| dc.description.abstract | Calcium sulfoaluminate cement (CSA) is a special type of cement mainly composed of calcium sulfoaluminate (also known as ye′elimite, abbreviated as C4A3S̅), dicalcium silicate (also known as belite, abbreviated as C2S), and tetracalcium aluminoferrite (C4AF). It is characterized by rapid early strength development, short setting time, good fluidity, and low drying shrinkage. Due to the high cost of the materials, research has proposed using CSA in combination with Portland I type cement (OPC) to maintain features such as high early strength and fast setting while improving economic feasibility.
This research is divided into three stages: ＂Preliminary Study of the Engineering Properties of the OPC-CSA System,＂ ＂Study on the Volume Stability and Sulfate Resistance of OPC-CSA System Cement Mortar,＂ and ＂Investigation of Strategies to Improve the Engineering Properties of the OPC-CSA System.＂ The first stage examines the effect of OPC/CSA ratios (1/9, 2/8, 3/7, 5/5, 7/3, 8/2, and 9/1) on workability and mechanical properties. The second stage investigates the impact of OPC/CSA ratios (same as the first stage) on volume stability and sulfate resistance, including short-term sulfate resistance tests for rapid analysis of deterioration phenomena. The third stage uses promising OPC-CSA systems (based on results from the first two stages) and applies different improvement strategies (adding silica fume, reducing water-to-cement ratio, and high-temperature curing) to enhance engineering properties, ultimately proposing an appropriate mix design for the OPC-CSA system and establishing the relationship between non-destructive testing methods (rebound hammer, ultrasonic, and maturity methods) and strength development.
The results indicate that the OPC-CSA system exhibits the fastest hydration development with an OPC ratio of 20% to 50%, achieving a strength of over 12 MPa in 1.5 hours and nearly 60 MPa in 28 days, demonstrating Very Rapid Hardening (VRH) performance. Among these, C30 (30% OPC) shows the best strength development, reaching 16 MPa in 1.5 hours and 62.9 MPa in 28 days. In terms of fresh properties, when the OPC ratio is 20% to 50%, the final setting time is within 10 minutes, and the flow is below 40%, not meeting ASTM C1600 specifications. Among these, C30 has the worst fresh properties, with a final setting time of only 5 minutes and mortar flow of 0%. For drying shrinkage, when the OPC ratio is below 50%, CSA can inhibit OPC shrinkage, maintaining the overall drying shrinkage below 0.02%, with C30 showing the lowest shrinkage of only 0.015%. When the OPC ratio is above 50%, the overall drying shrinkage significantly changes with the OPC ratio, reaching 0.047% to 0.064% in 28 days, with C70 showing the most significant shrinkage at 0.060%. Regarding sulfate resistance, when the OPC ratio is 10% to 30%, the system shows good sulfate resistance, with expansion maintained below 0.030% after 168 days of immersion in sulfate solution, and no significant appearance changes. Among these, C30 has the best sulfate resistance, with an expansion of only 0.025%. When the OPC ratio is 50% to 90%, the system exhibits high expansion characteristics, with expansion reaching 0.049% to 0.101% after 168 days of immersion, with C50 showing the worst sulfate resistance at 0.101%.
Based on the results of the first two stages, C30 (high strength and good durability), C50, C70 (high strength but poor durability), and C80 (low strength but minor durability issues) were selected for improvement strategy research using silica fume (SF) to replace OPC, reducing the water-to-cement ratio, and high-temperature curing. Using SF to replace OPC for durability improvement, the best effect is achieved when SF replaces 30% of the OPC volume. After improvement, C50SF30, C70SF30, and C80SF30 all show a length reduction of 0.01% to 0.03% of the initial length, but the fresh properties deteriorate due to accelerated fluidity and slump loss caused by SF, with zero fluidity 30 minutes after mixing. In terms of enhancing mechanical properties, the combination of reducing the water-to-cement ratio and high-temperature curing showed the best performance, improving early and late strength by 58% to 215% and 21% to 37%, respectively. Next was only changing the water-to-cement ratio, with high-temperature curing being the least effective. After mix improvement, C50SF30 with a water-to-cement ratio of 0.35 and cured at room temperature or for 6 hours at 40℃ meets the ultra-rapid hardening cement engineering application requirements. C50SF30 with a water-to-cement ratio of 0.35 and cured at room temperature reaches a compressive strength of 21.8 MPa in 1.5 hours and 73.9 MPa in 28 days. C50SF30 with a water-to-cement ratio of 0.35 and cured for 6 hours at 40℃ reaches a compressive strength of 28.4 MPa in 1.5 hours and 75.2 MPa in 28 days, showing no expansion and no significant appearance changes in short-term sulfate resistance tests, rated as good. Non-destructive testing using the maturity method, ultrasonic pulse velocity method, and rebound hammer method to evaluate the compressive strength of cement mortar shows R2 values greater than 0.7, indicating good feasibility of non-destructive testing methods.
Compared to the common sulfate resistance test ASTM C1012, which has a test period of up to one year, this study proposes a short-term sulfate resistance test. By evaluating length changes and appearance morphology, similar results to the ASTM C1012 six-month test can be obtained in just 56 days. Based on length changes and appearance morphology, durability ratings are defined as ＂good,＂ ＂poor,＂ and ＂very poor.＂ When the durability rating is ＂poor＂ or ＂very poor,＂ it indicates poor sulfate resistance of the mix, and this test is suitable for OPC-CSA systems with OPC ratios of 50% to 90%. | en_US |