Effect of gypsum content on the compressive strength of LC3 cement

ABSTRACT The cement industry has been required to minimize the CO2 emissions caused by the Portland cement manufacturing process. The use of supplementary cementitious materials (MCS's) replacing part of the clinker is one of the most viable options to reduce the emission of this pollutant. LC3 cement is a new type of cement that uses high MCS contents, counting on the synergistic reaction between calcined clay and limestone to obtain mechanical strength similar to that of traditional cements. This research intends to investigate the influence of the gypsum content on the compressive strength of a LC3 cement at 1, 3, 7, 28 and 91 days, considering that the effects of the gypsite content on the mechanical resistance of the LC3 cement are little understood. For the accomplishment of this study six mortars were produced. In three of them the reference cement (clinker + gypsum) was produced with 3.5% gypsum, without additions. In three mortars an LC3 cement with 45% clinker replacement (30% calcined clay and 15% limestone) was used. The proportion of gypsum in these cements was 3, 5 and 7% in relation to the total mass. The properties in the fresh state were evaluated by means of the spreading test, taking into account the superplasticizer content used in each mortar. For the hardened state analysis, the compressive strengths were compared at the ages of 1, 3, 7, 28 and 91 days. A statistical analysis (ANOVA and DUNCAN test) was performed on the results. From the results obtained it was possible to verify that the gypsite content influences the compressive strength of the LC³ cement. In the early ages, from 1 to 7 days, the greatest results of compressive strength were obtained by cements with gypsum contents of 5%. At these three ages the 5% gypsum LC3 cement had comparable results with the reference cements. At 28 and 91 days LC3 cements with 7% gypsum led to the highest values of compressive strength, and the highest value at 91 days was 40.0 MPa, comparable to the reference cements.

摘要 水泥行业已被要求尽可能降低波特兰水泥生产过程产生的二氧化碳排放。采用辅助胶凝材料（supplementary cementitious materials，简称MCS's）替代部分熟料，是降低该类污染物排放的最可行方案之一。LC3水泥是一类新型水泥，其采用高比例辅助胶凝材料，依托煅烧黏土与石灰石之间的协同反应，获得与传统水泥相当的力学强度。本研究旨在探究石膏掺量对LC3水泥在1、3、7、28及91天龄期抗压强度的影响，鉴于目前关于石膏掺量对LC3水泥力学性能的影响机制尚不明晰。为完成本研究，共制备六组砂浆试样。其中三组采用基准水泥（熟料+石膏），其石膏掺量为3.5%，未添加任何辅助胶凝材料。另外三组采用经45%熟料替代的LC3水泥（其中煅烧黏土占30%、石灰石占15%），该组试样中石膏占总质量的比例分别为3%、5%及7%。针对新拌砂浆的性能，通过扩展度试验进行评价，同时兼顾各组砂浆所掺加的高效减水剂用量。对于硬化砂浆的性能分析，对比了各龄期（1、3、7、28及91天）的抗压强度。对试验结果开展了统计分析（方差分析ANOVA与邓肯DUNCAN检验）。试验结果表明，石膏掺量会对LC3水泥的抗压强度产生显著影响。在早期龄期（1至7天），石膏掺量为5%的LC3水泥展现出最优的抗压强度。在该三个龄期内，5%石膏掺量的LC3水泥性能与基准水泥相当。在28天及91天龄期，石膏掺量为7%的LC3水泥抗压强度最高，其中91天龄期的最高抗压强度达40.0 MPa，与基准水泥性能持平。