Na-bentonite was lower,and the Ca-bentonite was lowest.The maximum impermeability pressure could reach 2.37 times,and the lowest impermeability pressure still could reach 2.02 times contrast to the reference.By comparing the influence of quartz particles of the same particle size on the macroscopic and microscopic properties of mortar,we concluded waterproofing mechanism of bentonite in cement-based materials as the following shown.On the one hand,the particle filling effect,water absorption characteristics and reaction with cement of bentonite could refine pore structure and decrease porosity,and those will contribute to the increase in impermeability.On the other hand,when the external water penetrated into the interior of the mortar,the bentonite in the pore would swell again,and sealed the pores to inhibit the penetration of water,which was the important reason for that the mortar incorporated into bentonite could behave the excellent waterproof performance.In addition,modified Mg-bentonite not only had the waterproof effect due to swelling,but also the schistose Mg?OH?₂ crystals formed near the bentonite due to ion exchange can also increase the tortuosity of the water permeation path.Therefore,Mg-bentonite can make mortar have better waterproof performance.Finally,the effects of different types of bentonite on the mechanical strength,impermeability and freeze-thaw resistance of concrete were considered.We concluded that bentonite could also enhance the compressive strength of concrete.The improvement of magnesium-based bentonite is stronger than sodium-based bentonite,and calcium-based bentonite has the least effect.The enhancement of the impermeability of concrete reinforced with bentonite was also excellent,mainly shown as the increase of impermeability pressure and the decrease of penetration depth.When 8%Mg-bentonite,Na-bentonite,Ca-bentonite were added,the impermeability pressure of concrete were increased by 266.7%,200%,and 100%compared to the reference concrete,respectively.In a frozen environment,bentonite would lose its ability to swell,so bentonite had less improvement in freeze-thaw resistance of concrete and may even had adverse effects when a large amount of Na-bentonite was incorporated into the concrete.The modified Mg-bentonite had a better enhancement on the freeze-thaw resistance of concrete.In summary,bentonite can make cement-based materials achieve excellent waterproof performance through its influence on the microstructure of cement hydration products and its swelling characteristics.There is widely distribution of bentonite resources,the cement-based material can achieve self-waterproof by incorporating bentonite in low-cost,and meanwhile,the harmful ions would not be introduced.The research can provide a new idea for concrete waterproofing technology.