How to validate dynamic radio button in ionic 3

I have a series of questions which have radio answer choices. I can’t figure out how to use angular validation to require the user to select all answer before clicking save & next.

NOTE: Please note that click save & next button gets the next 5 question from the controller depending on what choice was made. It’s basically a dynamic questionnaire. Below is my code

.<ion-content id="testmodeCont" >
 <div class="midCont mrgn0" >
   <form #f="ngForm" class="login">
       <div class="formCont">
         <div *ngIf = "questionCategory == 'Diabetes'">
           <div class="boxInCont" *ngFor="let question of AllQuestions | slice:numberOfQuestionsToDisplay:numberOfQuestionsToleft; let i = index">
            <div *ngIf="mLanguageFlag === 'English'" >
              <div class="heading" *ngIf="question.QuestionType=='dropdown'||question.QuestionType=='radio'||question.QuestionType=='toggle'|| question.QuestionType=='multiselect' ">
                Q.&nbsp;{{question.QuestionText}}
                </div>
            </div>

          <div *ngIf="mLanguageFlag === 'English'">
            <div class="row mrgnB20"  *ngIf="question.QuestionType=='radio'">
             <div>
               <div class="quetxt">                       
                <ion-list radio-group  [ngModelOptions]="{standalone: true}" [(ngModel)]="AllGroupAnswers[i].ExpectedAnswerId" >
                                     <ion-item  class="opteach" *ngFor="let answer of question.ExpectedAnswers">
                  <ion-label class="radioBox fr">{{answer.Answer}}
                  </ion-label>
                <ion-radio item-left  [value]="answer.ExpectedAnswerId" (ionSelect)="ValueChanged(answer)">
                  </ion-radio>
                 </ion-item>
                </ion-list>                    
               </div>
             </div>
            </div>
          </div>

          <div *ngIf="mLanguageFlag === 'English'">
                 <ion-item class="row" *ngIf="question.QuestionType=='dropdown'">
                   <ion-select formControlName="option" okText="Okay" cancelText="Nah" placeholder= "{{'personal.select'|translate}}" interface = "alert">
                     <ion-option *ngFor="let answer of question.ExpectedAnswers" [value]="answer.Answer" (ionSelect)="ValueChanged(answer)">{{answer.Answer}}
                 </ion-option>
                   </ion-select>
                 </ion-item>
             </div>
        </div>
      </div>
    </div> 
    </form>                 
  </div>
  <div class ="testbtn">
    <ion-grid>
      <ion-row>
       <ion-col align-self-center>
        <button ion-button  block color="button-color" class="prevBtn" type="submit" (click) = "previous()" *ngIf="previousState" >{{"Previous" | translate}}</button>
       </ion-col>
       <ion-col align-self-end>
        <button ion-button block color="button-color"  class="saveBtn" type="submit" (click) = "next()" *ngIf="isDataLoad" >{{"Save & Next" | translate}}</button>
       </ion-col>
      </ion-row>
    </ion-grid>
      </div>

Please help me to sort out the problem.