The following papers include a number of randomized trials in which calcium supplements increased the risk of cardiovascular disease events, including heart attacks:
Bolland MJ, Grey A, Avenell A, Gamble GD, Reid IR. Calcium supplements with or without vitamin D and risk of cardiovascular events: reanalysis of the Women’s Health Initiative limited access dataset and meta-analysis. BMJ. 2011 Apr 19;342:d2040.
Bolland MJ ,Barber PA ,Doughty RN ,Mason B ,Horne A ,Ames R ,et al. Vascular events in healthy older women receiving calcium supplementation: randomised controlled trial. BMJ 2008;336:262
Li S, Na L, Li Y, Gong L, Yuan F, Niu Y, Zhao Y, Sun C. Long-term calcium supplementation may have adverse effects on serum cholesterol and carotid intima-media thickness in postmenopausal women: a double-blind, randomized, placebo-controlled trial. Am J Clin Nutr. 2013 Nov;98(5):1353-9.
Bolland M.J., Avenell A., Baron J.A. et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. BMJ 2010; 341:c3691.