Effect Of Calcium Carbide Residue On The Sulfate Resistance Of Fly Ash-metakaolin Based Geopolymer

Durability of geopolymers plays a vital role in the service life of structure.Sulfate erosion is one of the key factors that cause the durability of concrete structures to fail.As a new type of green building material,the performance and degradation mechanism of geopolymers in sulfate environment are not fully studied,especially for bridges,tunnels,foundations and slope protection and other types of buildings that are partially buried in corrosive sea water and salt lakes.In this case,the situation is more complicated.The purpose of this research is to realize the utilization of industrial solid waste,using fly ash as the main raw material to prepare geopolymers to explore its deterioration mechanism in sulfate environments,which has important engineering significance.Fly ash,metakaolin,and calcium carbide residue were used as cementing materials,and a mixture of liquid water glass and Na OH liquid was used as an activator to prepare fly ash-based geopolymers.First,the influence of materials ratio and curing conditions on the properties of geopolymers were investigated.Subsequently,the damage process of mortars semi-immersed in sulfate solution was investigated,and the influence of mineral admixture（calcium carbide residue）and the nature of erosion solution on the damage degree of semi-immersed mortars were explored.Finally,the deterioration process of mortars fully immersed in sulfate solution under the conditions of temperature and humidity changes was studied,and was compared with the semi-immersed mortars.The main conclusions are as follows:Molar ratios of oxides are the key factors affecting the compressive strength of geopolymers.The best oxide molar ratio in this study was:n（Si O₂）/n（Al₂O₃）=1.97,n（Ca O）/n（Si O₂）=0.4,and n（Na₂O）/n（H₂O）=0.18.The compressive strengths obtained at 3 d,7 d,28 d,and 150 d were 41.2,63.7,74.2 and 90.0 MPa,respectively.Curing conditions are also an important factor affecting the compressive strength of geopolymers.Increasing the curing temperature can significantly increase the early strength of geopolymers,but the later strength will shrink.The content of calcium carbide residue（CCR）and the nature of corrosion solution have a great influence on the damage degree of mortars.For mortas semi-immersed in Na₂SO₄ and Mg SO₄ solutions,the immersed part was only subject to sulfate chemical attack,while the water evaporation area above the liquid level was simultaneously subject to sulfate chemical and physical attack.The 10%CCR-admixed mortars showed the best sulfate corrosion resistance.However,excessive CCR（>20%）will reduce the sulfate resistance of the mortar.Compared with the Mg SO₄ solution,the mortar semi-immersed in the Na₂SO₄ solution corroded faster because the mortar semi-immersed in the Na₂SO₄ solution were more severely corroded by salt crystals.The damage degrees of the three sulfate environmental conditions to the mortars are:cycle soaking under temperature and humidity change>cycle soaking under temperature change>natural soaking.When the mortar was fully immersed in the sulfate solution,the sulfate ion was transmitted from the high-concentration solution to the interior of mortars mainly through diffusion.The main erosion mechanism of the mortars was sulfate chemical attack.On the contrary,for the semi-immersed mortar in sulfate solution,the sulfate ions were transported through capillary adsorption.The semi-immersed mortars were not only attacked by chemical sulfate,but also attacked by physical sulfate.