Add a new attribute columns.command.materialIconClass and render it e.g. as <i class="material-icons">contact_mail</i> 

Example: 

var grid = $("#grid").kendoGrid({
                        dataSource: {
                           pageSize: 20,
                           data: createRandomData(50)
                        },
                        pageable: true,
                        height: 550,
                        columns: [
                            { field: "FirstName", title: "First Name", width: "140px" },
                            { field: "LastName", title: "Last Name", width: "140px" },
                            { field: "Title" },
                            { command: { materialIconClass: "contact_mail", text: "View Details", click: showDetails }, title: " ", width: "180px" }]
                    }).data("kendoGrid");