Performance optimization and mechanism research of C20 coal gangue concrete based on response surface and water resistance

The use of coal gangue as aggregates to prepare concrete is an effective way to promote the sustainable development of building materials and coal industries. This study first explores the influence of water–binder ratios, cement dosage, and sand–aggregate ratios on the compressive strength of full coal gangue aggregates through response surface methodology. Then, the influence of water–binder ratio and superplasticizer synergy on the slump, compressive strength, compression characteristics, and water resistance of full coal gangue concrete was explored and its degradation mechanism was revealed. Finally, the influence of substitution rate on slump, compressive strength, hydration products, and microstructure under different water–binder ratios was explored by replacing natural stones with coal gangue coarse aggregates. The results showed that the order of influencing factors on the compressive strength of full coal gangue aggregate concrete is water–binder ratio > cement dosage > sand–aggregate ratio. Taking the maximum compressive strength as the optimization objective, the optimal mix ratio was obtained as follows: cement dosage of 450 kg/m3, water–binder ratio of 0.7, sand–aggregate ratio of 33%, and compressive strength of 8.6 MPa after 28 days of sealed curing, with an error of less than 5% compared to the predicted value; Full coal gangue aggregate concrete is limited by its own compressive strength and cannot improve its compressive strength by reducing the water–binder ratio. The lower the water–binder ratio, the higher are its water absorption rate, strength loss rate, and mass loss rate. This is mainly because coal gangue contains minerals such as water calcium zeolite and montmorillonite, which have water absorption and expansion properties; using coal gangue coarse aggregate to replace some natural crushed stone can effectively improve the water resistance of concrete. When the water–binder ratio is 0.4 and the coal gangue coarse aggregate substitution rate is 50%, it meets the requirement of compressive strength >20 MPa after 28 days of water immersion in concrete.