Firstly, depending on which LCI source (IMPACT or eTool default) your project is using, the equipment options will vary.
For the default dataset, we generally use ‘Concrete Pump’ for concrete mixer trucks. With the IMPACT dataset, you can use concrete pump OR ‘large electrical equipment (eg. mixer)’ although that sounds more like those floor mixers rather than a whole truck.
Please refer to this support post for the comprehensive list of equipment available in eToolLCD: https://support.etoollcd.com/index.php/knowledgebase/people-equipment-list/